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Pr and Ph are preferentially oxidized compared to n C 17 and n C 18 , leading to decrease in Pr/ n C 17 and Ph/ n C 18 ratios with thiaadamantanes concentrations, a measure of the extent of TSR in Tazhong oils and condensates. • TSR does not preferentially deplete triterpanes and α,β hopanes over Ts. • Pr and Ph are preferentially oxidized over n C 17 and n C 18. • i C 4 was preferentially depleted over n C 4 only during earlier TSR stage. • Relative oxidization sequence during TSR overlaps significantly. Tazhong and four other well-known TSR-altered petroleum reservoirs from around the world were analyzed for their molecular and carbon isotopic compositions of saturated hydrocarbons, thiaadamantanes concentrations or H 2 S contents to determine the relative stability of saturated hydrocarbons during thermochemical sulfate reduction (TSR). We find that TSR does not preferentially deplete tricyclic terpanes and 17α, 21β hopanes with the "exposed" tertiary carbon atoms compared to 18α (H)-22, 29, 30-trisnorhopane (Ts) and C 29 Ts that have no tertiary carbon atoms, as proposed by Xia and Gong [35]. However, pristane (Pr) and phytane (Ph) seem to have been preferentially oxidized over normal C 17 and C 18 alkanes, respectively, resulting in decreasing Pr/ n C 17 and Ph/ n C 18 ratios with increasing extent of TSR for the Tazhong and Nisku oils and condensates. i C 4 appears to be preferentially depleted relative to n C 4 during early TSR stage. At later TSR stage both n C 3 and n C 4 are oxidized more readily than i C 4 in the Mobile Bay Norphlet Formation condensates. Thus, we tentatively propose that relative oxidization order during TSR may start with long chain iso -alkanes followed by n -alkanes, then cycloalkanes, C 2 -C 4 alkanes and finally methane; however, it seems that oxidization of these compounds overlaps significantly. [ABSTRACT FROM AUTHOR]

Chemistry, H, O and Sr isotopes of formation waters were determined from the Ordovician carbonate reservoirs in the Tazhong area, Tarim basin. The aim is to elucidate their origin and migration and their alteration by thermochemical sulfate reduction (TSR) and water-rock interactions. The waters were originated from evaporated seawater, and are enriched in Ca and Sr, and depleted in Mg and SO 4 compared with the seawater evaporation trajectory (SET), thus are considered to result from dolomitization, anhydrite dissolution and in situ TSR. δ 18 O values show positive shift due to water-rock interactions, and δD values show negative shift with increasing TSR extents or total thiaadamantanes concentrations of the associated oils, likely resulting from hydrogen isotope exchange with TSR-H 2 S. However, the δD-δ 18 O relationship was mainly controlled by the mixing with paleo-meteoric water with δD of −50‰ and δ 18 O of −7.5‰. Paleo-meteoric water may have leached the Silurian siliciclastic rocks and thus has 87 Sr/ 86 Sr up to 0.715. Upper Ordovician (O 3 l) formation water shows more negative δD and δ 18 O values, higher 87 Sr / 86 Sr ratios and lower total dissolved solids (TDS) than the O 1–2 y water, which may have resulted from mixing of paleo-meteoric water. Lower Ordovician (O 1–2 y) formation water shows 87 Sr / 86 Sr ratio of 0.7095–0.7105, TDS from 160 to 240 g/l, and Sr concentrations from 300 to 1000 mg/L, which may have resulted from mixing with hydrothermal fluid up-migrated from Ediacaran and Lower Cambrian siliciclastic rocks. Significant mixings with paleo-meteoric water and hydrothermal fluid occurred at paleo-highland at the east of No.1 Fault-Slope Zone and active faulting zone near No.10 Structural Belt, respectively. Hydrothermal fluid and oils probably shared similar migration pathways in No.10 Structural Belt. Formation water geochemistry may thus provide clues to constraint on petroleum migration. [ABSTRACT FROM AUTHOR]

Intense thermochemical sulfate reduction (TSR) and up to 18% H 2 S are found in the Upper Permian Changxing Formation (P 3 ch) in the northeast (NE) Sichuan Basin, China, despite that rare gypsum or anhydrite was found in this formation. Here, we present new concentration data of carbonate-associated sulfate (CAS) from carbonate host rocks, C, O, and Sr isotope data for TSR-related calcites, and S isotope data for sulfur compounds obtained during this study. These data along with spatial-temporal changes in palaeogeopressure conditions, hydraulic conductivity and the physical capacity indicate that the H 2 S was generated locally from TSR within the P 3 ch reservoirs. We propose that the reactive sulfates were derived from CAS released during dolomitization and recrystallization of earlier dolomite within the P 3 ch Fm. and from the cross-formational migration of evaporative brines from the Lower Triassic Feixianguan Formation (T 1 f) to P 3 ch Fm. Our calculation shows that the two sources could provide enough SO 4 2− for the generation of H 2 S within the P 3 ch reservoirs. Early downward migration of sulfate-rich evaporative brines from the T 1 f formation occurred in near-surface and shallow burial diagenetic settings (mainly <1000 m). The evaporative brines seeped into porous grainstones and displaced preexisting seawater, causing pervasive dolomitization within the P 3 ch Fm. Subsequently, TSR calcites precipitated from the pore water have high Sr concentrations (up to 7767 ppm), close to the T 1 f TSR calcites, and 87 Sr/ 86 Sr ratios mainly from 0.7074 to 0.7078, which are significantly higher than those of Late Permian seawater but within the range of early Triassic seawater. [ABSTRACT FROM AUTHOR]

Abstract: Thermochemical sulfate reduction is considered to result in H2S >10% and high dryness coefficient values in the Lower Triassic and Upper Permian carbonate reservoirs in the NE Sichuan Basin. The gases produced from gas–water transition and water intervals were measured to have H2S higher than 30%, and are calculated to have significantly higher H2S and CO2 contents than a gas produced from a gas interval, and thus are not used to reflect TSR extents. Methane and ethane were shown to shift positively in carbon isotopes as a result of TSR. However, the fractionation has not quantitatively described. A logarithmic relationship is found to give the best fit for methane δ13C1 and [1 − H2S/(∑C1–6+H2S)] for all the gases with an equation of δ13C1, t = −16.6lnx − 33.0, indicating a closed system Rayleigh distillation with a kinetic fractionation factor αC of 1.0166. Ethane shows similar δ13C2 shift to methane (6.1‰ vs 5.5‰) for H2S/[H2S + ∑C1–6] = 0.2 in the NE Sichuan Basin. The δ13C2 deviation is significantly less than that of Mobile Bay Jurassic Norphlet Fm TSR-altered ethane (+16‰) for H2S/[H2S + ∑C1–6] = 0.1 (Mankiewicz et al., 2009). The ethane in association with high H2S (>10%) in NE Sichuan Basin shows slightly lighter δ13C values than those of the potential source rocks. Thus, it is possible for small amounts of gas derived from cracking of the source rocks to have mixed with TSR-altered gas in the high H2S pools. [Copyright &y& Elsevier]

It is hard to distinguish pyrites generated by microbial sulfate reduction (MSR) from thermochemical sulfate reduction (TSR). The upper Ediacaran Dengying Formation (ca 551–541 Ma) of Sichuan basin is chosen to study for this purpose by means of petrological, bulk and micrometer-scale analyses on barite, pyrite, calcite, and dolomite for carbon and sulfur isotope and rare earth elements and yttrium (REY) compositions. We find that MSR-derived pyrites precipitated from near-surface to shallow burial settings are characterized by seawater-like REY patterns and superchondritic Y/Ho ratios (74.3 ± 9.5), and have δ34S pyrite values in a wide range from −36.8‰ to 34.8‰, positively correlated with arsenic contents, likely through cryptic sulfur cycles driven by arsenic. In contrast, TSR-derived pyrites generated during deep burial settings have REY characterized by non-seawater-like patterns and chondritic Y/Ho ratios (33.6 ± 5.1), and show small sulfur isotopic fractionations (~10.5‰). TSR-pyrites yield both U-shaped and bell-shaped δ34S transects compared with U-shaped distributions of MSR-pyrites. The calcites associated with the TSR-pyrites show LREE enrichment, elevated Y/Ho ratios (85.8 ± 14.2) and negative δ13C values, likely resulting from decomposition of organic matter. This study highlights a combination method from REY patterns, δ13C and δ34S values from analyses on micro-sized pyrite and the associated calcites can be used to effectively distinguish MSR from TSR. [Display omitted] • MSR-pyrites have different petrology, REY and δ34S patterns from TSR-pyrites. • Cryptic sulfur cycles are likely driven by redox-sensitive metals such as arsenic. • TSR-calcites show LREE enrichment, elevated Y/Ho ratios and negative δ13C values. [ABSTRACT FROM AUTHOR]

Deep (4000–6000 m) and ultra-deep burial (>6000 m) carbonate reservoirs are becoming important petroleum exploration targets of China and worldwide basin. Thermochemical sulfate reduction (TSR) may be pervasive in the deep and ultra-deep burial petroliferous carbonate strata associated with evaporite. However, there is still considerable confusion on the effect of TSR on reservoir quality. Petrography, geochemistry and fluid inclusion data from carbonate reservoirs of Tarim Basin were integrated to resolve the disagreement. Based on distribution of sulfur-bearing mineral, SO 4 2− concentrations and SO 4 2−/Cl− ratios in both formation water and fluid trapped in fluid inclusion, we propose that SO 4 2− in formation water is likely major reactant involved in TSR reaction of North Slope, whereas coarsely diagenetic anhydride is likely major sulfate reactant involved in TSR reaction of East Burial Hill Belt. Silurian fracture-filling pyrite has δ34S values similar to those of Ordovician fracture-filling pyrite, H 2 S and sulfur, suggesting that TSR dissolved sulfate involved in (North Slope) may occur in a partly open system for H 2 S and CO 2. However, TSR diagenetic anhydride involved in (East Burial Hill Belt) may occur in a closed diagenetic system. TSR in a partly open-system, associated with gas charge and flow of cross formational fluids, probably increased fluid pressure, and further transported H 2 S, CO 2 and solute from dissolution through fracture. A closed system of anhydrite-bearing strata seems to hinder transports of TSR-derived CO 2 or bicarbonate, and result in precipitation of TSR calcite, pyrite and bitumen close to the TSR sites. In combination with petrographic observations, intense dissolution in North Slope have released more inorganic CO 2 leading to TSR calcite and CO 2 in the natural gas showing δ13C values much heavier relative to δ13C values of TSR calcites in East Burial Hill Belt as light as −13.4‰. TSR-induced dissolution of carbonate minerals during oil-dominated TSR may be related to the reaction of dolomite with anhydride and release of H+ due to pyrite precipitation, rather than new formed water during TSR. Similar to geological settings of Ordovician TSR, anhydrite-poor Lower Cambrian dolostones underlying evaporites seals may thus be the most promising target for future exploration success in ultra-deep Cambrian reservoirs of Tarim Basin. The controversy about the effects of TSR on reservoir quality can partly be ascribed to differently geological settings where TSR occurred. • Sulfate reactants can be speculated by mineralogy, and geochemistry of formation water and fluid trapped in fluid inclusion. • Comparison of δ34S value of pyrite and H 2 S from Silurian and Ordovician imply partly open diagenetic systems during TSR. • Petrology, 13C-rich TSR calcite and present-day CO 2 suggest H 2 S, CO 2 and solute were transported from dissolution sites. [ABSTRACT FROM AUTHOR]

Evidence of dolomite dissolution during deep burial domain: (A) Corrosion of saddle dolomite (red stippled line indicates pore (P) boundary), well GS103, depth 5,305.43 m. (B) Pore formed around stylolites (red arrows), well GS32, depth 5,433.89 m. (C) Solid bitumen (Bit) in the center of dissolved saddle dolomite (red stippled lines), well MX108, depth 5,336.08 m. • Primary porosity is overprinted by meteoric diagenesis and dolomitization. • Six of seven dolomite types show dolomitization processes known from Phanerozoic. • Deep burial dissolution is confirmed by petrographic evidence. • Hydrothermal pulses and thermochemical sulfate reduction induce late corrosion. The deeply buried (>7 km) upper Ediacaran (Sinian) Dengying Formation (ca. 551.1–541 Ma) in the Sichuan Basin, China, is the largest Precambrian dolostone gas reservoir worldwide. Gas exploration from the Dengying Formation, however, is hampered by a limited understanding of its complex dolomitization history and porosity evolution. New petrological, geochemical and petrophysical analyses were performed and are discussed here to develop a better understanding of the formation's complexities. Microbialite, dolo-grainstone, and crystalline dolostone lithologies from the platform margin have high primary porosities relative to dolo-mudstone and less common microbialite lithologies from the low-energy platform interior. Spatially-variable primary porosity was subsequently overprinted by meteoric dissolution and dolomitization, often inducing secondary porosity. The various forms of dolomitization (sabkha, reflux, and burial dolomitization, and cementation) increased the resistivity of these rocks to chemical and physical compaction and porosity destruction. In the deep burial domain, medium- to coarse-crystalline dolomite cements and saddle dolomite precipitated at fluid temperatures of 120–160 °C and 160–220 °C, respectively. The significance of deep burial dissolution is critically discussed and constrained by: (i) corrosion of late diagenetic minerals, (ii) pores cross-cutting (or forming around) stylolites, and (iii) the occurrence of solid bitumen in the center of secondary pores. Vuggy, inter- and intracrystalline pores developed during a late and deep burial stage, and comprise ca. 20% of the overall Dengying reservoir porosity. Mechanisms which induced corrosion at this burial stage include hydrothermal pulses and thermochemical sulfate reduction. Similar to many carbonate reservoirs in the Phanerozoic, this study documents that the platform margin has better porosities and higher gas production compared to the platform interior due to its specific carbonate rock properties and diagenesis. Data shown here document the complex multiphase dolomitization history and high potential for gas production from deeply buried Precambrian dolostone reservoirs. [ABSTRACT FROM AUTHOR]

Molecular and compound specific isotope analysis of volatile organic sulfur compounds (VOSC), alkanes and CO 2 in natural gases from the Ordovician Majiagou Formation of the Daniudi field (Ordos basin, China) was performed. The analysis of VOSC demonstrated abundant thiols, sulfides and thiophenes in several samples while others had low concentrations of sulfides only. Molecular analysis of main alkanes and non-hydrocarbons showed the samples vary by the nitrogen and H 2 S concentrations. Thiols in the gas were at isotopic equilibrium with H 2 S bearing isotopic signature similar to that of Ordovician anhydrites within the reservoirs, hence the source of H 2 S was likely thermochemical sulfate reduction (TSR) of these anhydrites. In contrast, gas samples that lacked thiols also lacked H 2 S. Since all gases came from the same formation and similar thermal regime, the lack of thiols and H 2 S indicates that the occurrence of TSR in the field was limited, likely by contact between hydrocarbons to the anhydrites. The isotopic signature (δ34S) of thiophenes in the gas samples suggests the source of the gas is unlikely to be Carboniferous or Permian coals which were previously suggested as the source rocks for the gas. The δ13C and δ2H of alkanes further supports this suggestion as it demonstrates Carboniferous and Permian gases are unlikely to undergo TSR and generate the gas present in the Ordovician reservoirs. Instead, the δ13C and δ2H of alkanes hint that the gas source is shale rather than coal. This study demonstrates the applicability of compound specific sulfur isotope analysis of VOSC in identification of H 2 S sources, performing gas-source rock correlation and acting as a complementary proxy in case where routine analysis of hydrocarbons (δ13C and δ2H) is insufficient for identification of the gas source. • Volatile organic sulfur compounds were measured in natural gas from the Ordovician Majiagou Formation in the Daniudi field. • Molecular and sulfur isotopic distribution of thiols indicate that the origin of H 2 S is TSR from Ordovician anhydrites. • The natural gas of the Majiagou Formation was likely generated in-situ from an Ordovician source rock. • Distribution of VOSC isotopes indicates TSR in Ordovician gas reservoirs was likely localized due to limited sulfate presence. [ABSTRACT FROM AUTHOR]

Improved understanding of the reservoir mineral effects on the occurrence of thermochemical sulfate reduction (TSR) and possible mechanisms is critical for petroleum exploration and production. A series of hydrous pyrolysis experiments were conducted with and without the presence of montmorillonite, illite and quartz (silicates), calcite and dolomite (carbonates) at 360 °C and 50 MPa for 12 h, 48 h and 312 h. Gaseous compounds, C 7–8 light hydrocarbons, n -C 9 + and organic sulfur compounds as well as compound-specific δ 34 S analysis were conducted subsequently. The results show that with the heating time increasing or TSR advancing, n –C 9 + decrease gradually accompanied by the increases of TSR products, H 2 S, alkylbenzenes, benzothiophenes (BTs) and dibenzothiophenes (DBTs). The δ 34 S values of BTs and DBTs progressively approached that of MgSO 4 and the difference in their average δ 34 S values (Δ 34 S BT-DBT ) decreased. Compared to experiments with the absence of minerals, the experiments with silicates, in particular montmorillonite, are depleted in n –C 9 + and enriched in H 2 S, light monoaromatics and organosulfur compounds and smaller Δ 34 S BT-DBT . Experiments with carbonates demonstrated the opposite trend. This indicates that silicates, in particular montmorillonite, accelerated TSR rates while carbonates reduced them. Under the experimental conditions, the exchange cations within montmorillonite and on the surface of clays and talc precipitation can increase the concentrations of H + and hence HSO 4 − and then catalyze the generation of isoalkanes, which are prone to be oxidized in TSR. This can reduce the activation energy required for the occurrence of TSR. It might therefore be possible for TSR to occur at relatively low temperatures in silicate reservoirs and source rocks in natural environments. Carbonate dissolution can reduce the concentrations of H + and HSO 4 − , and hence reduce the TSR rates. This study demonstrates the importance of minerals in regulating the TSR occurrence and suggests that TSR can occur in siliciclasitic as well as carbonate formations in nature. Therefore, the significance of minerals should be considered in the evaluation of TSR and sour gas risk in TSR-altered hydrocarbon accumulations in different geological contexts. [ABSTRACT FROM AUTHOR]

Re -Os isotope analyses were conducted on solid bitumens from Lower Cambrian Longwangmiao and topmost Ediacaran Dengying formations in Anyue gas-field, Central Sichuan basin to date the main gas accumulation. Chemical and C-S isotopic analyses indicate that the bitumen samples have experienced thermochemical sulfate reduction (TSR), which, however, did not disrupt Re-Os isotope systematics. Re-Os isotope analyses on the five bitumen samples outside the gas field give an isochron age of 138.6 ± 2.7 Ma (MSWD = 4.0, n = 5, ±2σ) with an Os i of 2.880 ± 0.022. This new age has a smaller error than ever before. We ascribe this result to the much greater spread in our 187Re/188Os and 187Os/188Os data, with ranges reaching 126.8 to 976.6 and 3.172 to 5.142, respectively. This is the first report that Re-Os isotopes can be used to date TSR-altered petroleum and solid bitumen for their generation and migration. Other samples within the gas field have lower initial 187Os/188Os values, may have much more oils contributed from the Lower Cambrian source rock, and/or result from water-oil interactions prior to TSR. Our dating indicates that a large-scale gas generation from the cracking of paleo-oils likely occurred simultaneously with the TSR, which led to increase in δ34S value, negative shift in δ13C value of solid bitumen in the Central Sichuan Basin and MVT mineralization in southeastern margin of the basin. • The solid bitumen was altered by TSR leading to the C-S isotopic relationship. • TSR in gas intervals does not disrupt Re-Os isotopic systematics in solid bitumen. • Bitumen Re-Os dating indicates the large scale gas accumulation at ∼139 Myr ago. [ABSTRACT FROM AUTHOR]