# Occurrence and Origin of H2S from Volcanic Reservoirs in Niudong Area of the Santanghu Basin, NW China

Authors: Xiangxian Ma; Guodong Zheng; Minliang Liang; Dianhe Xie; Giovanni Martinelli; Wasim Sajjad; Wang Xu; Qiaohui Fan; Liwu Li; Li Du; Yidong Zhao
Journal: Geofluids
 (2019)

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## Abstract

A series of samples including natural gas, formation water, and rocks were collected from volcanic rock reservoirs in the Niudong area of the Santanghu Oilfields and analyzed for their mineral and/or chemical compositions and sulfur and carbon isotopes in order to investigate the occurrence and origin of hydrogen sulfide (H2S). H2S was mostly dissolved in the formation water along with petroleum production in the study area. The δ34S values of on-well H2S samples varied in a range of 9.2‰ to 20.5‰, probably indicating thermochemical sulfate reduction (TSR) and/or thermal decomposition of organic sulfur-bearing compounds (TDS) as the genetic process for H2S. However, the chemical composition of formation waters from the Kalagang Formation (C2k) and their coefficient of desulfurization also revealed that TSR could be the main principle for H2S formation. Considering the regional geological background, especially the tectonic structures and thermal evolution features of the basin, it was concluded that H2S in the study area was dominantly produced by thermal genesis with TSR as a domain through interactions between hydrocarbons and aqueous sulfate dissolved from sulfate minerals.


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## Body

## 1. Introduction

Hydrogen sulfide (H2S) is one of the most perilous constituents of natural gases and also hazardous in several aspects such as diluting the proportion of hydrocarbons in natural gas, gravely altering its economic vitality, and being extremely lethal and corrosive to equipment used for oil and gas exploration and development [1, 2]. It is significantly important to study and get a better understanding of H2S’s occurrence and its origin for the reduction of health-related risks and to ensure safety, proper management of reservoirs, suitable construction facilities, and drilling-well design [3–5]. According to the literature, there are four major sources in geology for H2S from a viewpoint of genesis: (1) inorganic (volcanic) source [6], (2) bacterial sulfate reduction (BSR) [7, 8], (3) thermal decomposition of organic sulfur-bearing compounds (TDS) in oil or kerogen [5, 9], and (4) thermochemical sulfate reduction (TSR) [7, 10, 11]. Inorganic (volcanic) H2S genesis always occurs through volcanic activities and leads to a higher concentration of H2S, whose sulfur isotope is normally in a range of −1‰ to −6-7‰ V-CDT [6, 11]. BSR is the typical and important source of H2S genesis in sediments and many gas and oil reservoirs, being common in a low proportion of H2S contribution (sulfur concentration<3–5%). The BSR source for H2S genesis is generally considered to be active at temperatures below 80–100°C, and their sulfur isotopic ratios are mostly in a range of -5‰ and +5‰ V-CDT [7, 8, 11]. TDS normally takes place in the heating periods of organic matters and following petroleum formation. However, the H2S contribution of TDS is very low and the isotopic ratios of sulfur range from 4‰ to 12‰ V-CDT, being always related to the secondary recovery of petroleum by using steam and/or hot water [7, 11]. Finally, the most influential source for H2S production is TSR, contributing massive proportions of H2S in natural gases and presenting in numerous petroliferous basins and even in some metal sulfide deposits [5, 7, 9, 12, 13]. Their sulfur isotopic ratios are normally between 8‰ and 25‰ V-CDT [3, 14]. The TSR reactions often occur along with interactions between hydrocarbons and aqueous sulfate which is always derived from the dissolution of sulfate minerals (primarily anhydrite and also barite and celestite). Both experimental studies and field inspections have revealed that the TSR process was kinetically practicable at outset temperatures>100°C. Suitable temperature and pH values are the important essential factors to control the reaction rate and degree of TSR [8, 15–21]. A common chemical reaction of TSR can be summarized as follows [22].
(1)sulphate+petroleum⟶calcite+H2S±H2O±CO2±S±alteredpetroleumWith the progression of petroleum exploitation, unconventional oil and gas resources are the vital sources for exploration. Besides clastic and carbonate rocks, volcanic rocks as reservoirs are a potential target for oil and gas exploration as well in recent times [23]. Therefore, in the current scenario, the occurrence and origin of H2S in volcanic reservoirs should be an important topic for exploration and as a supplement to the basic theory. About 100 ppm of H2S was found in several wellheads in the Niudong area, Santanghu Basin, during the oil and gas development in 2010. Since then, growing concerns on the influence of H2S on workers’ health have led to the study on H2S and measures were set up to reduce the negative influences on health and equipment. However, before controlling H2S in these reservoirs, its occurrence and origin should be properly revealed. Therefore, a series of samples, including oil and gas, formation water, drilling core rocks, and on-well H2S precipitant were collected and analyzed for this purpose.


## 2. Geological Settings

The Santanghu Basin, bordered by the Junggar Basin to the west, the Tuha Basin to the south, and the Mongolia Gebi to the northeast, is a special petroleum-bearing basin with volcanic rocks as reservoirs in NW China (Figure1(a)). The basin in an area of 2.3×104km2 and with accumulative petroleum reserves of more than 3×108 tons has been discovered at present. Since the initial formation in the Silurian period, this basin has experienced several tectonic movements, inducing well-developed regional faults and strongly weathered local rocks [23–25]. Volcanic eruptions frequently occurred in and around the basin during the late Palaeozoic era which probably produced some fracture channels by faulting. However, the process of volcanism was extremely intensive during the Middle Permian period. A basalt layer in a thickness of 200–600 m was developed in several parts of the basin, mainly due to crustal thinning, lithospheric subsidence, and asthenosphere upwelling [25]. The present tectonic units of the Santanghu Basin can be divided into the northern thrust uplift zone, central depression zone, and southern thrusting nappe zone. The Niudong area belongs to the central depression in the front edge of the Tiaoshan uplift and pitches the Malang depression in the northwest-southeast direction with an area of 260 km2.Figure 1
(a) Location of the study area, structure units, and section map of the Santanghu Basin. (b) Lithological column and sedimentary evolution of the Carboniferous (modified after Song et al. [24]).
(a)
(b)The Carboniferous sequences in the study area can be classified into five formations (from bottom to top, Figure1(b)): the Donggulubasitao Formation (C1d), Jiangbasitao Formation (C1j), Bashan Formation (C2b), Harjiawu Formation (C2h), and Kalagang Formation (C2k). The Kalagang Formation is primarily composed of fundamental intermediate volcanic lava that is incorporated within basalt, andesite, and transitional rock types [24]. Beneath the Kalagang Formation is the Haerjiawu Formation which contains hydrocarbon source rocks. Hydrocarbons from these source rocks migrated vertically into the weathered volcanic crust through faults. Reservoirs developed oil wells in the areas where there were abundant faults, and the hydrocarbons are distributed mainly in the weathered volcanic crust near the faults. The volcanic rocks of the Haerjiawu Formation alternate with source rocks. Hydrocarbons were generated from these source rocks, migrated into the weathered volcanic crust directly or through the faults, and then accumulated to construct the petroleum reservoir in the study area [23].The maximum burial depth of the Kalagang Formation reached 1637 m in the study area, and the present strata temperature is about 55°C [26]. Since the Santanghu Basin is just located at the junction of several major tectonic belts such as the Tianshan and Altai Mountains tectonic zones, the geothermal conditions should be changed in geological evolution history. The ancient geothermal gradient could be higher than that at present [27]. The geothermal history of the study area indicates the Middle Permian magmatism which caused locally thermal anomalies, and both the Late Indosinian movement (Late Triassic and Early Jurassic period) and Late Yanshan movement (Late Jurassic period) contributed to the increase of ancient geothermal gradients in the basin and led to the maturation of organic matters in the source rocks [26]. The temperature in oil and gas reservoirs of the Kalagang Formation (C2k) reached more than 110°C during the Permian-Triassic period [26, 27].


## 3. Samples and Analysis

### 3.1. Sample Collection

Seventeen rock samples were selected from drilling cores of the Niudong area for the present study on geochemical and mineralogical characteristics of the volcanic rocks (Table1). All the cored samples were rocks in the reservoir of Kalagang formation (C2k), including 9 basalts, 4 andesites, 2 volcanic breccia, and 2 tuff samples. The basalts were characterized by greyish-green and brownness and contained oil spots, air holes, and cracks. The andesites contained few and small purple oil spots and holes. The volcanic breccia and tuff were pyroclastic rocks with greyish-green and taupe, respectively. Six samples with clear veins were selected, and the vein fillings were extracted by indoor drilling for sulfur and carbon isotope analysis (Table 2 and Figure 2). The veins were in white color and transparent, some of which were characterized in an X-type of joints.Table 1
General property of study samples from the Kalagang Formation in the Santanghu Basin.
Sample no.
Depth (m)
Description
NDYX-01
1401.59-1401.80
Dark grey basalt
NDYX-02
1402.25-1402.39
Dark grey basalt with oil infected
NDYX-03
1403.38-1403.51
Tauro fluorescence basalt
NDYX-04
1410.81-1411.07
Tauro fluorescence basalt
NDYX-05
1416.44-1416.59
Purple basalt
NDYX-06
1420.84-1420.94
Brown basalt with oil infected
NDYX-07
1423.48-1423.62
Greyish-green breccia
NDYX-08
1429.38-1429.61
Purple oil spot andesite
NDYX-09
1429.61-1429.72
Purple oil spot andesite
NDYX-10
1430.05-1430.20
Purple oil spot andesite
NDYX-11
1433.82-1433.91
Taupe oil spot basalt
NDYX-12
1437.36-1437.51
Purple oil spot andesite
NDYX-13
1439.38-1439.49
Greyish-green breccia
NDYX-14
1444.68-1444.85
Brown-grey fluorescent basalt
NDYX-15
1447.23-1447.37
Brown-grey fluorescent basalt
NDYX-16
1508.63-1508.75
Taupe tuff
NDYX-17
1509.48-1509.56
Taupe tuffTable 2
General properties of fillings in the crack of core samples obtained from the Kalagang Formation in the Santanghu Basin.
Sample no.
Depth (m)
Filling feature
Core
NDCT-01
1457.56-1457.68
Taupe
Dark grey basalt
NDCT-02
1552.85-1553.00
Transparent
Greyish-green basalt
NDCT-03
1540.69-1540.93
Greyish-green
Grey-brown basalt
NDCT-04
1508.63-1508.75
White
Taupe tuff
NDCT-05
1509.48-1509.56
White vein
Taupe tuff
NDCT-06
1430.05-1430.20
Milky white
Purple andesiteFigure 2
Photos showing the veins of cored samples.The samples for H2S isotopes were obtained from twelve production wells and one gas gathering station. For sampling, the mixture of natural gases was introduced into a pre-prepared cadmium acetate solution (Cd (CH3COO)2·3H2O), and then the precipitant of cadmium sulfide (CdS) was collected in situ by filtration for laboratory analysis.


### 3.2. Analytical Methods

All the rock samples without chemical pretreatment were crushed into powder by using an agate mortar and pestle. Mineralogical measurements were carried out by the powder X-ray diffraction (XRD) method, using a D/Max-3B X-ray diffraction (XRD) Bruker diffractometer equipped with a graphite monochromator and operated at 40 kV and 100 mA using Cu Kα radiation. A small portion of finely powdered sample crushed again in a clean mortar was properly mounted on a plastic holder (ϕ25 mm, depth 1 mm). Scanning of samples was performed over an interval of 2–60° (2θ) at a scanning speed of 2°/min for every 0.03° (2θ) step. Divergence, scattering, and receiving slits were 0.5°, 0.5, and 0.10 mm, respectively.Sulfur K-edge XANES analysis of well-crushed either cored rock or drilled vein samples were performed at beamline 4B7A of the Beijing Synchrotron Radiation Facility (BSRF), Beijing. Both the beam path and samples were positioned in a vacuum to overcome the scattering of X-ray and their absorption by air when the crushed samples were exposed to the X-ray beam with an incident angle of 45°. The X-ray fluorescence emitted during analysis was quantified by using a solid-state detector (Si). As a reference, the spectrum of the blank filter was also measured for the comparison purpose and the absorption of sulfur was negligible [28].The isotopic composition of sulfur (S) was analyzed by using an isotope mass spectrograph (Model MAT271) produced by Finnigan MAT Company. The mass range is 1 to 700 amu, and the mass resolution is 220 to 3000 used for gas composition and stable isotope analysis. CdS and CuO (copper oxide) obtained through precipitation were mixed in a 1 : 3 weight ratio into a quartz tube and rapidly heated to 850°C inside the reaction furnace for 30 minutes. Subsequently, the released gas was collected inside the cold trap by using liquid nitrogen for freezing the gas, and the vacuum pump was started to eliminate the impurities and pure SO2 was released. The obtained gas was introduced into the instrument for isotope analysis and the δ34S value of sulfide was ultimately obtained by using an international standard (CDT), with an accuracy of ±0.5‰.Calcite presented inside the vein fillings of rock samples was extracted by using a small driller and was subjected to carbon and oxygen isotopic analysis. About 50–100 mg of calcite samples was treated with pure phosphoric acid for 4 hours at 72°C under a vacuumed container. The released CO2 was analyzed for carbon and oxygen isotopes by using a Finnigan MAT253 plus mass spectrometer standardized with GBW04416. The obtained data of carbon and oxygen were reported in units/mL relative to the V-PDB standard. The precision for both δ13C and δ18O measurements was ±0.5‰.The composition of formation water and the concentration of H2S were measured by the Sangtanghu Oil Production Plant of Tuha Oilfield Company. A portable hydrogen sulfide detector was used to monitor the H2S concentration at the wellhead of Oilfield, the precision being better than 5%.


## 3.1. Sample Collection

Seventeen rock samples were selected from drilling cores of the Niudong area for the present study on geochemical and mineralogical characteristics of the volcanic rocks (Table1). All the cored samples were rocks in the reservoir of Kalagang formation (C2k), including 9 basalts, 4 andesites, 2 volcanic breccia, and 2 tuff samples. The basalts were characterized by greyish-green and brownness and contained oil spots, air holes, and cracks. The andesites contained few and small purple oil spots and holes. The volcanic breccia and tuff were pyroclastic rocks with greyish-green and taupe, respectively. Six samples with clear veins were selected, and the vein fillings were extracted by indoor drilling for sulfur and carbon isotope analysis (Table 2 and Figure 2). The veins were in white color and transparent, some of which were characterized in an X-type of joints.Table 1
General property of study samples from the Kalagang Formation in the Santanghu Basin.
Sample no.
Depth (m)
Description
NDYX-01
1401.59-1401.80
Dark grey basalt
NDYX-02
1402.25-1402.39
Dark grey basalt with oil infected
NDYX-03
1403.38-1403.51
Tauro fluorescence basalt
NDYX-04
1410.81-1411.07
Tauro fluorescence basalt
NDYX-05
1416.44-1416.59
Purple basalt
NDYX-06
1420.84-1420.94
Brown basalt with oil infected
NDYX-07
1423.48-1423.62
Greyish-green breccia
NDYX-08
1429.38-1429.61
Purple oil spot andesite
NDYX-09
1429.61-1429.72
Purple oil spot andesite
NDYX-10
1430.05-1430.20
Purple oil spot andesite
NDYX-11
1433.82-1433.91
Taupe oil spot basalt
NDYX-12
1437.36-1437.51
Purple oil spot andesite
NDYX-13
1439.38-1439.49
Greyish-green breccia
NDYX-14
1444.68-1444.85
Brown-grey fluorescent basalt
NDYX-15
1447.23-1447.37
Brown-grey fluorescent basalt
NDYX-16
1508.63-1508.75
Taupe tuff
NDYX-17
1509.48-1509.56
Taupe tuffTable 2
General properties of fillings in the crack of core samples obtained from the Kalagang Formation in the Santanghu Basin.
Sample no.
Depth (m)
Filling feature
Core
NDCT-01
1457.56-1457.68
Taupe
Dark grey basalt
NDCT-02
1552.85-1553.00
Transparent
Greyish-green basalt
NDCT-03
1540.69-1540.93
Greyish-green
Grey-brown basalt
NDCT-04
1508.63-1508.75
White
Taupe tuff
NDCT-05
1509.48-1509.56
White vein
Taupe tuff
NDCT-06
1430.05-1430.20
Milky white
Purple andesiteFigure 2
Photos showing the veins of cored samples.The samples for H2S isotopes were obtained from twelve production wells and one gas gathering station. For sampling, the mixture of natural gases was introduced into a pre-prepared cadmium acetate solution (Cd (CH3COO)2·3H2O), and then the precipitant of cadmium sulfide (CdS) was collected in situ by filtration for laboratory analysis.


## 3.2. Analytical Methods

All the rock samples without chemical pretreatment were crushed into powder by using an agate mortar and pestle. Mineralogical measurements were carried out by the powder X-ray diffraction (XRD) method, using a D/Max-3B X-ray diffraction (XRD) Bruker diffractometer equipped with a graphite monochromator and operated at 40 kV and 100 mA using Cu Kα radiation. A small portion of finely powdered sample crushed again in a clean mortar was properly mounted on a plastic holder (ϕ25 mm, depth 1 mm). Scanning of samples was performed over an interval of 2–60° (2θ) at a scanning speed of 2°/min for every 0.03° (2θ) step. Divergence, scattering, and receiving slits were 0.5°, 0.5, and 0.10 mm, respectively.Sulfur K-edge XANES analysis of well-crushed either cored rock or drilled vein samples were performed at beamline 4B7A of the Beijing Synchrotron Radiation Facility (BSRF), Beijing. Both the beam path and samples were positioned in a vacuum to overcome the scattering of X-ray and their absorption by air when the crushed samples were exposed to the X-ray beam with an incident angle of 45°. The X-ray fluorescence emitted during analysis was quantified by using a solid-state detector (Si). As a reference, the spectrum of the blank filter was also measured for the comparison purpose and the absorption of sulfur was negligible [28].The isotopic composition of sulfur (S) was analyzed by using an isotope mass spectrograph (Model MAT271) produced by Finnigan MAT Company. The mass range is 1 to 700 amu, and the mass resolution is 220 to 3000 used for gas composition and stable isotope analysis. CdS and CuO (copper oxide) obtained through precipitation were mixed in a 1 : 3 weight ratio into a quartz tube and rapidly heated to 850°C inside the reaction furnace for 30 minutes. Subsequently, the released gas was collected inside the cold trap by using liquid nitrogen for freezing the gas, and the vacuum pump was started to eliminate the impurities and pure SO2 was released. The obtained gas was introduced into the instrument for isotope analysis and the δ34S value of sulfide was ultimately obtained by using an international standard (CDT), with an accuracy of ±0.5‰.Calcite presented inside the vein fillings of rock samples was extracted by using a small driller and was subjected to carbon and oxygen isotopic analysis. About 50–100 mg of calcite samples was treated with pure phosphoric acid for 4 hours at 72°C under a vacuumed container. The released CO2 was analyzed for carbon and oxygen isotopes by using a Finnigan MAT253 plus mass spectrometer standardized with GBW04416. The obtained data of carbon and oxygen were reported in units/mL relative to the V-PDB standard. The precision for both δ13C and δ18O measurements was ±0.5‰.The composition of formation water and the concentration of H2S were measured by the Sangtanghu Oil Production Plant of Tuha Oilfield Company. A portable hydrogen sulfide detector was used to monitor the H2S concentration at the wellhead of Oilfield, the precision being better than 5%.


## 4. Results and Discussion

### 4.1. Occurrence of H2S

During the first recovery of oil in the Santanghu Oilfield, the H2S content was reported in the oil-wellhead as 10–120 ppm/m3air in 2010, and the relative content of H2S in the dissolved air of formation water reached 0.19%. The average pressure value of reservoir strata C2k in the study area was 10.5 MPa, of which the H2S-bearing wells were mostly located in the area with low pressure (3–9 MPa) [29], and fractures were strongly developed in the surrounding rocks. Furthermore, the oil-wells containing H2S more than 100 ppm were always characterized with a higher water cut (65–100%) while other oil-wells in lower H2S concentration (<30 ppm) were assigned to a lower water cut (<60%). Thus, H2S was mainly distributed in oil-wells with low formation pressure, high water cut, and fractures developed and mostly dissolved in formation water.


### 4.2. Minerals and Sulfur Characteristics of Reservoir Rocks

The reservoir rocks of the Santanghu Oilfields are typical volcanic rocks of the Carboniferous Kalagang Formation, which are dominated by basalt, andesite, volcanic breccia, and tuff [23]. The SiO2 content of the volcanic rocks is 44% to 65%. There are a number of air holes and almond constructs in the rocks so that these rocks have relatively high porosity and proper connectivity; the porosity is 6% to 11% [23]. Weathering and leaching during the cessation of deposition should be the major controlling factor of favorable reservoirs in the Carboniferous volcanic rocks, and the secondary minerals such as clay minerals could be largely formed. The hydrolysis belt was mainly composed of mudstone and tiny volcanic grains, most of which were broken down into clay minerals [23]. The cracks or vein fillings in the core of volcanic rocks were widely developed and mainly consisted of volcanic zeolites, quartz, and/or calcite (Figure 3).Figure 3
Powder X-ray diffraction patterns for the study samples.Sulfur species were herein referred to as the chemical status of sulfur, which was identified using XANES in this study by comparison of the obtained spectra for samples with those of selected sulfur-containing reference materials such as pyrite (FeS2), pyrrhotite (FeS), calcium sulfate (CaSO4), and sulfur (S0). The impact of self-absorption associated with reference materials was investigated by using well-crushed mixtures of boron nitride in various concentrations of sulfur. The self-absorption impact was not detected in sulfur concentration<0.5wt.% [28]. The peak positions obtained from the cored samples were mostly positioned at two series of comparable energy range to those presented in the spectra of pyrite, pyrrhotite, and calcium sulfate, signifying that the foremost sulfur species were sulfide and sulfate, respectively. The absorbing peaks presented on the left (in range of 2.470–2.474 keV) indicate the presence of S2-, S22-, and also possibly elemental sulfur (S0). On the other hand, the peak presented on the right (in range of 2.481–2.484 keV) denotes SO42- species (Figure 4). The spectra thus obtained for all samples are shown in Figure 4, in which several composite variations within numerous sulfur species are displaced based on their spectrum structures as well as the location of the peak that is signified as sulfides and/or sulfate, respectively. In this study, it was found that both the sulfate (more than 90%) and sulfide minerals widely coexisted in the reservoir rocks, which could provide an effective sulfur source for the genesis of H2S gas within the reservoirs.Figure 4
Spectra of sulfur species in the study samples. (a) Filling materials in the veins of cored rocks and (b) core samples. The absorbing peak on the left with a lower energy range corresponds to sulfides whereas the peak on the right with a higher energy range corresponds to sulfate.
(a)
(b)


### 4.3. Characteristics of Formation Water

The chemical compositions of formation water are shown in Table3 and Figure 5. Relatively most samples belong to the group of Na(K)-Cl or Ca-Cl2 water while the existence of Ca(Mg)-SO4 waters indicates possible evolutionary trends toward SO4-enriched waters. There are no samples belonging to the Ca(Mg)-HCO3 and Na(K)-HCO3 groups in the study area. In particular, the Na++K+ concentration of formation water in Kalagang Formation (C2k) was in a range of 69–1008 mg/L and the Cl− concentration in a range of 309–1955 mg/L, respectively. The range of Mg2+ concentration of C2k formation water was from 1 to 291 mg/L with an average of 56.6 mg/L. Generally, the higher the Mg2+ concentration, the higher would be the H2S yields, because ion Mg2+ could play a catalysis role in TSR [30–32]. The range of SO42− concentration was from 27 to 1288 mg/L. The coefficient of desulfurization (100×r SO42-/Cl-) was in a range of 2–160 with a mean value of 30. This coefficient could be used to reflect consumption of SO42- during TSR and the redox conditions of formation water. The low mark means reduced condition and active TSR reactions [32]. The H2S contraction rose with a decrease in the coefficient of desulfurization (Figure 6), exhibiting that TSR really occurred in the study reservoirs. The CO32- concentration in most water samples was low (mostly not detected), and the average of HCO3- was 335.8 mg/L. In addition, the concentration of HCO3- decreased with the process of TSR reactions, strongly indicating the dilution of additional water under TSR conditions. This kind of dilution might be lowering the concentration of CO32- and HCO3- in formation waters of volcanic reservoirs.Table 3
Hydrochemistry of the formation water from the Niudong area, Santanghu Basin.
Sample no.
Stratum
Sampling date
H2S (ppm/m3 air)
mg/L
Type
pH
K++Na+
Ca2+
Mg2+
Cl-
SO42-
HCO3-
CO32-
Total salinity
ND8-14
C2k
2008/8/7
n.d.
410
864
291
1755
920
710
268
5218
CaCl2
9.3
ND78-9
C2k
2008/8/6
115
592
545
1
1652
32
164
n.d.
2986
NaCl2
7.3
ND78-812
C2k
2010/8/7
6
371
123
10
512
283
168
n.d.
1467
NaCl
6.0
ND89-10
C2k
2010/10/8
44
647
442
24
1375
329
397
n.d.
3214
CaCl2
7.0
ND8-11
C2k
2008/8/7
35
1008
683
67
1955
434
1171
n.d.
5318
NaCl
6.5
ND89-9
C2k
2008/11/4
31
592
521
14
1544
349
123
n.d.
3143
NaCl2
6.0
ND8-111
C2k
2010/5/1
13
194
566
98
1272
47
482
n.d.
2659
CaCl2
7.0
ND89-812
C2k
2010/8/24
25
69
315
21
309
495
85
n.d.
1294
CaSO4
6.0
ND9-9
C2k
2010/3/7
50
547
384
15
1393
188
57
n.d.
2584
CaCl2
7.0
ND89-91
C2k
2010/6/5
44
869
369
15
1324
495
596
n.d.
3668
NaCl
6.0
ND 8-71
C2k
2007/12/21
120
386
396
7
1264
27
36
10
2126
CaCl2
9.0
ND8-10
C2k
2007/10/16
13
394
873
122
1475
1288
143
n.d.
4295
CaCl2
7.7
ND8-8
C2k
2008/8/7
10
537
404
51
1368
255
234
n.d.
2849
NaCl2
6.5
Note: these data were provided by the Santanghu Oil Production Plant of Tuha Oilfield Company. pH was tested at room temperature.Figure 5
Pipe diagram for ion composition of study formation waters.Figure 6
Relationship between the coefficient of desulfurization ratios and H2S concentrations of oil wells.


### 4.4. Sulfur Isotopes of H2S

The sulfur isotopes of sulfates and/or sulfides presented in the volcanic rocks from wells ND89-9 and ND89-10 are quite close to each other, being 9.2‰ and 10.3‰, respectively. The natural gases at the Niudong pumping station have undergone a series of technological operations such as separation of oil and gas, whose sulfur isotopic ratios reached 20.5‰ (Table4). As shown in Figure 7, the δ34SV-CDT values for TSR-H2S in this study were comparable with those in the Sichuan Basin (main frequency: 8‰ to 26‰) [14], the Tarim Basin (14‰ to 19‰) [17, 33], and the Ordos Basin (16‰ to 20‰) [14]. Such comparison may indicate that H2S presented in the oil wells of the Niudong area could probably be derived from TSR. δ34SV-CDT of TDS-H2S in crude oils is usually from 3‰ to 15‰ [5]; thus, H2S in the Niudong area could also be due to TDS. However, there is lack of meaningful evidence for the possibility of inorganic (volcanic) origin and BSR involvement for H2S in the study area.Table 4
Sulfur and carbon isotope of samples obtained from the Kalagang Formation in the Santanghu Basin.
Well or sample no.
Type
δ
34SV-CDT‰
δ
13CV-PDB‰
δ
18OV-PDB‰
Note
Gas pumping station
Gas
20.5
ND89-9
Gas in water
9.2
ND89-10
Gas in water
10.3
NDCT-03
Rock
-10.7
-20.6
Calcite
NDCT-04
Rock
-5.8
-21.8
Calcite
NDCT-05
Rock
-5.7
-18.6
Calcite
NDCT-06
Rock
-3.3
-21.4
CalciteFigure 7
Correlation betweenδ34S values and H2S contents of samples from the Santanghu Basin and other referenced TSR H2S-bearing basins in China (data of Sichuan Basin, Tarim Basin, and Ordos Basin are from Zhu et al. [14] and Jiang et al. [22]).


### 4.5. Carbon Isotopes of Calcite

The isotopic data of the calcite matrix from the Kalagang Formation exhibited a relatively wider range of carbon isotopic values between -10.7‰ and -3.3‰ V-PDB and a narrower range of oxygen isotopic values between -21.8‰ and -18.6‰ V-PDB, respectively (Table4). These relatively negative carbon isotopic values may suggest a possible contribution of organic carbon into calcite, but the carbon and oxygen isotopic values failed on the boundary of TSR calcite [22] which was formed through TSR reactions. In addition, these carbon and oxygen isotopic values of vein calcite were also similar to those of primarily igneous carbonatite (δ13CV-PDB‰: -5.0‰ to -8.0‰, δ18OV-PDB‰: -21.7‰ to -23.7‰) [34], indicating the calcite formation with fluids from deep layers under low hydrothermal temperatures (<167°C) [35]. Therefore, calcite in the rock veins was mostly of magmatic origin and very less produced due to TSR reactions.


### 4.6. Origin of H2S

As for the low concentration of H2S in crude oils and natural gases, the BSR origin is always regarded as the first originating mechanism that may be caused by bacterial loadings during water injection. The sulfur isotopes of H2S in the study area were 9.2‰ to 20.5‰, suggesting that BSR was probably not the most possible source in the Niudong Oilfield, where H2S was found in the first recovery of oil without water injection in early oil and gas exploration. Additionally, the values of sulfur isotopes are not in favor of volcanic sources for the genesis of H2S. Therefore, TSR and TDS should be their possible origins that were supported by the sulfur isotopic data. However, TDS usually occurred during the secondary recovery of oil under a relatively higher temperature than under the initial temperature of TSR [5], but no steam and/or hot water was utilized during the secondary recovery of heavy oils in the study reservoir and the heavy oils was typically low in sulfur content (about 0.17%). Based on these facts, either TDS should not be the origin or slightly contributed if TDS occurred. As for TSR, although gypsum and/or anhydrite in the cored rocks was not observed by naked eyes, trace sulfate and sulfide were detected by XANES. Such sulfur-bearing materials in the reservoir rocks could supply enough sulfur for TSR reactions due to the huge volume of rocks. Secondly, the local geothermal temperature (>110°C) reached the reaction conditions for TSR during the Permian-Triassic period as for the volcanic activity [27]. Finally, some increase in H2S concentration was observed with a decrease in desulfurization coefficient, which confirmed the occurrence of TSR. Such formed H2S in the reservoir of volcanic rocks is easily dissolved in the formation water. On the other hand, CO32- and HCO3- concentrations in most samples were relatively low, indicating that the additional water formed along with TSR may dilute the concentrations of both CO32- and HCO3-. Considering all the evidences above, TSR should be the main source for H2S in the natural gases of the Niudong Oilfield, but the TDS origin could not also be excluded.


## 4.1. Occurrence of H2S

During the first recovery of oil in the Santanghu Oilfield, the H2S content was reported in the oil-wellhead as 10–120 ppm/m3air in 2010, and the relative content of H2S in the dissolved air of formation water reached 0.19%. The average pressure value of reservoir strata C2k in the study area was 10.5 MPa, of which the H2S-bearing wells were mostly located in the area with low pressure (3–9 MPa) [29], and fractures were strongly developed in the surrounding rocks. Furthermore, the oil-wells containing H2S more than 100 ppm were always characterized with a higher water cut (65–100%) while other oil-wells in lower H2S concentration (<30 ppm) were assigned to a lower water cut (<60%). Thus, H2S was mainly distributed in oil-wells with low formation pressure, high water cut, and fractures developed and mostly dissolved in formation water.


## 4.2. Minerals and Sulfur Characteristics of Reservoir Rocks

The reservoir rocks of the Santanghu Oilfields are typical volcanic rocks of the Carboniferous Kalagang Formation, which are dominated by basalt, andesite, volcanic breccia, and tuff [23]. The SiO2 content of the volcanic rocks is 44% to 65%. There are a number of air holes and almond constructs in the rocks so that these rocks have relatively high porosity and proper connectivity; the porosity is 6% to 11% [23]. Weathering and leaching during the cessation of deposition should be the major controlling factor of favorable reservoirs in the Carboniferous volcanic rocks, and the secondary minerals such as clay minerals could be largely formed. The hydrolysis belt was mainly composed of mudstone and tiny volcanic grains, most of which were broken down into clay minerals [23]. The cracks or vein fillings in the core of volcanic rocks were widely developed and mainly consisted of volcanic zeolites, quartz, and/or calcite (Figure 3).Figure 3
Powder X-ray diffraction patterns for the study samples.Sulfur species were herein referred to as the chemical status of sulfur, which was identified using XANES in this study by comparison of the obtained spectra for samples with those of selected sulfur-containing reference materials such as pyrite (FeS2), pyrrhotite (FeS), calcium sulfate (CaSO4), and sulfur (S0). The impact of self-absorption associated with reference materials was investigated by using well-crushed mixtures of boron nitride in various concentrations of sulfur. The self-absorption impact was not detected in sulfur concentration<0.5wt.% [28]. The peak positions obtained from the cored samples were mostly positioned at two series of comparable energy range to those presented in the spectra of pyrite, pyrrhotite, and calcium sulfate, signifying that the foremost sulfur species were sulfide and sulfate, respectively. The absorbing peaks presented on the left (in range of 2.470–2.474 keV) indicate the presence of S2-, S22-, and also possibly elemental sulfur (S0). On the other hand, the peak presented on the right (in range of 2.481–2.484 keV) denotes SO42- species (Figure 4). The spectra thus obtained for all samples are shown in Figure 4, in which several composite variations within numerous sulfur species are displaced based on their spectrum structures as well as the location of the peak that is signified as sulfides and/or sulfate, respectively. In this study, it was found that both the sulfate (more than 90%) and sulfide minerals widely coexisted in the reservoir rocks, which could provide an effective sulfur source for the genesis of H2S gas within the reservoirs.Figure 4
Spectra of sulfur species in the study samples. (a) Filling materials in the veins of cored rocks and (b) core samples. The absorbing peak on the left with a lower energy range corresponds to sulfides whereas the peak on the right with a higher energy range corresponds to sulfate.
(a)
(b)


## 4.3. Characteristics of Formation Water

The chemical compositions of formation water are shown in Table3 and Figure 5. Relatively most samples belong to the group of Na(K)-Cl or Ca-Cl2 water while the existence of Ca(Mg)-SO4 waters indicates possible evolutionary trends toward SO4-enriched waters. There are no samples belonging to the Ca(Mg)-HCO3 and Na(K)-HCO3 groups in the study area. In particular, the Na++K+ concentration of formation water in Kalagang Formation (C2k) was in a range of 69–1008 mg/L and the Cl− concentration in a range of 309–1955 mg/L, respectively. The range of Mg2+ concentration of C2k formation water was from 1 to 291 mg/L with an average of 56.6 mg/L. Generally, the higher the Mg2+ concentration, the higher would be the H2S yields, because ion Mg2+ could play a catalysis role in TSR [30–32]. The range of SO42− concentration was from 27 to 1288 mg/L. The coefficient of desulfurization (100×r SO42-/Cl-) was in a range of 2–160 with a mean value of 30. This coefficient could be used to reflect consumption of SO42- during TSR and the redox conditions of formation water. The low mark means reduced condition and active TSR reactions [32]. The H2S contraction rose with a decrease in the coefficient of desulfurization (Figure 6), exhibiting that TSR really occurred in the study reservoirs. The CO32- concentration in most water samples was low (mostly not detected), and the average of HCO3- was 335.8 mg/L. In addition, the concentration of HCO3- decreased with the process of TSR reactions, strongly indicating the dilution of additional water under TSR conditions. This kind of dilution might be lowering the concentration of CO32- and HCO3- in formation waters of volcanic reservoirs.Table 3
Hydrochemistry of the formation water from the Niudong area, Santanghu Basin.
Sample no.
Stratum
Sampling date
H2S (ppm/m3 air)
mg/L
Type
pH
K++Na+
Ca2+
Mg2+
Cl-
SO42-
HCO3-
CO32-
Total salinity
ND8-14
C2k
2008/8/7
n.d.
410
864
291
1755
920
710
268
5218
CaCl2
9.3
ND78-9
C2k
2008/8/6
115
592
545
1
1652
32
164
n.d.
2986
NaCl2
7.3
ND78-812
C2k
2010/8/7
6
371
123
10
512
283
168
n.d.
1467
NaCl
6.0
ND89-10
C2k
2010/10/8
44
647
442
24
1375
329
397
n.d.
3214
CaCl2
7.0
ND8-11
C2k
2008/8/7
35
1008
683
67
1955
434
1171
n.d.
5318
NaCl
6.5
ND89-9
C2k
2008/11/4
31
592
521
14
1544
349
123
n.d.
3143
NaCl2
6.0
ND8-111
C2k
2010/5/1
13
194
566
98
1272
47
482
n.d.
2659
CaCl2
7.0
ND89-812
C2k
2010/8/24
25
69
315
21
309
495
85
n.d.
1294
CaSO4
6.0
ND9-9
C2k
2010/3/7
50
547
384
15
1393
188
57
n.d.
2584
CaCl2
7.0
ND89-91
C2k
2010/6/5
44
869
369
15
1324
495
596
n.d.
3668
NaCl
6.0
ND 8-71
C2k
2007/12/21
120
386
396
7
1264
27
36
10
2126
CaCl2
9.0
ND8-10
C2k
2007/10/16
13
394
873
122
1475
1288
143
n.d.
4295
CaCl2
7.7
ND8-8
C2k
2008/8/7
10
537
404
51
1368
255
234
n.d.
2849
NaCl2
6.5
Note: these data were provided by the Santanghu Oil Production Plant of Tuha Oilfield Company. pH was tested at room temperature.Figure 5
Pipe diagram for ion composition of study formation waters.Figure 6
Relationship between the coefficient of desulfurization ratios and H2S concentrations of oil wells.


## 4.4. Sulfur Isotopes of H2S

The sulfur isotopes of sulfates and/or sulfides presented in the volcanic rocks from wells ND89-9 and ND89-10 are quite close to each other, being 9.2‰ and 10.3‰, respectively. The natural gases at the Niudong pumping station have undergone a series of technological operations such as separation of oil and gas, whose sulfur isotopic ratios reached 20.5‰ (Table4). As shown in Figure 7, the δ34SV-CDT values for TSR-H2S in this study were comparable with those in the Sichuan Basin (main frequency: 8‰ to 26‰) [14], the Tarim Basin (14‰ to 19‰) [17, 33], and the Ordos Basin (16‰ to 20‰) [14]. Such comparison may indicate that H2S presented in the oil wells of the Niudong area could probably be derived from TSR. δ34SV-CDT of TDS-H2S in crude oils is usually from 3‰ to 15‰ [5]; thus, H2S in the Niudong area could also be due to TDS. However, there is lack of meaningful evidence for the possibility of inorganic (volcanic) origin and BSR involvement for H2S in the study area.Table 4
Sulfur and carbon isotope of samples obtained from the Kalagang Formation in the Santanghu Basin.
Well or sample no.
Type
δ
34SV-CDT‰
δ
13CV-PDB‰
δ
18OV-PDB‰
Note
Gas pumping station
Gas
20.5
ND89-9
Gas in water
9.2
ND89-10
Gas in water
10.3
NDCT-03
Rock
-10.7
-20.6
Calcite
NDCT-04
Rock
-5.8
-21.8
Calcite
NDCT-05
Rock
-5.7
-18.6
Calcite
NDCT-06
Rock
-3.3
-21.4
CalciteFigure 7
Correlation betweenδ34S values and H2S contents of samples from the Santanghu Basin and other referenced TSR H2S-bearing basins in China (data of Sichuan Basin, Tarim Basin, and Ordos Basin are from Zhu et al. [14] and Jiang et al. [22]).


## 4.5. Carbon Isotopes of Calcite

The isotopic data of the calcite matrix from the Kalagang Formation exhibited a relatively wider range of carbon isotopic values between -10.7‰ and -3.3‰ V-PDB and a narrower range of oxygen isotopic values between -21.8‰ and -18.6‰ V-PDB, respectively (Table4). These relatively negative carbon isotopic values may suggest a possible contribution of organic carbon into calcite, but the carbon and oxygen isotopic values failed on the boundary of TSR calcite [22] which was formed through TSR reactions. In addition, these carbon and oxygen isotopic values of vein calcite were also similar to those of primarily igneous carbonatite (δ13CV-PDB‰: -5.0‰ to -8.0‰, δ18OV-PDB‰: -21.7‰ to -23.7‰) [34], indicating the calcite formation with fluids from deep layers under low hydrothermal temperatures (<167°C) [35]. Therefore, calcite in the rock veins was mostly of magmatic origin and very less produced due to TSR reactions.


## 4.6. Origin of H2S

As for the low concentration of H2S in crude oils and natural gases, the BSR origin is always regarded as the first originating mechanism that may be caused by bacterial loadings during water injection. The sulfur isotopes of H2S in the study area were 9.2‰ to 20.5‰, suggesting that BSR was probably not the most possible source in the Niudong Oilfield, where H2S was found in the first recovery of oil without water injection in early oil and gas exploration. Additionally, the values of sulfur isotopes are not in favor of volcanic sources for the genesis of H2S. Therefore, TSR and TDS should be their possible origins that were supported by the sulfur isotopic data. However, TDS usually occurred during the secondary recovery of oil under a relatively higher temperature than under the initial temperature of TSR [5], but no steam and/or hot water was utilized during the secondary recovery of heavy oils in the study reservoir and the heavy oils was typically low in sulfur content (about 0.17%). Based on these facts, either TDS should not be the origin or slightly contributed if TDS occurred. As for TSR, although gypsum and/or anhydrite in the cored rocks was not observed by naked eyes, trace sulfate and sulfide were detected by XANES. Such sulfur-bearing materials in the reservoir rocks could supply enough sulfur for TSR reactions due to the huge volume of rocks. Secondly, the local geothermal temperature (>110°C) reached the reaction conditions for TSR during the Permian-Triassic period as for the volcanic activity [27]. Finally, some increase in H2S concentration was observed with a decrease in desulfurization coefficient, which confirmed the occurrence of TSR. Such formed H2S in the reservoir of volcanic rocks is easily dissolved in the formation water. On the other hand, CO32- and HCO3- concentrations in most samples were relatively low, indicating that the additional water formed along with TSR may dilute the concentrations of both CO32- and HCO3-. Considering all the evidences above, TSR should be the main source for H2S in the natural gases of the Niudong Oilfield, but the TDS origin could not also be excluded.


## 5. Conclusions

The volcanic reservoir rocks of the Carboniferous Kalagang Formation are dominated by basalt and andesite, whose cracks or vein fillings in the core are mainly composed of volcanic zeolites, quartz, and/or calcite. Both sulfate (more than 90%) and sulfide minerals extensively existed in the reservoir rocks, which could provide a potential sulfur source for H2S gas formation through TSR in the reservoirs.H2S was distributed in the producing wells with low formation pressure and high water cut of the oil wells, which was mostly dissolved in the formation water. The sulfur isotopic data and ion compositions of formation water indicated that TSR should be the main source for H2S genesis. The geological structures of the basin, the mineral compositions of the reservoir rocks, and the evolution characteristics of the paleogeothermal temperatures provided effective conditions for TSR reactions in the study area.



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*Source: 1279658-2019-06-02.xml*