川东北地区飞仙关组油气藏H_2S由TSR和TDR两种方式形成

Origin of H_2S from TSR and TDR reactions in oil/gas reservoirs in the Feixianguan Formation,the northeastern Sichuan Basin

川东北地区下三叠统飞仙关组油气藏的油气来自上二叠统龙潭(吴家坪)组,在印支晚期—燕山早期(T3—J1)温度达到80～120℃时烃源岩进入生油阶段。残余H2S启动了油、原生水与硫酸盐的热化学还原反应(TSR),产生大量H2S,基本耗尽了烃源岩中的硫酸盐。该阶段形成的H2S,其同位素δ34S值接近于二叠纪硫酸盐的δ34S值,为10‰左右。油气运移到储层后,在燕山中晚期(J2—K1)储层温度达到了116℃左右,又发生硫酸盐热化学还原反应,产生的H2S其同位素δ34S值约为15‰～16‰,其量不超过50%。至喜马拉雅期,储层温度达到170℃左右,一直比较稳定的含硫有机化合物(噻吩、环硫烷烃等)发生热裂解反应(TDR)产生CH4,H2S,CO2等。生成的H2S气不断进入储层与储层中的气态烃形成现在的混合型气藏。这就是H2S形成的两阶段、两方式(TSR,TDR)模式。

In northeastern Sichuan Basin,oil and gas in reservoirs in the Lower Triassic Feixianguan Formation are sourced from the Permian Longtan/Wujiaping Formation.From the Late Indo-Chinese epoch to the Early Yanshanian epoch（T3 -J1）,source rocks in the Longtan Formation entered into oil window when their temperature reached 80 -120℃.Residual H2S activated the thermochemical sulfate reduction among oil,connate water and sulfates,generating large amount of H2S,which almost consuming all the sulfates in source rocks.H2S gene-rated during this stage has a δ34S isotope similar to that of the Permian sulfates,i.e.,10‰.After hydrocarbons migrated into the reservoirs,reservoir temperature reached about 116℃ in the late Yanshanian epoch （J2 -K1）,when the thermochemical sulfate reduction （TSR） happened again.The H2S generated at this time has a δ34S isotope of 15‰ -16‰ and accounts for no more than 50%.During the Himalayan period,reservoir temperature reached about 170℃,sulfur-bearing organic compounds （thiophene,cycloalkanes,etc） which kept stable previously experienced thermochemical decomposition reaction （TDR） and generated CH4,H2S,CO2,etc.The H2S generated from TDR continuously entered into the reservoirs and mixed with the gaseous hydrocarbon there,creating current mixed gas reservoirs.This is the two-stage and two-pattern （TSR and TDR） genetic model of H2S.