Carbon sequestration for geological negative emissions of the shale gas value chain in China

Domain: neuroscience
Published: 2026-01-26
Editorial outcome: major_revision

Abstract
Carbon sequestration in shale gas operations represents a crucial pathway to achieve Geological
Negative Emissions, which is essential for global 1 .5℃ targets. However, the emissions reduction
potential and economic viability of this approach in China’s shale gas value chain remain unclear.
This study quantifies the potential for transforming China’s shale gas value chain from an emission
source to a carbon sink, while revealing spatial heterogeneity in economic feasibility. Results
demonstrate that synergistic deployment of CO -based technologies can achieve Geological
Negative Emissions across the value chain, with national carbon sink potential reaching 66 billion
tonnes of CO -equivalents and shale gas production increasing by 4,518 billion cubic meters.
Multi-scenario economic assessments reveal that marine shale in the Sichuan Basin exhibits
inherent profitability, whereas continental reservoirs require carbon credit integration or optimized
production. Current Chinese carbon market prices provide insufficient incentives, implementing
region-specific subsidies and enhancing carbon pricing frameworks could unlock this potential,
thereby contributing to national carbon neutrality goals and multiple Sustainable Development
Goals.

Introduction
China’s shale gas sector has grown remarkable, expanding from 0.025 to 25 billion cubic meters
(bcm) between 2012 and 2023 (i.e., a more than 1,000-fold increase)1. This growth reduces China’s
42% natural gas import dependency2 while simultaneously driving technological innovation and
economic growth3,4. Globally, China’s shale gas development holds particular significance as it
provides an alternative model to the predominantly studied United States (U.S.) experience. As
the third country to establish commercial-scale shale gas operations, China offers invaluable
insights into overcoming unique geological, technological, and regulatory challenges in a different
context. Shale gas is actively contributing to China’s transition towards carbon neutrality by
providing a cleaner alternative to coal, with an increasing impact on reducing carbon emissions in
power generation and industrial processes5.

Applying CO -based technologies in shale gas operations can generate substantial carbon sink
effects, contributing significantly to global carbon neutrality goals6,7. However, large-scale
deployment is constrained by formation-specific challenges including economic feasibility
barriers8,9, compounded by risks of CO leakage10 and induced seismicity11. The U.S., as the
world’s largest shale gas producer, has conducted reservoir-specific simulations and laboratory-
scale explorations of carbon sink potential, evaluating techniques such as CO fracturing and
geological sequestration12–15. Nevertheless, existing research predominantly examines isolated
processes rather than implementing full value-chain integration assessments, and systematic
analysis of CO -based technologies’ applicability across diverse shale reservoir types remains
underdeveloped. China, facing more complex geological conditions, is pioneering integrated CO
based technologies including CO fracturing16,17, CO huff-n-puff18, electrification of operations19,
carbon sequestration8,12, and enhanced leak detection programs20,21, demonstrating enhanced
effectiveness in its diverse shale formations. However, systematic assessment of emission-
reduction potential across shale gas value chains under varying geological contexts remains
challenging.

Assessing the carbon footprint of the shale gas value chain requires consideration of emissions
across its entire lifecycle, as greenhouse gas (GHG), predominantly CH and CO , are released
during exploration, production, processing, transportation, and final use22–25. Fugitive methane
emissions, associated gas flaring, energy consumption in compression and processing facilities,
and downstream activities all contribute to the overall climate-forcing potential of shale gas
development. While carbon capture, utilization, and storage technologies have been proven
feasible in the shale gas industry26, the negative carbon potential and economic viability of their
large-scale application remain unclear. Consequently, the net emissions from a complete life-cycle
perspective and the cumulative climate impacts of the complete shale gas value chain are poorly
understood. This knowledge gap hampers the accurate assessment of the industry’s climate
impacts and impedes strategic carbon sequestration planning.

This study analyzes the climate impacts of the entire shale gas value chain from 2012 to 2023
using life cycle assessment (LCA) and climate impact models, identifying key intervention points
and technologies for emission reduction strategies across the entire value chain in the future.
Integrating Discounted Cash Flow (DCF) analysis with LCA, this study assesses the economic
feasibility of multiple emission-reduction scenarios in diverse shale gas reservoirs, with particular
emphasis on carbon sequestration potential and enhanced gas recovery efficiency across CO
based technologies (Fig. 1). The results demonstrate how technological solutions can advance both
environmental and economic objectives in energy transitions. The findings inform decarbonization
strategies that address multiple Sustainable Development Goals (SDGs): energy security through
cleaner fuels (SDG 7), industrial innovation (SDG 9), and climate mitigation through negative
emissions pathways (SDG 13). Given the representativeness of China’s diversified geological
contexts of shale gas reservoirs, the findings of this study can provide scientific foundations for
achieving Geological Negative Emissions of the shale gas value chain in many regions around the
world.

Results
Synergy of carbon sequestration and production enhancement
China’s shale gas sector demonstrates exceptional Geological Negative Emissions potential across
major basins, advancing climate action goals (SDG 13). Unlike passive carbon sinks, shale gas
carbon sequestration provides verifiable active CO removal through geological sequestration,
meeting the strict definition of anthropogenic removals outlined by Allen et al. (2024) for effective
climate mitigation27. Three primary technologies enable carbon sequestration in the shale gas
industry: CO fracturing (before production), CO huff-n-puff (in production), and CO
sequestration (in depleted wells). CO fracturing and CO huff-n-puff serve dual purposes:
enhancing production while sequestering carbon. Based on our modeling analysis, CO fracturing
can achieve a carbon sink of 1,406.5 million tonnes (Mt) while increasing shale gas production by
4,018 bcm. Similarly, our simulations indicate that the CO huff-n-puff technique can create a
610.2 Mt carbon sink with a 499.8 bcm production increase. CO
sequestration in depleted wells
demonstrates the highest potential, reaching a 49,092 Mt carbon sink (Supplementary Fig. 1 and
Fig. 2). The Carbon Neutrality Coefficient (CNC) measures the carbon reduction potential of CO
based technologies compared to conventional shale gas extraction baseline emissions. The baseline
represents current extraction practices without carbon mitigation measures, with emissions varying
by regions depending on geological conditions and extraction parameters. Combined
implementation of these technologies achieves negative carbon intensity of -437.0 g CO e m-3,
with national CNC exceeding 110%. This transformation from emission source to carbon sink
aligns with China’s carbon neutrality goals, with an estimated cumulative carbon sink potential of
66,455.9 Mt CO e – equivalent to 4.8 years of China’s carbon emissions (13,940 Mt CO e in
2022)28 – while increasing shale gas production by 4,518 bcm, approximately China’s total energy
consumption in 2023 (Fig. 2)29. This is attributed to CO fracturing and CO huff-n-puff
technologies enhancing the estimated ultimate recovery (EUR) per well, thereby providing more
space for subsequent CO sequestration.

Regional implementation of these technologies shows distinct effectiveness patterns, with the
Ordos Basin demonstrating the highest potential. Marine-continental transition shales in the Ordos
Basin achieve a CNC of 157.5%, while terrestrial shales reach a CNC of 162.8%. These results
indicate 57.5% and 62.8% reduction beyond carbon neutrality, respectively. These exceptional
results are attributed to the basin's unique geological characteristics and optimal reservoir
conditions. The Sichuan Basin shows particularly favourable conditions due to its superior
reservoir characteristics, higher EUR values, and low carbon intensity of upstream power. The
basin's performance is further enhanced by its well-developed infrastructure and operational
efficiency.

These results highlight the substantial potential for synergy between carbon sequestration and
production enhancement in China's shale gas industry. Regional variations in effectiveness
underscore the importance of geology-specific implementation strategies. The integration of these
three key carbon sequestration technologies with additional systematic emissions reduction
measures can further reduce the carbon footprint of the shale gas value chain. The synergistic
emission reduction benefits will next be quantitatively assessed through multi-scenario simulations.

Historical carbon footprint and multi-scenario Geological Negative Emissions potential
assessment
China’s shale gas value chain generated cumulative emissions of 3.1 Mt CH and 341.5 Mt CO
during 2012–2023, contributing to a maximum global temperature increase of 4.1 × 10-4 K over a
100-year integration period (Fig. 3). Downstream consumption dominates this impact, accounting
for 71.3% of temperature increase, with CO emissions contributing 76.6%. These figures
encompass the full life-cycle emissions spectrum, from initial extraction and processing to
transportation and final consumption. To assess future emission trajectories, we established a
baseline scenario representing business-as-usual operations without additional carbon reduction
measures. This baseline analysis reveals that the national carbon intensity reaches 3,214.3
(3,014.6–3,417.3) gCO e m-3, projecting total value chain emissions of 42,525.1 (39,883.4–
45,210.4) Mt CO e under business-as-usual operations.

Five strategic scenarios were developed and evaluated to reduce the carbon footprint of the shale
gas value chain. The carbon intensity of the shale gas value chain for the relevant scenarios is
shown in Fig. 4. The optimized management (OM) scenario focuses on improved operational
efficiency, better equipment maintenance, and advanced monitoring systems. This approach could
reduce carbon intensity from the current baseline of 3,214.3 to 2,907.3 (2,724.4–3,091.7) g CO e
per cubic meter of gas produced, achieving a reduction of 4,062.2 (427.1–7,749.3) Mt CO e or
roughly 9.6(1.1–17.4)% of the sector’s total emissions. Building on these management
improvements, the optimized production (OP) scenario implements technical innovations and
production optimization methods. It delivers an additional reduction of approximately 1,952.3
(1,787.3–2,199.3) Mt CO e, further decreasing the sector’s emissions by 4.6 (4.1–5.2)%.

The handling depleted wells (HW) scenario demonstrates breakthrough potential for negative
emissions. By injecting CO into depleted wells for carbon sequestration, this strategy transforms
the entire shale gas value chain: each cubic meter of shale gas produced results in a net
sequestration of 300.8 (32.4–556.8) gCO e underground. The impact is substantial, delivering a
reduction of 46,504.7 (43,212.3–48,874.6) Mt CO e and a national CNC of 109.3 (99.0–118.1)%.
This success underscores the potential of well-focused interventions in achieving significant
carbon reductions.

The combined scenario (CS), which integrates all three reduction measures, demonstrates even
more significant potential. This comprehensive approach achieves a net sequestration of 601.1
(281.7–847.4) g CO e per cubic meter of gas produced across the value chain. This indicates that
the shale gas industry becomes a net carbon sink (i.e., Geological Negative Emissions).
The
cumulative national reduction reaches 53,751.6 (4,7539.1–58749.1) Mt CO e, with the national
CNC attaining 118.7 (108.7–126.5)%. These results validate that the strategic integration of
management optimization, production enhancement, and depleted well handling can transform the
shale gas industry from a carbon source to a significant carbon sink.

Economic viability across shale gas reservoirs and scenarios
The economic feasibility of carbon reduction measures exhibits distinct regional patterns based on
geological conditions and operational environment (SDG 8). These basin-specific characteristics
create distinct patterns in investment requirements and potential returns. Marine shale in Sichuan
Basin shows superior economic viability, driven by favorable geological conditions, established
infrastructure, and optimized operational parameters.

Sichuan Basin’s marine shale demonstrates robust economic performance, achieving an Internal
Rate of Return (IRR) of 20.1(13.6–27.1)% and a Unit Net Present Value (UNPV) of US$0.052
(0.036–0.066) m-3 under baseline scenario. This indicates the strong profitability in this region. In
contrast, other basins show negative IRR and UNPV values, reflecting geological constraints. The
optimized management (OM) scenario, while reducing emissions per cubic meter of gas to 2,907.3
(2,724.4–3,091.7) gCO e and achieving a 9.6 (1.1–17.4)% carbon neutrality contribution, yields
minimal economic gains with IRR increases of 0.01–0.03%. This suggests limited financial
benefits from management optimization alone. However, the optimized production (OP) scenario
demonstrates more promising results for continental deposits, with significant IRR increases of
4.5% to 7.4% and UNPV improvements of US$0.042 to 0.056 m-3. Profitability was achieved only
in continental shale formations within the Tarim Basin and Ordos Basin, where favorable
geological conditions and CO source proximity offset the technology’s higher implementation
costs.

Carbon pricing demonstrates transformative effects on project economics. Current market
conditions present significant challenges. The industrial CO prices at US$40.40 t-1 far exceed the
Chinese carbon market price of US$13.23 t-1, limiting technology adoption incentives30. With
Chinese carbon market prices, the profitability remains limited to Sichuan Basin marine shale,
Tarim Basin continental, and Ordos continental blocks. European carbon market prices enable
broader profitability across regions, with IRRs ranging from 0.2% to 29.8% and UNPVs between
US$0.001 to 0.400 m-3 in profitable basins. The Tarim and Ordos Basins continental blocks show
particularly strong potential under European carbon pricing conditions, with IRRs of
approximately 13.4% and 14.2%, respectively. The launch of the global carbon market at COP29
may accelerate China's carbon market internationalization, unlocking emission reduction
potential31.

The combined scenario integrates multiple technologies and management strategies across the
value chain. Without carbon pricing benefits, only the Sichuan Basin marine shale remains
profitable. Chinese carbon market prices enable viability in several continental blocks, particularly
in the Tarim and Ordos Basins (IRRs around 5.8%). European carbon pricing ensures profitability
across all regions (IRRs 4.7%–29.8%). In particular, continental shale projects achieve returns of
9.3% to 14.2% (except for Songliao Basin: 5.8%). Rising carbon market prices in China could
enhance the economic viability of carbon reduction measures. Fig. 5 presents the UNPV under
varying carbon prices for the combined scenario. Supplementary Fig. 3 and 4 present UNPV
analyses for other scenarios and regional variations, respectively.

These financial indicators must be balanced against the substantial climate-related costs observed
historically. During 2012–2023, cumulative climate losses ranged from US$0.55 billion to
US$4.88 billion based on EPA's social costs of methane (SCM) and carbon (SCC). Recent research
indicates that actual costs may be 3.6 times higher than the current SCC values, emphasizing the
need to consider both economic returns and broader societal impacts when evaluating the viability
of carbon reduction measures in shale gas development32.

Methods
Assessment of carbon sink potentials in various shale gas reservoirs
Carbon sinks in shale gas reservoirs are achieved through three technologies: CO fracturing
(before production), CO huff-n-puff (in production), and CO sequestration (in depleted wells).
These technologies integrate throughout the life cycle of the shale gas well. We assess their carbon
sink potential both independently and in combination across diverse shale gas reservoirs. Crucially,
CO fracturing and CO huff-n-puff technologies enhance shale gas production while serving
carbon sink purposes. This is due to the phase changes of CO and acidic chemical reactions that
can enhance fracture networks34,35. Shale’s superior CO adsorption capacity displaces methane
effectively36,37, with particular benefits in water-sensitive formations38.

CO fracturing. During well completion, high-pressure CO is injected to replace water for
fracturing, creating fracture networks that facilitate gas recovery. Approximately 70% of injected
CO returns during the production, while 30% remains sequestered17. The cumulative production
increase percentage (CPIP) varies with geological formations. In water-sensitive continental and
transitional shale gas reservoirs, significant enhancements in shale gas production can be achieved.
In contrast, the production increase effect in non-water-sensitive marine shale gas reservoirs is
generally modest39,40. Based on these, we set CPIP at 60% (55%–65%) for transitional and
continental shale and 20% (18%–22%) for marine shale16. The production increase can be
calculated using equation (1):
∆𝐸 =𝐸𝑈𝑅∙CPIP (1)
𝑖
where EUR (m3) varies across different shale gas reservoirs, with the specific calculation
methodology detailed in Supplementary Note 1.

The carbon sink for a single shale gas well using CO fracturing is given by equation (2):
𝑀 =𝑉 ∙𝑛 ∙𝜌 ∙30% (2)
𝐶𝑂2,𝐹 𝐶𝑂2 𝑠𝑡𝑎𝑔𝑒 𝐶𝑂2,𝐶
where V is the volume of CO required for a single fracturing stage, n is the number of
CO2 stage
fracturing stages, and ρ is the density of CO during transportation and storage.
CO2,C

CO huff-n-puff. During the typical 20-year production cycle, shale gas wells exhibit a
characteristic production decline curve until reservoir depletion occurs. To mitigate this decline
and increase the EUR, the CO huff-n-puff technology can be employed. The CO huff-n-puff
stimulation is executed over a 6-year period (within years 13 to 18 of the well lifecycle), repeated
for 12 cycles. Based on existing experiment and field studies41,42, each cycle comprises one month
CO injection (20 t per day), followed by a one-month soaking, and a four-month production.
Critically, 50% of the injected CO undergoes permanent geological sequestration16. The
CO huff-n-puff yields a CPIP of 3% (2%–4%) for marine shale and 6% (4%–8%) for transitional
and continental shale. The method for calculating production increase is shown in equation (1).

CO sequestration. Injecting CO into depleted shale gas wells achieves significant carbon sinks
while leveraging existing infrastructure. The sequestration mechanisms in shale gas wells include
structural, adsorption, and mineralization sequestration.

Structural sequestration, the primary method, involves CO injection into deep shale formations,
where geological impermeability prevents upward migration of CO . The structural sequestration
capacity is calculated using equation (3):
𝑀 =(EUR+∆𝐸)∙𝑅 ∙𝜌 ∙𝑋 (3)
𝐶𝑂2,𝑆 𝐶𝑂2/𝐶𝐻4 𝐶𝑂2
where ρ denotes the density of CO at standard conditions, 1.98 kg m-3; X indicates the proportion
of free CH in the reservoir, and R refers to the molar ratio of CO to CH at a specific
4 CO2/CH4 2 4
reservoir depth. The calculation method for R is presented in equation (4), where D is the
CO2/CH4
well depth, m.
𝑅 =2×10−7𝐷2−0.015𝐷+4.1707 (4)
𝐶𝑂2/𝐶𝐻4

Adsorption sequestration occurs through competitive adsorption of CO displacing CH , as
calculated using equation (5):
𝑀 =(EUR+∆𝐸)∙𝛼 ∙𝜌 ∙(1−𝑋) (5)
𝐶𝑂2,𝐴 𝐶𝑂2/𝐶𝐻4 𝐶𝑂2
The adsorption selectivity coefficient (α ), which reflects the potential of CO to displace
CO2/CH4 2
CH , depends on the total organic carbon content (TOC) of the reservoir and the clay mineral
4
content (C ). Based on these parameters (see Supplementary Table 1 for specific reservoir data),
clay
it is calculated using equation (6):
𝛼 =0.0153∙TOC∙𝐶 +0.7081∙TOC (6)
𝐶𝑂2/𝐶𝐻4 𝑐𝑙𝑎𝑦

Mineralization sequestration (M ), involving CO reaction with formation minerals, is
CO2,M 2
assumed to be 1.5% of the structural sequestration capacity mine (M ). Therefore, the total
CO2,S
sequestration capacity is given by equation (7):
𝑀 =𝑀 +𝑀 +𝑀 (7)
𝐶𝑂2,𝑇 𝐶𝑂2,𝑆 𝐶𝑂2,𝐴 𝐶𝑂2,𝑀
Critically, the total CO sequestration potential is driven by the technically recoverable shale gas
reserves, rather than the total original gas-in-place. This approach aligns with the net utilization
rate concept, as the pore space associated with unrecoverable gas remains inaccessible for CO
injection and thus does not contribute to the practical storage capacity.

Life cycle assessment of value chain carbon emissions
This study employs life cycle assessment to quantify carbon emissions (including CH and CO )
4 2
across China’s shale gas value chain. The system boundary of the life cycle inventory model
includes upstream production, midstream transportation and distribution, and downstream
consumption (Supplementary Figure 5). The functional unit selected for this study is one cubic
meter of shale gas produced at the well pad over its full life cycle. The carbon emissions stem from
energy inputs, raw materials production, methane leakage, and CO emissions, with the latter
primarily arising from downstream shale gas combustion. The GHG emission intensities for
materials are based on life cycle assessment literature17,43–46, while site data are derived from recent
studies17,47,48. The carbon footprint of the shale gas value chain for different shale gas reservoirs
under different scenarios is assessed by carbon intensity (in g CO e m-3), which is defined by
equation (8):
𝐼 =𝐼 +𝐼 +𝐼 (8)
𝑖,𝑗 𝑃,𝑖,𝑗 𝑇,𝑖,𝑗 𝐶,𝑖,𝑗
where I , I , and I represent the carbon intensity of shale gas production, transportation and
P,i,j T,i,j C,i,j
distribution, and consumption, respectively, across shale gas reservoir i and scenario j. Upstream
production emissions are calculated through detailed bottom-up modeling. The production carbon
intensity I , encompassing five distinct operational phases, is calculated using equation (9):
P,i,j
𝐼 =𝐼 +𝐼 +𝐼 +𝐼 +𝐼 (9)
𝑃,𝑖,𝑗 𝑃1,𝑖,𝑗 𝑃2,𝑖,𝑗 𝑃3,𝑖,𝑗 𝑃4,𝑖,𝑗 𝑃5,𝑖,𝑗
where I , I , I , I and I represent the carbon intensity during site preparation, drilling
P1,i,j P2,i,j P3,i,j P4,i,j P5,i,j
and cementation, well completion, production, and decommissioning phase under shale gas
reservoir i and scenario j, detailed in Supplementary Note 1.

Midstream and downstream emissions employ literature-based approaches due to infrastructure
standardization. Transportation and distribution carbon intensity I is derived from pipeline
T,i,j
network studies specific to China’s shale gas infrastructure49. Carbon intensity of consumption
I
C,i,j
is calculated using sector-weighted emission factors, accounting for end-use distributions
across industry, transportation, and power generation sectors50–52. These values are summarized in
Supplementary Table 14.

Given China’s diverse shale gas distribution across basins (Sichuan, Tarim, and Ordos, among
others) and varying reservoir types (marine, transitional, and continental), the relevant parameters
vary significantly. Taking the marine shale gas in Sichuan Basin as an example, which is currently
the only commercial source in China, this region has an average vertical depth of 3 km for shale
gas wells, with a lifespan of 20 years and a total production of 118.1 million cubic meters per
well53. Each well pad consists of six shale gas wells, with additional parameters for other shale gas
reservoirs detailed in the Supplementary Note 1. Subsequent assessments evaluate the life cycle
carbon footprints of shale gas in different reservoirs under different scenarios to determine
reduction potential of China’s shale gas industry.

Cumulative climate impact assessment of the shale gas value chain
Based on the life cycle assessment results, we develop a GHG emission model (including CH and
4
CO ) for China’s shale gas value chain from 2012 to 2023. Production data comes from the China
2
Statistical Yearbook, with missing city-level consumption data supplemented by natural gas supply
ratios (detailed in Supplementary Table 2). Model uncertainties primarily stem from upstream
wells parameters midstream and downstream emission factors.

To assess cumulative climate impact, we use change in global average temperature rather than
global warming potential as the indicator. This is because the former can better evaluate long-lived
GHG impacts and reveal temporal trajectories. This study only considers the climate effects of
CH and CO and does not include transient chemical species (CO, NO , and VOCs) and
4 2 X
particulate matter. The temperature change is quantified through the convolution of the emission
model and the average global temperature potential (AGTP). As shown in equation (10), the AGTP
represents temperature change from pulse emission, comprising the radiative forcing from the
emission and the temperature response per unit of radiative forcing.
𝑡
AGTP(𝑡)=∫IRF (𝑠)𝑅 (𝑡−𝑠)d𝑠 (10)
𝑖 𝑖 𝑇
0
To reflect the uncertainty in AGTP values, a Monte Carlo simulation approach is used, as detailed
in the Supplementary Note 2.

For climate damage quantification, we use the US EPA’s social costs: social costs of methane
(SCM) ranging from US$400–3,500 t-1 CH , and social cost of carbon (SCC) ranging from
4
US$11–132 t-1 CO 54,55, asdetailed in Supplementary Table 16. Cost ranges reflect discount rate
2
choices, which reflect the relative emphasis on current versus future costs. These estimates may
be conservative, as recent studies have estimated SCC values much higher than the upper bound
used here32.

Assessment of emission reduction potential throughout the shale gas value chain
China holds the world’s largest shale gas resources, with geological reserves exceeding 100×1012
m3 and technically recoverable resources of 10–32×1012 m3, distributed across Sichuan, Ordos,
and Tarim Basins56. Established commercial fields (Changning, Weiyuan, Zhaotong, Fuling)
currently produce 23.8 bcm annually57. Given the potential for large-scale development, we
construct four scenarios to explore GHG emission reduction potential, with a baseline scenario
reflecting current technological standards and operational practices.

(1) Optimized Management (OM) scenario. Shale gas leakage occurs throughout the value chain
- from upstream equipment (tanks, compressors, flares)25 to midstream infrastructure (compressor
stations, pipelines)58–60, and downstream consumer points. Given the similarity in shale gas
production workflows, facility specifications and leakage rate between China and the U.S., we
derive China's methane leakage reduction potential from empirically validated U.S. leak detection
and repair (LDAR) programs. Documented implementations in U.S. shale basins demonstrate 36–
44% leakage reductions20,21. Through technology applicability adjustments accounting for China’s
regulatory maturity and infrastructure gaps, we project a 40% (30–50%) reduction for advanced
LDAR deployment in China, directly generating CO -equivalent emission reductions derived via
the 100-year global warming potential conversion framework defined in IPCC AR6. This
estimation approach follows established methodologies for assessing China's methane mitigation
potential61.

(2) Optimized Production (OP) scenario. CO fracturing and CO huff-n-puff technologies deliver
synergistic benefits through dual mechanisms of geological CO₂ sequestration and production
enhancement, wherein sequestered CO offsets value chain carbon emissions (technical parameters
detailed in preceding sections) 9,62,63. Facility Electrification further contributes to emission
reductions, leveraging the inherent advantage of centrally supplied electricity which exhibits
markedly lower carbon footprints than on-site diesel generators19. Significant regional variations
in electricity carbon intensity across sedimentary basins, driven by heterogeneity in local grid
structures, are documented quantitatively in Supplementary Table 3.

(3) Handled Wells (HW) scenario. Depleted wells offer CO sequestration potential through
structural, adsorption, and mineralization mechanisms. CO
injection into deep formations,
secured by impermeable strata, enables potential net-zero or negative emissions. The total
sequestration capacity of shale gas wells is calculated as shown in equation (7).

(4) Combined strategies scenario. This scenario integrates the above management strategies
through nonlinear synergistic interactions, not merely arithmetic addition. Specifically, CO
fracturing and huff-n-puff enhance well EUR, thereby expanding the total geological carbon
sequestration capacity. However, the finite geological storage capacity creates trade-offs: CO
injected during fracturing and huff-n-puff operations reduces the volume available for
sequestration in depleted wells, requiring optimization to maximize overall carbon reduction
benefits.

The carbon emission reduction (Mt CO e) across different shale gas reservoirs under various
scenarios versus the baseline scenario is calculated using equation (11):
𝛥𝐶 =(𝐶 +𝐶 +𝐶 )−(𝐶 +𝐶 +𝐶 ) (11)
𝑖,𝑗 𝑃,𝑖,𝑏 𝑇,𝑖,𝑏 𝐶,𝑖,𝑏 𝑃,𝑖,𝑗 𝑇,𝑖,𝑗 𝐶,𝑖,𝑗
where C , C , and C represent the carbon emissions of shale gas production, transportation
P,i,b T,i,b C,i,b
and distribution, and consumption under the baseline scenario. Then, the total national carbon
emission reduction across scenarios is calculated using equation (12):
𝑛
𝛥𝐶 =∑𝛥𝐶 (12)
𝑗 𝑖,𝑗
𝑖=1
To assess the carbon emission reduction potential across different shale gas reservoirs under
various scenarios, the carbon neutrality coefficient (CNC) is defined by equation (13):
CNC =(𝐼 −𝐼 )⁄𝐼 (13)
𝑖,𝑗 𝑖,𝑏 𝑖,𝑗 𝑖,𝑏
where I is the carbon intensity of the shale gas life cycle for different shale gas reservoirs under
i,b
the baseline scenario. When the CNC is between 0 and 100%, it indicates that the shale gas value
chain has not achieved carbon neutrality; when the CNC equals 100%, the carbon footprint of the
shale gas value chain is zero; and when the CNC is greater than 100%, the shale gas value chain
can achieve negative emissions. It is important to note that this negative emission status is defined
specifically to the value chain itself, meaning that the amount of CO sequestered within the system
boundary exceeds the direct emissions generated by the chain.

Techno-economic analysis of emission reduction scenarios
The configuration of multiple scenarios delineates technological pathways for decarbonizing the
shale gas value chain. Crucially, systematic quantification of the dynamic interactions between
regional resource endowments and technological costs remains imperative. Through techno-
economic analysis, this methodology reveals regionally heterogeneous economic viability of
emission-reduction technologies. Consequently, it mitigates potential resource misallocation
arising from uniform policy implementation and establishes spatially differentiated cost-optimized
deployment pathways.

A typical shale gas development project encompasses a 21-year lifecycle, comprising one year of
construction followed by 20 years of production. Under the baseline scenario, the cash outflows
during the initial year primarily arise from construction costs, including expenditures for drilling,
hydraulic fracturing, and surface facility engineering. Throughout the production phase, cash
inflows originate from shale gas sales revenue and government subsidies, while outflows consist
of continuously incurred business costs and tax costs.

Relative to the baseline scenario, emission reduction scenarios necessitate distinct cash flow
considerations. The OM scenario employs LiDAR and Optical Gas Imaging technology for LDAR,
incurring an incremental cost of approximately US$0.098 m-3 64. This investment, however,
generates compensatory revenue through recovered methane sales by mitigating fugitive emissions.
Concurrently, the OP scenario implements CO fracturing and CO huff-n-puff during Year 1 and
Years 13–18, while the HW sub-scenario involves CO sequestration in abandoned wells, requiring
expenditures for CO procurement and ancillary energy inputs. The comprehensive CS scenario,
integrating all aforementioned technologies, consequently demands holistic cash flow modeling
that internalizes these interdependent cost-benefit structures. To evaluate economic viability across
different shale gas reservoirs, we conduct policy sensitivity analyses incorporating variable carbon
credit revenues.

The DCF method is a financial valuation approach grounded in the time value of money theory.
Its core logic involves forecasting the free cash flow generated by an asset or project and
discounting it to its present value using a risk-adjusted discount rate, thereby quantifying its
intrinsic value65. Financial profitability metrics such as Net Present Value (NPV), UNPV, IRR, and
Dynamic Payback Period are all established based on this method. In this study, the DCF method
is adopted to evaluate the techno-economic feasibility of multiple emission-reduction scenarios
across the shale gas value chain. The UNPV and IRR are selected as profitability indicators to
assess the techno-economic performance of different scenarios. UNPV, a core metric for evaluating
the profitability per unit of shale gas, is defined as the project's NPV divided by its total production
(Q ). It is calculated as follows in equation (14):
total
UNPV = NPV⁄𝑄 (14)
𝑡𝑜𝑡𝑎𝑙
The NPV is calculated by discounting the annual net cash flows (NCF) within the shale gas value
t
chain using the industry benchmark discount rate (r ) and summing the discounted present values.
0
This metric characterizes the absolute profitability of the shale gas value chain. Its calculation is
given by equation (15):
𝑡=𝑛
NPV = ∑ NCF ∙(1+𝑟 )−𝑡 (15)
𝑡 0
𝑡=0
The NCF is calculated using equation (16):
t
NCF = 𝑄 ∙𝑓∙(𝑃+𝑆 )−𝑇 −𝐼 (16)
𝑡 𝑡 𝑠𝑢𝑏𝑠𝑖𝑑𝑦 S𝑡 𝑡
where Q denotes shale gas production in year t; f represents the proportion of marketable gas
t
output; P is the shale gas wellhead price; S signifies the shale gas development subsidy, a
subsidy
fiscal incentive provided by the Chinese government, quantified at US$0.013 m-3 66. The variable
T
t
represents the tax cost, encompassing value-added tax, income tax, resource tax, and other
applicable levies. I denotes the business cost in year t, where I specifically refers to well pad
t I 1
construction investment, including expenditures for drilling, fracturing, and completion activities.
The computational specifics regarding T and I are comprehensively detailed in the Supplementary
t t
Note 3.

The IRR refers to the discount rate at which a project’s cumulative NPV equals zero, reflecting the
project's investment return rate. The IRR accounts for the time value of money. A higher IRR
indicates better project profitability, making it a key indicator for evaluating project profitability.
Its calculation is given by equation (17):
𝑡=𝑛
∑ 𝑁𝐶𝐹 ∙(1+𝐼𝑅𝑅)−𝑡 = 0 (17)
𝑡
𝑡=0

Uncertainty analysis
This study employs Monte Carlo simulation involving 100,000 iterations to conduct uncertainty
analysis of the negative emissions potential and economic viability across the shale gas value chain.
Random values were generated following normal, triangular, and uniform distributions, with
detailed parameter settings provided in Supplementary Table 19, and the calculation of
uncertainties adheres to the principles and methods outlined in the 2006 IPCC guidelines67.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors' exploration of the potential for geological negative emissions within China's shale gas value chain, specifically the transformation of shale gas extraction into a carbon sink via CO₂ fracturing and sequestration technologies, is an interesting study. They assess the shale gas industry value chain and conduct a detailed economic feasibility analysis using statistical data. However, several points need attention.\nOn page 4，in the section \"Synergy of carbon sequestration and production enhancement\": Line 88, “CO2 fracturing and CO2 huff-n-puff serve dual purposes: enhancing production while sequestering carbon. CO2 fracturing can achieve a carbon sink of 1,406.5 Mt while increasing shale gas production by 4,018 bcm. The CO2 puff-n-huff technique can create a 610.2 Mt carbon sink with a 499.8 bcm production increase.” Missing data sources, add relevant references.\nOn page 5，in the section \"Synergy of carbon sequestration and production enhancement\": Line 101, The Carbon Neutrality Coefficient (CNC) is used to measure the carbon reduction potential in comparison with baseline emissions. What does baseline emissions refer to, and what is the specific amount of emissions?\nOn page 6，in the section \"Value chain assessment for carbon footprint and Geological Negative Emissions \": (1) Lines 131 and 137, the CO2 emission reductions under the Optimized management scenario (OM) and Optimized production scenario are described respectively, but the subsequent calculation schemes only present the leakage reduction of CH4 (OM) and the incremental production of shale gas (OP), and how CO2 emission reductions are derived? (2) Line 148, Is the comprehensive plan simply the sum of the CO2 emission reductions of the other three plans, or is there a different calculation method? If it's a simple sum, will the three methods interfere with or affect each other?\nOn page 9，in the section \"Economic viability across shale gas reservoirs and scenarios \": Line 173, the article mentions that optimizing the production plan can significantly improve economic benefits, but the high costs of CO2 fracturing technology and CO2 injection may not be matched by the increased production of shale gas, potentially affecting economic feasibility. Additionally, CO2 fracturing technology has certain limitations. Can the appropriate regions for implementing the optimized production plan be identified based on the formation conditions?\nOn page 14, in the section \"Life cycle assessment of value chain carbon emissions\": (1) Line 334, The statement \"The carbon emissions stem from energy inputs, raw materials production, methane leakage, and CO2 emissions\" should clarify whether it includes the CO2 emissions from the final combustion of shale gas. (2) Line 339, Regarding the calculation of carbon intensity, it is recommended that the authors provide a detailed explanation of how the proportion coefficients for the three stages (upstream, midstream, and downstream) are established. This would help readers better understand the methodology and assumptions behind the carbon intensity calculations.\nOn page 16, in the section \"Assessment of emission reduction potential throughout the shale gas value chain,\" the authors propose four scenarios for reducing greenhouse gas emissions, two things need to be noted:(1) For the OM scenario, it is suggested that the authors elaborate on the specific process of estimating that methane leakage in China can be reduced by about 40% based on the success of US LDAR program. This would enhance the clarity and credibility of the estimation.(2) In the OP scenario, the authors distinguish the cumulative shale gas production percentage (CPIP) for CO2 fracturing technology under different geological conditions (marine, terrestrial, and transitional). This distinction enhances the credibility of the data and reveals the CO2 sequestration potential in different regions. However, the discussion lacks an exploration of specific challenges that may be encountered during the implementation of these technologies. It is recommended that the authors supplement the discussion with relevant challenges to enhance the study's practicality and reliability.\nOn page 18，in the section \"Techno-economic analysis of emission reduction scenarios\": Lines 421 and 427, formulas 9 and 10 are used to calculate the Internal Rate of Return (IRR) and net present value (NPV). Were these formulas derived through deduction or do they cite previous research? If they were derived, please provide the derivation process; if they cite previous research, please specify the relevant references.", "reviewer_2": "This article is focused on China. At a minimum, the title should include that.\nThe abstract description I read while agreeing to review does not match the content of the manuscript.\nI did read the paper and it is difficult to understand. The authors use many buzz words that I am not familiar with. These could be terms that apply to China.\nThe figure fonts and figure size are very small. When reading the text and referring to the paper, it is hard to understand and interpret the figures. There is also a lot of information in each figure.\nIn the methods section, I don’t under why equation (1) is using 30% sequestered while later the authors use 50% sequestered. And the reference is from 2016, which seems like a time when people were not thinking about storage CO2 in shale gas reservoirs.\nThe authors should also be consistent when using sequestration versus storage through out the text.", "reviewer_3": "The article puts forward a very interesting proposal: transforming shale gas formations themselves into net carbon sinks by applying techniques such as CO₂ hydraulic fracturing, the huff-n-puff method (enhanced recovery with CO₂ injection), and storage in depleted wells. The approach is innovative and is framed within the idea of achieving geological negative emissions—something that could have a transformative impact on the carbon balance of the unconventional gas industry.\nOne of the manuscript’s most striking claims is that the potential for net emissions reduction could reach 1.981 billion tons of CO₂ (1.981 Gt CO₂) between 2024 and 2060, which would represent a spectacular climate benefit if validated. However, this ambition contrasts sharply with the scientific execution of the article, which presents significant shortcomings both in its conceptual framing and in its development. The main issues identified are outlined below in an orderly and reasoned manner.\n1) The introduction lacks solid and specific references to support its key claims. The concept of geological negative emissions is mentioned as emerging, but no prior scientific literature from countries with mature shale gas industries—such as the US or Canada—is properly cited. As a result, the reader lacks the necessary context to situate this work within the current state of knowledge.\n2) Throughout the article, highly ambitious figures for the potential CO₂ sequestration are presented, yet the traceability of those calculations is not clearly or transparently explained. For example, the authors state that a total of 1.98 Gt CO₂ could be stored by 2060 using a combination of technologies, but the assumptions behind this estimate—such as injection coefficients or average recovery factors—are not broken down or justified. This undermines the credibility of the conclusions.\n3) The article fails to address emissions associated with methane, both in terms of fugitive leaks and full lifecycle impacts (e.g., indirect emissions from supply chain activities). This is a critical omission, as recent studies have consistently shown that methane leakage is often the key obstacle to achieving carbon neutrality—or net negativity—in shale gas systems. Here, methane is barely mentioned, and certainly not quantified.\n4) There is no distinct conclusion section or a clear and concise summary of the key findings. The final ideas are scattered across various parts of the discussion, making it difficult for the reader to extract the article’s main contributions, limitations, or policy implications.\n5) The structure of the manuscript is confusing. There is no clear delineation between the methodology, results, and discussion sections. Technical concepts, emissions calculations, and interpretative commentary are blended without smooth transitions. This lack of structure makes it harder to follow the line of reasoning and diminishes the overall impact of the work.\n6) The mathematical methodology is underdeveloped. While a few formulas are included, there are no worked examples or validation cases to demonstrate the robustness of the models used. For instance, the net capture cost (NCC), net present value (NPV), and internal rate of return (IRR) are all mentioned, but not a single complete numerical example is provided using real or assumed data.\n7) The figures are mostly low-resolution maps focused exclusively on China. Although it is logical for the study to focus on that country, it would be beneficial to include at least some international comparison or broader context, given the global relevance of the topic. Moreover, the maps do not provide sufficient technical detail—such as geological characteristics, pressure, depth, or estimated storage capacity.\n8) Internationally comparable economic units are not used. All costs are expressed in yuan per cubic meter, with no conversions to dollars or other widely used benchmarks. This limits the article’s accessibility to international audiences and hinders comparison with other studies.\n9) More comprehensive visual tools are missing. There are no summary tables that compare the different technologies (CO₂ fracturing, huff-n-puff, injection into depleted wells), nor graphs illustrating CO₂ capture over time or economic profitability curves. This weakens the communicative value of the analysis.\n10) Although the manuscript mentions the use of Monte Carlo simulations to assess uncertainty, it does not explain the underlying assumptions, input distributions, or the interpretation of the results. No probabilistic outputs, sensitivity analyses, or uncertainty ranges are shown. This part of the analysis remains underdeveloped and unconvincing.\n11) From an editorial perspective, the manuscript requires a thorough review. There are spelling errors, confusing phrases, and misused terms (e.g., “puff-n-huff” instead of “huff-n-puff”). Furthermore, there is some repetition of ideas that could be avoided with more careful editing.\n12) The article does not critically analyze the risks, limits, or barriers of the proposal. For example, the possibility of CO₂ leaks, infrastructure costs, social acceptance, or regulatory implications are not discussed. The presentation is overly optimistic and lacks a balanced reflection on potential flaws or uncertainties.\n13) The structure of the article urgently needs improvement. There is no hierarchical numbering of sections and subsections that allows the reader to easily orient themselves. The discussion mixes several thematic lines without an easily identifiable order. It is strongly recommended that authors include an outline or \"content map\" at the beginning of the document, specifying the main sections of the manuscript (e.g., methodology → results → economic discussion → climate implications), and that consistent numbering be used throughout the text (e.g., 1, 1.1, 1.2, 2, 2.1, etc.). This formatting improvement would greatly facilitate understanding of the argument and critical reading of the document.\nIn summary, the article starts with a very promising idea and addresses a crucial problem with remarkable ambition. However, it needs a thorough revision to fulfill its promise. It lacks methodological rigor, clarity of presentation, risk analysis, and solid references that situate the work within the existing international framework. If these aspects are corrected, the manuscript could make a valuable contribution to the debate on negative emissions in the shale gas industry. In its current state, however, the most reasonable recommendation is a major revision."}, "author_response": "Dear Reviewers,\nWe sincerely appreciate the thorough and constructive reviews from you. Your\ninsightful comments have been invaluable in strengthening our manuscript. We have\nundertaken substantial revisions to comprehensively address all concerns raised,\nparticularly regarding methodological rigor, clarity of presentation, risk analysis, and\npositioning within the international framework.\nMajor Revisions Overview:\n1. Enhanced Methodological Rigor and Transparency:\nWe have comprehensively revised the Methods section with detailed equations and\nparameter justifications for CO₂ sequestration calculations, added explicit breakdown\nof assumptions behind the 1.98 Gt CO₂ sequestration estimate, clarified the rationale\nfor different sequestration rates (30% for CO₂ fracturing vs. 50% for huff-n-puff),\nincorporated comprehensive uncertainty quantification through Monte Carlo\nsimulations (100,000 iterations), developed transparent calculation frameworks\nshowing how EUR values vary across geological formations, and provided detailed\ntechno-economic modeling with worked examples in supplementary materials to\nensure full reproducibility of our results.\n2. Strengthened Risk Analysis and Economic Assessment:\nThe manuscript now integrates comprehensive risk analysis including technical\nchallenges (CO₂ leakage, induced seismicity), operational barriers (infrastructure\nrequirements, source-sink matching), and market uncertainties. We have expanded\neconomic modeling to include sensitivity analyses under different carbon pricing\nscenarios (Chinese market: $13.23/t CO₂; European market: $78.19/t CO₂),\nquantified how geological heterogeneity affects economic viability across basins,\nand added detailed discussions of implementation challenges specific to marine,\nterrestrial, and transitional shale formations, demonstrating that profitability critically depends on both\ngeological conditions and carbon pricing mechanisms.\n3. Improved Clarity and Structure:\nWe have completely reorganized the manuscript with clear separation between Results\nand Discussion sections, added a visual content map (Figure 1) outlining the three-\nmodule research framework, redesigned all figures with enhanced readability and high-\nresolution graphics, standardized terminology throughout (e.g., \"well pad\" instead of\n\"well site,\" consistent use of \"sequestration\"), converted all economic units to US$ for\ninternational comparability, and created clear subsections in the Discussion addressing\nimplications, barriers, and policy recommendations, ensuring logical flow and easy\nnavigation throughout the manuscript.\n4. Solid International Context and References:\nThe literature review has been substantially expanded to incorporate global\nperspectives, particularly from U.S. and Canadian shale gas industries. We have added\nfoundational references including Lyu et al. (2021) comprehensive review in Energy &\nEnvironmental Science, Edwards et al. (2015) on CO₂ storage capacity in Marcellus\nshale, Tao & Clarens (2013) estimating U.S. shale sequestration potential, and the U.S.\nDOE NETL methodology for standardized assessment. The work is now properly\npositioned within international climate frameworks and CCUS research, with 85\nadditional recent publications incorporated to ensure our China-specific findings are\ncontextualized within global knowledge.\n5. Comprehensive Methane Emissions Analysis:\nResponding to critical feedback about methane accounting, we have explicitly\nintegrated fugitive methane emissions throughout the life cycle assessment, with all\nCH₄ emissions converted to CO₂-equivalent using IPCC AR6 GWP₁₀₀ values. Detailed\nemission factors for upstream, midstream, and downstream activities are provided in\nSupplementary Table 13, methane leakage reduction potential through LDAR programs\nis quantified (40% reduction estimated for China based on U.S. empirical data), and the\ncritical role of methane control in achieving net-negative emissions is emphasized\nthroughout the analysis.\n6. Enhanced Visual Communication and Technical Details:\nAll figures have been recreated at high resolution with improved readability,\ncomprehensive technology comparison tables have been added showing key parameters\nfor CO₂ fracturing, huff-n-puff, and depleted well sequestration, sensitivity analysis\ntornado charts reveal that shale gas price and carbon credit value are primary economic\ndrivers, and uncertainty ranges are visualized for all major results. Maps now include\ntechnical details such as formation depths and geological characteristics requested by\nreviewers, significantly improving the manuscript's ability to communicate complex\ntechnical and economic relationships.\nWe have provided detailed point-by-point responses to all comments in the\naccompanying document, with all changes in the manuscript clearly marked using track\nchanges. Where specific requests could not be fully addressed, we have provided\nthorough explanations and alternative approaches.\nWe believe these substantial revisions have transformed the manuscript into a more\nrigorous, clear, and impactful contribution that advances understanding of carbon\nmanagement opportunities in shale gas systems. The work now provides a solid\nfoundation for evidence-based policy decisions on achieving carbon-negative fossil\nfuel operations, with clearly articulated uncertainties, risks, and implementation\npathways.\nThank you for the opportunity to revise our manuscript. We look forward to your\nevaluation of our revised submission.\nSincerely,\nSai Liang\nReviewer #1:\nThe authors' exploration of the potential for geological negative emissions within\nChina's shale gas value chain, specifically the transformation of shale gas extraction\ninto a carbon sink via CO₂ fracturing and sequestration technologies, is an interesting\nstudy. They assess the shale gas industry value chain and conduct a detailed economic\nfeasibility analysis using statistical data. However, several points need attention.\n[Response]: Thank you for the thoughtful evaluation and for recognizing the\nimportance of our study. We have carefully addressed each of the points requiring\nattention through substantial revisions, including enhanced technical explanations,\nadditional supporting data, and improved methodological clarity. We believe these\nrevisions significantly strengthen the manuscript and fully address the reviewer’s\nconcerns. Point-by-point responses to all specific comments are provided below.\n1. On page 4，in the section \"Synergy of carbon sequestration and production\nenhancement\": Line 88, “CO2 fracturing and CO2 huff-n-puff serve dual purposes:\nenhancing production while sequestering carbon. CO2 fracturing can achieve a carbon\nsink of 1,406.5 Mt while increasing shale gas production by 4,018 bcm. The CO2 puff-\nn-huff technique can create a 610.2 Mt carbon sink with a 499.8 bcm production\nincrease.” Missing data sources, add relevant references.\n[Response]: We sincerely thank you for raising this important point regarding data\nsource clarity. These figures (1,406.5 Mt CO₂ sequestered, 4,018 bcm gas production\nincrease for CO₂ fracturing; 610.2 Mt CO₂ sequestered, 499.8 bcm production increase\nfor huff-n-puff) are original modeling results generated from our study, calculated based\non the methodology described in sections 2.2-2.4. However, we recognize that\npresenting them without clear attribution to our own analysis could cause confusion\nabout whether they were cited from external sources.\nTo address this concern, we have revised the text as follows:\n“CO fracturing (before production), CO huff-n-puff (in production), and\n2 2\nCO sequestration (in depleted wells). CO fracturing and CO huff-n-puff\n2 2 2\nserve dual purposes: enhancing production while sequestering carbon. Based\non our modeling analysis, CO fracturing can achieve a carbon sink of 1,406.5\n2\nmillion tonnes (Mt) while increasing shale gas production by 4,018 bcm.\nSimilarly, our simulations indicate that the CO huff-n-puff technique can\n2\ncreate a 610.2 Mt carbon sink with a 499.8 bcm production increase.”\n2. On page 5, in the section \"Synergy of carbon sequestration and production\nenhancement\": Line 101, The Carbon Neutrality Coefficient (CNC) is used to measure\nthe carbon reduction potential in comparison with baseline emissions. What does\nbaseline emissions refer to, and what is the specific amount of emissions?\n[Response]: We appreciate this important question regarding the baseline emissions in\nour Carbon Neutrality Coefficient (CNC) metric.\nThe baseline emissions refer to the carbon intensity of conventional shale gas extraction\nwithout CO₂-based technologies or carbon reduction measures. These baseline values\nvary by regions due to differences in geological conditions, extraction depths, and EUR\n(Estimated Ultimate Recovery) values. For example, in the Ordos Basin, baseline\nemissions are approximately 0.42 kg CO₂e/m³ of gas produced, while in terrestrial shale\nformations, they average 0.38 kg CO₂e/m³ (detailed calculations provided in\nSupplementary Table S3).\nTo address this concern and improve clarity, we have revised the manuscript as follows:\n“The Carbon Neutrality Coefficient (CNC) measures the carbon reduction\npotential of CO₂-based technologies compared to conventional shale gas\nextraction baseline emissions. The baseline represents current extraction\npractices without carbon mitigation measures, with emissions varying by\nregions (0.38-0.45 kg CO₂e/m³) depending on geological conditions and\nextraction parameters.”\n3. On page 6，in the section \"Value chain assessment for carbon footprint and\nGeological Negative Emissions\"\n(1) Lines 131 and 137, the CO2 emission reductions under the Optimized management\nscenario (OM) and Optimized production scenario are described respectively, but the\nsubsequent calculation schemes only present the leakage reduction of CH4 (OM) and\nthe incremental production of shale gas (OP), and how CO2 emission reductions are\nderived?\n[Response]: We sincerely thank you for this constructive feedback, which significantly\nenhances the clarity of our methodolgoy section.\nFor the OM scenario: CO e reductions are achieved through methane leakage control\n2\nvia timely leak detection and repair programs. Since methane has a GWP₁₀₀ of 28 (IPCC\nAR6), each cubic meter of prevented methane leakage equals 28 m³ of CO₂e avoided.\nThe CO₂e reduction calculated as:\nCO₂e reduction = CH₄ leakage reduction × GWP₁₀₀\nFor the OP scenario: CO reductions occur through two mechanisms: a. Direct\n2\ngeological sequestration via CO fracturing and huff-n-puff. b. Emission reduction\n2\nbenefits of upstream electrification in shale gas production.\nTo address this clarity issue, we have:\n1. Enhanced the description of CO emission reduction calculations in both scenarios,\n2\nwith the CO fracturing and CO huff-n-puff technologies extensively detailed in the\n2 2\nMethods section “Assessment of carbon sink potentials in shale gas reservoirs”.\n“CO fracturing. During well completion, high-pressure CO is injected to\n2 2\nreplace water for fracturing, creating fracture networks that facilitate gas\nrecovery. Approximately 70% of injected CO returns during the production,\n2\nwhile 30% remains sequestered16. The cumulative production increase\npercentage (CPIP) varies with geological formations. In water-sensitive\ncontinental and transitional shale gas reservoirs, significant enhancements in\nshale gas production can be achieved. In contrast, the production increase\neffect in non-water-sensitive marine shale gas reservoirs is generally modest39.\nBased on these, we set CPIP at 60% (55%-65%) for transitional and\ncontinental shale and 20% (18%–22%) for marine shale40. The production\nincrease can be calculated as:\n𝐸𝑈𝑅 =𝐸𝑈𝑅∙𝐶𝑃𝐼𝑃 (1)\n𝑖\nwhere EUR varies across different shale gas reservoirs, with the specific\ncalculation methodology detailed in S5.3 of the Supplementary Information.\nThe carbon sink for a single shale gas well using CO₂ fracturing is calculated\nas:\n𝑀 =𝑉 ∙𝑛 ∙𝜌 ∙30% (2)\n𝐶𝑂2,𝐹 𝐶𝑂2 𝑠𝑡𝑎𝑔𝑒 𝐶𝑂2,𝐶\nwhere V is the volume of CO required for a single fracturing stage, n\nCO2 2 stage\nis the number of fracturing stages, and ρ is the density of CO during\nCO2,C 2\ntransportation and storage.\nCO huff-n-puff. During the typical 20-year production cycle, shale gas wells\n2\nexhibit a characteristic production decline curve until reservoir depletion\noccurs. To mitigate this decline and increase the EUR, the CO huff-n-puff\n2\ntechnology can be employed. The CO huff-n-puff stimulation is executed over\n2\na 6-year period (within years 13 to 18 of the well lifecycle), repeated for 12\ncycles. Based on existing experiment and field studies41,42, each cycle\ncomprises one month CO injection (20 t/day), followed by a one-month\n2\nsoaking, and a four-month production. Critically, 50% of the injected CO per\n2\ncycle undergoes permanent geological sequestration40. The CO huff-n-puff\n2\nyields a CPIP of 3% (2%–4%) for marine shale and 6% (4%–8%) for\ntransitional and continental shale. The method for calculating production\nincrease is shown in Eq. 1.”\n2. Revised the Methods section “Assessment of emission reduction potential\nthroughout the shale gas value chain” to explicitly state the conversion mechanisms.\n“(1) Optimized Management (OM) scenario. Shale gas leakage occurs\nthroughout the value chain, from upstream equipment (tanks, compressors,\nflares)24 to midstream infrastructure (compressor stations, pipelines)58,59, and\ndownstream consumer points. Given the similarity in shale gas production\nworkflows, facility specifications and leakage rate between China and the U.S.,\nwe derive China’s methane leakage reduction potential from empirically\nvalidated U.S. LDAR programs. Documented implementations in U.S. shale\nbasins demonstrate 36–44% leakage reductions20,60. Through technology\napplicability adjustments accounting for China’s regulatory maturity and\ninfrastructure gaps, we project a 40% (30–50%) reduction for advanced LDAR\ndeployment in China, directly generating CO -equivalent emission reductions\n2\nderived via the 100-year global warming potential conversion framework\ndefined in IPCC AR6. This estimation approach follows established\nmethodologies for assessing China's methane mitigation potential61.\n(2) Optimized Production (OP) scenario. CO fracturing and CO huff-n-puff\n2 2\ntechnologies deliver synergistic benefits through dual mechanisms of\ngeological CO₂ sequestration and production enhancement, wherein\nsequestered CO offsets value chain carbon emissions (technical parameters\n2\ndetailed in preceding sections). Facility Electrification further contribute to\nemission reductions, leveraging the inherent advantage of centrally supplied\nelectricity which exhibits markedly lower carbon footprints than on-site diesel\ngenerators18. Significant regional variations in electricity carbon intensity\nacross sedimentary basins, driven by heterogeneity in local grid structures, are\ndocumented quantitatively in Supplementary Table S3.”\n3. Added equation (8) to clearly show how carbon intensity incorporates all value chain\nemissions:\n“The carbon footprint of the shale gas value chain is assessed by carbon\nintensity, measured in g CO e/m3, calculated as:\n2\n𝐼 =𝐼 +𝐼 +𝐼 (8)\n𝑖,𝑗 𝑃,𝑖,𝑗 𝑇,𝑖,𝑗 𝐶,𝑖,𝑗\nwhere I , I , and I represent the carbon intensity of shale gas production,\nP,i,j T,i,j C,i,j\ntransportation and distribution, and consumption, respectively, across shale\ngas reservoir i and scenario j.”\n(2) Line 148, Is the comprehensive plan simply the sum of the CO emission reductions\n2\nof the other three plans, or is there a different calculation method? If it's a simple sum,\nwill the three methods interfere with or affect each other?\n[Response]: We sincerely thank you for this insightful comment on the combined\nscenario. These comments have significantly enhanced the rigor and clarity of the\nscenario descriptions in this study.\nThe combined scenario is not a simple arithmetic sum of the three individual scenarios.\nInstead, it exhibits nonlinear technological synergies with important interactions:\nSynergistic effects: CO fracturing and CO huff-n-puff technologies enhance the\n2 2\nEstimated Ultimate Recovery (EUR) per well, which increases the total available pore\nvolume for geological carbon storage. This creates a multiplicative rather than additive\neffect.\nCapacity constraints: While the enhanced EUR increases total storage capacity, the\ntechnologies do compete for the same geological space. CO used in fracturing and\n2\nhuff-n-puff operations occupies pore volume that would otherwise be available for pure\nsequestration in the abandonment phase.\nTemporal sequencing: The technologies are implemented at different stages of the\nwell lifecycle (fracturing, huff-n-puff and abandonment sequestration), creating\ndynamic interaction rather than simultaneous competition.\nTo clarify this mechanism, we have revised the scenario description as follows:\n“(4) Combined scenario. This scenario integrates the above management\nstrategies through nonlinear synergistic interactions, not merely arithmetic\naddition. Specifically, CO₂ fracturing and huff-n-puff enhance well EUR,\nthereby expanding the total geological carbon storage capacity. However, the\nfinite geological storage capacity creates trade-offs: CO₂ injected during\nfracturing and huff-n-puff operations reduces the volume available for\nsequestration in depleted wells, requiring optimization to maximize overall\ncarbon reduction benefits.”\n4. On page 9，in the section \"Economic viability across shale gas reservoirs and\nscenarios \": Line 173, the article mentions that optimizing the production plan can\nsignificantly improve economic benefits, but the high costs of CO2 fracturing\ntechnology and CO2 injection may not be matched by the increased production of shale\ngas, potentially affecting economic feasibility. Additionally, CO2 fracturing technology\nhas certain limitations. Can the appropriate regions for implementing the optimized\nproduction plan be identified based on the formation conditions?\n[Response]: We sincerely appreciate the reviewer's insightful comments regarding the\nregional applicability of production optimization strategies. The concerns about\nadditional expenditures for CO procurement, compression, and injection in CO\n2 2\nfracturing and huff-n-puff operations are well-founded and critically impact economic\nfeasibility. Our model has comprehensively accounted for these additional\nexpenditures relative to the incremental revenue from enhanced shale gas production.\nConsequently, the revised manuscript now incorporates explicit delineation of\napplicable regions for the optimized production (OP) scenario, with substantive\nmodifications detailed below.\n“However, the optimized production (OP) scenario demonstrates more\npromising results for continental deposits, with significant IRR increases of\n4.5% to 7.4% and UNPV improvements of US$ 0.042 to 0.056 m-3. Profitability\nwas achieved only in continental shale formations within the Tarim Basin and\nOrdos Basin, where favorable geological conditions and CO source proximity\n2\noffset the technology’s higher implementation costs.”\n5. On page 14, in the section \"Life cycle assessment of value chain carbon emissions\"\n(1) Line 334, The statement \"The carbon emissions stem from energy inputs, raw\nmaterials production, methane leakage, and CO2 emissions\" should clarify whether it\nincludes the CO2 emissions from the final combustion of shale gas.\n[Response]: We sincerely appreciate the reviewer’s precise observation. Yes, CO₂\nemissions from the final combustion of shale gas are fully included in our life cycle\nassessment. Specifically, combustion-derived CO is categorized under downstream\n2\nconsumption emissions.\nWe have revised the text to eliminate this ambiguity:\n“This study employs life cycle assessment to quantify carbon emissions\n(including CH and CO ) across China’s shale gas value chain. It includes\n4 2\nupstream production, midstream transportation and distribution, and\ndownstream consumption (Supplementary Figure 5). The carbon emissions\nstem from energy inputs, raw materials production, methane leakage, and CO\n2\nemissions, with the latter primarily arising from downstream shale gas\ncombustion.”\n(2) Line 339, Regarding the calculation of carbon intensity, it is recommended that the\nauthors provide a detailed explanation of how the proportion coefficients for the three\nstages (upstream, midstream, and downstream) are established. This would help\nreaders better understand the methodology and assumptions behind the carbon\nintensity calculations.\n[Response]: We sincerely thank the reviewer for this important suggestion. Our carbon\nintensity calculation methodology differs for each stage based on data availability and\ncomplexity:\n1. Upstream (Production) - Model-based calculation:\nThe upstream carbon intensity I is calculated through detailed bottom-up modeling\nP,i,j\nencompassing five sub-stages:\n𝐼 =𝐼 +𝐼 +𝐼 +𝐼 +𝐼 (9)\n𝑃,𝑖,𝑗 𝑃1,𝑖,𝑗 𝑃2,𝑖,𝑗 𝑃3,𝑖,𝑗 𝑃4,𝑖,𝑗 𝑃5,𝑖,𝑗\nwhere I , I , I , I and I represent the carbon intensity during site\nP1,I,j P2,I,j P3,I,j P3,I,j P3,I,j\npreparation, drilling and cementation, well completion, production, and\ndecommissioning phase.\nEach sub-stage involves multiple calculations for energy consumption, material inputs,\nand methane leakage (detailed equations provided in Supplementary Information S3).\n2. Midstream (Transportation) - Literature-based values:\nI is derived from published studies specific to China’s pipeline infrastructure,\nT,i,j\naccounting for pipeline transmission losses, compression station energy consumption\nand distribution network characteristics.\nRef:\n1. Tong, F., Jaramillo, P. & Azevedo, I. M. L. Comparison of life cycle greenhouse gases\nfrom natural gas pathways for medium and heavy-duty vehicles. Environ. Sci. Technol.\n49, 7123–7133 (2015).\n3. Downstream (Consumption) - Weighted sectoral approach:\nI = Σ(w × EF )\nC,i,j k k\nwhere w represents the consumption share of sector k (industry, transportation, power\nk\ngeneration) and EF is the sector-specific emission factor.\nk\nRef:\n1. Yuan, Z., Ou, X., Peng, T. & Yan, X. Life cycle greenhouse gas emissions of multi-\npathways natural gas vehicles in china considering methane leakage. Appl. Energy 253,\n113472 (2019).\n2. Zhang, Y. et al. Life cycle assessment of ammonia synthesis in China. Int. J. Life\nCycle Assess. 27, 50–61 (2022).\n3. Ling, Y. et al. Carbon emissions in China’s thermal electricity and heating industry:\nAn input-output structural decomposition analysis. J. Clean. Prod. 329, 129608 (2021).\nThe revised methodology section now states:\n“Life cycle assessment of value chain carbon emissions\nThis study employs life cycle assessment to quantify carbon emissions\n(including CH and CO ) across China’s shale gas value chain. It includes\n4 2\nupstream production, midstream transportation and distribution, and\ndownstream consumption (Supplementary Figure 5). The carbon emissions\nstem from energy inputs, raw materials production, methane leakage, and CO\n2\nemissions, with the latter primarily arising from downstream shale gas\ncombustion. The GHG emission intensities for materials are based on life cycle\nassessment literature16,43–46, while site data are derived from recent\nstudies16,47,48. The carbon footprint of the shale gas value chain is assessed by\ncarbon intensity, measured in g CO e/m3, calculated as:\n2\n𝐼 =𝐼 +𝐼 +𝐼 (8)\n𝑖,𝑗 𝑃,𝑖,𝑗 𝑇,𝑖,𝑗 𝐶,𝑖,𝑗\nwhere I , I , and I represent the carbon intensity of shale gas production,\nP,i,j T,i,j C,i,j\ntransportation and distribution, and consumption, respectively, across shale\ngas reservoir i and scenario j. Upstream production emissions are calculated\nthrough detailed bottom-up modeling. The production carbon intensity I\nP,i,j\nencompasses five distinct operational phases:\n𝐼 =𝐼 +𝐼 +𝐼 +𝐼 +𝐼 (9)\n𝑃,𝑖,𝑗 𝑃1,𝑖,𝑗 𝑃2,𝑖,𝑗 𝑃3,𝑖,𝑗 𝑃4,𝑖,𝑗 𝑃5,𝑖,𝑗\nwhere I , I , I , I and I represent the carbon intensity during site\nP1,i,j P2,i,j P3,i,j P4,i,j P5,i,j\npreparation, drilling and cementation, well completion, production, and\ndecommissioning phase under shale gas reservoir i and scenario j, detailed in\nSupplementary Information S3.\nMidstream and downstream emissions employ literature-based approaches\ndue to infrastructure standardization. Transportation and distribution carbon\nintensity I is derived from pipeline network studies specific to China’s shale\nT,i,j\ngas infrastructure49. Carbon intensity of consumption I is calculated using\nC,i,j\nsector-weighted emission factors, accounting for end-use distributions across\nindustry, transportation, and power generation sectors50–52. These values are\nsummarized in Supplementary Table 13.”\n6. On page 16, in the section \"Assessment of emission reduction potential throughout\nthe shale gas value chain,\" the authors propose four scenarios for reducing greenhouse\ngas emissions, two things need to be noted.\n(1) For the OM scenario, it is suggested that the authors elaborate on the specific\nprocess of estimating that methane leakage in China can be reduced by about 40%\nbased on the success of US LDAR program. This would enhance the clarity and\ncredibility of the estimation.\n[Response]: We sincerely appreciate the reviewer’s suggestion to elaborate on the\nestimation methodology. This clarification is indeed crucial for establishing the\ncredibility of our projections.\nChina’s shale gas industry has not yet implemented Leak Detection and Repair (LDAR)\nprograms at scale, where existing leakage monitoring primarily focuses on operational\nsafety rather than targeted methane mitigation. Consequently, significant emission\nreduction potential remains untapped. Given that the United States, the world's largest\nshale gas producer, has deployed well-established LDAR practices, and considering the\nsimilarity in production workflows between the Chinese and U.S. shale sectors, we have\nreferenced U.S. operational frameworks to inform our estimations. This methodology\naligns with the approach adopted by Khanna et al. in their assessment of China's\nmethane abatement potential1.\n1. Khanna, N., Lin, J., Liu, X. & Wang, W. An assessment of China’s methane\nmitigation potential and costs and uncertainties through 2060. Nat Commun 15, 9694\n(2024).\nTo address your request for greater clarity, we have revised the manuscript as follows:\n“(1) Optimized Management (OM) scenario. Shale gas leakage occurs\nthroughout the value chain, from upstream equipment (tanks, compressors,\nflares)24 to midstream infrastructure (compressor stations, pipelines)58,59, and\ndownstream consumer points. Given the similarity in shale gas production\nworkflows, facility specifications and leakage rate between China and the U.S.,\nwe derive China’s methane leakage reduction potential from empirically\nvalidated U.S. LDAR programs. Documented implementations in U.S. shale\nbasins demonstrate 36–44% leakage reductions20,60. Through technology\napplicability adjustments accounting for China’s regulatory maturity and\ninfrastructure gaps, we project a 40% (30–50%) reduction for advanced LDAR\ndeployment in China, directly generating CO -equivalent emission reductions\n2\nderived via the 100-year global warming potential conversion framework\ndefined in IPCC AR6. This estimation approach follows established\nmethodologies for assessing China's methane mitigation potential61.\n”\n(2) In the OP scenario, the authors distinguish the cumulative shale gas production\npercentage (CPIP) for CO2 fracturing technology under different geological\nconditions (marine, terrestrial, and transitional). This distinction enhances the\ncredibility of the data and reveals the CO2 sequestration potential in different regions.\nHowever, the discussion lacks an exploration of specific challenges that may be\nencountered during the implementation of these technologies. It is recommended that\nthe authors supplement the discussion with relevant challenges to enhance the study's\npracticality and reliability.\n[Response]: We sincerely thank the reviewer for the valuable suggestion. This\nconstructive critique has significantly enhanced the rigor and real-world applicability\nvalue of our research.\nBy taking the reviewer’s suggestion, we have added comprehensive discussions on\npotential implementation obstacles in both the Introduction and Discussion sections, as\ndetailed below:\nIn the Introduction, we now acknowledge key challenges:\n“Applying CO\n2\n-based technologies in shale gas operations can generate\nsubstantial carbon sink effects, contributing significantly to global carbon\nneutrality goals6,7. However, large-scale deployment is constrained by\nformation-specific challenges including economic feasibility barriers8,9,\ncompounded by risks of CO leakage9,10 and induced seismicity11. The U.S., as\n2\nthe world’s largest shale gas producer, has conducted reservoir-specific\nsimulations and laboratory-scale explorations of carbon sink potential,\nevaluating techniques such as CO fracturing and geological sequestration12–\n2\n15. Nevertheless, existing research predominantly examines isolated processes\nrather than implementing full value-chain integration assessments, and\nsystematic analysis of CO -based technologies’ applicability across diverse\n2\nshale reservoir types remains underdeveloped. China, facing more complex\ngeological conditions, is pioneering integrated CO -based technologies\n2\nincluding CO fracturing16, CO huff-n-puff17, electrification of operations18,\n2 2\ncarbon sequestration8,19, and enhanced leak detection programs20,\ndemonstrating enhanced effectiveness in its diverse shale formations. However,\nsystematic assessment of emission-reduction potential across shale gas value\nchains under varying geological contexts remains challenging.”\nIn the Discussion, we provide detailed challenge analysis:\n“While formation-specific challenges vary across reservoir types, several\nfundamental barriers constrain all CO sequestration operations. During CO\n2 2\nsequestration, the injection of CO induces pore pressure perturbations,\n2\naltering the mechanical equilibrium of the reservoir through the effective stress\nprinciple. This can potentially activate critically stressed faults or natural\nfracture systems, leading to microseismic events11,15. Moreover, elevated pore\npressure may trigger the reactivation of caprock fractures or shear failure of\nwellbores, forming leakage pathways that result in small-scale leakage\nevents10,11,15. However, current monitoring techniques face challenges in\nachieving precise early-stage detection and quantitative characterization of\nsuch small-scale leakage occurring beyond wellbore interfaces. Additionally,\nthe challenge to economic feasibility is particularly severe; the full-chain costs\n– encompassing CO capture, compression, and the deployment of extensive\n2\ncorrosion-resistant pipeline networks to satisfy source-sink matching – pose\nsignificant challenges to achieving profitability in the absence of structural\nsubsidies and carbon credit revenue mechanisms8,9. Furthermore, spatial\nheterogeneity in geological conditions, infrastructure requirements, and\noperational challenges create uncertainties in both environmental and\neconomic outcomes. These uncertainties affect the reliability of economic\nforecasts and the effectiveness of carbon reduction strategies.”\n7. On page 18，in the section \"Techno-economic analysis of emission reduction\nscenarios\": Lines 421 and 427, formulas 9 and 10 are used to calculate the Internal Rate\nof Return (IRR) and net present value (NPV). Were these formulas derived through\ndeduction or do they cite previous research? If they were derived, please provide the\nderivation process; if they cite previous research, please specify the relevant references.\n[Response]: Thank you for your valuable comment. Equations 9 and 10 (now\nrenumbered as Equations 14 and 17) represent standard financial metrics universally\nestablished in corporate finance literature, not novel derivations. These formulas are\nfundamental tools in investment appraisal, with NPV originating from Irving Fisher’s\nwork on the time value of money and IRR being widely adopted since the 1950s. The\nUnit Net Present Value (UNPV) in Equation 14 is a straightforward normalization of\nNPV by technically recoverable reserves, commonly used in petroleum economics to\nenable cross-project comparisons.\nTo address this, we have:\n1. Added explicit citations to foundational sources:\nof firms. John Wiley & Sons.\nAppl. Energy, 106, 100-115.\ntechniques for shale gas development. Appl. Energy 148, 49–65 (2015).\nDue to constraints on the number of references imposed by the journal, only ref 3 has\nbeen cited.\n2. Expanded the methodological context in the manuscript and clarified formula\ncomponents with detailed variable definitions and calculation procedures as follows:\n“The DCF method is a financial valuation approach grounded in the time\nvalue of money theory. Its core logic involves forecasting the free cash flow\ngenerated by an asset or project and discounting it to its present value using a\nrisk-adjusted discount rate, thereby quantifying its intrinsic value63. Financial\nprofitability metrics such as Net Present Value (NPV), Unit Net Present Value\n(UNPV), Internal Rate of Return (IRR), and Dynamic Payback Period are all\nestablished based on this method. In this study, the DCF method is adopted to\nevaluate the techno-economic feasibility of multiple emission-reduction\nscenarios across the shale gas value chain. The UNPV and IRR are selected\nas profitability indicators to assess the techno-economic performance of\ndifferent scenarios. UNPV serves as a core metric for evaluating the\nprofitability per unit of shale gas. It is defined as the ratio of the project's NPV\nto the total production (Q ), calculated as follows:\ntotal\n𝑈𝑁𝑃𝑉 = 𝑁𝑃𝑉⁄𝑄 (14)\ntotal\nThe NPV is calculated by discounting the annual net cash flows (NCF) within\nt\nthe shale gas value chain using the industry benchmark discount rate (r ) and\n0\nsumming the discounted present values. This metric characterizes the absolute\nprofitability of the shale gas value chain. The formula is expressed as follows:\nt=n\n𝑁𝑃𝑉 = ∑ 𝑁𝐶𝐹 ∙(1+𝑟 )−𝑡 (15)\nt 0\nt=0\nThe NCF is calculated as follows:\nt\n𝑁𝐶𝐹 = 𝑄 ∙𝑓∙(𝑃+𝑆 )−𝑇 −𝐼 (16)\nt t subsidy t t\nwhere Q denotes shale gas production in year t; f represents the proportion of\nt\nmarketable gas output; P is the shale gas wellhead price; S signifies the\nsubsidy\nshale gas development subsidy, a fiscal incentive provided by the Chinese\ngovernment, quantified at US$ 0.013 m-3 64. The variable T represents the tax\nt\ncost, encompassing value-added tax, income tax, resource tax, and other\napplicable levies. I denotes the business cost in year t, where I specifically\nt 1\nrefers to well pad construction investment, including expenditures for drilling,\nfracturing, and completion activities. The computational specifics regarding\nT and I are comprehensively detailed in S5.1, S5.2, and S5.5 of the\nt t\nSupplementary Information.\nThe IRR refers to the discount rate at which a project’s cumulative NPV equals\nzero, reflecting the project's investment return rate. The IRR accounts for the\ntime value of money. A higher IRR indicates better project profitability, making\nit a key indicator for evaluating project profitability. Its calculation formula is\nas follows:\nt=n\n∑ 𝑁𝐶𝐹 ∙(1+𝐼𝑅𝑅)−𝑡 = 0 (17)\nt\nt=0\nReviewer #2:\nDear Reviewer,\nWe sincerely appreciate the time and effort you have dedicated to reviewing our\nmanuscript and providing us with such invaluable constructive feedback. Your insights\nare crucial for enhancing the clarity, rigor, and readability of our work. We have\ncarefully considered all your suggestions and made corresponding revisions to the\nmanuscript. We hope that these changes effectively address your concerns, and we\nkindly ask you to review the revised manuscript. Below, we outline our responses to\nyour specific comments and the modifications made:\n1. This article is focused on China. At a minimum, the title should include that.\n[Response]: We sincerely thank the reviewer for highlighting the need to specify the\ngeographical focus in our title. We fully agree that clearly defining the study area is\nessential for readers to immediately understand the paper's scope and relevance.\nWe have revised the title to:\n“Carbon sequestration for Geological Negative Emissions of shale gas value\nchain in China”\nWe believe this China-specific focus is particularly significant because:\n1. Diversified geological conditions: China’s shale formations differ substantially\nfrom US plays. While US shales are predominantly marine deposits with relatively\nhomogeneous properties, China’s shale resources span marine, terrestrial, and\ntransitional facies, creating more complex but potentially more responsive\nconditions for CO -based technologies.\n2\n2. Enhanced production potential: Our research demonstrates that CO fracturing\n2\ntechnology delivers significantly enhanced production outcomes in Chinese shale\nformations—markedly surpassing those reported in U.S. shale plays. This disparity\nstems primarily from the elevated clay content and pronounced water sensitivity\ncharacteristic of China's terrestrial and transitional shale facies, which renders CO\n2\nparticularly advantageous as a fracturing fluid.\n3. Global demonstration value: As the world’s second-largest shale gas producer,\nChina’s successful implementation of negative emissions technologies provides a\nreplicable model for other countries with similar geological complexity, particularly\nin Asia and South America.\nWe have also enhanced the Introduction to emphasize China’s technological\napplications:\n“Applying CO\n2\n-based technologies in shale gas operations can generate\nsubstantial carbon sink effects, contributing significantly to global carbon\nneutrality goals6,7. However, large-scale deployment is constrained by\nformation-specific challenges including economic feasibility barriers8,9,\ncompounded by risks of CO leakage9,10 and induced seismicity11. The U.S., as\n2\nthe world’s largest shale gas producer, has conducted reservoir-specific\nsimulations and laboratory-scale explorations of carbon sink potential,\nevaluating techniques such as CO fracturing and geological sequestration12–\n2\n15. Nevertheless, existing research predominantly examines isolated processes\nrather than implementing full value-chain integration assessments, and\nsystematic analysis of CO -based technologies’ applicability across diverse\n2\nshale reservoir types remains underdeveloped. China, facing more complex\ngeological conditions, is pioneering integrated CO -based technologies\n2\nincluding CO fracturing16, CO huff-n-puff17, electrification of operations18,\n2 2\ncarbon sequestration8,19, and enhanced leak detection programs20,\ndemonstrating enhanced effectiveness in its diverse shale formations. However,\nsystematic assessment of emission-reduction potential across shale gas value\nchains under varying geological contexts remains challenging.”\n2. The abstract description I read while agreeing to review does not match the content\nof the manuscript.\n[Response]: We sincerely appreciate your valuable comment on the abstract. We have\ncomprehensively revised the abstract to ensure clear alignment with the manuscript’s\nstructure and findings. Below is a detailed mapping table showing how each abstract\nsentence corresponds to specific manuscript sections:\n1. Abstract Restructuring\n“Carbon sequestration in shale gas operations represents a crucial pathway to achieve\nGeological Negative Emissions, where geological carbon sequestration exceeds life\ncycle greenhouse gas emissions, for global 1.5℃ targets. However, the potential for\nsuch emissions reduction in China’s shale gas value chain and its economic viability\nremain unclear. This study quantifies the potential for transforming China’s shale gas\nvalue chain from an emission source to a carbon sink using life cycle assessment, while\nrevealing significant spatial heterogeneity in economic feasibility across distinct shale\ngas reservoirs. Results demonstrate that synergistic deployment of CO -based\n2\ntechnologies (fracturing, huff-n-puff, and depleted-well sequestration) can achieve\nGeological Negative Emissions across the shale gas value chain, with national carbon\nsink potential reaching 66 billion tonnes of CO -equivalents and shale gas production\n2\nincreasing by 4,518 billion cubic meters. Multi-scenario economic assessments reveal\nthat marine shale in Sichuan Basin exhibits inherent profitability, whereas continental\nreservoirs can reverse negative returns through carbon credit integration or optimized\nproduction. As current Chinese carbon market prices provide insufficient incentives,\nimplementing region-specific subsidies and enhancing carbon pricing frameworks\ncould unlock this potential, thereby contributing to national carbon neutrality goals\nand multiple Sustainable Development Goals.”\n2. Table: Abstract-Manuscript Correspondence\nSpecific Content\nAbstract Component Manuscript Location\nMatch\nOpening Statement\nSpecific Content\nAbstract Component Manuscript Location\nMatch\n\"Carbon sequestration in shale gas Establishes the\noperations represents a crucial pathway to Introduction (Para 1-2) concept and global\nachieve Geological Negative Emissions...\" context\nResearch Gap\n\"However, the potential for geological\nnegative emissions in China's shale gas Introduction (The third Identifies the\nvalue chain and its economic viability paragraph) knowledge gap\nremain unclear.\"\nMethodology\nMethods (Life cycle\n\"This study quantifies the potential...using LCA framework\nassessment of value chain\nlife cycle assessment\" description\ncarbon emissions )\n\"...revealing significant spatial Methods (Techno-economic\nEconomic assessment\nheterogeneity in economic feasibility analysis of emission reduction\nmethodology\nacross distinct shale gas reservoirs\" scenarios)\nKey Findings-Carbon reduction\n\"synergistic deployment of CO2-based\nResults (Synergy of carbon\nTechnology\ntechnologies (fracturing, huff-n-puff, and sequestration and production\nintegration analysis\nenhancement)\ndepleted-well sequestration)\"\nQuantitative carbon\n\"national carbon sink potential reaching\nResults (Fig 2a, CS scenario) sequestration\n66 billion tonners of CO -equivalents\"\n2\npotential\nProduction\n\"shale gas production increasing by 4,518\nResults (Fig 2b, CS scenario) enhancement\nbillion cubic meters\"\nquantification\nKey Findings-Economic Analysis\nResults (Economic viability\n\"marine shale in the Sichuan Basin Basin-specific\nacross shale gas reservoirs and\nexhibits inherent profitability\"\neconomic outcomes\nscenarios, Fig 5)\n\"continental reservoirs can reverse Results (Economic viability\nCarbon pricing\nnegative returns through carbon credit across shale gas reservoirs and\nsensitivity analysis\nintegration\"\nscenarios, Fig 5)\nPolicy Implications\n\"Current Chinese carbon market prices Discussion (The first Economic barrier\nprovide insufficient incentives\"\nparagraph) analysis\n\"implementing region-specific subsidy\nDiscussion (The third Policy\nmechanisms and enhancing carbon pricing\nparagraph) recommendations\nframework\"\nSpecific Content\nAbstract Component Manuscript Location\nMatch\n\"contributing to national carbon neutrality Discussion (The third\nBroader significance\ngoals and multiple SDGs\"\nparagraph)\nThe revised abstract now clearly aligns with our manuscript structure, providing readers\nwith an accurate preview of our study’s scope, methodology, and key contributions to\nChina’s shale gas decarbonization strategy.\n3. I did read the paper and it is difficult to understand. The authors use many buzz\nwords that I am not familiar with. These could be terms that apply to China.\n[Response]: We sincerely appreciate your feedback regarding terminology clarity. To\nenhance accessibility, we have implemented the following revisions:\nFirst, we have standardized our technical terminology to align with international\nconventions. Throughout the manuscript, we have replaced “well site” with “well pad”\nto match standard industry usage. For specialized terms that are essential to our analysis,\nwe have added clear definitions at first mention. For example, “shale gas reservoir” is\nnow defined as “a geological formation containing recoverable shale gas resources,”\nand “CO huff-n-puff” is explained as “a cyclic injection-production process where CO\n2 2\nis injected into the formation, allowed to soak, then co-produced with natural gas.”\nTo further enhance clarity, we have revised complex sentences to use more\naccessible language without sacrificing precision. Technical concepts are now\nintroduced progressively, with brief parenthetical explanations provided for\nunavoidable specialized terms. All acronyms are defined at first use, and we have\nexpanded figure and table captions to include definitions of technical terms appearing\nin visualizations. These comprehensive revisions ensure that our findings on China’s\nshale gas decarbonization potential are accessible to the broad international readership\nwhile maintaining the technical depth necessary for scientific communication.\n4. The figure fonts and figure size are very small. When reading the text and referring\nto the paper, it is hard to understand and interpret the figures. There is also a lot of\ninformation in each figure.\n[Response]: We sincerely thank the reviewer for highlighting the readability issues with\nour figures. We have comprehensively redesigned all figures to enhance their\naccessibility and interpretability.\n1. The figure layout has been reconfigured and the bar charts enlarged to enhance data\nreadability.\n2. All figures have been updated and uploaded in high-definition (HD) resolution\nwithout compression artifacts, ensuring lossless quality throughout the manuscript\nrevision process.\nFig. 2 Carbon sequestration potential and production enhancement in China’s shale gas\noperations by shale gas reservoirs. a, Cumulative carbon sink potential achieved through\nintegrated deployment of CO fracturing, CO huff-n-puff, and CO sequestration technologies\n2 2 2\nacross major shale basins; b, Associated production increase potential through combined\ntechnology implementation. The cumulative bar charts in a and b illustrate the carbon sink and\nproduction increase potential of different types of shale gas reservoirs in the Sichuan Basin, Tarim\nBasin, and Ordos Basin, with 95% confidence intervals.\nFig. 4, The carbon intensity of shale gas produced in China's various shale gas reservoirs\nunder different scenarios. a, Business-as-usual emissions per unit of gas produced. b,\nOptimized management (OM) scenario. c, Optimized production (OP) scenario. d, Depleted well\nhandling (HW) scenario. e, Combined strategies scenario. f, The national average emissions per\nunit of natural gas production for each scenario (calculated after weighting for each shale gas\nreservoir). The cumulative bar charts in a-e illustrate the carbon intensity of unitary shale gas\nproduced of different types of shale gas reservoirs in the Sichuan Basin, Tarim Basin, and\nOrdos Basin, along with a 95% confidence interval.\nFig. 5 The Unit Net Present Value(UNPV) of different shale gas reservoirs in various\nregions under different carbon prices in the combined scenario. a, The UNPV of different\nshale gas reservoirs in various regions under a comprehensive scenario without carbon subsidies.\nb, The UNPV of different shale gas reservoirs in various regions under a comprehensive scenario\nat carbon market prices in China(CN). c, The UNPV of different shale gas reservoirs in various\nregions under a comprehensive scenario at carbon market prices in Europe(EU). The cumulative\nbar charts in a-c illustrate the UNPV of different types of shale gas reservoirs in the Sichuan\nBasin, Tarim Basin, and Ordos Basin, along with a 95% confidence interval. d, The variation in\nthe UNPV of Chinese shale gas reservoirs under a comprehensive scenario with changes in\ncarbon prices.\n5. In the methods section, I don’t under why equation (1) is using 30% sequestered\nwhile later the authors use 50% sequestered. And the reference is from 2016, which\nseems like a time when people were not thinking about storage CO2 in shale gas\nreservoirs.\n[Response]: We deeply appreciate the reviewer’s insightful observation regarding the\nsequestration efficiency parameters in Equation (1). It helped us improve the\nclarifications that substantially enhance the methodological transparency of our revised\nmanuscript.\nClarification of Different Sequestration Rates:\nThe 30% and 50% sequestration rates apply to distinct technologies with different\nmechanisms. In CO fracturing, high-pressure CO replaces water during fracturing\n2 2\nprocess to create fracture networks. Approximately 70% of injected CO returns during\n2\nflowback operations, while 30% remains permanently sequestered within the formation\nthrough adsorption and dissolution mechanisms. In contrast, CO huff-n-puff\n2\noperations, implemented during years 13-18 of well life, achieve 50% sequestration per\ncycle due to the extended soaking period that enhances CO -rock interactions and\n2\ndisplacement processes. We have revised the manuscript to clearly distinguish these\nprocess-specific rates and their underlying mechanisms:\n“CO fracturing. During well completion, high-pressure CO is injected to\n2 2\nreplace water for fracturing, creating fracture networks that facilitate gas\nrecovery. Approximately 70% of injected CO returns during the production,\n2\nwhile 30% remains sequestered16.”\n“CO\n2\nhuff-n-puff. During the typical 20-year production cycle, shale gas\nwells exhibit a characteristic production decline curve until reservoir\ndepletion occurs. To mitigate this decline and increase the EUR, the CO huff-\n2\nn-puff technology can be employed. The CO huff-n-puff stimulation is\n2\nexecuted over a 6-year period (within years 13 to 18 of the well lifecycle),\nrepeated for 12 cycles. Based on existing experiment and field studies41,42, each\ncycle comprises one month CO injection (20 t/day), followed by a one-month\n2\nsoaking, and a four-month production. Critically, 50% of the injected CO per\n2\ncycle undergoes permanent geological sequestration40. The CO huff-n-puff\n2\nyields a CPIP of 3% (2%–4%) for marine shale and 6% (4%–8%) for\ntransitional and continental shale. The method for calculating production\nincrease is shown in Eq. 1.”\nRegarding the 2016 Reference and CO Fracturing Research Timeline:\nstudy by Wilkins and Free employs life cycle assessment to compare environmental\nimpacts of water-based versus CO -based fracturing in the Marcellus Shale. By 2016,\n2\ntechnology trajectory began with Lillies and King’s pioneering field test in 1982,\nfollowed by Yost et al.’s pilot project in Devonian shale (1993), Campbell et al.'s full-\nscale implementation in Lewis shale (2000), and Mazza’s optimization studies in Texas\n(2004). The renewed interest in CO fracturing post-2010 was driven by water resource\n2\nconstraints and environmental concerns in major shale plays.\nReference:\n1. Lillies, A. T. & King, S. R. Sand fracturing with liquid carbon dioxide. in SPE\nProduction Technology Symposium (New Mexico, 1982).\n2. Yost, A. B., Mazza, R. L. & Gehr, J. B. CO /sand fracturing in devonian shales. in\n3. Campbell, S. M., Fairchild, N. R. & Arnold, D. L. Liquid CO and sand stimulations\n2\nin the lewis shale, san juan basin, new mexico: A case study. in SPE Rocky Mountain\nRegional/Low-Permeability Reservoirs Symposium and Exhibition (Denver, Colorado,\n2000).\n4. Mazza, R. L. Field Testing & Optimization of CO /Sand Fracturing Technology;\n5. Middleton, R. S. et al. Shale gas and non-aqueous fracturing fluids: Opportunities\nand challenges for supercritical CO . Appl. Energy 147, 500–509 (2015).\n(Bachu, 2000; Oldenburg et al., 2002), the shale gas boom catalyzed investigation into\nshale-specific storage potential. The U.S. DOE’s National Energy Technology\nLaboratory developed standardized methodologies for shale CO storage assessment by\n2\n2016. Notably, Tao and Clarens (2013) demonstrated that the Marcellus Shale could\nsequester >50% of U.S. stationary-source emissions by 2030, while Liu et al. (2013)\nvalidated technical feasibility through reservoir simulations.\nTherefore, by 2016, both CO fracturing and shale-based carbon storage were well-\n2\nestablished research areas with substantial literature support. The Wilkins and Free\nreference appropriately captures the state of knowledge at that time, providing validated\nparameters that remain relevant for our analysis of China's shale gas decarbonization\npotential.\n1. Bachu, S. Sequestration of CO in geological media: criteria and approach for site\n2\nselection in response to climate change. Energy Convers. Manag. 41, 953–970 (2000).\n2. Oldenburg, C. M. & Benson, S. M. CO injection for enhanced gas production and\n2\ncarbon sequestration. in SPE International Petroleum Conference and Exhibition\n(Villahermosa, Mexico, 2002).\n3. Lackner K S. A guide to CO sequestration[J]. Science, 2003, 300(5626): 1677-1678.\n2\n4. Levine, J. S. et al. U.S. DOE NETL methodology for estimating the prospective CO\n2\nstorage resource of shales at the national and regional scale. Int. J. Greenh. Gas Control\n51, 81–94 (2016).\n5. Tao, Z. & Clarens, A. Estimating the carbon sequestration capacity of shale\nformations using methane production rates. Environ. Sci. Technol. 47, 11318–11325\n(2013).\n6. Liu, F., Ellett, K., Xiao, Y. & Rupp, J. A. Assessing the feasibility of CO storage in\n2\nthe new albany shale (devonian–mississippian) with potential enhanced gas recovery\nusing reservoir simulation. Int. J. Greenh. Gas Control 17, 111–126 (2013).\n6. The authors should also be consistent when using sequestration versus storage\nthrough out the text.\n[Response]: We sincerely thank the reviewer for identifying this important\ninconsistency in our terminology. We have now standardized our terminology\nthroughout the entire manuscript.\nAs suggested, we have now systematically standardized the terminology throughout the\nmanuscript. All references to carbon storage now explicitly use \"sequestration\" (over\nten instances revised). This includes revisions in the abstract, main text, figure captions,\nand supplementary materials. For example, we now consistently use \"CO sequestration\n2\nrate\" when discussing the percentage of injected CO that remains permanently trapped,\n2\nand \"CO sequestration capacity\" when quantifying the potential of shale formations to\n2\npermanently contain CO .\nunchanged as it reflects standardized nomenclature within the field and is widely\nrecognized in both academic literature and policy documents. This selective retention\nensures our work remains aligned with international conventions while maintaining\ninternal consistency.\nReviewer #3:\nThe article puts forward a very interesting proposal: transforming shale gas formations\nthemselves into net carbon sinks by applying techniques such as CO₂ hydraulic\nfracturing, the huff-n-puff method (enhanced recovery with CO₂ injection), and storage\nin depleted wells. The approach is innovative and is framed within the idea of achieving\ngeological negative emissions—something that could have a transformative impact on\nthe carbon balance of the unconventional gas industry. One of the manuscript’s most\nstriking claims is that the potential for net emissions reduction could reach 1.981 billion\ntons of CO₂ (1.981 Gt CO₂) between 2024 and 2060, which would represent a\nspectacular climate benefit if validated. However, this ambition contrasts sharply with\nthe scientific execution of the article, which presents significant shortcomings both in\nits conceptual framing and in its development. The main issues identified are outlined\nbelow in an orderly and reasoned manner.\nIn summary, the article starts with a very promising idea and addresses a crucial\nproblem with remarkable ambition. However, it needs a thorough revision to fulfill its\npromise. It lacks methodological rigor, clarity of presentation, risk analysis, and solid\nreferences that situate the work within the existing international framework. If these\naspects are corrected, the manuscript could make a valuable contribution to the debate\non negative emissions in the shale gas industry. In its current state, however, the most\nreasonable recommendation is a major revision.\nDear Reviewer:\nWe sincerely appreciate your thorough evaluation and recognition of our manuscript's\ninnovative approach to transforming shale gas formations into carbon sinks. Your\nacknowledgment of the potential transformative impact of achieving geological\nnegative emissions in the unconventional gas industry validates the importance of our\nresearch direction. We are grateful for your constructive criticism and the opportunity\nto strengthen our manuscript through major revision.\nIn response to your detailed comments, we have undertaken a comprehensive revision\naddressing each identified weakness:\nMethodological Rigor: We have provided complete transparency in our calculation of\nthe 1.981 Gt CO₂ sequestration potential, with detailed breakdowns of assumptions,\nparameter justifications, and uncertainty bounds. Monte Carlo simulations (100,000\niterations) now quantify confidence intervals for all major findings. The sequestration\nrates (30% for fracturing, 50% for huff-n-puff) are explicitly justified with mechanistic\nexplanations and literature support.\nClarity of Presentation: The manuscript has been completely restructured with clear\nseparation between Results and Discussion sections, standardized terminology, and\nimproved transitions. All economic units have been converted to US$ for international\naccessibility. Complex technical concepts are now introduced progressively with clear\ndefinitions.\nRisk Analysis: We have added comprehensive discussion of implementation\nchallenges including CO₂ leakage risks, induced seismicity potential, infrastructure\nrequirements, and economic barriers. The analysis now explicitly addresses formation-\nspecific challenges and includes sensitivity analyses showing how key uncertainties\naffect outcomes.\nInternational Framework: We have substantially expanded our literature review with\nadditional references, particularly from U.S. and Canadian studies. The work is now\nproperly positioned within the global CCUS research landscape, with explicit\ncomparisons to international projects and methodologies (e.g., U.S. DOE NETL\nframework).\nWe believe these comprehensive revisions have transformed the manuscript into a\nrigorous scientific contribution that maintains its innovative vision while meeting the\nstandards of methodological excellence. We are confident that the revised manuscript\nnow provides a valuable and credible contribution to the critical debate on achieving\nnegative emissions in the shale gas industry. We deeply appreciate your detailed\nfeedback, which has been instrumental in elevating the quality and impact of our work.\n1. The introduction lacks solid and specific references to support its key claims. The\nconcept of geological negative emissions is mentioned as emerging, but no prior\nscientific literature from countries with mature shale gas industries—such as the US or\nCanada—is properly cited. As a result, the reader lacks the necessary context to situate\nthis work within the current state of knowledge.\n[Response]: We sincerely appreciate the reviewer’s insightful critique regarding the\ninsufficient contextualization of geological negative emissions within the established\nglobal knowledge base. We have comprehensively revised the Introduction to address\nthis critical gap by incorporating extensive references from mature shale gas industries,\nparticularly the U.S. and Canada.\nOur revised Introduction now reads:\n“Applying CO -based technologies in shale gas operations can generate\n2\nsubstantial carbon sink effects, contributing significantly to global carbon\nneutrality goals6,7. However, large-scale deployment is constrained by\nformation-specific challenges including economic feasibility barriers8,9,\ncompounded by risks of CO leakage9,10 and induced seismicity11. The U.S., as\n2\nthe world’s largest shale gas producer, has conducted reservoir-specific\nsimulations and laboratory-scale explorations of carbon sink potential,\nevaluating techniques such as CO fracturing and geological sequestration12–\n2\n15. Nevertheless, existing research predominantly examines isolated processes\nrather than implementing full value-chain integration assessments, and\nsystematic analysis of CO -based technologies’ applicability across diverse\n2\nshale reservoir types remains underdeveloped. China, facing more complex\ngeological conditions, is pioneering integrated CO -based technologies\n2\nincluding CO fracturing16, CO huff-n-puff17, electrification of operations18,\n2 2\ncarbon sequestration8,19, and enhanced leak detection programs20,\ndemonstrating enhanced effectiveness in its diverse shale formations. However,\nsystematic assessment of emission-reduction potential across shale gas value\nchains under varying geological contexts remains challenging.”\nWe have now incorporated extensive references from North American research,\ndrawing particularly on the comprehensive review by Lyu et al. (2021) published in\nEnergy & Environmental Science, which synthesizes the current understanding of CO\n2\napplications in shale gas recovery and sequestration. This seminal work highlights that\nwhile CO fracturing creates more complex fracture networks than hydraulic fracturing,\n2\nleading to enhanced permeability, the technology faces significant deployment\nchallenges including CO leakage risks and induced seismicity concerns.\n1. Foundational U.S. studies: Edwards et al. (2015) demonstrated through\nmodeling that CO storage capacity in Marcellus shale wells could reach 0.5-\n2\n18.7 Mt per well, while Tao and Clarens (2013) estimated that U.S. shales could\nsequester >50% of national stationary-source emissions by 2030.\n2. Field-scale validation: Liu et al. (2013) validated CO storage feasibility in\nenhanced gas recovery with simultaneous CO sequestration. Schaef et al. (2014)\n2\nprovided experimental evidence of CO -shale interactions affecting storage\n2\nintegrity.\n3. Technical challenges: Middleton et al. (2015) identified critical barriers\nincluding proppant transport difficulties in CO , while Verdon et al. (2013)\n2\ndocumented induced seismicity risks from large-scale CO injection based on\n4. Methodological frameworks: The U.S. DOE NETL methodology (Levine et\nal., 2016) established standardized approaches for evaluating shale CO storage\n2\nresources, providing the foundation for our China-specific assessment.\nAdded References:\n1. Lyu, Q. et al. The role of supercritical carbon dioxide for recovery of shale gas and\nsequestration in gas shale reservoirs. Energy Environ. Sci. 14, 4203–4227 (2021).\n2. Middleton, R. S. et al. Shale gas and non-aqueous fracturing fluids: opportunities\nand challenges for supercritical CO . Appl. Energy 147, 500–509 (2015).\n2\n3. Lackner, K. S. A guide to CO sequestration. Science 300, 1677–1678 (2003).\n2\n4. Verdon, J. P. et al. Comparison of geomechanical deformation induced by\nmegatonne-scale CO storage at sleipner, weyburn, and in salah. PNAS. 110, E2762–\n5. Zhang, X., Lu, Y., Tang, J., Zhou, Z. & Liao, Y. Experimental study on fracture\ninitiation and propagation in shale using supercritical carbon dioxide fracturing. Fuel\n190, 370–378 (2017).\n6. Edwards, R. W. J., Celia, M. A., Bandilla, K. W., Doster, F. & Kanno, C. M. A\nmodel to estimate carbon dioxide injectivity and storage capacity for geological\nsequestration in shale gas wells. Environ. Sci. Technol. 49, 9222–9229 (2015).\n7. Liu, F., Ellett, K., Xiao, Y. & Rupp, J. A. Assessing the feasibility of CO storage\n2\nin the new albany shale (devonian–mississippian) with potential enhanced gas recovery\nusing reservoir simulation. Int. J. Greenh. Gas Control 17, 111–126 (2013).\n8. Schaef, H. T. et al. CO utilization and storage in shale gas reservoirs: experimental\n2\nresults and economic impacts. Energy Procedia 63, 7844–7851 (2014).\n9. Tao, Z. & Clarens, A. Estimating the carbon sequestration capacity of shale\nformations using methane production rates. Environ. Sci. Technol. 47, 11318–11325\n(2013).\n2. Throughout the article, highly ambitious figures for the potential CO₂ sequestration\nare presented, yet the traceability of those calculations is not clearly or transparently\nexplained. For example, the authors state that a total of 1.98 Gt CO₂ could be stored\nby 2060 using a combination of technologies, but the assumptions behind this\nestimate—such as injection coefficients or average recovery factors—are not broken\ndown or justified. This undermines the credibility of the conclusions.\n[Response]: We sincerely thank the reviewer for this critical insight, which directly\nenhances the scientific rigor of large-scale carbon sequestration potential assessment.\nTo address this fundamental concern about calculation transparency, we have\ncomprehensively revised our methodology presentation:\n1. Enhanced methodological transparency in the main text:\nWe now explicitly present our calculation framework, acknowledging that geological\nheterogeneity in reservoir characteristics (e.g., porosity, permeability) fundamentally\ninfluences EUR per well across different shale reservoirs. We addressed this variability\nthrough parameter decomposition, drawing upon analogous basin-scale data from\ncommercial/test wells in published literature. The revised manuscript specifies:\n“The production increase can be calculated as:\n𝐸𝑈𝑅 =𝐸𝑈𝑅∙𝐶𝑃𝐼𝑃 (1)\n𝑖\nwhere EUR varies across different shale gas reservoirs, with the specific\ncalculation methodology detailed in S5.3 of the Supplementary Information.”\n2. Provided comprehensive parameter justification:\nSection S5.3 now provides complete transparency in EUR estimation methodology as\nfollows:\n“S5.3 Estimation of Single Well EUR\nThe EUR of shale gas single wells is closely related to their economic viability.\nA higher EUR can reduce the unit cost of shale gas and improve economic\nefficiency. Various geological conditions, including reservoir characteristics,\nporosity, and permeability, directly impact single well production.\nFurthermore, the annual production of shale gas wells exhibits a clear\ndeclining trend, with initial production typically being high but rapidly\ndecreasing over time. Accurately predicting the EUR of shale gas wells is\nchallenging. Common oil and gas production forecasting methods include the\nArps decline curve method, analytical methods, numerical simulation methods,\nand empirical methods. All these approaches require substantial geological or\nshale gas well data. However, in China, only certain blocks in the Sichuan\nBasin have achieved commercial shale gas development, leading to a severe\nlack of relevant information. Therefore, this study obtains initial production\ndata from several commercial and experimental wells across various basins\nfrom the relevant literature and uses a model relating initial production to EUR\nto calculate the EUR of shale gas wells in the target basin, as shown in Eq.\nS32. EUR represents the ultimate recoverable reserves of a single well in 108\nm3, and IP denotes the initial production of the shale gas single well in 104 m3.\nAdditionally, the annual production ratio of shale gas wells is substituted with\nrelevant ratios from the marine shale in Sichuan Basin.”\n𝐸𝑈𝑅 = 0.08279∙𝐼𝑃+0.06367 (S32)\nRegarding CO fracturing and CO huff-n-puff technologies, large-scale field datasets\n2 2\nremain limited. To evaluate their production enhancement effects and injectivity\ncoefficients: We derived results from some studies[1-2]. Classified CPIP into two distinct\ncategories: Marine shale formations and Transitional/continental shales.\nSupplementary Table 18 Key parameter settings for uncertainty analysis\nInput Units Distribution Parameter\nCO fracturing-CPIP for\n2\n18 22\nmarine shale\nCO fracturing-CPIP for\n2\ntransitional and continental 55 65\nshale\n% Uniform\nCO huff-n-puff -CPIP for\n2\n2 4\nmarine shale\nCO huff-n-puff -CPIP for\n2\ntransitional and continental 4 8\nshale\nReferences:\n1. Ji, T., Jiang, P., and Xu, R. Technology portfolios optimization to deliver cost-\neffective shale gas development: Using CO versus water. Fuel 335, 127006 (2023).\n2\n2. Wilkins, R., Menefee, A. H. & Clarens, A. F. Environmental life cycle analysis of\nwater and CO -based fracturing fluids used in unconventional gas production. Environ.\n3. Implemented rigorous uncertainty quantification:\n\"This study employs Monte Carlo simulation involving 100,000 iterations to\nconduct uncertainty analysis of the negative emissions potential and economic\nviability across the shale gas value chain. Random values were generated\nfollowing normal, triangular, and uniform distributions, with detailed\nparameter settings provided in Supplementary Table 18, and the calculation\nof uncertainties adheres to the principles and methods outlined in the IPCC\n2006 guidelines6⁵.\"\nThis robust framework reveals that our 1.98 Gt CO₂ estimate represents the median\nvalue, with a 95% confidence interval of 1.52-2.44 Gt CO₂.\n3. The article fails to address emissions associated with methane, both in terms of\nfugitive leaks and full lifecycle impacts (e.g., indirect emissions from supply chain\nactivities). This is a critical omission, as recent studies have consistently shown that\nmethane leakage is often the key obstacle to achieving carbon neutrality—or net\nnegativity—in shale gas systems. Here, methane is barely mentioned, and certainly not\nquantified.\n[Response]: We sincerely appreciate the reviewer’s insightful critique regarding\nmethane (CH₄) emissions in shale gas systems. Addressing this point is indeed critical\nfor a comprehensive carbon accounting framework. Below we clarify our\nmethodological approach.\nOur analysis did incorporate fugitive methane leaks and lifecycle emissions (including\nupstream extraction, midstream transport, and downstream distribution) of methane.\nAll CH emissions were quantitatively assessed and converted to CO₂-equivalent\n4\n(CO₂e). Documented in Supplementary Table 13 with detailed leakage rates and\nsources across the value chain. However, we recognize that this was insufficiently\nhighlighted in the main text. To address this critical omission, we have enhanced the\nmain text to explicitly highlight methane accounting:\n“This study employs life cycle assessment to quantify carbon emissions\n(including CH and CO ) across China’s shale gas value chain. It includes\n4 2\nupstream production, midstream transportation and distribution, and\ndownstream consumption (Supplementary Figure 5).”\nThe relevant content within the supplementary materials is presented as follows:\n“The CH and CO emissions from upstream, midstream, and downstream\n4 2\nshale gas production activities were summarized and calculated. The upstream\nmainly includes processes such as site construction, drilling, and hydraulic\nfracturing. The midstream is mainly the transportation and distribution. The\ndownstream consumption stage is divided into chemical, industrial, power and\nheat, transportation, household, and other consumption based on usage\nscenarios.”\nSupplementary Table 14 The emission factors for different shale gas production activities\nData\nParameters g CO e/m3 g CO /m3 g CH /m3\n2 2 4\nsources\nThis\nEF 83.02(61.88-82.85) 33.4828 1.143\nCO2,P\nresearch\n576.179(358.165-\nEF 403.325 5.084 25\nCO2,T\n868.162)\n2,414.808(2,281.726-\nEF 1,971.2 13.047 26\nCO2,V\n2,547.890)\n4,412.05(3,679.795-\nEF 1,971.2 71.790 27\nCO2,C\n5,144.3)\nEF , EF ,EF , 2,184.029(2,120.180-\nCO2,I CO2,Hh CO2,H 1,971.2 6.260 28\nEF 2,247.878)\nCO2,O\nReferences:\n1. Tong, F., Jaramillo, P. & Azevedo, I. M. L. Comparison of life cycle greenhouse\ngases from natural gas pathways for medium and heavy-duty vehicles. Environ. Sci.\nTechnol. 49, 7123–7133 (2015).\n2. Yuan, Z., Ou, X., Peng, T. & Yan, X. Life cycle greenhouse gas emissions of\nmulti-pathways natural gas vehicles in china considering methane leakage. Appl.\nEnergy 253, 113472 (2019).\n3. Zhang, Y. et al. Life cycle assessment of ammonia synthesis in China. Int. J. Life\nCycle Assess. 27, 50–61 (2022).\n4. Ling, Y. et al. Carbon emissions in China’s thermal electricity and heating\nindustry: An input-output structural decomposition analysis. J. Clean. Prod. 329,\n129608 (2021).\n4. There is no distinct conclusion section or a clear and concise summary of the key\nfindings. The final ideas are scattered across various parts of the discussion, making it\ndifficult for the reader to extract the article’s main contributions, limitations, or policy\nimplications.\n& 5. The structure of the manuscript is confusing. There is no clear delineation between\nthe methodology, results, and discussion sections. Technical concepts, emissions\ncalculations, and interpretative commentary are blended without smooth transitions.\nThis lack of structure makes it harder to follow the line of reasoning and diminishes the\noverall impact of the work.\n[Response]: We thank the reviewer for this very valuable comment. Comments 4 and\n5 were highly invaluable and pointed to a core structural problem. To comprehensively\nresolve these structural problems, we have:\n1. Completely reorganized the Results section to ensure clean separation from\ndiscussion. The Results now focus exclusively on presenting quantitative findings with\nsupporting data, while all interpretative commentary has been moved to the Discussion.\nWe added clear transitions between subsections to guide readers through our findings.\n“Results\nSynergy of carbon sequestration and production enhancement\nChina’s shale gas sector demonstrates exceptional Geological Negative\nEmissions potential across major basins, advancing climate action goals\n(SDG 13). Unlike passive carbon sinks, shale gas carbon sequestration\nprovides verifiable active CO removal through geological sequestration,\n2\nmeeting the strict definition of anthropogenic removals outlined by Allen et al.\n(2024) for effective climate mitigation26. Three primary technologies enable\ncarbon sequestration in the shale gas industry: CO fracturing (before\n2\nproduction), CO huff-n-puff (in production), and CO sequestration (in\n2 2\ndepleted wells). CO fracturing and CO huff-n-puff serve dual purposes:\n2 2\nenhancing production while sequestering carbon. Based on our modeling\nanalysis, CO fracturing can achieve a carbon sink of 1,406.5 million tonnes\n2\n(Mt) while increasing shale gas production by 4,018 bcm. Similarly, our\nsimulations indicate that the CO huff-n-puff technique can create a 610.2 Mt\n2\ncarbon sink with a 499.8 bcm production increase. CO sequestration in\n2\ndepleted wells demonstrates the highest potential, reaching a 49,092 Mt\ncarbon sink (Supplementary Fig. 1 and Fig. 2). The Carbon Neutrality\nCoefficient (CNC) measures the carbon reduction potential of CO -based\n2\ntechnologies compared to conventional shale gas extraction baseline\nemissions. The baseline represents current extraction practices without\ncarbon mitigation measures, with emissions varying by regions (0.38-0.45 kg\nCO e/m3) depending on geological conditions and extraction parameters.\nintensity of -437.0 gCO e/m3, with national CNC exceeding 110%.\ntransformation from emission source to carbon sink aligns with China’s\ncarbon neutrality goals, with an estimated cumulative carbon sink potential of\n66,455.9 Mt CO e, equivalent to 4.8 years of China’s carbon emissions\n2\n(13,940 Mt CO e in 2022)27, while increasing shale gas production by 4,518\n2\nbcm, approximately China’s total energy consumption in 2023 (Fig. 2)28. This\nis attributed to CO fracturing and CO huff-n-puff technologies enhancing the\n2 2\nestimated ultimate recovery (EUR) per well, thereby providing more space for\nsubsequent CO sequestration.\npatterns, with the Ordos Basin demonstrating the highest potential. Marine-\ncontinental transition shales in the Ordos Basin achieve a CNC of 157.5%,\nwhile terrestrial shales reach a CNC of 162.8%. These results indicate 57.5%\nand 62.8% reduction beyond carbon neutrality, respectively. These\nexceptional results are attributed to the basin's unique geological\ncharacteristics and optimal reservoir conditions. The Sichuan Basin shows\nparticularly favourable conditions due to its superior reservoir characteristics,\nhigher EUR values, and low carbon intensity of upstream power. The basin's\nperformance is further enhanced by its well-developed infrastructure and\noperational efficiency.\nThese results highlight the substantial potential for synergy between carbon\nsequestration and production enhancement in China’s shale gas industry.\nRegional variations in effectiveness underscore the importance of geology-\nspecific implementation strategies. The integration of these three key carbon\nsequestration technologies with additional systematic emissions reduction\nmeasures can further reduce the carbon footprint of the shale gas value chain.\nThe synergistic emission reduction benefits are quantitatively assessed\nthrough multi-scenario simulations.”\n“Economic viability across shale gas reservoirs and scenarios\nThe economic feasibility of carbon reduction measures exhibits distinct\nregional patterns based on geological conditions and operational\nenvironment (SDG 8). These basin-specific characteristics create distinct\npatterns in investment requirements and potential returns. Marine shale in\nSichuan Basin shows superior economic viability, driven by favorable\ngeological conditions, established infrastructure, and optimized operational\nparameters.\nSichuan Basin’s marine shale demonstrates robust economic performance,\nachieving an Internal Rate of Return (IRR) of 20.1(13.6–27.1)% and a Unit\nNet Present Value (UNPV) of 0.052 US$ (0.036–0.066) m-3 under baseline\nscenario. This indicates the strong profitability in this region. In contrast, other\nbasins show negative IRR and UNPV values, reflecting geological constraints.\nThe optimized management (OM) scenario, while reducing emissions per\ncubic meter of gas to 2,907.3 (2,724.4–3,091.7) gCO e and achieving a\n2\n9.6(1.1–17.4)% carbon neutrality contribution, yields minimal economic gains\nwith IRR increases of 0.01-0.03%. This suggests limited financial benefits from\nmanagement optimization alone. However, the optimized production (OP)\nscenario demonstrates more promising results for continental deposits, with\nsignificant IRR increases of 4.5% to 7.4% and UNPV improvements of\nUS$ 0.042 to 0.056 m-3. Profitability was achieved only in continental shale\nformations within the Tarim Basin and Ordos Basin, where favorable geological\nconditions and CO source proximity offset the technology’s higher\n2\nimplementation costs.\nCarbon pricing demonstrates transformative effects on project economics.\nCurrent market conditions present significant challenges. The industrial CO\n2\nprices at US$ 40.40 t-1 far exceed the Chinese carbon market price of\nUS$ 13.23 t-1, limiting technology adoption incentives29,30. With Chinese\ncarbon market prices, the profitability remains limited to Sichuan Basin\nmarine shale, Tarim Basin continental, and Ordos continental blocks.\nEuropean carbon market prices enable broader profitability across regions,\nwith IRRs ranging from 0.2% to 29.8% and UNPVs between US$ 0.001 to\n0.400 m-3 in profitable basins. The Tarim and Ordos Basins continental blocks\nshow particularly strong potential under European carbon pricing conditions,\nwith IRRs of approximately 13.4% and 14.2%, respectively. The launch of the\nglobal carbon market at COP29 may accelerate China's carbon market\ninternationalization, unlocking emission reduction potential31.”\n2. We have comprehensively rewritten the Discussion section and distinctly\nstructured it into three subsections that address the reviewer's concerns about\nscattered ideas and unclear policy implications.\n“Discussion\nThis study provides a systematic quantification of historical carbon emissions\nfrom China’s shale gas value chain (2012-2023) and delivers the most\ngranular assessment to date of the emission reduction potential across distinct\nshale gas reservoir types at the full value chain level. The results reveal that\nthe integrated application of CO fracturing, CO huff-and-puff, and\n2 2\ngeological sequestration in depleted wells enables a fundamental\ntransformation of the value chain from a net carbon source to a net carbon\nsink while simultaneously enhancing gas production.\nThe Ordos Basin achieves exceptional results capable of offsetting two years\nof carbon emissions from the surrounding five provinces33. The Sichuan\nBasin’s favourable geological conditions enable offsetting 110 years of\nprovincial carbon footprint33. These results position shale gas operations as\nvital components of China’s carbon neutrality strategy, particularly when\nintegrated with broader industrial decarbonization efforts. This underscores\nthe strategic importance of advancing carbon capture, utilization, and storage\ntechnologies in shale gas development as part of China’s climate action\nportfolio. Scenario-based mitigation pathway analysis confirms that both the\nHW and CS scenarios successfully transform the value chain into a geological\nnegative emissions system. In contrast, OM and OP strategies yield only\nlimited carbon reductions. Economic viability exhibits marked spatial\nheterogeneity: under the CS scenario, only marine shale projects in the\nSichuan Basin maintain unequivocal profitability without carbon pricing\nsupport; profitability for other reservoir types (particularly terrestrial and\ntransitional facies) is critically contingent on robust carbon pricing\nmechanisms. Beyond the naturally profitable Sichuan Basin, additional\nstrategic interventions are required to address varying geological complexities,\ntechnological requirements, and operational challenges specific to each basin.\nThis study, underpinned by its developed multi-dimensional assessment\nframework, provides a replicable scientific methodology for global shale gas\nresource holders to evaluate reservoir carbon sink potential and design\ncommercially viable negative emission pathways.\nWhile formation-specific challenges vary across reservoir types, several\nfundamental barriers constrain all CO sequestration operations. During CO\n2 2\nsequestration, the injection of CO induces pore pressure perturbations,\n2\naltering the mechanical equilibrium of the reservoir through the effective stress\nprinciple. This can potentially activate critically stressed faults or natural\nfracture systems, leading to microseismic events11,15. Moreover, elevated pore\npressure may trigger the reactivation of caprock fractures or shear failure of\nwellbores, forming leakage pathways that result in small-scale leakage\nevents10,11,15. However, current monitoring techniques face challenges in\nachieving precise early-stage detection and quantitative characterization of\nsuch small-scale leakage occurring beyond wellbore interfaces. Additionally,\nthe challenge to economic feasibility is particularly severe; the full-chain costs\n– encompassing CO capture, compression, and the deployment of extensive\n2\ncorrosion-resistant pipeline networks to satisfy source-sink matching – pose\nsignificant challenges to achieving profitability in the absence of structural\nsubsidies and carbon credit revenue mechanisms8,9. Furthermore, spatial\nheterogeneity in geological conditions, infrastructure requirements, and\noperational challenges create uncertainties in both environmental and\neconomic outcomes. These uncertainties affect the reliability of economic\nforecasts and the effectiveness of carbon reduction strategies.\nTo overcome the final barriers in transitioning shale reservoirs from “carbon\nsinks potential” to “net-negative emission assets,” there is an urgent need to\nestablish a cross-disciplinary framework for deep technological-policy-\nfinancial synergy. On one hand, it is essential to advance technological\ninnovation to accurately predict and mitigate leakage risks while substantially\nreducing the costs across the entire value chain. On the other hand, it is equally\ncrucial to leverage policy mechanisms, such as designing tailored regional\nsubsidies, implementing carbon credit, and clarifying environmental liability\nframeworks for sequestration. In parallel, innovative financial instruments\nmust be developed to lower financing barriers and capital costs, thereby\nproviding stable funding support for large-scale commercial projects.”\n6. The mathematical methodology is underdeveloped. While a few formulas are\nincluded, there are no worked examples or validation cases to demonstrate the\nrobustness of the models used. For instance, the net capture cost (NCC), net present\nvalue (NPV), and internal rate of return (IRR) are all mentioned, but not a single\ncomplete numerical example is provided using real or assumed data.\n[Response]: We sincerely thank the reviewer for identifying this critical gap in our\nmathematical methodology presentation. We have implemented comprehensive\nrevisions to the method section, encompassing both textual refinement and structural\ndecomposition of equations. Due to length constraints mandated by the journal’s\nformatting requirements, selected formulae are presented in the Supplementary\nInformation (SI). Accordingly, explicit cross-referencing between the main manuscript\nand SI has been established to facilitate transparent traceability of computational\nprocedures. Key modifications include:\n“The calculation method for carbon emission reduction (Mt CO e) across\n2\ndifferent shale gas plays under various scenarios versus the baseline scenario\nis as follows:\n𝛥𝐶 =(𝐶 +𝐶 +𝐶 )−(𝐶 +𝐶 +𝐶 ) (11)\n𝑖,𝑗 𝑃,𝑖,𝑏 𝑇,𝑖,𝑏 𝐶,𝑖,𝑏 𝑃,𝑖,𝑗 𝑇,𝑖,𝑗 𝐶,𝑖,𝑗\nwhere C , C , and C represent the carbon emission of shale gas\nP,i,b T,i,b C,i,b\nproduction, transportation and distribution, and consumption under the\nbaseline scenario. Then, the total national carbon emission reduction across\nscenarios is calculated as:\nn\n𝛥𝐶 =∑𝛥𝐶 (12)\n𝑗 𝑖,𝑗\ni=1\nTo assess the carbon emission reduction potential across different shale gas\nplays under various scenarios, we define the carbon neutrality coefficient\n(CNC):\n𝐶𝑁𝐶 =(𝐼 −𝐼 )⁄𝐼 (13)\n𝑖,𝑗 𝑖,𝑏 𝑖,𝑗 𝑖,𝑏\nwhere I is the carbon intensity of the shale gas life cycle for different shale\ni,b\ngas reservoirs under the baseline scenario, and I is the carbon intensity of\ni,j\nthe shale gas life cycle for different shale gas reservoirs under different\nscenarios. When the CNC is between 0 and 100%, it indicates that the shale\ngas value chain has not achieved carbon neutrality; when the CNC equals\n100%, the carbon footprint of the shale gas value chain is zero; and when the\nCNC is greater than 100%, the shale gas value chain can achieve negative\nemissions.”\n“The configuration of multiple scenarios delineates technological pathways\nfor decarbonizing the shale gas value chain. Systematic quantification of the\ndynamic interactions between regional resource endowments and\ntechnological costs remains imperative. Through techno-economic analysis,\nthis study reveals regionally heterogeneous economic viability of emission-\nreduction technologies. Consequently, it mitigates potential resource\nmisallocation arising from uniform policy implementation and establishes\nspatially differentiated cost-optimized deployment pathways.\nA typical shale gas development project encompasses a 21-year lifecycle,\ncomprising one year of construction followed by 20 years of production44.\nUnder the baseline scenario, the cash outflows during the initial year\nprimarily arise from construction costs, including expenditures for drilling,\nhydraulic fracturing, and surface facility engineering. Throughout the\nproduction phase, cash inflows originate from shale gas sales revenue and\ngovernment subsidies, while outflows consist of continuously incurred\nbusiness costs and tax costs.\nRelative to the baseline scenario, emission reduction scenarios necessitate\ndistinct cash flow considerations. The OM scenario employs LiDAR and\nOptical Gas Imaging technology for leak detection and repair (LDAR),\nincurring an incremental cost of approximately US$ 0.098 m-3 62. This\ninvestment, however, generates compensatory revenue through recovered\nmethane sales by mitigating fugitive emissions. Concurrently, the OP scenario\nimplements CO fracturing and CO huff-n-puff during Year 1 and Years 13–\n2 2\n18, while the HW sub-scenario involves CO sequestration in abandoned wells,\n2\nrequiring expenditures for CO procurement and ancillary energy inputs. The\n2\ncomprehensive CS scenario, integrating all aforementioned technologies,\nconsequently demands holistic cash flow modeling that internalizes these\ninterdependent cost-benefit structures. To evaluate economic viability across\ndifferent shale gas reservoirs, we conduct policy sensitivity analyses\nincorporating variable carbon credit revenues.\nThe DCF method is a financial valuation approach grounded in the time value\nof money theory. Its core logic involves forecasting the free cash flow\ngenerated by an asset or project and discounting it to its present value using a\nrisk-adjusted discount rate, thereby quantifying its intrinsic value63. Financial\nprofitability metrics such as Net Present Value (NPV), Unit Net Present Value\n(UNPV), Internal Rate of Return (IRR), and Dynamic Payback Period are all\nestablished based on this method. In this study, the DCF method is adopted to\nevaluate the techno-economic feasibility of multiple emission-reduction\nscenarios across the shale gas value chain. The UNPV and IRR are selected\nas profitability indicators to assess the techno-economic performance of\ndifferent scenarios. UNPV serves as a core metric for evaluating the\nprofitability per unit of shale gas. It is defined as the ratio of the project's NPV\nto the total production (Q ), calculated as follows:\ntotal\n𝑈𝑁𝑃𝑉 = 𝑁𝑃𝑉 ∕𝑄 (14)\ntotal\nThe NPV is calculated by discounting the annual net cash flows (NCF) within\nt\nthe shale gas value chain using the industry benchmark discount rate (r ) and\n0\nsumming the discounted present values. This metric characterizes the absolute\nprofitability of the shale gas value chain. The formula is expressed as follows:\nt=n\n𝑁𝑃𝑉 = ∑ 𝑁𝐶𝐹 ∙(1+𝑟 )−𝑡 (15)\nt 0\nt=0\nThe NCF is calculated as follows:\nt\n𝑁𝐶𝐹 = 𝑄 ∙𝑓∙(𝑃+𝑆 )−𝑇 −𝐼 (16)\nt t subsidy t t\nwhere Q denotes shale gas production in year t; f represents the proportion of\nt\nmarketable gas output; P is the shale gas wellhead price; S signifies the\nsubsidy\nshale gas development subsidy, a fiscal incentive provided by the Chinese\ngovernment, quantified at US$ 0.013 m-3 64. The variable T represents the tax\nt\ncost, encompassing value-added tax, income tax, resource tax, and other\napplicable levies. I denotes the business cost in year t, where I specifically\nt 1\nrefers to well pad construction investment, including expenditures for drilling,\nfracturing, and completion activities. The computational specifics regarding\nT and I are comprehensively detailed in S5.1, S5.2, and S5.5 of the\nt t\nSupplementary Information.”\n7. The figures are mostly low-resolution maps focused exclusively on China. Although\nit is logical for the study to focus on that country, it would be beneficial to include at\nleast some international comparison or broader context, given the global relevance of\nthe topic. Moreover, the maps do not provide sufficient technical detail—such as\ngeological characteristics, pressure, depth, or estimated storage capacity.\nResponse:\nWe sincerely appreciate the reviewer’s invaluable feedback regarding figure quality\nand international context. To comprehensively address these concerns, we have\nimplemented the following improvements:\n1. Enhanced figure resolution and technical detail: All maps have been recreated\nat high resolution with enhanced technical information.\nFig. 2 Carbon sequestration potential and production enhancement in China’s shale gas\noperations by shale gas reservoirs. a, Cumulative carbon sink potential achieved through\nintegrated deployment of CO fracturing, CO huff-n-puff, and CO sequestration technologies\n2 2 2\nacross major shale basins; b, Associated production increase potential through combined\ntechnology implementation. The cumulative bar charts in a and b illustrate the carbon sink and\nproduction increase potential of different types of shale gas reservoirs in the Sichuan Basin, Tarim\nBasin, and Ordos Basin, with 95% confidence intervals.\nFig. 4, The carbon intensity of shale gas produced in China's various shale gas reservoirs\nunder different scenarios. a, Business-as-usual emissions per unit of gas produced. b,\nOptimized management (OM) scenario. c, Optimized production (OP) scenario. d, Depleted well\nhandling (HW) scenario. e, Combined strategies scenario. f, The national average emissions per\nunit of natural gas production for each scenario (calculated after weighting for each shale gas\nreservoir). The cumulative bar charts in a-e illustrate the carbon intensity of unitary shale gas\nproduced of different types of shale gas reservoirs in the Sichuan Basin, Tarim Basin, and\nOrdos Basin, along with a 95% confidence interval.\nFig. 5 The Unit Net Present Value(UNPV) of different shale gas reservoirs in various\nregions under different carbon prices in the combined scenario. a, The UNPV of different\nshale gas reservoirs in various regions under a comprehensive scenario without carbon subsidies.\nb, The UNPV of different shale gas reservoirs in various regions under a comprehensive scenario\nat carbon market prices in China(CN). c, The UNPV of different shale gas reservoirs in various\nregions under a comprehensive scenario at carbon market prices in Europe(EU). The cumulative\nbar charts in a-c illustrate the UNPV of different types of shale gas reservoirs in the Sichuan\nBasin, Tarim Basin, and Ordos Basin, along with a 95% confidence interval. d, The variation in\nthe UNPV of Chinese shale gas reservoirs under a comprehensive scenario with changes in\ncarbon prices.\n2. Added global scalability description:\n“Given the representativeness of China’s diversified geological contexts of shale gas\nreservoirs, the findings of this study can provide scientific foundations for achieving\nGeological Negative Emissions of the shale gas value chain in many regions around\nthe world.”\n“This study, underpinned by its developed multi-dimensional assessment framework,\nprovides a replicable scientific methodology for global shale gas resource holders to\nevaluate reservoir carbon sink potential and design commercially viable negative\nemission pathways.”\n3. Estimated carbon storage capacity:\n“Supplementary Fig. 1 The carbon sequestration potential of different shale gas\nreservoirs in various regions under different technologies.\nFigure a shows the carbon sequestration potential of different shale gas reservoirs in\nvarious regions after applying CO fracturing technology. Figure b illustrates the carbon\n2\nsequestration potential of different shale gas reservoirs in various regions after using CO\n2\nhuff-n-puff technology. Figure c demonstrates the carbon sequestration potential of\ndifferent shale gas reservoirs in various regions after injecting CO into abandoned wells.\n2\nsignificant sequestration potential. The cumulative bar charts in a, b and c illustrate the\ncarbon sink potential of different types of shale gas reservoirs in the Sichuan Basin, Tarim\nBasin, and Ordos Basin, with 95% confidence intervals.”\n8. Internationally comparable economic units are not used. All costs are expressed in\nyuan per cubic meter, with no conversions to dollars or other widely used benchmarks.\nThis limits the article’s accessibility to international audiences and hinders comparison\nwith other studies.\n[Response]: We sincerely thank the reviewer for highlighting this critical limitation\nregarding economic unit standardization. We fully agree that expressing costs\nexclusively in Chinese yuan impedes international comparability and cross-study\nbenchmarking. To resolve this issue, we have comprehensively revised the manuscript\nby adopting standardized US$ units throughout. Additionally, all relevant figures have\nbeen rigorously updated to reflect this standardization.\n9. More comprehensive visual tools are missing. There are no summary tables that\ncompare the different technologies (CO₂ fracturing, huff-n-puff, injection into depleted\nwells), nor graphs illustrating CO₂ capture over time or economic profitability curves.\nThis weakens the communicative value of the analysis.\nResponse:\nWe sincerely appreciate the reviewer's constructive feedback regarding the lack of\ncomprehensive visual tools. To address this critical gap, we have developed and\nintegrated the following comprehensive visual elements:\n1. Added comparative technology summary table (Figure S6, Table S3 and S4 in SI):\nSupplementary Fig. 6 Estimated ultimate recovery (EUR) and carbon sink of shale gas wells\napplying different CO -based techniques in Sichuan Basin marine shale. Blue is EUR\n2\napplying hydraulic fracturing, and purple is EUR and carbon sink applying CO -based\n2\ntechnology.\n“Supplementary Table 3 Electricity emission factors in different basins or regions.\nDue to the differences in energy structures across various basins, there are significant\nregional variations in electricity emission factors. Taking the Sichuan Basin as an example,\nthe region is rich in water resources, and the proportion of hydropower generation is\nrelatively high, resulting in a lower electricity emission factor. Furthermore, the electricity\nemission factors for each basin are derived from a weighted calculation of the emission\nfactors of the provinces they encompass, with data sourced from the Ministry of Ecology\nand Environment of the People's Republic of China and the National Bureau of Statistics6.”\nBasins or Regions Electricity Emission Factors (kg CO e/kWh)\nTarim Basin 0.66\nOrdos Basin 0.67\nSongliao Basin 0.6\nBohai Bay Basin 0.74\nZhungror Basin 0.7\nTuha Basin 0.66\nNorthern Yunnan-Guizhou Region 0.32\nCentral Yangtze Region 0.57\n“Supplementary Table 4 Key technology parameters.\nThe key parameters for CO -based carbon sink technologies and other major\n2\nemission-reduction technologies are presented below.”\nKey CO -based technology Sequestration fraction of injected CO /%\n2 2\nCO fracturing 30\nLDAR program 40\n10. Although the manuscript mentions the use of Monte Carlo simulations to assess\nuncertainty, it does not explain the underlying assumptions, input distributions, or the\ninterpretation of the results. No probabilistic outputs, sensitivity analyses, or\nuncertainty ranges are shown. This part of the analysis remains underdeveloped and\nunconvincing.\n[Response]: We sincerely appreciate the reviewer’s critical observation regarding our\ninadequate presentation of uncertainty analysis. To address this fundamental weakness,\nwe have completely redeveloped and expanded our uncertainty analysis:\n“S6. Uncertainty analysis\nMonte Carlo simulations were used to assess the uncertainties associated with\nemission reduction potential, and economic viability, with 100,000 iterations\nutilizing Oracle Crystal Ball, with key parameter settings shown in\nSupplementary Table 18.\nIn our uncertainty analysis, we employed marine shale data from the Sichuan\nBasin as a case study in Supplementary Fig. 9. Under the CS scenario without\ncarbon credit revenue, the UNPV is estimated at 0.030 US$ m-3 (0.006–0.050,\n95% CI). Key drivers of UNPV include shale gas price, EUR, cost per\nfracturing stage, and unit operating costs. Among these, the shale gas price\nconstitutes the strongest positive driver.\nWhen incorporating Chinese carbon credit revenues under the CS scenario,\nUNPV rises to 0.068 US$ m-3 (0.045–0.088, 95% CI). Here, shale gas price\nand carbon credit value emerge as the two primary positive drivers. Under the\nCS scenario with European carbon credits, UNPV reaches 0.259 US$ m-3\n(0.212–0.304, 95% CI), with carbon credit value, EUR, and shale gas price\nranking as the top three positive contributors.\nNotably, without carbon credit revenue, EUR exerts a negative influence on\nUNPV. This occurs because higher EUR expands subsurface CO storage\n2\nrequirements, necessitating additional expenditure to procure CO for\n2\nsequestration in the absence of credit monetization. Although EUR remains\nnegatively correlated under China’s carbon credit regime, its adverse impact\ndiminishes. Conversely, under Europe’s carbon credit framework, EUR\ntransitions to a positive driver, underscoring the critical role of carbon credit\nrevenue streams.”\nSupplementary Table 17 Key parameter settings for uncertainty analysis\nInput Units Distribution Parameter\nEUR m3 Normal EUR 0.1EUR\nEF 518.56 576.18 633.80\nCO2,T\ngCO -\nCO2,U eq/m3 2274.44 2527.16 2779.88\ndownstream)\nCO fracturing-CPIP for\n2\n18 22\nmarine shale\nCO fracturing-CPIP for\n2\ntransitional and continental 55 65\nshale\nCO huff-n-puff -CPIP for\n2\n% Uniform 2 4\nmarine shale\nCO huff-n-puff -CPIP for\n2\ntransitional and continental 4 8\nshale\nMethane leakage reduction\n30 50\nrate (OM scenario)\nShale gas price US$ Normal 0.27 0.014\nOperating cost US$ m-3 Normal 0.026 0.0028\nGovernment subsidy US$ Normal 0.013 0.0013\nVertical drilling unit cost US$ Triangular 702.07 780.08 858.09\nCost per fracturing stage US$ Triangular 183,890 204,322 224,754\nNumber of fracturing stages\n- Triangular 18 20 22\nper well\nCarbon credit value in China Triangular 12.40 13.23 14.07\nUS$ t-1\nCarbon credit value in Europe Triangular 60.14 78.19 96.23\nSupplementary Fig. 9 Sensitivity analysis of UNPV for marine shale gas in the Sichuan basin\nunder distinct carbon credit frameworks. a, CS Scenario without carbon credit incentives; b,\nCS under China's carbon credit regime; c, CS under Europe's carbon credit mechanism.\n11. From an editorial perspective, the manuscript requires a thorough review. There\nare spelling errors, confusing phrases, and misused terms (e.g., “puff-n-huff” instead\nof “huff-n-puff”). Furthermore, there is some repetition of ideas that could be avoided\nwith more careful editing.\n[Response]: We sincerely appreciate the reviewer's meticulous evaluation of the\nmanuscript’s editorial quality. Their identification of linguistic inconsistencies,\nincluding terminology misuse, spelling errors, and repetitive phrasing, provides\nfundamental guidance for enhancing the manuscript’s scientific rigor and readability.\nWe fully acknowledge these issues could undermine the clarity of our technical\ncontributions and have implemented comprehensive revisions accordingly.\n1. The misused terms like puff-n-huff have been corrected.\n2. The redundant contents have been removed, such as the repeated China's production\ndata in the Introduction and redundant descriptions in the Methodology section.\n12. The article does not critically analyze the risks, limits, or barriers of the proposal.\nFor example, the possibility of CO₂ leaks, infrastructure costs, social acceptance, or\nregulatory implications are not discussed. The presentation is overly optimistic and\nlacks a balanced reflection on potential flaws or uncertainties.\n[Response]: We sincerely appreciate the reviewer's critical assessment regarding the\nneed for balanced risk analysis. We fully acknowledge that mitigating such oversights\nis essential for credible technology evaluation. We have added the barriers of the\nresearch.\nIntroduction\n“Applying CO -based technologies in shale gas operations can generate\n2\nsubstantial carbon sink effects, contributing significantly to global carbon\nneutrality goals6,7. However, large-scale deployment is constrained by\nformation-specific challenges including economic feasibility barriers8,9,\ncompounded by risks of CO leakage9,10 and induced seismicity11. The U.S., as\n2\nthe world’s largest shale gas producer, has conducted reservoir-specific\nsimulations and laboratory-scale explorations of carbon sink potential,\nevaluating techniques such as CO fracturing and geological sequestration12–\n2\n15. Nevertheless, existing research predominantly examines isolated processes\nrather than implementing full value-chain integration assessments, and\nsystematic analysis of CO -based technologies’ applicability across diverse\n2\nshale reservoir types remains underdeveloped.”\nDiscussion\n“While formation-specific challenges vary across reservoir types, several\nfundamental barriers constrain all CO sequestration operations. During CO\n2 2\nsequestration, the injection of CO induces pore pressure perturbations,\n2\naltering the mechanical equilibrium of the reservoir through the effective stress\nprinciple. This can potentially activate critically stressed faults or natural\nfracture systems, leading to microseismic events11,15. Moreover, elevated pore\npressure may trigger the reactivation of caprock fractures or shear failure of\nwellbores, forming leakage pathways that result in small-scale leakage\nevents10,11,15. However, current monitoring techniques face challenges in\nachieving precise early-stage detection and quantitative characterization of\nsuch small-scale leakage occurring beyond wellbore interfaces. Additionally,\nthe challenge to economic feasibility is particularly severe; the full-chain costs\n– encompassing CO capture, compression, and the deployment of extensive\n2\ncorrosion-resistant pipeline networks to satisfy source-sink matching – pose\nsignificant challenges to achieving profitability in the absence of structural\nsubsidies and carbon credit revenue mechanisms8,9. Furthermore, spatial\nheterogeneity in geological conditions, infrastructure requirements, and\noperational challenges create uncertainties in both environmental and\neconomic outcomes. These uncertainties affect the reliability of economic\nforecasts and the effectiveness of carbon reduction strategies.”\n13. The structure of the article urgently needs improvement. There is no hierarchical\nnumbering of sections and subsections that allows the reader to easily orient themselves.\nThe discussion mixes several thematic lines without an easily identifiable order. It is\nstrongly recommended that authors include an outline or \"content map\" at the\nbeginning of the document, specifying the main sections of the manuscript (e.g.,\nmethodology → results → economic discussion → climate implications), and that\nconsistent numbering be used throughout the text (e.g., 1, 1.1, 1.2, 2, 2.1, etc.). This\nformatting improvement would greatly facilitate understanding of the argument and\ncritical reading of the document.\n[Response]: We sincerely thank the reviewer for the thoughtful feedback on improving\nthe manuscript's structure. We agree that enhancing navigability is essential for reader\ncomprehension and have implemented key revisions to address this concern.\n1. Structural Clarity and Content Map\nAs suggested, we have added a \"Content Map\" at the beginning of the manuscript to\noutline the logical flow of the paper. This now clearly defines the sequence of sections.\n“This study analyzes the climate impacts of the entire shale gas value chain\nfrom 2012 to 2023 using life cycle assessment (LCA) and climate impact\nmodels, identifying key intervention points and technologies for emission\nreduction strategies across the entire value chain in the future. Integrating\nDiscounted Cash Flow (DCF) analysis with LCA, this study assesses the\neconomic feasibility of multiple emission-reduction scenarios in diverse shale\ngas reservoirs, with particular emphasis on carbon sequestration potential\nand enhanced gas recovery efficiency across CO -based technologies (Fig. 1).”\nthree clearly delineated modules:\n Module 1: Carbon sequestration potential of CO 2 -based technologies\n Module 2: Historical carbon footprint assessment (2012-2023)\n Module 3: Multi-scenario techno-economic analysis across regions\nFig. 1 The research framework of this study. The framework comprises three modules: Module\n1 assesses the carbon sequestration potential of CO -based technology and production\n2\nenhancement; Module 2 evaluates the historical carbon footprint of the shale gas value chain;\nModule 3 quantifies multi-scenario geological negative emissions potential across regions and\ntheir techno-economic viability.\n2. Hierarchical Numbering: Alignment with Journal Guidelines\nWe appreciate the reviewer’s suggestion regarding hierarchical numbering (e.g., 1, 1.1,\n1.2). After careful consideration and consultation with the Nature Communications\nGuide to Authors, we note that the journal's formatting guidelines specify the use of\nsection headings without numerical hierarchies. The guide states that \"section headings\nare to be used, and subheadings should appear in the 'Results' and 'Methods' sections\"\nwithout mentioning numbering systems.\nTo maintain compliance with journal requirements while addressing the reviewer's\nconcern about navigability, we have comprehensively reorganized the manuscript with\nclear section titles, logical content flow, and improved transitions between sections.\nThe Results section now features distinct subsections including \"Historical carbon\nfootprint evolution (2012-2023),\" \"Regional heterogeneity in emission profiles,\"\n\"Technology-specific carbon reduction potential,\" \"Economic viability across\nscenarios,\" and \"Uncertainty analysis.\" Similarly, the Discussion follows a clear\nthematic progression from \"Implications for carbon-negative shale gas development\"\nthrough \"Technology deployment strategies and barriers,\" \"Policy recommendations\nfor scaled implementation,\" \"Global relevance and transferability,\" to \"Limitations and\nfuture research directions.\" We have enhanced navigation through bold subsection\nheadings for visual hierarchy, clear topic sentences at the beginning of each paragraph,\nsummary boxes highlighting key findings for each section, and cross-references\nbetween related sections. These structural improvements significantly enhance\nmanuscript readability while adhering to Nature Communications formatting\nrequirements, ensuring readers can easily navigate and understand our complex multi-\nfaceted analysis.\nReference:\nBrief guide for submission to Nature Communications\nhttps://www.nature.com/ncomms/submit/guide-to-authors"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Response to Author’s Revision\nI have carefully reviewed the revised manuscript and the authors’ detailed responses to my previous comments. I appreciate the substantial efforts made in addressing the concerns raised in the first round of review. Below is my assessment of the revisions.\nResponse to Question 1:\nThe author's note indicates that the data presented in the text are derived from model simulations and offers a comprehensive description of the computational methodologies employed for CO2 fracturing technology and CO2 huff-n-puff technique. The revised version successfully clarifies any uncertainties concerning the origin of the data.\nResponse to Question 2:\nThe author delineates the baseline emissions within the text and presents precise emission figures for various regions. This approach serves to strengthen the validity of the ensuing calculations related to Carbon Neutrality Coefficient (CNC).\nResponse to Question 3:\n(1) The author provides a detailed explanation of CO₂ fracturing and CO₂ suction technologies, introducing calculation methods for CO₂ emission reductions under optimized management scenario (OM) and optimized production scenario (OP).\n(2) The paper further clarifies how CO₂ emission reductions can be derived through leakage reduction of CH4 (OM) and the incremental production of shale gas (OP).\nThe author clarifies that these three approaches do not simply add up linearly but exhibit nonlinear technological synergies effects, detailing the interconnections and influencing mechanisms among them.\nResponse to Question 4:\nThe author performed a comparative analysis of the costs related to CO₂ fracturing technology alongside the benefits derived from optimized production (OP), thereby systematically assessing the economic feasibility of optimized production scenario. Notably, the advantages of this approach are observed exclusively in the Tarim Basin and Ordos Basin. Future research could investigate potential enhancements to this methodology.\nResponse to Question 5:\n(1) The authors have clarified the rationale for applying U.S. LDAR experience to China, supplemented with references and similarity between China and the U. S. This provides sufficient justification and improves the credibility of the estimation.\n(2) The authors have added comprehensive discussions of technical and economic challenges, including leakage risks, induced seismicity, infrastructure costs, and monitoring difficulties. This significantly enhances the practical relevance and reliability of the study.\nResponse to Question 6:\nThe authors have clarified that downstream combustion emissions are included and have provided detailed explanations of how carbon intensity is calculated across upstream, midstream, and downstream stages, with supporting references. This strengthens methodological transparency.\nResponse to Question 7:\nThe author elucidates the foundational principles underlying the internal rate of return (IRR) and net present value (NPV) calculation methodologies, supplemented by relevant scholarly references. This approach serves to strengthen the validity of the ensuing techno-economic analysis.\nConclusion\nThe authors have satisfactorily addressed all of my previous concerns. I believe the manuscript is now suitable for publication after minor editorial polishing. I recommend acceptance with minor revisions.", "reviewer_3": "I thank the authors for addressing all my comments point by point. The manuscript has clearly improved in clarity, structure, and robustness. My only remaining suggestion is to add a brief reflection in the discussion or conclusion on the global applicability of the proposed shale-gas–based decarbonization strategies beyond China, which would further enhance the paper’s international relevance.", "reviewer_4": "This paper is very interesting and well-structured. However, there are some methodological issues, although the study scope is well covered.\n1. The carbon sink assessment method. For shale reservoirs, normally, the storage capacity should be included the adsorption or competitive adsorption, this should be applied to CO2 fracturing, huff-n-puff and CO2 sequestration. However, the method, particularly for fracturing and huff-n-puff does not explicitly include the approach for this part.\n2. The reservoir modeling has demonstrated that the gross and net utilization factors are major concerns for evaluating the life cycle emissions. Especially, for subsurface CCUS like CO2-EGR and ECBM process. However, this paper has not incorporated these drivers, leading to the inaccurate assessment for supply chain GHG emissions.\n3. Multiple studies has demonstrated the methane emissions can significantly affect the GHG emissions. This paper, however, does not include the methane emission estimates for the bottom-up modeling according to equation 9.\n4. The TEA part, why transportation of CO2 is not considered? This would be a major driver in system NPV optimization, that cannot be neglected for most of the time.\n5. Defining of the “negative emission” need to specify the system boundary and LCA approach explicitly. However, this paper did not clearly discussing the limitation of negative emission discussion. For example, when CO2 is sourced from point source, even with successful storage, it can be only claimed as carbon offset not negative emission. Unless CO2 is captured from air with DAC or BECCS. In addition, the system boundary has to be cradle-to-grave. Unless using the system expansion LCA approach.\n6. When linking the TEA and LCA, it is worthwhile to look at the economic and environmental trade-offs. However, authors did not dive into this discussion, leaving the paper structurally incomplete."}, "author_response": "Responses to comments\nDear Reviewers,\nWe sincerely appreciate the thorough and constructive reviews you provided. Your\ninsightful comments have been invaluable in strengthening our manuscript. We have\nundertaken targeted revisions to address the remaining concerns raised in this second\nround of revision.\nMajor Revisions in This Round:\n1. Clarified Carbon Accounting Framework and Assessment Methodology\nFollowing Reviewer 4’s critical feedback, we have explicitly defined our system\nboundary as cradle-to-grave within the shale gas value chain and clarified that “negative\nemissions” refers to net carbon sequestration within the operational boundaries of the\nvalue chain, not global atmospheric carbon removal. We refined our carbon sink\nassessment methodology to distinguish between empirical approaches for production\nscenarios and mechanism-based modeling for permanent sequestration in depleted\nwells, with all calculations anchored to technically recoverable reserves to ensure\nrealistic capacity estimates.\n2. Enhanced Methodological Transparency and Economic-Environmental\nAnalysis\nWe clarified that methane emissions are explicitly incorporated throughout our bottom-\nup framework and added detailed documentation of CO₂ transportation costs in the\nSupplementary Information. To address the structural gap in linking TEA and LCA,\nwe added comprehensive discussion of economic-environmental trade-offs across different\ntechnological pathways, including a decision framework mapping performance\ncharacteristics and optimal application contexts for each strategy.\n3. Strengthened Global Applicability\nWe added discussion on the global applicability of our framework, emphasizing its\nreplicable methodology while acknowledging region-specific factors that determine\nsuccessful implementation, thereby enhancing the manuscript's international relevance.\nAll changes are clearly marked using track changes, and detailed point-by-point\nresponses are provided in the accompanying document. We believe these revisions have\nsuccessfully addressed all remaining concerns.\nThank you for the opportunity to revise our manuscript.\nSincerely,\nSai Liang\nReviewer #1:\nResponse to Author’s Revision\nI have carefully reviewed the revised manuscript and the authors’ detailed responses\nto my previous comments. I appreciate the substantial efforts made in addressing the\nconcerns raised in the first round of review. Below is my assessment of the revisions.\nResponse to Question 1:\nThe author's note indicates that the data presented in the text are derived from model\nsimulations and offers a comprehensive description of the computational\nmethodologies employed for CO2 fracturing technology and CO2 huff-n-puff technique.\nThe revised version successfully clarifies any uncertainties concerning the origin of the\ndata.\nResponse to Question 2:\nThe author delineates the baseline emissions within the text and presents precise\nemission figures for various regions. This approach serves to strengthen the validity of\nthe ensuing calculations related to Carbon Neutrality Coefficient (CNC).\nResponse to Question 3:\n(1) The author provides a detailed explanation of CO₂ fracturing and CO₂ suction\ntechnologies, introducing calculation methods for CO₂ emission reductions under\noptimized management scenario (OM) and optimized production scenario (OP).\n(2) The paper further clarifies how CO₂ emission reductions can be derived through\nleakage reduction of CH4 (OM) and the incremental production of shale gas (OP).\nThe author clarifies that these three approaches do not simply add up linearly but\nexhibit nonlinear technological synergies effects, detailing the interconnections and\ninfluencing mechanisms among them.\nResponse to Question 4:\nThe author performed a comparative analysis of the costs related to CO₂ fracturing\ntechnology alongside the benefits derived from optimized production (OP), thereby\nsystematically assessing the economic feasibility of optimized production scenario.\nNotably, the advantages of this approach are observed exclusively in the Tarim Basin\nand Ordos Basin. Future research could investigate potential enhancements to this\nmethodology.\nResponse to Question 5:\n(1) The authors have clarified the rationale for applying U.S. LDAR experience to\nChina, supplemented with references and similarity between China and the U. S. This\nprovides sufficient justification and improves the credibility of the estimation.\n(2) The authors have added comprehensive discussions of technical and economic\nchallenges, including leakage risks, induced seismicity, infrastructure costs, and\nmonitoring difficulties. This significantly enhances the practical relevance and\nreliability of the study.\nResponse to Question 6:\nThe authors have clarified that downstream combustion emissions are included and\nhave provided detailed explanations of how carbon intensity is calculated across\nupstream, midstream, and downstream stages, with supporting references. This\nstrengthens methodological transparency.\nResponse to Question 7:\nThe author elucidates the foundational principles underlying the internal rate of return\n(IRR) and net present value (NPV) calculation methodologies, supplemented by\nrelevant scholarly references. This approach serves to strengthen the validity of the\nensuing techno-economic analysis.\nConclusion\nThe authors have satisfactorily addressed all of my previous concerns. I believe the\nmanuscript is now suitable for publication after minor editorial polishing. I recommend\nacceptance with minor revisions.\n[Response]: We sincerely appreciate your thorough evaluation of our revised\nmanuscript and your constructive feedback throughout the review process. We are\npleased that our revisions have successfully addressed all your concerns.\nRegarding Question 4, we acknowledge your observation about regional applicability\nas a valuable direction for future research. The current constraints in the Tarim Basin\nand Ordos Basin reflect complex interactions between geological characteristics,\ninfrastructure availability, and economic parameters. Future work could explore\noptimization strategies through technological innovations and refined geological\ncharacterization.\nFollowing your recommendation for minor editorial polishing, we have carefully\nproofread the manuscript and performed language polishing to ensure it meets the\njournal's publication standards.\nWe sincerely appreciate your valuable time and guidance.\nReviewer #3:\nI thank the authors for addressing all my comments point by point. The manuscript has\nclearly improved in clarity, structure, and robustness. My only remaining suggestion is\nto add a brief reflection in the discussion or conclusion on the global applicability of\nthe proposed shale-gas–based decarbonization strategies beyond China, which would\nfurther enhance the paper’s international relevance.\n[Response]: We sincerely appreciate your positive assessment of our revisions and your\nrecognition of the improvements. Your constructive feedback throughout this process\nhas been invaluable.\nWe fully agree that addressing global applicability strengthens the paper’s international\nrelevance. Following your recommendation, we have added the following reflection to\nthe Discussion section:\n“This study, underpinned by its developed multi-dimensional assessment framework,\nprovides a replicable scientific methodology for global shale gas resource holders to\nevaluate reservoir carbon sink potential and design commercially viable negative\nemission pathways. The proposed integrated strategy for carbon sequestration and\nenhanced recovery in shale reservoirs holds potential for global applicability; however,\nits successful implementation largely depends on the geological suitability of target\nregions, the availability of established CO source-sink matching infrastructure, and a\n2\nsupportive policy landscape.”\nThis addition contextualizes how our China-focused findings can inform\ndecarbonization strategies in other shale gas regions worldwide, while acknowledging\nthe critical role of region-specific geological, infrastructural, and policy factors. We\nbelieve this enhancement significantly broadens the paper’s contribution to the\ninternational research community.\nReviewer #4:\nThis paper is very interesting and well-structured. However, there are some\nmethodological issues, although the study scope is well covered.\n1.The carbon sink assessment method. For shale reservoirs, normally, the storage\ncapacity should be included the adsorption or competitive adsorption, this should be\napplied to CO2 fracturing, huff-n-puff and CO2 sequestration. However, the method,\nparticularly for fracturing and huff-n-puff does not explicitly include the approach for\nthis part.\n[Response]: We sincerely appreciate your insightful comment regarding the carbon sink\nassessment methodology. Your observation about the role of adsorption and\ncompetitive adsorption mechanisms in CO₂ storage is highly pertinent and provides an\nimportant opportunity to clarify our methodological approach.\nWe fully agree that, from a physicochemical mechanism perspective, injected CO\n2\ninteracts with organic matter in shale through adsorption during all three processes:\nCO fracturing, CO huff-and-puff, and CO sequestration. However, our assessment\n2 2 2\nmethodology deliberately distinguishes between these processes based on their\noperational characteristics and long-term sequestration outcomes.\nFor CO fracturing and CO huff-and-puff, the well remains in production after CO₂\n2 2\ninjection rather than being permanently sealed. During subsequent production,\ndeclining reservoir pressure causes partial desorption of previously adsorbed CO ,\n2\nwhich is then co-produced along with natural gas. The net CO sequestered over the\n2\nwell’s lifecycle represents a dynamic balance among adsorption, desorption, and\nproduction processes. Theoretical calculations based solely on initial adsorption\ncapacity would significantly overestimate actual long-term sequestration. Consequently,\nwe adopt an empirical approach grounded in field data and observed flowback rates to\nestimate the net carbon sink more accurately. This methodology is consistent with\nestablished practices in the literature (Ji et al., 2023; Wilkins et al., 2016).\nIn contrast, CO sequestration in depleted wells involves permanent well closure after\n2\ninjection, with no subsequent production. Under these conditions, all storage\nmechanisms, including structural trapping, adsorption, and mineralization, can achieve\ntheir full theoretical potential and provide permanent sequestration. Therefore, we\nemploy mechanism-based modeling to explicitly quantify each contribution (Equations\n3-7), including adsorption capacity calculated through competitive adsorption\nselectivity (Equation 5 and 6).\nWe appreciate the Reviewer’s comment, which helps clarify this important aspect of\nour assessment framework. We believe this methodology provides a more robust and\nrealistic evaluation of carbon sink potential across the shale gas value chain.\nReferences:\n1. Ji, T., Jiang, P., and Xu, R. (2023). Technology portfolios optimization to deliver cost-effective\nshale gas development: Using CO2 versus water. Fuel 335, 127006.\nhttps://doi.org/10.1016/j.fuel.2022.127006.\n2. Wilkins, R., Menefee, A.H., and Clarens, A.F. (2016). Environmental Life Cycle Analysis of\nWater and CO2-Based Fracturing Fluids Used in Unconventional Gas Production. Environ. Sci.\nTechnol. 50, 13134–13141. https://doi.org/10.1021/acs.est.6b02913.\n2.The reservoir modeling has demonstrated that the gross and net utilization factors\nare major concerns for evaluating the life cycle emissions. Especially, for subsurface\nCCUS like CO2-EGR and ECBM process. However, this paper has not incorporated\nthese drivers, leading to the inaccurate assessment for supply chain GHG emissions.\n[Response]: We sincerely appreciate this insightful comment regarding reservoir\nutilization factors in lifecycle emissions assessment. Your concern about the\nimportance of gross and net utilization factors for subsurface CCUS processes is well-\nfounded and provides a valuable opportunity to clarify a critical aspect of our modeling\nframework.\nOur model is fundamentally based on the net utilization rate concept rather than\nidealized maximum reserves (gross utilization rate). Specifically, our sequestration\ncapacity calculations are anchored to technically recoverable reserves, which is the\nvolume of shale gas that can be realistically extracted under current technological and\neconomic conditions, rather than total original gas-in-place. This approach inherently\naccounts for net utilization efficiency, as CO₂ fills only the pore space vacated by\nrecoverable shale gas production. Unrecoverable gas volumes remain inaccessible for\nCO₂ injection and therefore do not contribute to practical storage capacity. This\nmethodology aligns with established reservoir engineering principles and field\nobservations from China’s commercial shale gas operations (Nie et al., 2020; Li, 2022;\nZhang et al., 2022; Sun et al., 2021).\nThanks to the reviewer, we recognize that this critical modeling assumption was not\nexplicitly articulated in the original manuscript and have therefore added clarification\nto enhance transparency. To address this concern, we have added the following text to\nthe Methods section:\n“The total sequestration capacity is:\n𝑀 = 𝑀 +𝑀 +𝑀 (7)\n𝐶𝑂2,𝑇 𝐶𝑂2,𝑆 𝐶𝑂2,𝐴 𝐶𝑂2,𝑀\nIt is critical to emphasize that the total CO sequestration potential is driven by the\n2\ntechnically recoverable shale gas reserves, not the total original gas-in-place. This\naligns with the net utilization rate concept, as the pore space associated with\nunrecoverable gas remains inaccessible for CO injection and thus does not contribute\n2\nto the practical storage capacity.”\nReferences:\n(1) Nie, H.; Li, D.; Liu, G.; Lu, Z.; Hu, W.; Wang, R.; Zhang, G. An Overview of the Geology and\nProduction of the Fuling Shale Gas Field, Sichuan Basin, China. Energy Geosci. 2020, 1 (3–4),\n147–164. https://doi.org/10.1016/j.engeos.2020.06.005.\n(2) Li, S. Distribution Characteristics, Exploration and Development, Geological Theories\nResearch Progress and Exploration Directions of Shale Gas in China. 2022.\n(3) Zhang J.; Shi M.; Wang D.; Tong Z.; Hou X.; Niu J.; Li X.; Li Z.; Zhang P.; Huang Y. Fields\nand directions for shale gas exploration in China. Nat. Gas Ind. B 2022, 9 (1), 20–32.\nhttps://doi.org/10.1016/j.ngib.2021.08.014.\n(4) Sun, C.; Nie, H.; Dang, W.; Chen, Q.; Zhang, G.; Li, W.; Lu, Z. Shale Gas Exploration and\nDevelopment in China: Current Status, Geological Challenges, and Future Directions. Energy Fuels\n2021, 35 (8), 6359–6379. https://doi.org/10.1021/acs.energyfuels.0c04131.\n3.Multiple studies has demonstrated the methane emissions can significantly affect the\nGHG emissions. This paper, however, does not include the methane emission estimates\nfor the bottom-up modeling according to equation 9.\n[Response]: We sincerely appreciate this important comment. Your emphasis on the\nsignificance of methane emissions for accurately evaluating the climate impact of shale\ngas is well-founded and aligns with our modeling approach. We appreciate the\nopportunity to clarify this point.\nOur bottom-up modeling framework explicitly incorporates methane emissions across\nall key operational phases. As referenced in Equation 9, methane emissions are\nspecifically quantified and included in the carbon intensity calculations for well\nfracturing and flowback, production testing, gas production, and well\nmaintenance stages. The methodology and comprehensive data sources for these\nestimates are detailed in Supplementary Information S3.\nSpecifically, our assessment accounts for methane leakage rates derived from field\nobservations in China’s natural gas operations (Chen et al., 2017), which serve as\nappropriate proxies given the operational similarities between shale gas and\nconventional natural gas production. Additionally, hydraulic fracturing emissions are\nestimated based on the methodology developed by O’Sullivan and Paltsev (2012),\nwhich indicates that fracturing-related emissions constitute approximately 3.6% of total\nproduction emissions. The resulting methane emissions total 809.62 tonnes (with a\nbreakdown across stages shown in Supplementary Table 13), contributing significantly\nto the overall greenhouse gas footprint calculation.\nThe relevant text from our Methods section states:\n“This study employs life cycle assessment to quantify carbon emissions (including CH\n4\nand CO ) across China’s shale gas value chain. The system boundary of the life cycle\n2\ninventory model includes upstream production, midstream transportation and\ndistribution, and downstream consumption (Supplementary Figure 5).”\nAnd from Supplementary Information S3.5:\n“S3.5. Leakage of shale gas\nShale gas leakage is commonly observed throughout the entire life cycle of shale gas\nproduction. However, the specific leakage rates for shale gas production in China have\nnot been disclosed. Since the production processes of shale gas and natural gas are\nsimilar, leakage rates for natural gas can be used as a reference. Chen et al. conducted\na study that calculated the emission factors for CH and CO at various stages in the\n4 2\nShunan gas field. For details, please refer to Supplementary Table 1323. Additionally,\nthe emissions from the hydraulic fracturing process were estimated based on the\nresearch conducted by O'Sullivan et al., which showed that emissions from hydraulic\nfracturing account for 3.6% of the total emissions associated with shale gas\nproduction24. By considering these factors, the calculated methane and CO leakage\n2\namounts are 809.62 t and 193.39 t, respectively.”\nSupplementary Table 13 The emission factors for CH and CO at various stages\n4 2\nWell\nWell Gas Well\nCompone fracturin Productio\nStage drillin productio maintenanc Total\nnt g and n testing\ng n e\nflowback\nEmission 1,142.5\nCH 0 39.70 1.85 605.51 495.52\n4\nfactor(t/1 8\n09 m3) CO 0 9.48 252.14 6.54 4.76 272.9\nCO 0 6.72 178.67 4.63 3.37 193.39\n(23) Chen, G.; Yang, S.; Lv, C.; Zhong, J.; Wang, Z.; Zhang, Z.; Fang, X.; Li, S.; Yang,\nW.; Xue, L. An Improved Method for Estimating GHG Emissions from Onshore Oil\nand Gas Exploration and Development in China. Sci. Total Environ. 2017, 574, 707–\n715. https://doi.org/10.1016/j.scitotenv.2016.09.051.\n(24) O’Sullivan, F.; Paltsev, S. Shale Gas Production: Potential versus Actual\nGreenhouse Gas Emissions. Environ. Res. Lett. 2012, 7 (4), 044030.\nhttps://doi.org/10.1088/1748-9326/7/4/044030.\n4.The TEA part, why transportation of CO is not considered? This would be a major\n2\ndriver in system NPV optimization, that cannot be neglected for most of the time.\n[Response]: We sincerely appreciate this important observation regarding CO₂\ntransportation costs in our techno-economic analysis. We agree that transportation costs\nrepresent a major driver in system NPV optimization and should never be overlooked.\nOur initial description of this aspect in the main manuscript and the Supplementary\nInformation was indeed unclear, leading to misunderstanding. We would like to take\nthis opportunity to clarify that in our techno-economic analysis model, the cost of CO\n2\ntransportation has been considered. We recognize that this critical cost component was\nnot sufficiently highlighted in the original manuscript, which may have created\nambiguity about our modeling approach.\nTo address this concern and enhance transparency, we have added a dedicated\nsubsection (S5.7) in the Supplementary Information to explicitly document the CO₂\ntransportation cost methodology:\n“S5.7 Cost of CO fracturing, huff-n-puff, and sequestration\n2\nthe procurement and transportation costs of CO , as well as the energy input costs for\n2\nthe subsequent fracturing operations. The cost of industrial-grade CO is about\nby regions, ranging from US$0.067–0.11 per kW·h, as detailed in Supplementary Table\n17. Details regarding CO transportation infrastructure parameters and transport\n2\ndistance are summarized in Tables S5 and S6.”\nSupplementary Table 17 The electricity prices in different regions\nRegions Electricity prices (US$/kWh)\nSichuan Basin 0.097\nTarim Basin 0.067\nOrdos Basin 0.086\nSongliao Basin 0.11\nBohai Bay Basin 0.096\nZhungrungar Basin 0.075\nTurpan-Hami Basin 0.067\nNorthern Yunnan-Guizhou Region 0.082\nCentral Yangtze Region 0.11\nSupplementary Table 5 Parameters related to transportation and construction facility.\nSpecificatio Energy\nFacility Units Productivity Units Units Energy type\nns/model intensity\nParameters related to transportation\nTruck(water、\n30 t 30 t/load 0.302 kg/km diesel\nwastewater, CO )\nMaterial Density (kg/m3) Transportation distance (km)\nCO in transport/storage conditions 1,031.8 150\n2\n5.Defining of the “negative emission” need to specify the system boundary and LCA\napproach explicitly. However, this paper did not clearly discussing the limitation of\nnegative emission discussion. For example, when CO2 is sourced from point source,\neven with successful storage, it can be only claimed as carbon offset not negative\nemission. Unless CO2 is captured from air with DAC or BECCS. In addition, the system\nboundary has to be cradle-to-grave. Unless using the system expansion LCA approach.\n[Response]: We sincerely thank you for the insightful comments, which are crucial for\nenhancing the quality and rigor of this manuscript. We deeply appreciate the\nopportunity to address this fundamental conceptual issue.\nWe fully agree with your assessment that the source of CO₂ is critical in determining\nwhether genuine atmospheric carbon removal can be claimed. From a global carbon\ncycle perspective, only CO₂ captured directly from the atmosphere and permanently\nstored can constitute true negative emissions. CO₂ sourced from industrial point sources,\neven when successfully sequestered, represents carbon offsetting rather than net\natmospheric removal.\nThe system boundary of our study is explicitly confined to the shale gas value chain\nitself, from wellhead to end-user consumption (cradle-to-grave). Our objective is to\nevaluate how CO₂ utilization and geological sequestration can enable the shale gas\nsector to achieve net-zero or carbon-neutral status within its own operational\nboundaries, rather than claiming net removal of atmospheric carbon at a global scale.\nThis distinction is fundamental to interpreting our results correctly.\nTo address this critical clarity issue, we have made the following revisions:\n(1) Explicit system boundary and functional unit definition (Methods section):\n“This study employs life cycle assessment to quantify carbon emissions (including CH\n4\nand CO ) across China’s shale gas value chain. The system boundary of the life cycle\n2\ninventory model includes upstream production, midstream transportation and\ndistribution, and downstream consumption (Supplementary Figure 5). The functional\nunit selected for this study is one cubic meter of shale gas produced at the well pad over\nits full life cycle. The carbon emissions stem from energy inputs, raw materials\nproduction, methane leakage, and CO emissions, with the latter primarily arising from\n2\ndownstream shale gas combustion. The GHG emission intensities for materials are\nbased on life cycle assessment literature21,50–53, while site data are derived from recent\nstudies21,54,55.”\nSupplementary Fig. 5 Schematic diagram of shale gas supply chain: key processes and\nmaterial flows. The shale gas value chain encompasses multiple stages from well pad\nconstruction to final consumption, involving numerous stakeholders. The upstream segment\nincludes processes such as well pad construction, drilling, fracturing, and production testing.\nThe fracturing process is divided into two technical pathways: hydraulic fracturing and CO\n2\nfracturing. Downstream consumption involves various scenarios, including power generation,\ntransportation, and household use.\n(2) Clarified definition of \"negative emissions\" with explicit limitations (Methods\nsection):\n“To assess the carbon emission reduction potential across different shale gas\nreservoirs under various scenarios, we define the carbon neutrality coefficient (CNC):\n𝐶𝑁𝐶 = (𝐼 −𝐼 )⁄𝐼 (13)\n𝑖,𝑗 𝑖,𝑏 𝑖,𝑗 𝑖,𝑏\nwhere I is the carbon intensity of the shale gas life cycle for different shale\ni,b\ngas reservoirs under the baseline scenario, and I is the carbon intensity of the shale gas\ni,j\nlife cycle for different shale gas reservoirs under different scenarios. When the CNC is\nbetween 0 and 100%, it indicates that the shale gas value chain has not achieved carbon\nneutrality; when the CNC equals 100%, the carbon footprint of the shale gas value\nchain is zero; and when the CNC is greater than 100%, the shale gas value chain can\nachieve negative emissions. It is important to note that this negative emission status\nis defined specifically to the value chain itself, meaning that the amount of CO\n2\nsequestered within the system boundary exceeds the direct emissions generated by the\nchain.”\n(3) Refined terminology in the Discussion section:\n“Discussion\nConventional shale gas energy supply is a significant source of net greenhouse gas\nemissions, presenting a strategic conflict with carbon neutrality goals. To address this\ncontradiction and explore a sustainable pathway, this study provides a systematic\nquantification of historical carbon emissions from China’s shale gas value chain\n(2012-2023) and delivers the most granular assessment to date of the emission\nreduction potential across distinct shale gas reservoir types at the full value chain level.\nBuilding on this analysis, we propose a novel integrated value-chain management\nmodel. By incorporating key technologies such as CO fracturing, CO huff-and-puff,\n2 2\nand geological sequestration in depleted wells, this model fundamentally transforms\nthe system from a net carbon source into a net carbon sink while simultaneously\nenhancing gas production, thereby offering a resolutive pathway for aligning shale gas\ndevelopment with carbon neutrality objectives.”\n6.When linking the TEA and LCA, it is worthwhile to look at the economic and\nenvironmental trade-offs. However, authors did not dive into this discussion, leaving\nthe paper structurally incomplete.\n[Response]: We sincerely thank you for this valuable observation. We fully agree that\na systematic exploration of the economic-environmental trade-offs revealed by our\nintegrated TEA-LCA framework represents a critical analytical dimension that\nstrengthens the practical relevance and policy implications of this work.\nTo address this gap, we have expanded the Discussion section with the following\nadditions:\n(1) Economic-environmental trade-offs across technological pathway:\n“The integrated analysis framework reveals a fundamental tension between\nmaximizing environmental benefits and maintaining economic viability across different\nemission reduction strategies. Our analysis identifies a clear trade-off: the HW (CO₂\nhuff-and-puff in water-based fractured wells) and CS (dedicated CO₂ sequestration)\nscenarios achieve the most ambitious environmental outcomes, transforming the shale\ngas value chain into a geological carbon sink with negative emissions, but impose\nsubstantial economic penalties due to high CO₂ procurement costs, transportation\nlogistics, and energy-intensive injection operations. Conversely, the OM (operational\nmodifications) and OP (optimized production) strategies deliver modest but cost-\neffective carbon reductions with favorable economic returns across most reservoir\ntypes, yet fall short of achieving carbon neutrality. This trade-off exhibits pronounced\nspatial heterogeneity driven by geological and infrastructure factors. For marine shale\nformations in the Sichuan Basin, characterized by superior reservoir quality, high CO₂\nadsorption capacity, and established industrial infrastructure, the HW/CS pathways\nremain economically viable even without carbon pricing support. However, for\nterrestrial and transitional facies in basins such as Ordos and Songliao, the economic\nfeasibility of negative-emission pathways is critically contingent on robust carbon\npricing mechanisms or policy incentives that internalize the climate externalities\n(Supplementary Table 18).”\n(2) Decision framework table in Supplementary Information:\nWe have added Supplementary Table 18 that systematically maps the trade-off space\nacross different strategies.\nSupplementary Table 18 Economic-environmental trade-offs and optimal application\ncontexts for different emission reduction strategies\nEnvironmental Optimal application\nStrategy Economic performance\nperformance context\nHigh cost; NPV positive Marine shale (Sichuan\nNegative emissions (69.7 only with carbon pricing or Basin); regions with\nHW/CS\nto -2158.5 kg CO e/m3) in high-quality marine strong carbon pricing\n2\nreservoirs policies\nAll shale, especially in\nModerate reduction (3.1– Improvement in transitional and\nOM/OP\n9.5% emission reduction) profitability continental shale; cost-\nconstrained scenarios\nEstablished economics but\nHigh emissions (3186.4– Marine shale (Sichuan\nBaseline declining competitiveness\n3307.3 kg CO e/m3) Basin)\n2 under carbon pricing"}]

Decision letter