鄂尔多斯盆地东南缘黄陵矿区中生代煤系烃源层构造—热演化过程与生物气生成

Tectonic-Thermal Evolution and Biogas Generation of Source Rocks from the Mesozoic Coal Measures at the Huangling Mining Area,Southeastern Margin of Ordos Basin

煤系烃源层的构造—热演化过程研究对揭示煤系气的生成机制具有重要意义。为揭示鄂尔多斯盆地东南缘黄陵矿区向斜核部中生代煤系烃源层的构造—热演化过程、生排烃阶段及煤系气成因类型,根据含煤地层岩性、剩余地层现今埋深、储层孔隙度、镜质组最大反射率、甲烷碳氢同位素组成和生物甲烷产率等数据,运用Petromod 1D模拟软件与回剥反演法及EASY%RO法对研究区煤系烃源岩的受热和生烃演化过程进行重建。研究结果表明:黄陵矿区自晚三叠世以来,煤系主要经历了3~4次“沉降—抬升”过程,其生烃演化过程可分为原生生物成因气、热成因气和次生生物成因气3个阶段。晚三叠世末期至中侏罗世早期,煤系烃源岩镜质组最大反射率演化至0.3%,对应原生生物成因气生成阶段;中侏罗世早期至早白垩世末期,煤系埋深及受热温度逐渐升高至最大值,镜质组最大反射率演化至0.67%~0.74%,热解生烃作用停止,对应热成因气生成阶段;自始新世早期至今,煤系发生抬升且埋深浅于2077~2148 m,受热温度低于75℃,对应次生生物成因气生成阶段。侏罗系延安组3号和2号煤层中甲烷碳、氢同位素组成数据和微生物降解煤岩生成生物气实验(甲烷累计产率为10.5~16.1μmol/g)为该区存在次生生物成因气提供了直接证据。

Research on the tectonic and thermal evolution of coal measures'source rocks is of great significance for revealing the generation mechanism of coal-measures gas.To reveal the process of tectonic-thermal evolution,the stage of hydrocarbon generation and expulsion,and the genetic types of the coal-measure gas,the thermal and hydrocarbon generation evolution histories of the source rocks in the Mesozoic coal measures in the Huangling mining area were studied in this paper,based on the data of lithology,buried depth,porosity,maximum vitrinite reflectance,carbon and hydrogen isotopic compositions of methane and biogas yield and with the use of the methods of Petromod 1D simulation,back-stripping inversion,and EASY%RO.The results show that the Mesozoic coal measures experience thrice-quartic"subsidence-uplift"stages in the Huangling mining area since the beginning sedimentation at the Late Triassic.The hydrocarbon generation process can be divided into three phases.The first stage is the primary biogas generation period during the Late Triassic to the early middle Jurassic.The maximum vitrinite reflectance evolved to 0.3%.In the second period of thermogenic gas generation during the early Middle Jurassic to late Early Cretaceous,the burial depth and thermal temperature of the coal measures continuously increased to the highest level at the end of the Early Cretaceous.The maximum vitrinite reflectance evolved to 0.67%~0.74%.In the third period of secondary biogas generation during Early Eocene so far,the coal measures'burial depth is uplifted to 2077~2148 m,and the thermal temperature is decreased to less than 75℃.The data of the carbon and hydrogen isotopic composition of methane and the accumulatively generated methane amount of 10.5~16.1μmol/g during the experiment of microbial degradation No.2 and No.3 coal provide evidence for the occurrence of secondary biogenic gas in the Huangling mining area.