Thermochemical sulphate reduction in Cambro–Ordovician carbonates in Central Tarim

Marine and Petroleum Geology 18 (2001) 729±741

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Thermochemical sulphate reduction in Cambro±Ordovician carbonates in Central Tarim Chunfang Cai a,*, Wangshui Hu b, Richard H. Worden c a

Institute of Geology and Geophysics, CAS, P.O. Box 9825 Beijing 100029, People's Republic of China b Jianghan Petroleum Institute, Jingzhou, Hubei 434102, People's Republic of China c Jane Herdman Laboratories, Department of Earth Sciences, University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP, UK Received 3 December 1999; received in revised form 24 May 2001; accepted 29 May 2001

Abstract H2S and CO2 are found in elevated concentrations in Palaeozoic reservoirs in the Tarim Basin in China. We have carried out analyses on gas, petroleum, mineral cement and bulk rock compositions and isotope ratios together with petrography and ¯uid inclusion to assess the origin of the H2S. A deep crustal (e.g. volcanic) origin of the H2S and CO2 is unlikely since the inert gases, N2 and He, have isotope ratios totally uncharacteristic of this source. Organic sources are also unlikely since the source rock has low a sulphur content and the sulphur isotope ratio of the petroleum correlates positively with the sulphur content, the opposite of what would be anticipated from petroleumderived H2S. Bacterial sulphate reduction is unlikely because temperatures are too high for bacteria to have survived. Thermochemical sulphate reduction of petroleum ¯uids by anhydrite in Lower Ordovician and Cambrian carbonate reservoirs is the most likely source of both the H2S and the CO2 causing isotopically characteristic pyrite, CO2 gas and calcite cement. H2S, and possibly CO2, migrated into Silurian sandstone reservoirs by cross formational ¯ow. The H2S, with the same sulphur isotope ratio as Ordovician anhydrite, was partially lost from the ¯uid phase by extensive growth of late diagenetic pyrite. Similarly the CO2 was partially lost from the ¯uid phase by precipitation of late diagenetic calcite. The H2S that resulted from TSR underwent reaction with the remaining petroleum resulting in locally elevated organic sulphur concentrations in the petroleum and the progressive adoption of the Ordovician anhydrite sulphur isotope ratio. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: H2S; CO2; Thermochemical sulphate reduction; Organic sulphur; Pyrite; Tarim basin

1. Introduction Elevated concentrations of dissolved H2S exist in oil®eld waters in Ordovician carbonate and Silurian sandstone petroleum reservoirs in Central Tarim, Tarim Basin, China (Cai & Hu, 1997). The origin, spatial distribution and relationship of the H2S with mineral diagenesis, porosity evolution and hydrocarbon alteration are of importance to petroleum exploration and development. Previous work has shown that high concentrations of H2S in petroleum ¯uids may be generated by thermochemical sulphate reduction (FTSR; e.g. Orr, 1977; Worden, Smalley, & Oxtoby, 1995). Correspondingly, chemistry and the isotopic compositions of light hydrocarbon gases (Connan, Lacrampe* Corresponding author. Present address: Jane Herdman Laboratories, Department of Earth Sciences, University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP, UK. Tel.: 144-0151-794-5200; fax: 144-0151-7945170. E-mail address: [email protected] (C. Cai).

Couloume, & Magot, 1996), petroleum (Sassen, 1988) and bituminous tar (Powell & MacQueen, 1984) become altered during TSR. Heydari (1997) and Heydari and Moore (1989) demonstrated the effects of TSR on burial diagenesis and porosity evolution. Integrated approaches to distinguish TSR from bacterial sulphate reduction (BSR) were suggested by Connan et al. (1996), Machel, Krouse, and Sassen (1995) and Worden et al. (1995). The lowest temperature at which TSR can occur is a matter of debate, but it is now clear that the kinetics of TSR depend upon a variety of factors including petroleum type, rock fabric and the amounts of pre-TSR water and reduced sulphur species (Worden, Smalley, & Cross, 2000). Most recent studies have paid much more attention to the origin of cements and the H2S gas than to the related CO2 gas. It has been suggested that CO2 may be one of the products of TSR as shown by CO2 that increasingly adopts reduced d 13C values as TSR proceeds, and the CO2 concentration in the gas phase seems to increase in some TSR systems (Krouse, Viau, Eliuk, Ueda, & Halas, 1988; Worden & Smalley, 1996).

0264-8172/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0264-817 2(01)00028-9

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Fig. 1. Map showing geological tectonics and location of major petroleum exploration wells (a) and West±East cross section AB of Central Tarim (b).

Elevated concentrations of dissolved H2S in oil®eld waters in the Tarim Basin have been reported and it has been suggested that they might result from TSR (Cai & Hu, 1997). Based upon biomarker parameters, Zhang et al. (2000) and Hanson, Zhang, Moldowan, Liang, and Zhang (2000) concluded that Central Tarim Paleozoic oils were derived from the Middle and Upper Ordovician source rock. Xiao, Song, Liu, Liu, and Fu (2000) demonstrated vertical secondary migration of oil and natural gases from Lower Palaeozoic source rock into overlying reservoirs in the Tarim Basin. Cai, Franks, and Aagaard (2001a) suggested that Ordovician oil®eld waters have migrated up to Carboniferous and Silurian reservoirs based on water geochemistry and 87Sr/ 86Sr ratios. This paper presents new data from cement, oil, gas and water samples, and attempts to account for the origin of H2S and CO2 and to prove the occurrence of TSR and in the Tarim Basin, northwest China. 2. Geological setting Central Tarim is located in the center of Tazhong Uplift,

Tarim Basin, northwest of China (Fig. 1a). It contains a large, NW trending reverse fault with a throw of 2200 m developed during the Caledonian orogeny and crossing the Ordovician and Silurian strata. An E±W cross section is shown in Fig. 1b. The faults and fractures are thought to be the main conduit for petroleum migration. The deepest exploration well (Tc1) in Central Tarim reached a depth of more than 7200 m and penetrated a complete section of the Cambrian. Cambrian strata do not outcrop and are considered to represent broadly continuous sedimentation. The Cambrian section is composed of tidal, platform and platform-marginal carbonate and evaporate rocks. The Middle Cambrian is a suite of supratidal anhydrite-bearing dolomite interbedded with dolomitic anhydrite or dolomite with thin anhydrite interlayers. According to seismic data, anhydrite and salt beds extend north to the Bachu Uplift (Fig. 1a) and to the west of North Tarim with an area of 200,000 km 2 and a thickness of 400±1400 m. The burial and geothermal history of Central Tarim shows that rapid sedimentation took place at the phase of the passive continental margin during the middle Cambrian to early Ordovician. Present-day and maximum temperatures of Cambrian

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in Cambrian, Lower Ordovician and Lower Carboniferous strata but not in Silurian rocks. 3. Sample collection and analysis

Fig. 2. Diagram showing a burial history constructed from well Tc1. Isotherms are constrained by ®ssion track data and vitrinite re¯ectance measurements. About 125±1758C occur in the Lower Ordovician and Cambrian.

and lower Ordovician strata range from 125 to 1758C based on bottom hole temperatures and thermal history analysis (Fig. 2: Zhang et al., 1999). The burial history was re-built by running ThermMod software using the chemical kinetic model of Burnham and Sweeney (1989). The Ordovician carbonate rocks show deepeningupward and then shoaling-upward sequences from bioclastic grainstone, packstone, and mudstone to sandstone. The Silurian sandstone sequence consists of tidal sandstone with extensive bitumen occurrence, and brown or red mudstone. Carboniferous sandstone strata consist of marine clastics while the Mesozoic and Cenozoic are mainly composed of terrestrial sandstones and mudstones (Cai et al., 2001a). Table 1 summarizes the source rocks, petroleum reservoirs, and the occurrence of various sulphur compounds in the Central Tarim Basin. Sulphate minerals were only found

Up to 150 mg samples of disaggregated sandstone were reacted overnight with 100% phosphoric acid at 258C under vacuum to release CO2 from calcite. The sample was then allowed to continue to react for 15 h at 758C to release CO2 from dolomite. Both calcite and dolomite sources of CO2 were analyzed for d 13C on a Finnigan MAT251 mass spectrometer standardized with NBS-18. Calcite cements in Ordovician limestone are present as vug-®lling, coarse and white crystals and contain numerous ¯uid inclusion. These cements were extracted from core using a dentist's drill and subject to stable isotope analysis using the same mass spectrometer as the bulk dissolved samples. All isotope data are reported relative to the PDB standard with a precision of ^0.1½. The Central Tarim oil®eld water samples that contain dissolved H2S from Ordovician carbonate and Silurian sandstone reservoirs were collected in 500 ml glass jars containing excess cadmium acetate (1.5 g) to precipitate dissolved sulphide as CdS. The SO2 gas for S isotope analyses was produced by combustion of a mixture of sulphide and Cu2O in a 1:10 at 11008C under vacuum. SO2 gas was collected in a sample tube by freezing. To transform sulphate to SO2 gas, barium chloride reagent was added to precipitate sulphate as BaSO4. The BaSO4 was mixed with V2O5 and SiO2 in a proportion of 1:3.5:3.5, and the mixture was placed in porcelain bottle and covered with a layer of copper wires. Sulphur isotope ratios were measured on a MAT 251 mass spectrometer and results are reported in the standard d notation relative to Canyon Diablo troilite (CDT). Reproducibility for d 34S is ^0.1± 0.3½. 3He/ 4He and d 15N were measured at VG-5400 and MAT271 mass spectrometer, respectively. The reproducibilities of the 3He/ 4He and d 15N are ^0.6% and ^1½, respectively. Microthermometry of ¯uid inclusions in the vuggy

Table 1 Main petrology, petroleum source and reservoir, and sulphur species in the sampled strata Strata

Main petrology

Genetic relation

Sulphur species

Carboniferous

Sandstone and mudstone with anhydrite-bearing dolostone Sandstone with extensive bitumen and mudstone

Petroleum reservoir

Plenty of anhydrite, but little pyrite and low H2S No anhydrite, but widespread pyrite, and high dissolved H2S in oil®eld water No anhydrite, little pyrite and no H2 S Plenty of anhydrite, pyrite and high dissolved H2S in oil®eld water

Silurian Middle and Upper Ordovician

Mudstone, shale and limestone

Cambrian and Lower Ordovician

Dolostone, shale, limestone and anhydrite beds

Petroleum reservoir Petroleum source rock and local reservoir Petroleum source and reservoir

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Table 2 Chemistry and isotopic composition of natural gases and H2S concentration in oil®eld water Well

Age

Depth a (m)

Natural gas mCO2

Tz101 Tz11 Tz12 Tz12 Tz12 Tz16 Tz161 c Tz161 c Tz162 c Tz162 Tz30 Tz4 TZ401 c Tz421 c Tz421 Tz421 Tz422 c Tz44 Tz44 Tz6 Tc1 Tz162 Tz43 Tz43 Tz49 a b c d

CIII S S S O CIII S O O O O C CIII CI CII CIII CII O112 O CIII O1 O122 O O O

3730 4425 4382 4412 4707 3800 4180 4293 5059 5991 5011 3632 3685 3259 3486 3573 3544 4827 4871 3726 6004 5984 5410 5697 6196

b

0.65 0.15 2.80 1.61 1.25 1.21 2.33 0.77 3.55 7.84 d 15.51 1.42 0.25 0.15 0.17 0.19 0.99 1.95 4.42 0.77 ± ± ± ± ±

Co-produced water mN2

b

6.47 7.11 7.06 6.83 2.15 49.06 2.96 18.97 19.97 0.61 1.51 21.55 14.08 19.02 13.89 11.59 39.42 3.42 3.93 6.26 ± ± ± ± ±

mCH4

13

b

89.24 87.01 88.88 90.69 95.45 44.88 93.13 71.88 72.06 88.72 82.25 75.29 81.43 77.63 82.17 80.6 55.56 92.91 90.28 91.51 ± ± ± ± ±

15

13

d Cco2

d N

d CCH4

H2S (ppm)

211.57 220.04 ± ± 210.94 218.65 ± ± ± ± 1.21 213.6 ± 217.59 ± 28.45 220.88 29.57 ± 212.01 ± ± ± ± ±

10.22 1.63 ± ± 4.23 2.53 ± ± ± ± 3.51 ± ± 4.04 3.89 2.42 1.05 2.97 ± 2.38 ± ± ± ± ±

246.3 ± ± ± 242.8 ± 241.8 241.8 ± ± 244.9 243.2 243.3 ± ± ± ± ± ± 242.3 ± ± ± ± ±

± ±

600 Intense smell of H2S 780 ± Intense smell of H2S ± 986 37500 d ± ± ± ± ± ± ± 1150 376 ± Intense smell of H2S 684 1175 Intense smell of H2S Intense smell of H2S

Depth is set as the middle point between perforations in meters. Gas composition in mol%. Wells Tz161, 162 are close to wells Tz16. Tz421, 422, Tz401 are close to well Tz4. After acidi®cation.

calcite cement in the Ordovician limestones was carried out using a Linkam THM600 heating±cooling stage with a precision of 0.18C. Measurements were made in strictly increasing temperature order so as to minimize inclusion re-equilibration.

Basic oil ®eld data and sulphur contents of petroleum and bitumen were collected from annually published reports on production from the Tarim Basin. Two bitumen extracted from sandstone were measured for sulphur content in Leeds University.

Fig. 3. Variations of the concentration of H2S dissolved in oil®eld waters, CO2 molar volume and d 13C value in natural gas vs. depth for wells from the Central Tarim. Note that high CO2 values occur at the depths of 4050±5100 m, and that most of the CO2 have d 13C values less than 210½, indicating an organic origin.

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Fig. 4. Relationship between d 34S and sulphur content in petroleum and bitumen from Ordovician, Silurian and Carboniferous. No signi®cant d 34S difference among the formation, and the sulphur-enriched samples have elevated d 34S values, indicating the same source of H2S has been incorporated into the organic matter.

4. Results 4.1. Reservoir ¯uid chemistry and isotopic composition Oil®eld waters range from being colorless, through to being yellow, or even dark brown. Most of the dark water samples are characterized by elevated dissolved H2S concentrations, ranging up to 1175 ppm (Table 2), whilst colorless and pale yellow water samples contain little H2S. The water samples with the highest H2S concentrations are associated with petroleum ®elds that have a gas cap or at least have high gas-oil ratios and have CO2 with low d 13C

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values. Conversely water samples with low H2S concentrations tend to have less associated gas (low gas-oil ratios). Of 96 analysed gas samples, about 60% have a CO2 concentration less than 1.2 mol% (Fig. 3, Table 2). There is no simple correlative relationship between CO2 and H2S concentrations. Another characteristic of Central Tarim natural gas accumulations is that CO2 concentration does not increase with depth. The highest CO2 concentrations occur between depths of 4050 and 5100 m (Fig. 3). Additionally, the CO2 d 13C values range from 28.5 to 220.9½ except for one sample which has a d 13C value of 11.2½ (Table 2). The relative volume of N2 in Paleozoic gas compositions is high, up to 57 vol% (49 mol%) but with a wide range (Table 2). The d 15N varies over a small range (11.0± 1 4.2½; Table 2). The 3He/ 4He ratio of four gas samples ranges from 4.0 to 4.6 £ 10 28. Ten petroleum samples and bitumen from Carboniferous, Silurian and Ordovician reservoirs have sulphur contents that vary from 0.015 to 1.7%. The petroleum and bitumen samples with the highest sulphur contents occur mainly in Silurian reservoirs. The oils and bitumen have d 34S values from 113.6 to 126.5½ with a mean value of 121.3 ^ 3.9½ …n ˆ 10†: There is a good correlation between the sulphur content of the petroleum and the sulphur isotope ratio. Those samples with elevated sulphur contents tend to have elevated d 34S values (Fig. 4). There is no signi®cant pattern between the age of the reservoir and the sulphur isotope ratio.

Fig. 5. Synthetic paragenetic sequences showing main stages in the diagenetic evolution of, (a) Cambrian and Ordovician carbonates (modi®ed after Chen, Sheng, Wang and Zhu (1994) and Ye (1994)), and (b) Silurian sandstones (modi®ed after Cai et al., 2001b).

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Fig. 6. Photomicrographs showing calcite cement occluding in vug of Ordovician grainstone of well Tz54 under plane light (a) and under cathodoluminesence (CL) (b), (scale bar ˆ 0.2 mm). The feature suggests that cementation took place during at least two periods, and that late-stage calcite characterized by orange or dark orange CL has low Fe/Mn ratios.

4.2. Paragenetic sequences, diagenetic minerals and mineral isotopic ratios 4.2.1. Cambrian and Ordovician limestones The main diagenetic events in the Cambrian and Ordovician grainstones during burial include gypsum dehydration to form anhydrite, early diagenetic calcite cementation, saddle dolomite cementation, late calcite cementation and anhydrite replacement by pyrite and calcite (Fig. 5a). Saddle dolomite was found to occur in Lower Ordovician limestone in well Tz2 and in well Tc1 in Cambrian dolomites and coexists with bitumen and heavy petroleum. More than two generations of calcite with different crystal shapes were identi®ed by cathodoluminesence, suggesting that calcite cementation took place during at least two episodes (Fig. 6a and b). Late stage calcite cement has orange and dark orange cathodoluminescence, indicating that the pore ¯uid had a relatively low Fe/Mn ratio. Calcite and pyrite have, at some localities, completely replaced anhydrite in the Middle Cambrian and Lower Ordovician gypsum-bearing carbonate rock or anhydrite beds. Calcite and pyrite occur as pseudomorphs after anhydrite in Ordovician micritic limestone at 4720 and 4723 m in well Tz12 (Fig. 7), in bioclastic grainstone at 5944 m in

well Tz54, and in anhydritic dolomite at 5800 m in well Tz1. The initial depths for replacement of anhydrite and calcite correspond to more than 1258C according to data from either bottom hole temperatures or from calculated geothermal gradients (Fig. 2). Drilled out calcite cements from Ordovician limestone have d 13C values from 26.0 to 29.4½. The d 13C of 18 bulk limestone samples ranges from 10.6 to 12.9½, averaging 11.8 ^ 0.7½ (Fig. 8). Therefore the d 13C of the cement in the limestones is much lighter than d 13C of the bulk limestone. Ordovician and Cambrian anhydrite from the Bachu Uplift, to the west of Central Tarim, have d 34S values of 126.1 and 133.7½, respectively, which are close to the contemporary seawater sulphate d 34S values reported by Claypool, Holser, Kaplan, Sakai, and Zak (1980). Late stage euhedral authigenic pyrite was found in small quantities in the Cambrian and Ordovician limestones. Bitumen was found within vugs (intercrystalline dissolution pores) after biosparite between 5757 and 5765 m in well Tz54, in dolomitized limestone in 5978.48 m in well Tz162, at .5800 m in dolomite of well Tz1, and in dissolution pores along stylolites and fractures in the Cambrian and Ordovician (Ye, 1994).

Fig. 7. Photomicrographs showing calci®cation of anhydrite in Lower Ordovician limestone (scale bar ˆ 0.2 mm): (a) TSR calcite occupying pre-anhydrite location under plane light; (b) anhydrite original con®guration under CL.

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Table 3 d 34S values of authigenic pyrite and anhydrite

Fig. 8. Distribution of d 13C of Ordovician bulk limestone and carbonate cements of Silurian sandstones and Ordovician limestone. Note that much lighter carbon occurs in cements than in bulk rock, indicating an organicderived carbon.

4.2.2. Silurian sandstones The paragenetic sequence in the Silurian sandstones (Fig. 5b) shows that there is no evidence for the existence of anhydrite (or other sulphate) cements in these sandstones. The paragenesis is dominated by early diagenetic calcite, quartz cement and late stage ferroan dolomite, as well as minor kaolinite cement and albitisation of K-feldspar. The later diagenetic carbonate cements in the Silurian sandstones are characterized by depleted d 13C values ranging from 23.8 to 221.5½ with an average of 29.9 ^ 4.8½. The last diagenetic mineral to form was pyrite which occurs as large, euhedral crystals (up to 2 mm, Fig. 9). The cubic habit of pyrite (and absence of framboidal pyrite) may indicate a non-bacterial origin at a relatively slow crystal growth rate during burial diagenesis at relatively high temperatures. Pyrite replaced detrital grains and calcite cement and represents up to 3% rock volume. The pyrite has a range of d 34S values from 117.7 to 134.4½ with an average of 125.2 ^ 7.4½ …n ˆ 6† (Table 3, Fig. 10). Solid bitumen was found within the intergranular porosity in Silurian sandstones, and was considered to precipitate during late-stage diagenesis due to an in¯ux of gas, increasing the gas oil ratio and leading to asphaltene exsolution

Fig. 9. Photograph showing cubic pyrite in bedding plane of Silurian sandstones. The pyrite has sizes up to 3 mm.

Well

Age

Depth (m)

Mineral

d 34S

Tz12 Tz12 Tz12 Tz11 Tz11 Tz11 Tz11 Tz161 Ma401 Fang 1

S S S S S S S O213 O1 [

4378 4384 4388 4390 4403 4450 4432 4243 2042 4602

FeS2 FeS2 FeS2 FeS2 FeS2 FeS2 FeS2 FeS2 CaSO4 CaSO4

34 34 20 9.5 25 13 20 18 26 34

from heavy oils (Cai et al., 1997; Cai, Gu, & Cai, 2001b; Zhang et al., 1999). The heavy oils were residues from the biodegradation of early emplaced petroleum (Xiao, Zhang, Zhao, & Zheng et al., 1997; Zhang et al., 1999) when the Silurian was uplifted and exposed to the surface and in®ltrated by paleometeoric water (Cai et al., 2001a). 4.3. Fluid inclusions in late calcite in Cambrian and Ordovician limestones 105 ¯uid inclusions in calcite cement in Cambrian and Ordovician limestone have a varied size of 1.5±60 mm, but are mostly smaller than 5 mm. The inclusions occur as single inclusions or in planar groups. All of the ¯uid inclusions are two-phase liquid-gas inclusions, with gas/liquid ratios from 2 to 40. They can be categorized as primary and secondary. The primary inclusions occur as a single inclusion in an otherwise inclusion-free crystal, or isolated

Fig. 10. Distribution of d 34S of anhydrite, pyrite and petroleum and bitumen from the Cambrian±Ordovician, Silurian and Carboniferous. No signi®cant difference among the values, indicating the sulphur in the pyrite and organic matter could have been derived from Cambrian±Ordovician anhydrite.

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Fig. 11. Histogram showing homogenization temperatures in ¯uid inclusions in the cements in the Cambrian and Ordovician carbonate rock with two peak values of 75±998C and 125±1508C.

away from other inclusions. Primary inclusions have a bimodal distribution of gas/liquid ratios with the two ranges falling between 4 and 8, and 15 and 40. The homogenization temperatures of the primary inclusions in calcite cement in the interval of 5074±5079 m of the Cambrian in well Tz1 can be divided into three groups: 80±908C, 140±1508C and up to 1808C. Calcite cement homogenization temperatures in the Ordovician are 75 ±1008C and 112±1498C in Tz24, and 135±1548C in Tz25. A histogram of homogenization temperatures shows that about 22% data (the second peak value) have a temperature range of 125±1508C (Fig. 11). The temperature is close to present-day reservoir temperature (125±1758C) so that resetting of homogenization temperatures, which may occur in calcite, is considered to have been negligible. Twenty two percent of the calcite cement growth therefore is interpreted to have occurred at temperatures greater than 1258C. 5. Discussion The association of relatively high molar volumes of CO2 in gases and high concentrations of H2S in associated waters suggest that they may be genetically related. However, there is no simple linear correlation between the two. The two gases are not inert to water-rock systems so that the lack of a simple relationship may be due to different degrees of loss of H2S and CO2 from the ¯uid phases (due, for example, to precipitation as carbonate and sulphide cements). 5.1. Origin of gas phase CO2 and carbonate cements Various criteria have been proposed to account for the generation of CO2 in oil®elds in the eastern China and the South China Sea (e.g. Dai, Ji, & Hao, 1989; Dai, Song, Dai, & Wang, 1996). It is thus possible to account for the origin of CO2 in the Tarim Basin where recent studies on petroleum source rock and ¯uid ¯ow of Palaeozoic reservoirs have been performed. In general, CO2 may be derived

from bacterial and thermal degradation of organic matter, decomposition of carbonates, and degassing of the mantle and atmosphere (e.g. Connan et al., 1996; Dai et al., 1989, 1996; Wycherley, Fleet, & Shaw, 1999). The d 13C values of HCO32(aq) from dissolved atmospheric CO2(g) in subsurface shallow waters are close to 0½ (Carothers & Kharaka, 1980). Atmospheric CO2 exerts an in¯uence upon carbonate precipitation within the zone of meteoric water activity during early diagenesis and close to unconformities or fault zones. Consequently, meteoric water carrying atmospheric CO2 is unlikely to be a source of the CO2 in the Tarim Basin reservoirs. d 13C values of mantle-derived CO2 are in the range from 24 to 27½ (Dai et al., 1989, 1996; Thrasher & Fleet, 1995). If CO2 is mainly mantle-derived, then the gas should also have entrained He and N2 from the same (mantle) source. The isotopes of He and N2 can be used to de®ne their origin (mantle, atmospheric, etc.) so that the isotopes of these gases can also be used to de®ne the source of CO2 (e.g. Dai et al., 1996; Sherwood-Lollar, Ballentine, & O'Nions, 1997; Wycherley et al., 1999; Zhu, Shi, & Fang, 2000). 3 He/ 4He ratio ranges in Central Tarim from 4.0 to 4.6 £ 10 28. This ratio is much lower than both the atmospheric helium isotope ratio (1.4 £ 10 26) and the mantlederived helium isotope ratio (3 £ 10 25) (Mamyrin & Tolstikhin, 1984). The helium isotope ratio may, instead, originate from a -decay of radioactive elements in organic matter (Xu et al., 1998) or the crystalline basement (Hiyagon & Kennedy, 1992). The N2 concentrations of gases in the Tarim Basin reach 57 vol%. This is thought to be typical for N2 generated from sedimentary organic matter at high temperatures in sedimentary basins (e.g. Littke, Krooss, Idiz, & Frielingsdorf, 1995; Zhu et al., 2000). The d 15N values from 11.0 to 14.2½ and are comfortably within the range for N2 derived from sedimentary organic matter (23 to 113½; Rigby & Batts, 1986). Together the N2 concentrations, the d 15N value and the 3 He/ 4He ratio, supported by the relatively low d 13C values of CO2, suggest little contribution from mantle-derived sources to the natural gases. Instead, the inert gas data suggest that the N2 and He had a predominantly organic source. In conclusion, the mantle (or any other very deep earth source) is discarded as a possible source of the CO2. Detrital limestone can be an important source of gas phase CO2 and carbonate in later minerals cements. Ordovician limestone has an average bulk rock d 13C value of 11.8½. If CO2 was mainly produced from the limestonewater interaction (as suggested by, for example, CoudrainRibstein, Gouze, & Marsily, 1998), then the d 13C value of CO2 would be expected to be close to that of limestone since there is minimal fractionation for the transformation of carbonate to CO2 at high temperatures. The gas phase CO2 has a range of d 13C values but they are almost exclusively much less than 11.8½. Thus, it seems to be very unlikely

C. Cai et al. / Marine and Petroleum Geology 18 (2001) 729±741

that CO2 (as well as calcite cement with the light d 13C values), could be derived solely from inorganic minerals. Moreover, Coudrain-Ribstein et al. (1998) and Smith and Ehrenberg (1989) have suggested that partial pressures of CO2 (Pco2, where Pco2 ˆ CO2 mol% £ subsurface pressure) may be controlled by chemical equilibration between carbonate minerals and associated water. Over the temperature range from 10 to 2008C, Pco2 values in some basins have been shown to increase with increasing depth of burial and thus temperature. However, in this study of the Tarim Basin, such a relationship does not feature. Pco2 and CO2 molar volumes do not increase with increasing depth of burial. Note that at depths greater than 5100 m, Pco2 values are less than those between 4050 and 5100 m. That is, Pco2 does not increase consistently with increasing temperature or depth, suggesting that temperature is not the only control on CO2 content. Thus an inorganic carbon contribution from the decomposition of carbonate or reactions between minerals and solutions controlled by temperature (CoudrainRibstein et al., 1998; Hutcheon & Abercrombie, 1990), is not signi®cant in the Tarim Basin. CO2 can be generated by oxidation, sulphate reduction or thermal decarboxylation of organic matter. These sources result in CO2 d 13C values of generally less than 220½ (Irwin, Curtis, & Coleman, 1977). Petroleum in Cambrian, Ordovician and Silurian reservoirs and kerogen in the Cambrian and Ordovician source rocks have an average d 13C value of 231.8½ (Zhao & Huang, 1996). If the CO2 derived from organic matter has a similar isotopic composition as the parent organic matter, then associated dissolved HCO32 would be about 10½ heavier at 508C and about 5½ heavier at 1008C (Wood & Boles, 1991). Therefore, the d 13C value of dissolved HCO32 from oxidised Cambrian and Ordovician organic matter would be anticipated to be about 230½ at temperatures of greater than 1258C. Since calcite cement in the Silurian sandstones has d 13C values as low as 220½, then this suggests an important contribution from the oxidation of organic matter to the overall CO2 and carbonate cement budget. 5.2. Fluid migration The geochemistry of petroleum source rocks and hydrocarbon inclusions (Xiao et al., 1997) has been used to show that the source of high maturity, light oil in the Silurian bitumen-bearing sandstone was the Middle and Upper Ordovician source rocks (Hanson et al., 2000; Zhang et al., 2000). Organic geochemical data have also shown that generation and emplacement took place during the Late Yanshan and Early Himalayan Orogeny (late Cretaceous to early Eocene) (Xiao et al., 1997, 2000). Emplacement of dry gas from Ordovician source rocks with R0 up to 1.45% in the Upper Paleozoic reservoir has also been suggested by Huang et al. (1996). The high (.1208C) homogenization temperatures in calcite cement in Silurian sandstones of well A (Xiao et al., 1997) are higher than

737

bottom-bole temperatures and are possibly indicative of upward migration of relatively hot, CO2-bearing gas or oil®eld water. This vertical migration might have released CO2 due to the consequent decrease in temperature and pressure. Cross formational ¯ow (Worden & Matray, 1995), upward migration and mixing of ¯uids are supported by water chemistry and 87Sr/ 86Sr ratios (Cai et al., 2001a). 5.3. Mechanism of H2S generation in the Tarim Basin Three mechanisms for H2S generation have been proposed: (1) thermal decomposition of organic sulphurcontaining petroleum and kerogen; (2) bacterial sulphate reduction (BSR) and (3) thermochemical sulphate reduction by hydrocarbons (TSR) (e.g. Aplin and Coleman, 1995; Orr, 1977). These mechanisms can be distinguished on the basis of temperature data, the characteristics of the diagenetic systems, ranges of H2S concentrations and d 34S of H2S and other reduced sulphur species (e.g. Cai et al., 1997; Machel et al., 1995). 5.3.1. Organic sources of H2S Temperatures of more than 1758C are thought to be required to cause the decomposition of organic matter to generate a signi®cant amount of H2S (e.g. Aplin & Coleman, 1995). Although a plot of burial history shows that temperatures of Cambrian strata can be up to 1758C, Lower Paleozoic petroleum has been considered to be derived from Middle and Upper Ordovician non-evaporite source rock (Zhang et al., 2000) with temperatures less than 1258C. Therefore, it is unlikely that a signi®cant quantity of reduced sulphur (total pyrite and dissolved H2S in the oil®eld waters) could be derived from the decomposition of organic matter. This is supported by the similar d 34S values of the pyrite, petroleum and bitumen. The explanation is as follows. In general, if sulphur in oil has been derived from the parent kerogen, then they typically have very similar sulphur stable isotope ratios. Similarly, organically-derived H2S is thought to have a d 34S value very close to parent S-enriched kerogen and oil (Orr, 1977). Marine kerogen contains sulphur that has a d 34S value that is typically about 15½ lower than the contemporary seawater. Thus, Ordovician kerogen (and resulting sulphur in oil and organically-derived H2S) should have a d 34S of about 111½ (Claypool et al., 1980). However, petroleum and bitumen in the Tarim Basin have a d 34S range mainly between 118.4 and 125.6½. The sulphur isotope data thus discount the possibility of the reduced sulphur (in sulphide compounds, e.g. pyrite) predominantly having an organic source. 5.3.2. Sulphate reduction sources Since we have argued against an organic source for the sulphur in the H2S, aqueous phase sulphate reduction is the likely origin of H2S in the Tarim Basin (e.g. Machel et al.,

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1995; Worden and Smalley, 1996): CaSO4 1 petroleum fluid ! CaCO3 1 H2 S 1 altered petroleum …or bitumen† 1 H2 O Anhydrite is typically replaced by calcite during sulphate reduction while the petroleum reactant will be altered since some compounds will be more reactive than others to sulphate. Organically-sourced CO2 and inorganicallysourced H2S are the gas phase products of sulphate reduction reactions. However, at least some of the CO2 is likely to react with aqueous calcium from the anhydrite source to produce solid phase calcite. Similarly H2S will react with any ferric mineral that are present in the system to precipitate pyrite. Fe may be derived from the clay minerals such as illite within carbonates in the Cambro±Ordovician (Worden et al., 2000), in Upper Ordovician mudstone, or from Febearing minerals in the Silurian sandstones. One of the consequnces of ¯ooding the system with H2S is that aqueous Fe becomes depleted (especially relative to Mn), resulting in low Fe/Mn ratios in formation water. Note that Fe-poor calcite typically is brightly luminescent, while calcite with more than 2 wt% siderite is non luminescent (e.g. SpoÈtl, Longstaff, Ramseyer, Kunk, & Wiesheu, 1998; Tucker & Wright, 1990). The late stage calcite in the Ordovician limestones is typically bright orange in a cathodoluminescence microscope (CL) (Fig. 6b) so that this stage of calcite is most likely to have been formed in Fe-poor formation water. The absence of Fe in the contemporary formation water is likely to have been due to the presence of H2S and the formation of pyrite. Although some evidence seems to support sulphate reduction in the Central Tarim, both BSR and TSR share a similar overall reaction, and it is necessary to distinguish these mechanisms. 5.3.2.1. Bacterial sulphate reduction. The growth and propagation of sulphate reducing bacteria (SRB) depend on the temperature, salinity, oxygen content and nutrient levels remaining favourable to the bacteria. Although it is possible for different kinds of SRB to grow in different environments, most SRB are active under restricted conditions: temperatures less than 60±808C, low salinities, strictly anoxic conditions and the maintenance of aqueous sulphide concentration below the threshold at which the SRB can survive. SRB are thought to consume hydrocarbons directly (Kirkland & Denison, 1995) or to consume byproducts of petroleum degradation by aerobic bacteria (e.g. Cai, Mei, Ma, Chen, & Liu, 1996; Jobson, Cook, & Westlake, 1979). The content of H2S produced by BSR is generally less than 3% of the natural gas composition. For example, dissolved H2S from BSR in Devonian oil®eld waters in the Alberta Basin ranges from 136 to 415 ppm (Connolly, Walter, Baadsgaard, & Longstaffe, 1990). The H2S concentration data from the Tarim Basin (up to 1175 ppm dissolved in water) seem to

be too high to come from a SRB source. SRB typically lead to a large sulphur isotope fractionation effect between sulphate and sulphide (e.g. Machel et al., 1995). The sulphur isotope data for anhydrite and pyrite in the Tarim Basin (Table 3) are within the same range and do not support the occurrence of BSR-induced isotope fractionation. Thus the data from temperatures, the H2S concentrations and sulphur isotopes do not seem to support the occurrence of BSR in Central Tarim. 5.3.2.2. Thermochemical sulphate reduction. TSR occurs in systems that undergo progressive burial (Cai et al., 1997). The temperature range for the initiation of TSR varies according to factors such as the presence of catalysts, the availability of anhydrite, the rock fabric (anhydrite crystal size; Worden et al., 2000), but is typically greater than 120± 1408C (Machel et al., 1995; Worden et al., 1995). TSR generally produces large volumes of H2S. However, the concentration of H2S is limited by the availability of Fe (or other transition metals) in the rock or formation waters as these metals lead to base metal sulphide precipitation and loss of ¯uid phase sulphide. Consequently, the total amount of sulphide (reduced sulphur) generated by TSR or by BSR is the best parameter to assess how much H2S has been produced and thus to distinguish between BSR and TSR. The proposal is supported by a case-study of North Sea oil ®elds, where the concentrations of H2S, thought to have been generated by TSR, are less than 1% by volume of the associated gas but late-stage pyrite is common (Worden & Smalley, 2001). In Central Tarim, the total sulphide quantity is high, with dissolved H2S ranging up to 1175 ppm in Ordovician and Silurian oil®eld waters, and as much as 3% late-stage cubic pyrite within Silurian sandstones. It is unlikely for such a large quantity of reduced sulphur to be generated in-situ in the Silurian sandstone during burial diagenesis, as no sulphate minerals have yet been found in these rocks. Together with the CO2, H2S must have undergone secondary migration (cross formational ¯ow: Worden & Matray, 1995) from Lower Paleozoic strata, similar to the case-study reported by Moldovanyi, Walter, and Land (1993). During TSR, there is commonly no signi®cant sulphur isotope fractionation in the process of transformation of sulphate to sulphide (e.g. Orr, 1977). Anhydrite can be used to constrain the seawater sulphur isotope ratio at the time of gypsum deposition (Worden, Smalley, & Fallick, 1997). This is supported by similarity between the measured Cambrian and Ordovician anhydrite and the contemporary seawater sulphate (Claypool et al., 1980). The six pyrite samples with a mean value of about 125½, thus probably result from TSR. However, two of the samples have relatively low d 34S values (19.5 and 113.4½, respectively), and could have a complex origin involving both eogenetic BSR-related pyrite and later TSR. For example, Riciputi, Cole, and Machel (1996) showed the pyrite in Devonian Nisku Formation has d 34S values ranging between 235

C. Cai et al. / Marine and Petroleum Geology 18 (2001) 729±741

and 120½, and were thought to be originated from both BSR and TSR. According to temperatures for the initiation of the replacement of anhydrite and bottom-hole temperatures, in conjunction with homogenization temperatures of ¯uid inclusions, sulphate reduction is considered to have occurred at a temperature greater than about 1258C. 5.4. Organic reactants involved in TSR During sulphate reduction processes, the organic reactants can be petroleum, kerogen or methane. Petroleum and kerogen (with d 13C values of about 231½ in the Tarim Basin) have d 13C values that are closer to the lowest CO2 or calcite cement d 13C values (about 221½) rather than methane (with an average of 243.3 ^ 1.6½, n ˆ 8; Table 2). However, the involvement of methane in TSR is well documented (Krouse et al., 1988; Worden & Smalley, 1996; Worden, et al., 1995), and CO2 carbon isotope values may become blurred if there has been mixing of different sources of CO2. Gas isotope and geochemistry data reveal that there is no relationship between the methane d 13C ratio and the CO2 d 13C ratio (Table 2). Since there is typically an inverse relationship between methane and CO2 carbon isotopes in systems that have undergone methane-sulphate reduction (Worden et al., 2000), it is unlikely that methane was the main reactant. It thus seems to be reasonable to assume that liquid phase petroleum was the reactant rather than the methane. 5.5. Location of TSR in the petroleum system and migration of H2S It is possible to conclude that TSR is responsible for the H2S and the CO2 in the reservoir ¯uids. It is thus pertinent to assess where TSR has occurred. The Ordovician and Cambrian marine limestones contain variable quantities of anhydrite cement. This has been partially replaced by calcite that is: (1) characterized by isotopically depleted carbon, (2) luminsecent (suggesting minimal aqueous iron and possibly enriched sulphide), (3) de®ned by ¯uid inclusion homogenisation temperatures typically greater than 1258C. They also contain late stage pyrite cement. It is thus very likely that TSR has occurred within these limestones involving anhydrite replacement by calcite and that once the local aqueous iron was exhausted due to the formation of pyrite, H2S could accumulate in the ¯uid phase. TSR is unlikely to have occurred in the Silurian sandstones because, seemingly, they contained no sulphate minerals capable of causing TSR to occur. It is thus likely that TSR occurred in other formations and migrated into the Silurian. The resulting elevated H2S concentrations in the Silurian reservoirs may have been due to cross formational ¯ow from Ordovician and Cambrian limestones, in which TSR was occurring. Cross formational ¯ow of gas phase CO2 must also have occurred since the depleted carbon

739

isotopes in the Silurian-hosted gas phase CO2 and late stage carbonate cements cannot have been locally generated. 5.6. TSR, H2S and sulphur in petroleum The petroleum and bitumen have similar d 34S values (mainly from 118.4 to 126.5½) and are similar for reservoirs of different ages (Fig. 10). This suggests that the organic sulphur has a common source. This is consistent with the result obtained by analysis of the petroleum system (Xiao et al., 2000), source rock (Zhang et al., 2000) and oil®eld water (Cai et al., 2001a). If the sulphur isotope enrichment in petroleum was due to fractional loss (of 34S-depleted sulphur) during cracking, then there would be a negative correlation between the sulphur weight percent and the d 34S (as sulphur is lost from the oil, the remainder would become progressively enriched in remaining 34S). However, there is a positive correlation between the sulphur content of petroleum and its d 34S (Fig. 4), suggesting that sulphur enriched in 34S has been added to the petroleum. The isotopically enriched sulphur is likely to have come from the H2S given the relative similarity of the petroleum sulphur and the sulphide. It is likely that there has been a reaction between the TSR-derived H2S and the petroleum in the reservoir. Thus there has been back-incorporation of some of the TSR H2S into the organic matter (e.g. Orr, 1974; Powell & MacQueen, 1984; Worden & Smalley, 2001). A reaction may be written to account for the process: Petroleum fluid 1 H2 S ! sulphur-rich altered petroleum and=or bitumen Thus the petroleum system ®rst undergoes TSR causing conversion of the sulphate into sulphide and consuming some of the petroleum itself. Subsequently, at least some of the H2S has then reacted with the remaining (or heavy) petroleum resulting in an increase in the sulphur content of the petroleum and bitumen and the progressive adoption of the sulphide (and thus anhydrite) sulphur isotope ratio. 6. Conclusions 1. CO2, N2 and He all have a mainly organic source in both Cambrian and Ordovician carbonate and Silurian sandstone reservoirs in the Tarim Basin. Meteoric water, deep crustal or mantle sources and inorganic water-rock interaction cannot explain the isotope ratios and distribution patterns of these gases 2. The sulphur in petroleum in both Cambrian and Ordovician carbonate and Silurian sandstone reservoirs in the Tarim Basin is at least partly derived from other than organic rich petroleum source rocks, as shown by the positive correlation between the relationship between

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3. 4.

5.

6.

C. Cai et al. / Marine and Petroleum Geology 18 (2001) 729±741

the sulphur content and the sulphur isotope ratio of the petroleum. The Tarim Basin system has too much sulphide and the reservoir temperatures are too hot for BSR to be the origin of the sulphide. Thermochemical sulphate reduction occurred in anhydritebearing Cambrian and Ordovician carbonate reservoirs, producing H2S, as shown by the occurrence of anhydrite pseudomorphed by calcite and pyrite, the high temperature of the replacive calcite growth and the sulphur isotope ratio of pyrite. TSR may also have produced the signi®cant volumes of CO2 seen in the Tarim Basin limestone reservoirs. Cross formational ¯ow of TSR-derived H2S and CO2 occurred from the Cambrian and Ordovician carbonate reservoirs to the Silurian sandstones as shown by the sulphur isotopes of diagenetically late abundant euhedral pyrite and the stable isotopes of gas phase CO2 and diagenetically late calcite cement. TSR could not have occurred in the Silurian sandstones since there were no sulphate mineral in these sandstones. TSR-derived H2S back-reacted with the remaining petroleum resulting in the addition of sulphur to the petroleum and the progressive adoption of the initial Cambrian and Ordovician anhydrite sulphur isotope ratio.

Acknowledgements The research is ®nancially supported by China's National Natural Sciences Foundation (49773198) and Royal Fellowship of the Royal Society, U.K. Dr S.C. George from Australia is thanked for valuable contributions through his review of the original manuscript. d 13C and d 34S measurement was performed in Open Laboratory for isotopic compositions of Geology and Mineral Agency, Land and Resource Ministry of China in Yichang city, Hubei province. 3He/ 4He and d 15N were measured at Open Laboratory for gas isotopic compositions of CAS, Lonzhou city, China. d 34S of petroleum and bitumen was measured in Leeds University, U.K. Keith Spence and Dave Hat®eld are thanked for their help with the sulphur isotope analyses. References Aplin, A. C., & Coleman, M. L. (1995). Sour gas and water chemistry of the Bridport sands reservoir Wytch Farm, UK. In J. M. Cubit & W. A. England, The Geochemistry of Reservoirs (pp. 303±314), Vol. 86. London: Special Publication of the Geological Society of London. Burnham, A. K., & Sweeney, J. (1989). A chemical kinetic model of vitrinite maturation and re¯ectance. Geochimica et Cosmochimica Acta, 53, 2649±2657. Cai, C., Hu, W., (1997). Bacterial and thermochemical sulfate reduction in Ordos, Sichuan, Tarim basins, western China. In Abstracts of Eighteenth International Conference on Organic Geochemistry, (pp. 33±34). The Netherlands. Cai, C., Mei, B., Ma, T., Chen, C., & Liu, C. (1996). Hydrocarbons-water-

rock interaction in the diagenetically altered system nearby the unconformities of Tarim Basin. Chinese Sciences Bulletin, 41, 1631±1635. Cai, C., Mei, B., Ma, T., Chen, C., Li, W., & Liu, C. (1997). Approach to Fluid-Rock Interaction in Tarim Basin, Beijing: Geological Publishing House, pp. 1±155, (in Chinese). Cai, C., Franks, S., & Aagaard, P. (2001a). Origin and migration of brines from Paleozoic strata in Central Tarim, China: constraints from 87 Sr/ 86Sr, dD, d 18O and water chemistry. Applied Geochemistry, 16, 1269±1284. Cai, C., Gu, J., & Cai, H. (2001b). Effect of hydrocarbon emplacement on diagenesis of Silurian sandstones of Central Tarim. Acta Sedimentological Sinica, 19, 60±65 (in Chinese). Carothers, W. W., & Kharaka, Y. K. (1980). Stable carbon isotopes of HCO32 in oil-®eld waters Ð Implications for the origin of CO2. Geochimica et Cosmochimica Acta, 44, 323±332. Chen, J., Sheng, Z., Wang, Z., Zhu, S., (1994). Analysis and prediction of Cambrian and Ordovician carbonate reservoirs in the Tarim Basin. China's National Key Project Report (Grant No. 85-101-01-05-06), (pp. 1±142). Xinan Petroleum Institute (in Chinese). Claypool, G. E., Holser, W. T., Kaplan, I. R., Sakai, K., & Zak, I. (1980). The age curves of sulfur and oxygen isotopes in marine sulfate and their mutual interpretation. Chemical Geology, 28, 199±260. Connan, J., Lacrampe-Couloume, G., Magot, M., (1996). Origin of gases in reservoirs. In D.A. Dolec (Ed.), Proceedings of the 1995 International Gas Research Conference, (pp. 21±61). Government Institutes, Inc. No. 1. Connolly, C., Walter, L. M., Baadsgaard, H., & Longstaffe, F. J. (1990). Origin and evolution of formation waters, Alberta Basin, Western Canada Sedimentary Basin: I. Chemistry. Applied Geochemistry, 5, 75±395. Coudrain-Ribstein, A., Gouze, P., & Marsily, G. (1998). Temperaturecarbon dioxide partial pressure trends in con®ned aquifers. Chemical Geology, 145, 73±89. Dai, J., Ji, H., & Hao, S. (1989). A general review on natural gas geology, Beijing: Petroleum Industry Publishing House pp. 10±30, (in Chinese). Dai, J., Song, Y., Dai, C., & Wang, D. (1996). Geochemistry and accumulations of carbon dioxide gases in China. American Association of Petroleum Geologists Bulletin, 80, 1615±1626. Hanson, A. D., Zhang, S., Moldowan, J. M., Liang, D., & Zhang, B. (2000). Molecular organic geochemistry of the Tarim basin, northwest China. American Association of Petroleum Geologists Bulletin, 84, 1109± 1128. Heydari, E. (1997). The role of burial diagenesis in hydrocarbon destruction and H2S accumulation, Upper Jurassic Smackover Formation, Black Creek ®eld, Mississippi. American Association of Petroleum Geologists Bulletin, 81, 26±45. Heydari, E., & Moore, C. H. (1989). Burial diagenesis and thermochemical sulfate reduction, Smackover Formation, Southeast Mississippi salt basin. Geology, 17, 1080±1084. Hiyagon, H., & Kennedy, B. M. (1992). Noble gases in CH4-rich gas ®elds, Alberta, Canada. Geochimica et Cosmochimica Acta, 26, 1569±1589. Huang, D., Liu, B., Wang, T., Xu, Y., Chen, S. J., & Zhao, M. (1996). Genetic types of natural gases and their maturity discrimination in the east of Tarim Basin. Sciences in China Bulletin. (D), 26, 365±372 (in Chinese). Hutcheon, I., & Abercrombie, H. (1990). Carbon dioxide in clastic rocks and silicate hydrolysis. Geology, 18, 541±544. Irwin, H., Curtis, C., & Coleman, M. (1977). Isotopic evidence for source of diagenetic carbonates formed during burial of organic-rich sediments. Nature, 69, 209±213. Jobson, A. M., Cook, F. D., & Westlake, D. W. S. (1979). Interaction of aerobic and anaerobic bacteria in petroleum biodegradation. Chemical Geology, 24, 355±465. Kirkland, D. W., & Denison, R. E. (1995). Diagenetic alteration of Permian strata at oil ®elds of south central Oklahoma, USA. Marine and Petroleum Geology, 12, 629±644. Krouse, H. R., Viau, C. A., Eliuk, L. S., Ueda, A., & Halas, S. (1988).

C. Cai et al. / Marine and Petroleum Geology 18 (2001) 729±741 Chemical and isotopic evidence of thermochemical sulfate reduction by light hydrocarbon gases in deep carbonate reservoirs. Nature, 33, 415± 419. Littke, R., Krooss, B., Idiz, E., & Frielingsdorf, J. (1995). Molecular nitrogen in natural gas accumulations: generation from sedimentary organic matter at high temperature. American Association of Petroleum Geologists Bulletin, 79, 410±430. Machel, H. G., Krouse, H. R., & Sassen, R. (1995). Products and distinguishing criteria of bacterial and thermochemical sulfate reduction. Applied Geochemistry, 8, 373±389. Mamyrin, B. A., & Tolstikhin, I. N. (1984). Helium isotopes in nature, New York: Elsevier. Moldovanyi, E. P., Walter, L. M., & Land, L. S. (1993). Strontium, boron, oxygen and hydrogen isotope geochemistry of brines from basal strata of the Gulf Coast Sedimentary basin, USA. Geochimica et Cosmochimica Acta, 57, 2083±2099. Orr, W. L. (1974). Changes in the sulfur content and isotopic ratios of sulfur during petroleum maturation-study of Big Horn Basin Paleozoic oil. American Association of Petroleum Geologists Bulletin, 58, 2295± 2318. Orr, W. L. (1977). Geologic and geochemical controls on the distribution of hydrogen sul®de in natural gas. In R. Campos & J. Goni, Advances in Organic Geochemistry, 1975, (pp. 571±597). Oxford: Pergamon Press. Powell, T. G., & MacQueen, R. W. (1984). Precipitation of sul®de ores and organic matter: sul®de reactions at Pine Point, Canada. Science, 224, 63±66. Riciputi, L. R., Cole, D. R., & Machel, H. G. (1996). Sul®de formation in reservoir carbonates of the Devonian Nisku Formation, Alberta, Canada: an ion microprobe study. Geochimica et Cosmochimica Acta, 60, 325±336. Rigby, D., & Batts, B. D. (1986). The isotopic composition of nitrogen in coals and oil shales. Chemical Geology, 58, 273±282. Sassen, R. (1988). Geochemical and carbon isotopic studies of crude oil destruction, bitumen precipitation and sulfate reduction in the deep Smackover Formation. Organic Geochemistry, 12, 351±361. Sherwood-Lollar, B., Ballentine, C. J., & O'Nions, R. K. (1997). The fate of mantle-derived carbon in a continental sedimentary basin: Integration of C/He relationships and stable isotopic signatures. Geochimica et Cosmochimica Acta, 61, 2295±2307. Smith, T. M., & Ehrenberg, S. N. (1989). Correlation of carbon dioxide abundance with temperature in clastic hydrocarbon reservoirs: relationship to inorganic chemical equilibrium. Marine and Petroleum Geology, 6, 129±135. SpoÈtl, C., Longstaff, F. L., Ramseyer, K., Kunk, M. L., & Wiesheu, R. (1998). Fluid-rock reactions in an evaporitic meÂlange, Permian Haselgebirge, Austrian Alps. Sedimentology, 45, 1019±1044. Thrasher, J., & Fleet, A. J. (1995). Predicting the risk of carbon dioxide pollution in petroleum reservoirs. In J. O. Girmalt, Organic Geochemistry: Developments and Applications to Energy, Climate, Environment and Human History, (pp. 1086±1088). San Sebastian: A.I.G.O.A. Tucker, M. E., & Wright, V. P. (1990). Carbonate Sedimentology, Oxford: Blackwell, pp. 1±482.

741

Wood, J. R., & Boles, J. R. (1991). Evidence for episodic cementation and diagenetic recording of seismic pumping events, North Coles Levee, California USA. Applied Geochemistry, 6, 509±521. Worden, R. H., & Matray, J. M. (1995). Cross formational ¯ow in the Paris basin. Basin Research, 7, 53±66. Worden, R. H., & Smalley, P. C. (1996). H2S-producing reactions in deep carbonate gas reservoirs: Khuff Formation Abu Dhabi. Chemical Geology, 133, 157±171. Worden, R. H., & Smalley, P. C. (2001). H2S in North Sea oil ®elds: importance of thermochemical sulphate reduction in clastic reservoirs. In R. Cidu, Proceedings of the 10th International Symposium on Water±Rock Interaction, Volume 2. Lisse: A.A. Balkema (pp. 659±662). Worden, R. H., Smalley, P. C., & Oxtoby, N. H. (1995). Gas souring by thermochemical sulfate reduction at 1408C. American Association of Petroleum Geologists Bulletin, 79, 854±863. Worden, R. H., Smalley, P. C., & Fallick, A. E. (1997). Sulfur cycle in buried evaporates. Geology, 25, 643±646. Worden, R. H., Smalley, P. C., & Cross, M. M. (2000). The in¯uence of rock fabric and mineralogy on thermochemical sulfate reduction: Khuff Formation, Abu Dhabi. Journal of Sedimentary Research, 70, 1210± 1221. Wycherley, H., Fleet, A., & Shaw, H. (1999). Some observations on the origins of large volumes of carbon dioxide accumulations in sedimentary basins. Marine and Petroleum Geology, 16, 489±494. Xiao, Z., Zhang, S., Zhao, M., & Zeng, Q. (1997). A brief analysis on forming periods of Silurian pools in the Tazhong A well. Acta Sedimentological Sinica, 15, 150±153 (in Chinese). Xiao, X., Song, Z., Liu, D., Liu, Z., & Fu, J. (2000). The Tazhong hybrid petroleum system, Tarim Basin, China. Marine and Petroleum Geology, 17, 1±12. Xu, Y., Shen, P., Liu, W., Tao, M., Sun, M., & Du, J. (1998). Noble gas geochemistry, Beijing: Publishing House of Sciences, pp. 1±231, (in Chinese). Ye, D. (1994). Deep dissolution of Cambrian±Ordovician carbonates in the Northern Tarim Basin. Acta Sedimentological Sinica, 12, 66±71 (in Chinese). Zhang, S., Liang, D., Wang, F., Zhang, B., Pian, L., Zhou, F., Zhang, M., Xie, Q., & Liu, L. (1999). A study on source rock and oil-rock correlation in Tarim Basin, China's National Key Project Report (Grant No. 96-111-01-03), Beijing: Institute of Petroleum Exploration and Development, pp. 80±158, (in Chinese). Zhang, S., Hanson, A. D., Moldowan, J. M., Graham, S. A., Liang, D., Chang, E., & Fago, F. (2000). Paleozoic oil-source rock correlations in the Tarim basin, NW China. Organic Geochemistry, 31, 273±286. Zhao, M., & Huang, D. (1996). Oil-source correlation of Paleozoic in Tarim Basin. In Q. Tong, D. Liang & C. Jia, Advance on Petroleum Geology in Tarim Basin, (pp. 300±310). Beijing: Publishing House of Sciences, (in Chinese). Zhu, Y., Shi, B., & Fang, C. (2000). The isotopic compositions of molecular nitrogen: implications on their origins in natural gas accumulations. Chemical Geology, 164, 321±330.