title: Evolution of coalbed methane: insights from stable and noble gas isotopes
Creators: name: Chen, Biying
Publisher: name: University of Glasgow
Year: 2021
Subjects: subject: Coalbed methane, Noble gas isotopes, Gas loss, Southeast Qinshui Basin, Liupanshui coal district.

description: Coalbed methane (CBM) is an important energy source in many countries. The origin and post-generation evolution of methane in coalbeds are rather poorly understood despite the importance of assessment of reserves and developing exploration strategies. Stable isotopes are the typical geochemical tools to analyse the gas origin and subsequent evolution. Noble gases are widely used for identifying the source of fluids in reservoirs and the impact of the tectonic and hydrologic activities on hydrocarbon generation and preservation. In this PhD, I have studied the composition of gases from the Southeast Qinshui Basin, the most CBM productive region in China, and the Liupanshui Coalfield, a CBM development target in southern China. Methane in these two regions was formed in the late Triassic during burial and heating driven by magmatic activity in the Late Jurassic to Early Cretaceous. Cenozoic basin inversion brought the coals to a shallow depth. This study aims to establish how the gas formed, to estimate the contribution of magmatic heating on methane generation and the influence of basin exhumation on gas storage. Nineteen gas samples were taken in April 2017 from methane-producing wellheads along an E-W transect perpendicular to the boundary fault in Southeast Qinshui Basin (SQB) in northern China. The stable isotope compositions (δ13C= -35.2 to -30.2‰, δD= -194 to -155‰) indicates a thermogenic origin with limited microbial input. They are, however, lighter than expected based on coal maturity, perhaps due to adsorption/desorption fractionation during commercial gas extraction. He-Ne-Ar isotopes are a mixture of crustal-radiogenic gas with air-derived noble gases from groundwater. 4He concentrations (0.52 to 33.25 ppmv) and 4He/40Ar* ratios (0.06–1.74) are unusually low. He-Ne-Ar concentrations are consistent with the open system Rayleigh fractionation of noble gases derived from air-saturated water (ASW) with 4He/40Ar*=1 during gas extraction. The low 4He/40Ar* compared with average crust (5) or local production (13) values, implies that radiogenic 4He produced in the coals was lost prior to equilibrium between gas and water phase in the reservoir. This likely occurred during rapid exhumation in Cenozoic. The event may have led to the loss of up to 44% of the methane from the coal seams. The depletion of Xe and Kr without significant enrichment of Ne suggests the strong retention of Kr and Xe in SQB coals. Ten CBM samples were collected in April 2018 from the four blocks containing different mature coals in Liupanshui Coalfield (LPC) in southern China. Two gas samples were collected from the same two wells in the Dahebian block in October 2018. This is the first gas geochemical work in LPC. Methane is mainly thermogenic in origin based on the C1/(C2+C3) ratio (16-971), δ13CCH4 (-42.9 to -34.9‰) and δDCH4 (-206 to -140‰). The difference in major gas and stable isotope compositions between the four blocks of LPC is mainly due to the different extent of maturation of coals in each block. Noble gases in LPC are a mixture of air-derived gases from ASW with crustal gases in the reservoir. The lower proportion of nucleogenic Ne and 4He/40Ar* ratios in gases from Qingshan block compared with other blocks implies an evident He and Ne loss. The gas loss is also inferred to be the result of basin inversion. 20Ne/36Ar ratios (0.23-0.70) are higher than local ASW (0.16), requiring a re-dissolution fractionation process similar to that proposed for CO2 gas reservoirs (Gilfillan et al., 2008). 84Kr/36Ar and 132Xe/36Ar ratios in LPC are mostly higher than the modelled fractionation lines. This is explained by desorption of initial trapped Xe and Kr in the coal matrix. The Qingshan block gases contain the same depleted Kr and Xe pattern as SQB, suggesting the stronger adsorption capacity of local coals to Kr and Xe. Over 93% of He and 9% of Ar budget of the coal basins is estimated to have been lost during the gas loss process. It explains the absence of mantle volatiles in the SQB and LPC CBM. This demonstrates the importance of basin inversion on gas preservation in shallow CBM and shows that the light noble gases are essential for tracing such a process.