Thermochemical Sulphate Reduction in Cambro±Ordovician Carbonates in Central Tarim
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Marine and Petroleum Geology 18 02001) 729±741 www.elsevier.com/locate/marpetgeo Thermochemical sulphate reduction in Cambro±Ordovician carbonates in Central Tarim Chunfang Caia,*, Wangshui Hub, Richard H. Wordenc aInstitute of Geology and Geophysics, CAS, P.O. Box 9825 Beijing 100029, People's Republic of China bJianghan Petroleum Institute, Jingzhou, Hubei 434102, People's Republic of China cJane 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 0e.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 petroleum- derived 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 2001Elsevier Science Ltd. All rights reserved. Keywords:H2S; CO2; Thermochemical sulphate reduction; Organic sulphur; Pyrite; Tarim basin 1. Introduction Couloume, & Magot, 1996), petroleum 0Sassen, 1988) and bituminous tar 0Powell & MacQueen, 1984) become altered Elevated concentrations of dissolved H2S exist in oil®eld during TSR. Heydari 01997) and Heydari and Moore 01989) waters in Ordovician carbonate and Silurian sandstone demonstrated the effects of TSR on burial diagenesis and petroleum reservoirs in Central Tarim, Tarim Basin, China porosity evolution. Integrated approaches to distinguish 0Cai & Hu, 1997). The origin, spatial distribution and rela- TSR from bacterial sulphate reduction 0BSR) were tionship of the H2S with mineral diagenesis, porosity evolu- suggested by Connan et al. 01996), Machel, Krouse, and tion and hydrocarbon alteration are of importance to Sassen 01995) and Worden et al. 01995). The lowest petroleum exploration and development. Previous work temperature at which TSR can occur is a matter of debate, has shown that high concentrations of H2S in petroleum but it is now clear that the kinetics of TSR depend upon a ¯uids may be generated by thermochemical sulphate reduc- variety of factors including petroleum type, rock fabric and tion 0FTSR; e.g. Orr, 1977; Worden, Smalley, & Oxtoby, the amounts of pre-TSR water and reduced sulphur species 1995). Correspondingly, chemistry and the isotopic compo- 0Worden, Smalley, & Cross, 2000). Most recent studies sitions of light hydrocarbon gases 0Connan, Lacrampe- 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 * Corresponding author. Present address: Jane Herdman Laboratories, 13 shown by CO2 that increasingly adopts reduced d C values Department of Earth Sciences, University of Liverpool, 4 Brownlow Street, as TSR proceeds, and the CO concentration in the gas phase Liverpool, L69 3GP, UK. Tel.: 144-0151-794-5200; fax: 144-0151-794- 2 5170. seems to increase in some TSR systems 0Krouse, Viau, E-mail address: [email protected] 0C. Cai). Eliuk, Ueda, & Halas, 1988; Worden & Smalley, 1996). 0264-8172/01/$ - see front matter q 2001Elsevier Science Ltd. All rights reserved. PII: S0264-8172001)00028-9 730 C. Cai et al. / Marine and Petroleum Geology 18 92001) 729±741 Fig. 1. Map showing geological tectonics and location of major petroleum exploration wells 0a) and West±East cross section AB of Central Tarim 0b). Elevated concentrations of dissolved H2S in oil®eld Tarim Basin, northwest of China 0Fig. 1a). It contains a waters in the Tarim Basin have been reported and it has large, NW trending reverse fault with a throw of 2200 m been suggested that they might result from TSR 0Cai & developed during the Caledonian orogeny and crossing the Hu, 1997). Based upon biomarker parameters, Zhang et al. Ordovician and Silurian strata. An E±W cross section is 02000) and Hanson, Zhang, Moldowan, Liang, and Zhang shown in Fig. 1b. The faults and fractures are thought to 02000) concluded that Central Tarim Paleozoic oils were be the main conduit for petroleum migration. The deepest derived from the Middle and Upper Ordovician source exploration well 0Tc1) in Central Tarim reached a depth of rock. Xiao, Song, Liu, Liu, and Fu 02000) demonstrated more than 7200 m and penetrated a complete section of vertical secondary migration of oil and natural gases from the Cambrian. Cambrian strata do not outcrop and are Lower Palaeozoic source rock into overlying reservoirs considered to represent broadly continuous sedimenta- in the Tarim Basin. Cai, Franks, and Aagaard 02001a) tion. The Cambrian section is composed of tidal, plat- suggested that Ordovician oil®eld waters have migrated form and platform-marginal carbonate and evaporate up to Carboniferous and Silurian reservoirs based on rocks. The Middle Cambrian is a suite of supratidal water geochemistry and 87Sr/86Sr ratios. This paper presents anhydrite-bearing dolomite interbedded with dolomitic new data from cement, oil, gas and water samples, and anhydrite or dolomite with thin anhydrite interlayers. attempts to account for the origin of H2S and CO2 and to According to seismic data, anhydrite and salt beds prove the occurrence of TSR and in the Tarim Basin, north- extendnorthtotheBachuUplift0Fig.1a)andtothe west China. west of North Tarim with an area of 200,000 km2 and a thickness of 400±1400 m. The burial and geothermal history of Central Tarim shows that rapid sedimentation 2. Geological setting took place at the phase of the passive continental margin during the middle Cambrian to early Ordovician. Central Tarim is located in the center of Tazhong Uplift, Present-day and maximum temperatures of Cambrian C. Cai et al. / Marine and Petroleum Geology 18 92001) 729±741 731 in Cambrian, Lower Ordovician and Lower Carboniferous strata but not in Silurian rocks. 3. Sample collection and analysis 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 d13C on a Finnigan MAT251mass spec- trometer 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 Fig. 2. Diagram showing a burial history constructed from well Tc1. Isotherms are constrained by ®ssion track data and vitrinite re¯ectance samples. All isotope data are reported relative to the PDB measurements. About 125±1758C occur in the Lower Ordovician and standard with a precision of ^0.1½. Cambrian. The Central Tarim oil®eld water samples that contain dissolved H2S from Ordovician carbonate and Silurian sand- stone reservoirs were collected in 500 ml glass jars contain- and lower Ordovician strata range from 125 to 1758C ing excess cadmium acetate 01.5 g) to precipitate dissolved based on bottom hole temperatures and thermal history sulphide as CdS. The SO2 gas for S isotope analyses was analysis 0Fig. 2: Zhang et al., 1999). The burial history produced by combustion of a mixture of sulphide and Cu2O was re-built by running ThermMod software using the in a 1:10 at 11008C under vacuum. SO2 gas was chemical kinetic model of Burnham and Sweeney collected in a sample tube by freezing. To transform 01989). sulphate to SO2 gas, barium chloride reagent was The Ordovician carbonate rocks show deepening- added to precipitate sulphate as BaSO4. The BaSO4 was upward and then shoaling-upward sequences from mixed with V2O5 and SiO2 in a proportion of 1:3.5:3.5, bioclastic grainstone, packstone, and mudstone to sand- and the mixture was placed in porcelain bottle and covered stone. The Silurian sandstone sequence consists of tidal with a layer of copper wires. Sulphur isotope ratios were sandstone with extensive bitumen occurrence, and measured on a MAT 251mass spectrometer and results are brown or red mudstone. Carboniferous sandstone strata reported in the standard d notation relative to Canyon consist of marine clastics while the Mesozoic and Ceno- Diablo troilite 0CDT). Reproducibility for d34Sis^0.1± zoic are mainly composed of terrestrial sandstones and 0.3½. 3He/4He and d15N were measured at VG-5400 and mudstones0Caietal.,2001a). MAT271mass spectrometer, respectively. The reproduc- Table 1summarizes the source rocks, petroleum reser- ibilities of the 3He/4He and d15N are ^0.6% and ^1½, voirs, and the occurrence of various sulphur compounds in respectively.