In Triassic Carbonate Reservoirs from the Sichuan Basin, China

Chemical Geology 202 (2003) 39–57 www.elsevier.com/locate/chemgeo

Thermochemical sulphate reduction and the generation of hydrogen sulphide and thiols (mercaptans) in Triassic carbonate reservoirs from the Sichuan Basin, China

Chunfang Caia,b, Richard H. Wordenb,*, Simon H. Bottrellc, Lansheng Wangd, Chanchun Yanga

a Institute of Geology and Geophysics, CAS, PO Box 9825, Beijing 100029, PR China b Jane Herdman Laboratories, Department of Earth Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK c School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK d Petroleum Exploration and Development Institute, Southwest Sichuan Petroleum Corporation, Chengdu, Sichuan Province, PR China Received 29 July 2002; accepted 20 June 2003

Abstract

The Sichuan Basin in China is a sour petroleum province. In order to assess the origin of H2S and other sulphur compounds 34 as well as the cause of petroleum alteration, data on H2S, thiophene and thiol concentrations and gas stable isotopes (d S and d13C) have been collected for predominantly gas phase petroleum samples from Jurassic, Triassic, Permian and Upper Proterozoic (Sinian) reservoirs. The highest H2S concentrations (up to 32%) are found in Lower Triassic, anhydrite-rich 34 carbonate reservoirs in the Wolonghe Field where the temperature has reached >130 jC. d S values of the H2S in the Wolonghe Triassic reservoirs range from + 22 to + 31xand are close to those of Triassic evaporitic sulphate from South China. All the evidence suggests that the H2S was generated by thermochemical sulphate reduction (TSR) locally within Triassic reservoirs. In the Triassic Wolonghe Field, both methane and ethane seem to be involved in thermochemical sulphate 13 reduction since their d C values become less negative as TSR proceeds. Thiol concentrations correlate positively with H2Sin the Triassic Wolonghe gas field, suggesting that thiol production is associated with TSR. In contrast, elevated thiophene concentrations are only found in Jurassic reservoirs in association with liquid phase petroleum generated from sulphur-poor source rocks. This may suggest that thiophene compounds have not come from a source rock or cracked petroleum. Rather they may have been generated by reaction between localized concentrations of H2S and liquid range petroleum compounds in the reservoir. However, in the basin, thiophene concentrations decrease with increasing vitrinite reflectance suggesting that source maturity (rather than source type) may also be a major control on thiophene concentration. D 2003 Elsevier B.V. All rights reserved.

Keywords: H2S; Thermochemical sulphate reduction; Thiophenes; Thiols; Mercaptans; Stable isotopes; Natural gas; Sichuan Basin

1. Introduction

* Corresponding author. Tel.: +44-151-794-5184; fax: +44-151- Elevated H2S concentrations (sour gas) have been 794-5196. found in many deep carbonate gas reservoirs around E-mail address: [email protected] (R.H. Worden). the world. The H2S is thought to originate from

0009-2541/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0009-2541(03)00209-2 40 C. Cai et al. / Chemical Geology 202 (2003) 39–57 thermochemical sulphate reduction (TSR); a process 2. Geological setting whereby sulphate minerals and petroleum react to- gether (e.g., Orr, 1977; Krouse et al., 1988; Sassen, The Sichuan Basin in southwest China (Fig. 1a) is 1988; Worden et al., 1995; Machel et al., 1995; a large, intracratonic basin with an area of about Heydari, 1997; Cai et al., 2001). Thermochemical 230,000 km2. A west–east cross section is shown in sulphate reduction has been studied extensively by Fig. 1b. The basement is Proterozoic continental examining H2S contents, and the sulphur and carbon crust. The Sichuan Basin represents one of China’s isotopic compositions of various gas phase com- largest natural gas provinces with gas found in pounds. Other sulphur-bearing compounds in sour Jurassic, Triassic, Permian, Carboniferous and Upper petroleum are only infrequently documented in geo- Proterozoic (Sinian) strata, and oil produced locally chemical studies, although a great number of sul- from Jurassic strata (e.g., Li et al., 1994). Three phur compounds have been reported in petroleum large-scale gas fields (reserves >300 108 m3) and and source rocks (e.g., Hughes, 1984; Orr and seventeen medium-scale gas fields (>50 108 but Sinninghe Damste´, 1990; Sinninghe Damste´ et al., < 300 108 m3) have been found in the basin (Li, 1990). 1996). Light hydrocarbon gases, condensates and gaso- Marine sedimentation dominated in the Sichuan line range petroleum have been shown to be in- Basin from the Upper Sinian to the Middle Triassic. volved in TSR (Krouse et al., 1988; Rooney, 1995; The Upper Sinian to Silurian sequence is composed of Worden and Smalley, 1996; Whiticar and Snowdon, 2000–4000 m of shallow marine carbonates, and 1999) although there are some who still consider black shale, with limited anhydrite in the Upper that light hydrocarbons in general, and methane in Sinian and Cambrian (Fig. 2). Marine sedimentation particular, are relatively unreactive during TSR was interrupted during the late Silurian Caledonian (Machel, 2001). Orogeny when the Sichuan Basin was uplifted and Sour gas has been reported in reservoirs from the exposed, resulting in minimal Devonian deposition Upper Proterozoic (Sinian) through to the Jurassic in (Fig. 1b). Middle Carboniferous sedimentation was the Sichuan Basin, China (Sheng et al., 1982; Dai, limited to the eastern part of the Sichuan Basin. 1986; Huang et al., 1995; Korsch et al., 1991; Wang, Middle Carboniferous anhydrite was found only near 1994; Sheng et al., 1997). However, H2S concentra- the Dachuan area, to the north of Linshui County and tions >10% have been found only in the Lower and the west of Kaijiang County (Fig. 1; Lu et al., 1996). Middle Triassic carbonates and evaporites. d34S val- Following the Caledonian Orogeny, marine trans- ues of the H2S are about + 25x (Sheng et al., gression occurred during the earliest Permian. The 1997), significantly more positive than those of Tri- Lower Permian is composed of platform carbonates assic seawater sulphates reported by Claypool et al. with a typical thickness of 300–500 m. Submarine (1980). From water chemistry and stable isotope data, basalt eruption occurred at the end of the Lower both water- and petroleum-bearing Lower and Middle Permian. The Upper Permian is composed of platform Triassic carbonate rocks are thought to be relatively carbonates with alternating marine and terrestrial coal- closed systems, with the saline formation waters bearing strata. being a residue following evaporite precipitation The Lower and Middle Triassic sequence is divid- (Zhou et al., 1997). ed into Feixianguan (T1f), Jialingjiang (T1j) and Lei- In this paper, we present data on the concentra- koupo Formations (T2l), and is composed predomi- tions of the sulphur-bearing organic compounds, nantly of platform carbonates and evaporites (Fig. 2). thiophenes and thiols (also known as mercaptans), Little anhydrite occurs in the Feixianguan Formation as well as H2S from the Sichuan Basin and explore in the whole basin (Lan et al., 1995) except in the relationships between their occurrence and TSR. We northeast part of the East Sichuan Basin (Yang et al., provide data from a Triassic gas field (Wolonghe) in 1999), in contrast to thick, basin-wide anhydrite beds the eastern part of the Sichuan Basin to address the in the Jialingjiang and Leikoupo Formations. The 34 origin of S-enriched H2S and the mechanism of Jialingjiang Formation includes five members. The TSR. Second, Fourth and Fifth Members contain 2- to 4-m- C. Cai et al. / Chemical Geology 202 (2003) 39–57 41

Fig. 1. Map showing (a) distribution of gas fields, (b) cross section of Wolonghe Field (modified from Tong, 1992; Li, 1996; Xu et al., 1998). thick anhydrite beds, but the First and Third Members posed of continental red sandstones, mudstones and contain little anhydrite (Tian and Wei, 1985). black shale with a thickness of 2000–5000 m (Huang As a result of the Yinzi Orogeny between the et al., 1995). The basin acquired its present structure Middle and Upper Triassic, the Sichuan Basin was after the Neogene Himalayan Orogeny. The burial and uplifted and exposed. Upper Triassic sediments are geothermal history of Well Zuo 1 in the East Sichuan freshwater lacustrine–alluvial clastics with local coal Basin (Figs. 1a and 3) shows that rapid sedimentation beds. Jurassic and Cretaceous sediments are com- took place during the Lower Triassic, Middle and 42 C. Cai et al. / Chemical Geology 202 (2003) 39–57

Fig. 2. Generalised stratigraphic column for the Sichuan Basin showing complex petroleum systems. Basin-scale anhydrite beds occur in the Lower and Middle Triassic while Sinian, Cambrian and Carboniferous strata contain anhydrite in local areas.

Upper Jurassic and that the Lower Triassic experi- natural gas in the Sinian strata of the Weiyuan Field is enced a maximum burial rate, and had the highest considered to have been generated in Lower Cambrian palaeo-temperature (>130 jC), at the end of Creta- source rocks while Carboniferous reservoirs have gas ceous (Fig. 3). Significant uplifts occurred at the end derived from Lower Silurian black shale (Huang et al., of the Middle Triassic and during the Tertiary. 1995, 1997; Song et al., 1997). In contrast, gas in Petroleum system analysis reveals that there are Lower Permian reservoirs is considered to have a numerous potential source rocks, reservoirs and cap- mixed origin from both Lower Permian carbonate rocks in the Sichuan Basin (Fig. 2; Table 1). Sinian to and Upper Permian coal source rocks (Huang et al., Middle Triassic reservoirs are predominantly carbon- 1995). Gas in Lower Triassic reservoirs is thought to ate while Upper Triassic and Jurassic reservoirs are have been generated from Triassic carbonate source mainly siliciclastic. The source rocks are commonly rocks (e.g., Zhang et al., 1991; Dai et al., 1997) while specific to reservoir horizons (Table 1); for example, gas in the Middle Triassic in the Moxi Field is thought C. Cai et al. / Chemical Geology 202 (2003) 39–57 43

3. Sampling and methods

Gas geochemistry data and concentrations of H2S dissolved in water have been collated from proprietary reports from the Sichuan Petroleum Bureau from 1965 through to the present (Table 2; Fig. 4). Petroleum and gas samples were collected and analysed using standard industry techniques. The concentrations of thiophene and thiol compounds as well as hydrocarbon gas d13C values have been integrated from data presented by Wang (1994), Huang (1990), Huang et al. (1995) and Xu et al. (1998). H2S-bearing natural gas samples from the Triassic Moxi and Wolonghe gas fields were bubbled slowly through a solution containing excess Zn acetate to Fig. 3. Diagram showing a typical burial and palaeo-temperature precipitate ZnS at the well-head. In the laboratory at history constructed from Well Zuo 1 in the East Sichuan Basin. the School of Earth Sciences, Leeds University, UK, Isotherms are constrained by vitrinite reflectance and fluid inclusion measurements (modified from Wang et al., 1998). ZnS was transformed to CuS by adding HCl and passing the evolved H2S through CuCl2 solution at a pH of 4. SO2 gas for sulphur isotope analysis was to have an Upper Permian coal source (e.g., Huang et produced by combustion of a mixture of the CuS and al., 1997). The Upper Triassic of the Zhongba Field Cu2O at 1070 jC in a vacuum (Robinson and Kusa- has gas from Upper Triassic coal. Oil and gas in kabe, 1975). The SO2 was cryogenically purified and Jurassic reservoirs are considered to have a sulphur- analysed on a VG SIRA10 gas source isotope ratio poor Jurassic lacustrine source (Sheng et al., 1991; mass spectrometer. Raw data were corrected using Zhang et al., 1991; Li et al., 1994; Wang, 1994; standard techniques (e.g., Craig, 1957) and reported Huang et al., 1997). relative to the V-CDT standard. Replicate analyses of

Table 1 Reservoir units with their interpreted source rock types, maturities and ages Reservoir Symbol Petroleum Source Regional Source age References for

type type vitrinite Ro,% source rock details

Jurassic J1t Oil and S-poor 0.9–1.4 Jurassic Sheng et al., 1991; gas lacustrine Zhang et al., 1991; shale Li et al., 1994; Wang, 1994

Upper T3 Gas Coal 0.9–1.4 Upper Huang et al., 1995, 1997 Triassic Triassic 1 3 Middle T2l ,T2l Gas Coal 1.0–2.2 Upper Huang et al., 1995, 1997 Triassic Permian

Lower T1j, T1f Gas Marine 1.2–2.0 Triassic Sichuan Petroleum Bureau, 1989; Triassic carbonate Zhang et al., 1991; Dai et al., 1997 Permian P Gas Marine carbonate 1.8–3.0 Lower and Huang et al., 1995, 1997 and coal Upper Permian Carboniferous C Gas Marine black 2.6 Lower Huang et al., 1995, 1997; shale Silurian Song et al., 1997 Sinian Z Gas Marine black 3.6–3.7 Lower Chen, 1992; shale Cambrian Huang et al., 1997 44 C. Cai et al. / Chemical Geology 202 (2003) 39–57

Table 2 Chemistry and d13C and d34S values of natural gases from Wolonghe, Weiyuan and Moxi fieldsa 13 13 13 34 Field Well Depth Age mC1 mC2 mC3 mCO2 100 C2–6/ mH2SmN2 d C1 d C2 d C3 dD d S Thiols name C1–6 1 Moxi Mo70 – T2l 98.1 0.09 0.004 0.16 0.096 0.80 0.87 – – – – + 13.3 – 1 Moxi Mo75-1 – T2l 98.7 0.07 0.004 0.15 0.075 0.38 0.72 – – – – 6.0 – 1 Moxi Mo17 – T2l 98.3 0.08 0.004 0.14 0.085 0.83 0.67 – – – – + 17.7 – 5 Wolonghe Wo2 1643 T1j1 96.3 0.46 0.080 0.16 0.558 2.61 0.32 – – – – + 22.2 1064 5 Wolonghe Wo3 1288 T1j1 96.4 0.45 0.076 0.14 0.543 2.36 0.51 32.7 28.9 24 – – 1102 5 Wolonghe Wo5 1799 T1j1 97.2 0.45 0.068 0.10 0.530 1.74 0.37 33.1 29.4 – – – – 5 Wolonghe Wo6 1588 T1j1 96.8 0.47 0.083 0.11 0.568 2.20 0.33 32.8 28.9 – – – – 5 Wolonghe Wo7 1541 T1j1 95.9 0.44 0.080 0.26 0.539 2.99 0.27 – – – – – – 5 Wolonghe Wo8 1188 T1j1 96.4 0.50 0.087 0.18 0.605 2.46 0.32 – – – – – – 5 Wolonghe Wo9 1977 T1j1 88.5 0.89 0.268 0.38 1.292 9.60 0.26 – – – – – – 5 Wolonghe Wo11 1492 T1j1 96.6 0.44 0.079 0.13 0.534 2.30 0.44 33.5 28.2 – – – 1000 5 Wolonghe Wo25 1676 T1j1 96.6 0.47 0.079 0.07 0.565 2.29 0.42 33 29 24 – – 1244 5 Wolonghe Wo27 1778 T1j1 96.5 0.46 0.080 0.12 0.556 2.44 0.31 33.1 29.2 – – – – 5 Wolonghe Wo33 2307 T1j1 95.3 0.50 0.081 0.43 0.606 3.23 0.38 – – – – + 26.5 – 5 b Wolonghe Wo45 2105 T1j1 95.6 0.53 0.099 0.29 0.654 2.97 0.47 – – – 136 + 24.7 – 5 4 Wolonghe Wo56 1464 T1j1 –j3 96.2 0.46 0.061 0.16 0.539 2.68 0.40 – – – – + 31 – 4 Wolonghe Wo28 2255 T1j3 96.6 0.45 0.079 0.13 0.545 2.36 0.29 – – – – – – 4 Wolonghe Wo63 2285 T1j3 77.4 0.23 0.041 0.75 0.349 18.83 2.69 – – – 100 + 30.4 – 4 Wolonghe Wo19 1741 T1j3 96.7 0.44 0.076 0.23 0.531 2.47 0.05 32.6 28.9 – – – – 4 3 Wolonghe Wo17 1652 T1j1 –j3 97.6 0.37 0.052 0.05 0.431 1.56 0.36 – – – – – 788 4 3 Wolonghe Wo37 1926 T1j1 –j3 96.9 0.65 0.087 0.12 0.755 1.40 0.81 34.5 29.9 26 – – – 4 3 Wolonghe Wo50 1902 T1j1 –j3 97.3 0.44 0.060 0.08 0.511 1.75 0.28 34.4 – – 141 – – 3 Wolonghe Wo38 1798 T1j 97.6 0.39 0.060 0.09 0.459 1.55 0.26 – – – – – – 3 Wolonghe Wo57 1860 T1j 98.7 0.26 0.022 0.04 0.285 0.20 0.77 – – – – – – Wolonghe Wo34 3066 P2 98.9 0.17 0.007 0.27 0.179 0.20 0.43 – – – – – – Wolonghe Wo47 3390 P1 99.2 0.12 0 0.15 0.121 0.37 0.12 – – – – – – Wolonghe Wo67 3291 P1 99.0 0.18 0.007 0.40 0.189 0.23 0.15 31.89 32.23 – – – – Wolonghe Wo68 4046 P1 99.1 0.10 0.004 0.34 0.105 0.05 0.37 – – – – – – b Wolonghe Wo83 3413 P1 99.2 0.15 0.007 0.19 0.158 0.26 0.15 31.69 32.79 – 140 + 5.7 – Wolonghe Wo48 3817 C2 98.9 0.25 0.018 0.41 0.270 0.09 0.31 32.35 35.72 – – – – Wolonghe Wo52 4594 C2 99.0 0.20 0.018 0.36 0.220 0.10 0.34 32.13 35.34 – – – – Wolonghe Wo58 3771 C2 99.0 0.23 0.018 0.35 0.250 0.11 0.25 32.25 35.69 – – – – Wolonghe Wo65 4138 C2 98.9 0.32 0.015 0.27 0.338 0.14 0.32 32.24 36.05 140 – – Wolonghe Wo85 4518 C2 98.4 0.35 0.026 0.35 0.381 0 0.87 32.13 36.26 – – – – b Wolonghe Wo96 3951 C2 99.8 0.17 0 0 0.170 0 0 32.98 35.46 – 140 + 5.8 – Weiyuan Wei100 3000 Z2 93.4 0.07 0 1.98 0.075 0.60 3.98 32.38 31.82 – 139 – – Weiyuan Wei109 2832 Z2 93.5 0.04 0 1.89 0.043 0.67 3.87 32.37 31.19 – 120 – – a Depth is set as the middle point between perforation, in m; Age ‘‘Z’’ represents late Proterozoic; ‘‘–’’ represents no measurement or no 3 13 13 13 34 sample. Thiols in mg/m ; other gas chemistry in mol% of total gas. d C1, d C2, d C3 in x(PDB) and d Sinx(CDT). b From Sheng et al. (1997) and Xu et al. (1998). standards confirmed the 2r uncertainty as F 0.2x. 4. Results 34 3 4 Other H2S d S data and gas sample He/ He data were 34 collected from material published by Sheng et al. 4.1. H2S concentration and d S data (1997) and Xu et al. (1998). The results of the d34S measurement in this current study are similar to those 4.1.1. Whole basin 34 by Sheng et al. (1997), who measured H2S d S Natural gas samples from the predominantly simultaneously with gas carbon isotope and chemistry, carbonate reservoirs of the Sinian to the Middle suggesting the results obtained by Sheng et al. (1997) Triassic contain variable quantities of H2S. The can be justifiably incorporated into the current study. maximum H2S concentrations in these reservoirs C. Cai et al. / Chemical Geology 202 (2003) 39–57 45

Fig. 4. Variation of CO2 molar percentage, H2S volume percentage of the natural gases and dissolved H2S concentrations in gas-field water versus depth in Wolonghe Field showing a similar variation of molar CO2 and H2S and elevated dissolved H2S in water. range from 0.6 to 32.0% by volume (Table 3). tions of H2S>10% by volume have only been found Sinian, Carboniferous and Permian reservoirs con- in the Lower and Middle Triassic. Relatively low tain < 5% by volume H2S by volume. Concentra- H2S concentrations ( V 0.6% by volume) are present

Table 3 34 3 4 Maximum H2S percentages, d S, He/ He, thiophene and thiol contents in natural gases and organic matter vitrinite reflectance Ro values of the corresponding strataa b Strata Ro H2S Thiophenes Thiols Whole basin Eastern part (%) maxi (%) (mg/m3) (mg/m3) (excluding the east) (including Wolonghe area) 3He/4He d34S 3He/4He d34S H2S H2S 10 8 (x) 10 8 (x) c d J1t 0.9–1.4/ 0.6 0.90–6.40 < DT – ––– 1.2 (n =8)

T3 –– 3 T2l 1.0–2.2/ 13.3 – – – – – – 1.6 (n =3) 1 T2l 2.7 – – 1.1 6.0 to + 17.7/ –– (n =1) + 8.3 (n =3)e

T1j 1.2–1.5 32.0 0.10–1.35 < DT to 1.1–3.6 + 6.8 to + 29.1/ 1.89–3.62 + 22.9 to + 24.7 (n =2) 1244 (n =4) + 13.6 (n = 23) (n =2) + 22.2 to + 31.0 / + 27.5 (n =4)e

T1f 1.1–2.0/ 2.5 – – – 6.0 to + 4.81/ –– 1.6 (n =4) 1.2 (n =2) P 1.8–3.0/ 3.4 0.03–0.32 0.11–2.00 1.6–3.0/2.2 + 20.4 to + 29.7/ 1.83–2.20 + 5.7 to + 12.8/ 2.2 (n =9) (n =4) + 24.1 (n = 17), (n =2) + 9.3 (n =2) + 13.3 C 2.6 (n = 1) 0.7 – – – – 2.09–2.72/2.50 9.6 to + 8.5/ (n =6) + 2.2 (n =7) Sinian 3.6–3.7 3.4 0.10–0.20 0.14–4.04 0.6–2.8/1.7 + 11.5 to + 14.4/ –– (n =2) (n =2) + 13.1 (n =4) a Data present in the form of range/average (number of samples). b From Huang et al. (1995, 1997) and Xu et al. (1998). c DT represents detection limit. d No data available. e From this study; others were from Xu et al. (1998), Sheng et al. (1997) and Dai (1986). 46 C. Cai et al. / Chemical Geology 202 (2003) 39–57

Fig. 5. Diagram showing that high H2S contents occur in reservoirs close to anhydrite beds in Lower Triassic Jialingjiang Formation, Wolonghe Field. Also d34S values of anhydrite measured in the basin and the interpreted seawater isotopic curves from different authors are plotted for comparison. in gas from Upper Triassic and Jurassic sandstone coproduced with gas with concentrations ranging reservoirs (Dai, 1986). from 106 to 2988 mg/l (Fig. 4). The Fourth and Fifth Gases from different systems have H2S with dif- Members of Jialingjiang Formation have natural gas 34 ferent d S values (Table 3). Gas samples from Sinian H2S concentrations of up to 32% and 18% by volume, reservoirs have a relatively narrow d34S range from respectively (Table 2). These two high values corre- + 11.5xto + 14.4x. Gas samples from Permian spond to the intervals with the greatest quantity of reservoirs are enriched with 34S and have d34S values anhydrite (Fig. 5). In contrast, the First and Third between + 20.4xand + 29.7x(Sheng et al., 1982; Members have relatively low H2S concentrations and Xu et al., 1998). Apart from the Wolonghe Field, discussed separately below, the majority of the gas samples from the Lower Triassic Jialingjiang Forma- 34 tion (T1j)haved S values from + 12xto + 16x with two anomalous values of + 6.8xand + 29.1x in the Naxi area in the southeast of the basin (Table 3; Fig. 1). Two samples from the Lower Triassic Feix- ianguan Formation (T1f) and the basal Middle Triassic 1 Leikoupo Formation (T2l ) have the same most negative d34S values ( 6.0x).

4.1.2. Wolonghe Field In the Wolonghe Field, H2S concentrations in Triassic reservoirs (Jialingjiang Formation, T1j) range predominantly from 5% to 10% by volume (Table 3). The most elevated H2S concentrations occur between 1900 and 2400 m, which is also where the highest 34 Fig. 6. H2S contents and d S values of natural gases from concentrations of CO2 are located (Fig. 4). As would Wolonghe Gas Field in the East and Moxi Gas Field in the Middle be expected, dissolved H2S was detected in water Sichuan. C. Cai et al. / Chemical Geology 202 (2003) 39–57 47

Thiophenes are cycloaromatic sulphur compounds whereas thiols are alkylsulphides (or mercaptans).

4.2.1. Thiophenes Thiophene concentrations (total of all compounds with thiophene structure) in natural gases range from 0.03 to 6.40 mg/m3 (at 1 atm pressure) in the Sichuan Basin (Fig. 7). Thiophene concentrations greater than 1.00 mg/m3 only occur in Lower Juras- sic freshwater–lacustrine sandstone reservoirs. Sinian, Permian and Triassic carbonate reservoirs routinely have gases with relatively low concentrations ( < 0.20 mg/m3) of thiophenes (Fig. 7). The Lower Jurassic sandstone reservoirs produce light oil, condensate and associated natural gas, but contain < 0.6% H2S. Gas in Jurassic reservoirs is wetter (higher ratios of SC2–6/SC1–6, Fig. 8) than gas in Triassic, Permian or Sinian reservoirs. Gases in the Sinian, Permian and most Triassic reservoirs are dominated by methane, as shown in Fig. 8.In summary, elevated thiophene concentrations tend to

Fig. 7. Thiophene and thiol contents in natural gases from different parts of the basin. Relatively high thiophene but zero thiol concentrations were found in the Jurassic reservoir in the Middle Sichuan (the x axis scatter is here introduced to clearly differentiate dots with the similar y values) (thiol contents of Triassic Wolonghe gases are sourced from this study, Table 2, while other data came from Huang, 1990). are essentially free of anhydrite (Sichuan Petroleum Bureau, 1989). We thus conclude that the local quantity of H2S seems to reflect directly the local quantity of anhydrite in the formation. 34 d S values from H2S from the Wolonghe Field range from + 22.4xto + 31.0x(Table 3; Fig. 6) and are close to those previously reported by Sheng et al. (1997) and Xu et al. (1998) (Table 3).

Fig. 8. Gaseous hydrocarbon wetness (SC2–6/SC1–6) ratios in 4.2. Thiophenes and thiols different areas showing the high ratios of gas in Jurassic reservoirs and much more negative values of gases in Triassic, Permian and Sinian reservoirs (the x axis scatter has been introduced to clearly Low-molecular-weight (LMW) thiophenes, thiols differentiate dots with the similar y values) (data of East Sichuan and even carbon disulphide have been detected in plot from Table 2, while data of other parts of the basin are from natural gases from the Sichuan Basin (Huang, 1990). Huang, 1990). 48 C. Cai et al. / Chemical Geology 202 (2003) 39–57

have the highest C2–6/C1–6percentages and gases from both Permian and Sinian are the lowest while gas from Triassic reservoirs is shown to have a broad range of C2–6/C1–6 percentages (Fig. 8). 13 d CCH4 values from Jurassic gas ranges from 37xto 44xPDB. Gas samples from Triassic, 13 Permian and Sinian reservoirs have d CCH4 values > 36xPDB. The majority of gas samples from 13 Triassic, Permian and Sinian reservoirs have d CC2H6 13 closer to d CCH4 than those from Jurassic reservoirs. 13 The gases with more negative d CCH4 generally 13 have a greater difference between d CC2H6 and 13 Fig. 9. Relationship between thiol and H2S content of gas in the d CCH4 (Table 2). Two gas samples from Permian Triassic part of the section in Wolonghe). 13 reservoirs have d CCH4 values less negative than 13 d CC2H6 . be found in wet gases associated with light oils and condensates while methane-dominated gases have 4.3.2. Wolonghe Field low thiophene concentrations. Wolonghe gas chemistry shows that gas samples from the Triassic have wetness values (C2–6/C1–6 4.2.2. Thiols percentages) ranging mainly from 0.1% to 0.7% Thiol compounds (also known as mercaptans) (Table 3). The values tend to be much higher than were not detected in the Jurassic natural gases, while those of gases from the Permian reservoir in high concentrations occur in Triassic, Sinian and Wolonghe which range from 0.1% to 0.2% (Fig. 8) Permian reservoirs, and especially in the Triassic and lie between those from the gases in the Jurassic in reservoirs of the Wolonghe Field. Thiol concentra- the Middle Sichuan Basin and those from the gases in tions are up to 1244 mg/m3 in the Wolonghe Field the Sinian in the Southwest Sichuan Basin (Fig. 8). (Fig. 9; Table 3) although they range from less than This suggests that the gas in the Triassic Wolonghe the detection limit to 6.11 mg/m3 in the rest of the Field has a different chemical composition from the Sichuan Basin. The stratigraphic distribution of thiol gas from other parts of the basin. compounds is thus totally different to that of the thiophenes. Low thiol concentrations tend to occur in gases with low H2S concentrations, whereas high concentrations are associated with the highest H2S concentrations in Lower and Middle Triassic strata (Huang, 1990), even though reservoir temperatures are similar. Five gas samples from the Wolonghe Field show that there is an approximately positive relationship between thiol and H2S concentrations (Fig. 9). This observation is similar to the observation of Ho et al. (1974) in which thiols in condensates were found to be associated with high H2S contents.

13 4.3. Gas chemistry and d C–H2S relationships

4.3.1. Whole basin Gas chemistry data (Table 2) show that among gases 13 13 13 Fig. 10. Relationship between d CCH4 and d CC2H6 d CCH4 for from different systems, those from Jurassic reservoirs samples from the whole basin and Wolonghe Field. C. Cai et al. / Chemical Geology 202 (2003) 39–57 49

5. Discussion

The Sichuan Basin affords us the opportunity to examine the relationships between source type and maturity, petroleum type and sulphur geochemistry. What is clear is that different parts of the stratigraphy have distinct sulphur geochemical characteristics. The crucial question is why? It is noteworthy that the highest H2S and thiol concentrations are found in Lower Triassic reservoirs containing petroleum sourced from sulphur-enriched marine carbonate source rocks (Tables 1 and 2). However, the maximum H2S concentrations are hugely in excess of what would be anticipated from an organic source and their relatively high d34S values are generally characteristic of an oxidized 13 Fig. 11. d C values of methane and ethane versus H2S/ (sulphate) sulphur source, rather than reduced sul- (H2S+SC1–6) for gases from the Wolonghe Field showing positive phur typical of petroleum source rocks. Note that 34 relationships. rare H2S d S values of 6x(Tables 2 and 3) may be indicative that the carbonate source rock has indeed generated a small quantity of H2S. It is most Gas samples from the Triassic section of the likely that these elevated concentrations of high d34S 13 x Wolonghe Field have d CCH4 values from 34.5 sulphide are the result of sulphate reduction. Given x 13 x to 32.6 PDB and d CC2H6 values from 29.4 the relatively high temperatures in the basin, reduc- x 13 to 28.2 PDB. d CC2H6 values are always less tion is more likely to have been thermochemical than 13 13 negative than d CCH4 values. d CCH4 values lie in biogenic (Sheng et al., 1982; Wang, 1994; Huang, between those of gases in Jurassic and Permian and 1990). The questions remain as to where sulphate Sinian reservoirs (Fig. 10). Relationships between reduction occurred in the basin (whether H2S migra- 13 13 d CCH4 and d CC2H6 show that the gas in the tion occurred), whether TSR has occurred between 13 Triassic has more negative d CCH4 values and less gas phase hydrocarbons (especially methane) and 13 negative d CC2H6 values than gases both from sulphate and about the link between sulphide and Permian reservoirs in the Wolonghe Field in the East thiol compounds. There is also the puzzling distri- Sichuan Basin and Sinian reservoirs in the Weiyuan bution of thiophene compounds to consider. Al- Field in the Southwest Sichuan Basin (Table 3). though they might be expected to follow the same A gas souring index [H2S/(H2S+SC1–6)] has been pattern as other sulphur compounds in the petroleum used previously to indicate the extent of thermochem- system, they, in fact, have a different pattern to both ical sulphate reduction (Worden and Smalley, 1996). thiols and H2S. 13 There are positive relationships between both d CCH4 These issues will be dealt with in the following 13 and d CC2H6 with H2S/(H2S+SC1–6) (Fig. 11). Thus, discussion. One persistent possibility for sour gas in gas samples with higher gas souring index values tend any crustal setting is that H2S has a primeval source to have alkane gases most enriched in 13C. (mantle or core). However, helium isotopes from the basin in general and the Wolonghe Field in particular 4.4. Noble gas isotope data indicate that the gas has been derived from sedimentary organic matter (Xu et al., 1998; Cai et al., 2001) Helium gas present in petroleum accumulations has and that there is negligible input of gas from mantle 3He/4He ratios ranging from 0.6 10 8 to 3.6 sources. This excludes a mantle or a deep crustal 10 8. Helium isotope ratios from the Wolonghe Field source of gas and implies that we must look for 8 8 range from 1.8 10 to 3.6 10 (Table 3). basinal, non-juvenile, sources of H2S. 50 C. Cai et al. / Chemical Geology 202 (2003) 39–57

5.1. Origin of H2S in the Wolonghe Field Bureau, 1989; Tong, 1992) from the Lower and Middle Triassic, which suggest that compartmentalisation is 5.1.1. A local source of H2S? predominant in the basin. Although anhydrite is abun- Formation testing showed that present bottom-hole dant within the Triassic section, it is concentrated temperatures of the Triassic reservoir in the Wolonghe within the Fourth and Fifth Members of the Jialingjiang Field are in the range 90–100 jC,andsoare Formation. Indeed, elevated H2S concentrations have apparently too low for the present-day occurrence of been found exclusively in the Fourth and Fifth Mem- TSR. Vitrinite reflectance (Ro) values range from bers (Fig. 5) and H2S loss due to reactions in the 1.17% to 1.54% in the vicinity of the Wolonghe Field reservoir with elements such as Fe and Zn is insignif- (Xu et al., 1998), but are mostly >1.35% (Huang et al., icant since practically no siliciclastics occur in the 1995; Wang, 1994). During the Neogene Himalayan Jialingjiang Formation. These factors thus support both Orogeny, uplift resulted in erosion of Tertiary and localized TSR and inhibited mixing of reservoir fluids. Cretaceous strata. Thus, based upon burial history, Both liquid and gas phase petroleum have been heat flow analysis and the Ro data, the base of the reported to be involved in TSR (e.g., Orr, 1974; Upper Triassic was concluded to have had a maxi- Krouse et al., 1988; Connan and Lacrampe-Cou- mum palaeo-temperature of not less than 130 jC(Fig. loume, 1993; Rooney, 1995; Worden et al., 1996; 3; Zeng, 1987; Wu et al., 1998; Liu et al., 2000). The Worden and Smalley, 1996; Cai et al., 2001). Only gas minimum temperature required for TSR has been the phase hydrocarbons are found in the Triassic of the subject of intense interest. In some basins, the mini- Wolonghe Field, suggesting that it is most likely for mum temperature for TSR is 140 jC or greater TSR to have been caused by the chemical oxidation of (Worden et al., 1995, 1998; Heydari, 1997), while short chain alkanes by sulphate. Reactions that have other basins have experienced TSR at temperatures as been reported include (e.g., Orr, 1974; Connan and low as 120 jC (Sassen, 1988; Rooney, 1995; Cai et Lacrampe-Couloume, 1993; Worden et al., 2000) the al., 2001; Worden and Smalley, 2001). It is clear that initial reduction of sulphate by pre-existing hydrogen there is no absolute universal minimum temperature. sulphide: This is probably because the extent of reaction is a 3H S SO2 Z4So 2H O 2OH R1 function of many controls including the time spent in 2 ðgÞ þ 4 ðaqÞ þ 2 þ ð Þ the reaction window (a protracted burial history and followed by the subsequent further reduction of ele- the consequent slow heating would lead to lower mental sulphur by hydrocarbons: minimum temperatures), petroleum type and compo- o sition, the rock fabric (e.g., anhydrite crystal size; 4S þ 1:33ðCH2Þþ2:66H2O Worden et al., 2000), timing of petroleum emplace- Z4H2SðgÞ þ 1:33CO2 ðR2Þ ment into the structure and wettability (where water- wet reservoirs are likely to undergo TSR more rapidly although the direct reaction between aqueous sulphate than petroleum wet systems; Worden and Heasley, and petroleum compounds has been suggested: 2000). With a maximum palaeotemperature of SO2 þ CH þ Hþ >130 jC in the Sichuan Basin in Lower Triassic strata 4 ðaqÞ 4 ðaqÞ ðaqÞ prior to Neogene uplift, TSR and H2S generation were ZHCO3 ðaqÞ þ H2SðgÞ þ H2O ðR3Þ thus perfectly possible given the range of minimum temperatures reported from around the world. The d34S values of anhydrite in the Lower Triassic The large, stratigraphically defined differences in Jialingjiang Formation in South China (Fig. 5) have sulphur isotope composition and H2S contents suggest been shown to range from + 24.7x to + 32.5x that H2S generation was localized within discrete (Chen et al., 1981; Chen and Chu, 1988) and even up stratigraphic reservoir intervals in the Sichuan Basin. to + 35.8x(Lin et al., 1998). Thus, the local early This possibility is supported by the chemistry and Triassic evaporites had sulphate d34S values (Strauss, isotope composition of the associated brines (Zhou et 1997) that are significantly more positive than those al., 1997) and formation pressure/depth data measured reported for Triassic oceans by Claypool et al. (1980) during drill-stem testing (DST) (Sichuan Petroleum (Fig. 5). Despite theoretical isotope fractionation of C. Cai et al. / Chemical Geology 202 (2003) 39–57 51

34S during the reduction of sulphate, TSR routinely conclusion is unlikely to be correct since there are leads to sulphide with similar or the same d34S values several contradictory lines of evidence: as the initial sulphate (e.g., Machel et al., 1995). The local anhydrite d34S values in the Lower Triassic (1) Using the global data from Claypool et al. (1980), Jialingjiang Formation are close to both the formation Permian seawater had d34S values from + 9xto 34 34 water d S (Lin et al., 1998) and the H2S d S values in +14x(Orr, 1974). The values are too low for the 34 the Wolonghe Field, suggesting strongly that the H2S d S values of the H2S found in the Triassic was generated by thermochemical sulphate reduction reservoirs. within the Triassic. (2) There is negligible anhydrite (and no signs of anhydrite replacement by TSR) in the Permian 5.1.2. Migration of H2S into the reservoir? section so that TSR is unlikely to have been In direct contrast to the idea that the H2S was extensive in Permian strata. Compared with the locally produced by TSR in the Triassic reservoirs, it gases reservoired in the Triassic, the gas in the has been suggested that the H2S migrated into the Permian reservoirs has relatively low H2S con- Triassic strata from Palaeozoic rocks (Sheng et al., centrations ( < 3.4%) and similar d34S values to 1997; Xu et al., 1998). Since sulphate minerals have the gas in the Triassic Wolonghe Field (Table 2), not been found in deeper Cambrian and Ordovician suggesting that the H2S in the Permian might be carbonate rocks in the East Sichuan Basin (Tong, derived from the Triassic (the opposite scenario to 1992), the two remaining possibilities for the primary the one suggested by Sheng et al. (1997) and Xu sources of the TSR H2S are the Permian and Carbon- et al. (1998). iferous carbonate reservoir rocks. 34 d S values of H2S resulting from TSR are usually 5.1.3. Summary of the evidence for the occurrence of close to those of the parent sulphates (e.g., Machel et TSR in lower Triassic reservoirs al., 1995). H2S in the Triassic Wolonghe Field has The evidence supporting the indigenous production 34 positive d Svalues(>+22x) that are close to of H2S in the Lower Triassic by TSR is: Carboniferous marine sulphate d34S values (approximately + 25x; Claypool et al., 1980), leading to the (1) H2S concentrations are highest where there is most possibility of a Carboniferous source of the H2S gas abundant anhydrite. 34 found in the Triassic. However, there are two strong (2) H2S has d S values similar to the local anhydrite lines of evidence against this: and aqueous sulphate. (3) There is a strong local compartmentalisation in the (1) The Carboniferous section contains very low stratigraphy revealed by water geochemistry and ( < 0.7%) H2S concentrations (Table 3). isotopes. Compartmentalisation would strongly 34 (2) Carboniferous H2S has low d S values relative inhibit input from external sources. to Carboniferous marine sulphate ( 9.6xto (4) Migration of H2S into Triassic reservoirs from the + 8.5x; Table 3), suggesting that TSR cannot Permian or Carboniferous is unlikely on the basis have caused the minor amount of H2S in the of geochemical evidence. Carboniferous section. (5) Locally modified carbon isotopes of alkane gas compounds correlate with the degree of TSR, 34 The low d S values and low H2S concentrations suggesting that TSR occurred in the reservoir to of Carboniferous H2S exclude the Carboniferous as the presently reservoired hydrocarbons. This idea a possible source for the H2S in Triassic reservoirs. is explored in Section 5.2. Based upon the elevated d34S values of the gas in Triassic reservoirs of the basin and their similarity to 5.2. Source, maturity and post-depositional alteration those of H2S in the Permian in the Sichuan Basin of natural gases in the Sichuan basin (Table 3), Sheng et al. (1997) concluded that H2Sin the Triassic in the basin might have originated in the In non-sour provinces the carbon isotope ratios of Permian and then migrated into the Triassic. This alkanes are thought to be affected by both source 52 C. Cai et al. / Chemical Geology 202 (2003) 39–57 rock type and source maturity (advanced maturation that the concentration of H2S in a reservoir may 13 can lead to increases in d CCH4; e.g., Tao and Chen, control the formation of thiol compounds. This 1989; Sheng et al., 1991; Wang, 1994). Hydrocarbon supports the conclusion that thiols can be formed gas carbon isotopes have been used to good effect to by reaction between H2S and the hydrocarbon com- reveal details of the source rock type, depositional pounds found in gas phase petroleum (Ho et al., environment and thermal maturity (e.g., Schoell, 1974). The generation of the most abundant H2Sby 1984; Tao and Chen, 1989; Sheng et al., 1991; TSR thus shows that there is a likely association Wang, 1994; Berner and Faber, 1996; Huang et al., between TSR and thiol production. One possible 1999). However, the range of gas isotope values in a specific association is that H2S reacts with petroleum single reservoir (Figs 10 and 11) mayalsobe compounds that remain after TSR to produce a new affected by secondary alteration after emplacement suit of thiol compounds (see also Orr, 1977; Worden in the reservoir (e.g., Krooss and Leythaeuser, 1988; and Smalley, 2001). Note that such neoformed thiols Prinzhofer and Huc, 1995; Cai et al., 2002). in particular, and organosulphur fraction in general, As TSR proceeds, d13C values of light hydro- would adopt the d34S of the original anhydrite as carbons have been shown in some basins to in- transmitted by the TSR H2S. crease progressively (Krouse et al., 1988; Rooney, 1995; Worden and Smalley, 1996; Whiticar and 5.3.2. Origin of thiophene Snowdon, 1999). Positive relationships exist be- Relatively high thiophene concentrations tend to 13 tween d CCH4 and H2S/(H2S+SC1–6) and between occur in association with light oil and condensate 13 d CC2H6 and H2S/(H2S+SC1–6) in the Wolonghe while low thiophene concentrations occur in dry gas gases in the Triassic reservoirs. The positive rela- (Figs. 7 and 8). The thiophene concentrations in the tionship between methane and ethane carbon iso- various petroleum fields are approximately inversely topes and the gas souring index values from the proportional to temperature and organic matter matu- Wolonghe Field (Fig. 11) may be a consequence of rity. The possible causes of the thiophene distribution TSR due to preferential reaction of 12C-hydrocar- in the Sichuan Basin include: bons, as a result of their weaker bond strengths (e.g., Krouse et al., 1988; Worden and Smalley, (1) Thermally controlled cracking of organosulphur- 1996), an example of kinetic isotope fractionation bearing materials (oil or kerogen). (e.g., Cramer et al., 2001). Fig. 11 demonstrates a (2) Back-reaction of H2S with hydrocarbons. general rule that hydrocarbon gas isotopes should (3) Intermediate TSR reactant. not be used for maturity or source characterisation if they have undergone sulphate reduction. Further- The source rocks of the natural gases in the Palae- more, Fig. 11 suggests that even methane, the most ozoic and Lower and Middle Triassic reservoirs, and thermodynamically stable of the alkanes, reacts with the Upper Triassic and Jurassic reservoirs in the sulphate during TSR. This result is seemingly in Sichuan Basin, are considered to be different (Table contradiction to the recent assertion that methane is 1). Gas in Jurassic reservoirs with the high thiophene largely unreactive during TSR (Machel, 2001). concentrations has been suggested to be derived from sulphur-poor type I kerogen while gas in the Lower 5.3. Origin of thiophene and thiols and Middle Triassic with relatively low thiophene concentrations are related to sulphur-rich type II 5.3.1. Origin of thiols carbonate and evaporite source rocks (Table 1; Huang, Thiol compounds were not detected in Jurassic 1990; Zhong et al., 1991; Wang, 1994; Dai et al., petroleum accumulations, while high concentrations 1997). If the thiophenes were generated directly occur in the gas-bearing Triassic, Sinian and Permian within the source rock as a function of the kerogen- reservoirs. Variable thiol concentrations occur within sulphur content, it might be expected that the thio- Triassic reservoirs with similar maturity but different phene distribution would be the opposite of that H2S contents. Thiol concentrations increase with found. However, the petroleum with the S-poor source increasing H2S concentrations (Fig. 9) suggesting rock has the highest thiophene concentrations. Thus, C. Cai et al. / Chemical Geology 202 (2003) 39–57 53 the difference in thiophene concentrations is unlikely ever, double bonds in hydrocarbons have been gen- to be a consequence of source rock type. However, erated during high temperature hydrous pyrolysis of there is a good inverse relationship between source n-alkanes (Leif and Simoneit, 2000; Seewald, 2001), rock maturity (as revealed by vitrinite reflectance, supporting the notion that sulphur can be incorporated Table 3) and thiophene concentration (Fig. 12). This into hydrocarbons during late diagenesis. suggests that thiophene concentrations may be a func- Some thiophene compounds have been shown to tion of source rock maturity rather than source rock be stable at elevated reservoir temperatures (e.g., type. However, the thiophene concentrations may Koopmans et al., 1995; Song et al., 1998), and also be a function of the post generation alteration significant breakdown of thiophenic structures to of petroleum. This possibility is explored below. H2S has not been reported at temperatures less than That thiophene compounds have higher concentra- about 200 jC (Aplin and Macquaker, 1993). Thermal tions in gases associated with light oils or condensates and thermocatalytic studies have established that non- than in single phase gas pools at relatively high thiophenic sulphur (aliphatic as in thiols, acyclic and temperature does not indicate that thiophenes are cyclic sulphides) evolve to produce H2S much more thermally unstable, as suggested by Huang (1990), easily than thiophenic sulphur (Orr and Sinninghe but may indicate that light oils and condensates are Damste´, 1990). The relative lack of thiophenes in more reactive to H2S, with thiophenes being the the Triassic and deeper reservoirs is thus unlikely to result. Evidence shows that isotopically distinct sul- be due to their higher temperatures than in the phur is routinely incorporated into petroleum at rela- shallower and cooler Jurassic reservoirs since thio- tively high temperatures in reservoirs (e.g., Powell phene compounds probably remain relatively stable in and Macqueen, 1984; Orr and Sinninghe Damste´, the deeper hotter reservoirs. 1990; Manzano et al., 1997; Betchel et al., 2001; Sheng et al. (1986) suggested that alkanes might Cai et al., 2001; Worden and Smalley, 2001). It is react with H2S or elemental sulphur to generate typical for sulphur to become incorporated into double thiolane. Thiolane compounds are thermally unstable bonds or functionalized radicals during the early stage and are thought to undergo dehydrogenation, thus of diagenesis of organic matter (Vairavamurthy and generating thiophenes (Sinninghe Damste´ et al., Mopper, 1987; Sinninghe Damste´ et al., 1990). How- 1990). Schmid et al. (1987) produced C18 2,5-dia-

Fig. 12. Relationship between vitrinite reflectance and thiophene concentration. The figure summarises a large volume of data but shows that thiophene concentrations seem to decrease in a systematic manner with increasing source rock maturity. 54 C. Cai et al. / Chemical Geology 202 (2003) 39–57 lkylthiophenes after heating n-octadecane in the pres- suggesting that these apparently unreactive alkanes ence of sulphur for a period of 65 h in a simulation are actively involved in the reduction of sulphate. experiment at 200–250 jC. The result supports the (5) In the Sichuan Basin, there is an apparent possibility that thiophenes can be generated by reac- connection between organosulphur species and tion between liquid phase alkanes and inorganic petroleum type. Thiophene compounds are asso- reduced sulphur compounds, as initially proposed by ciated with liquid petroleum in the Jurassic Orr (1974). Based on relative bond strengths, H2S can reservoirs while thiol compounds are associated theoretically react more easily with higher molecular with gas phase petroleum in Triassic reservoirs. weight hydrocarbon chains than with methane to The greatest quantities of thiophenes are found in generate (thiolanes and thus) thiophenes. This is petroleum generated by the lowest maturity source consistent with our observation that higher thiophene rocks. concentrations occur in wet gas associated with oils in (6) The link between phase and thiophene compounds Jurassic reservoirs and lower thiophene concentrations is uncertain, but may be a consequence of liquid occur in dry gas dominated by methane in Lower and phase thermochemical sulphate reduction or Middle Triassic, Permian and Sinian reservoirs. Thus, primary generation controlled by source maturity. an alternative mechanism to generation from source The least mature source rocks may have produced rocks as an inverse function of temperature (Fig. 12) the greatest quantity of thiophenes per unit of is to produce high thiophene concentrations in Juras- petroleum generation. sic reservoirs by reaction of longer chain alkanes with (7) It is possible that thiol compounds were generated H2S or elemental sulphur. Longer-chain alkanes are either during, or as a byproduct of, gas-phase only abundant in liquid phase petroleum and wet thermochemical sulphate reduction in the Triassic gases, so that more thiophene will be generated in carbonates. In the Triassic Wolonghe Field, thiol Jurassic reservoirs than in the dry gases in Sinian, concentrations correlate positively with the locally Permian and Triassic reservoirs. The origin of the produced TSR–H2S. This suggests that thiol thiophenes remains unresolved. compounds are the result of reaction between H2S and remaining post-TSR petroleum compounds. The coincidence of H2S and thiol 6. Conclusions compounds is thus genetic but limited, in the first case, by the occurrence of TSR. (1) There is up to 32% H2S in the natural gas accumulations in the Triassic carbonates and evaporites of the Wolonghe Field, which is Acknowledgements distinctly different from the relatively low H2S concentrations found in older and younger strata The research was financially supported by the UK in the Sichuan Basin. Royal Society, UK and the National Natural Sciences Foundation of China (grant no. 40173023). Ezat (2) The H2S in Triassic reservoirs in the Wolonghe Field, with a maximum palaeotemperature of Heydari is warmly thanked for constructive comments about 130 jC, has very high d34S values, close on an earlier version of this manuscript. Melodye Rooney and Simon George are thanked for critical to those of its indigenous anhydrite, and H2Sis concluded to have been generated by thermo- comment, which helped to improve the manuscript. chemical sulphate reduction. [LW] (3) The H2S content, sulphur isotope and reported petroleum source rock data show that the H Sin 2 References the Triassic Wolonghe Field has not migrated from Palaeozoic strata but was generated in situ Aplin, A.C., Macquaker, J.H.S., 1993. C–S–Fe geochemistry of by thermochemical sulphate reduction. some modern and ancient anoxic marine muds and mudstones. (4) The carbon isotope ratios of methane and ethane Philosophical Transactions of the Royal Society of London, A increase to higher values with our TSR parameter 15 (344), 89–100. C. Cai et al. / Chemical Geology 202 (2003) 39–57 55

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