Origin Of Hydrogen Sulfide In The Ediacaran And Cambrian In The Central Sichuan Basin

The largest hydrogen sulfide?H₂S?-bearing gas field in China has been discovered in the Ediacaran Dengying and Cambrian Longwangmiao formations in the Central Sichuan Basin.The H₂S concentrations in both reservois are generally lower than 3%,which can be formed by a variety of natural processes.Considerable controversy exists regarding the occurrence of thermochemical sulfate reduction?TSR?in the Dengying Formation,and few studies have investigated the origin of H₂S in the Longwangmiao Formation.An integrated approach to determining the origin of H₂S,involving the sulfur isotope analysis of various sulfur species,the chemical and isotopic analyses of natural gas,and the optical and geochemical analyses of solid bitumen,is demonstrated here for the two reservoirs.H₂S in the Dengying and Longwangmiao formations generally has?³⁴S values of greater than 19‰,and thus is enriched in ³⁴S relative to the sulfides derived from the mantle and bacteria sulfate reduction?BSR?.The contribution of mantle sulfur source and BSR to the H₂S in both reservoirs can be negligible.The?³⁴S values of H₂S in the Longwangmiao Formation are significantly heavier than those of the parent kerogen in the Qiongzhusi Formation.Similarly,some H₂S in the Dengying Formation is also isotopically heavier than the parent kerogen in the Qiongzhusi Formation and Z₂dn³.The highly ³⁴S-enriched H₂S in the two reservoirs cannot be generated via thermal decomposition of organosulfur compounds in either kerogen or crude oil.It is worth noting that the?³⁴S values of H₂S are close to those of the associated coeval seawater sulfate as represented by carbonate-associated sulfate?CAS?in the same strata.The small isotope fractionation between H₂S and parent seawater sulfate can be interpreted as a result of nearly complete reduction of dissolved sufate during TSR.Petrographic examination reveals that barite has been replaced by ³⁴S-enriched pyrite in the two reservoirs.The heavy sulfur isotope signatures of the replacive pyrite suggest a thermochemical origin for the replacement process;i.e.,the replacement of barite by pyrite has proceeded via in situ TSR.In addition,the?¹³C values of ethane in the Dengying Formation increase roughly with increasing gas sourness,indicating that ethane has been involved in TSR.The petrographic and geochemical data noted above indicate that H₂S in the two reservoirs was primarily derived from in situ TSR.Fluid inclusions in the saddle dolomite and quartz in the Dengying Formation and in the barite in the Longwangmiao Formation are characterized by high homogenization temperatures and salinities,suggesting that these minerals were derived from hydrothermal fluids.The barite can act as a source of reactive sulfate for TSR,as indicated by its replacement by the ³⁴S-enriched pyrite.The?³⁴S values of both the TSR-derived H₂S and pyrite are close to those of the hydrothermal barite in the same strata,further indicating that the reactive sulfate for TSR was primarily derived from hydrothermal fluids.Two types of altered solid bitumen are recognized in both reservoirs:one was altered by hydrothermal fluids and the other by TSR.Solid bitumen can be optically anisotropic and enriched in ¹³C during hydrothermal alteration.On the other hand,both the S/C ratios and?³⁴S values of solid bitumen increased as TSR proceeded.The TSR-derived H₂S in the Dengying Formation can back-react with the¹³C-depleted ethane to generate ¹³C-depleted ethanethiol,which may be subsequently incorporated into the bitumen.Consequently,the TSR-altered bitumen becomes isotopically lighter with increasing S/C ratios and is depleted in ¹³C relative to the remaining ethane in the Dengying Formation.Correlating the optical and geochemical characteristics of solid bitumen reveals that TSR was probably initiated by hydrothermal activity in both reservoirs.The hydrothermal fluids can provide both the necessary high-temperature environment and reactive sulfate for TSR to proceed.In addition,petrographic and geochemical data indicate that the reduced sulfur for Pb-Zn mineralization in the Dengying Formation was probably also derived from the in situ reduction of hydrothermal sulfate by hydrocarbons.