摘要
鄂西渝东区下寒武统水井沱组（牛蹄塘组）、上奥陶统五峰组和下志留统龙马溪组海相富有机质页岩是我国南方页岩气勘探的重要层位。研究表明，研究区水井沱组经历的最高古地温介于280~300 ℃，现今为3.90%~4.50%；五峰组和龙马溪组最高古地温介于200~220 ℃，现今为2.45%~3.00%，有机质处于高演化过成熟阶段。然而，页岩样品氯仿抽提物饱和烃组成仍较为完整，存在碳数分布多具双峰型的正构烷烃、部分类异戊二烯烷烃和甾萜类化合物。该现象说明在地层条件下页岩中较严格受限的液态烃向气态烃的热力转化即环化缩聚和加氢裂解受到限制，从而造成常见饱和烃生物标志化合物的基本碳骨架在地温为200~300 ℃和热力作用时间为50~180 Ma的条件下仍然可以保存。研究表明，过成熟页岩中的饱和烃生物标志化合物没有完全失去其指示成熟度和生源的意义。根据页岩样品中的正构烷烃碳数和双峰型分布特征以及C三环萜烷/C藿烷指标判断，水井沱组页岩有机质成熟度更高，有机质来源中以原核生物的母质贡献占优，而五峰组和龙马溪组页岩有机母质来源中主要为细菌和藻类。
Abstract
Organic-rich marine shales in the Lower Cambrian Shuijingtuo (Niutitang) Formation,the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation of western Hubei-eastern Chongqing area are very attractive targets for shale gas exploration in southern China.This research shows that the maximum paleogeotemperature experienced by the Shuijingtuo Formation ranges from 280℃ to 300℃,while the maximum paleogeotemperature of the Wufeng Formation and the Longmaxi Formation ranges from 200℃ to 220℃.The organic matter of the three formations is over-mature,with the vitrinite reflectance () for the Shuijingtuo Formation varying from 3.90% to 4.50%,while that for the Wufeng Formation and the Longmaxi Formation from 2.45% to 3.00%.However,nearly a complete series of n-alkanes mostly with bimodal distributions,isoprenoid alkenes,steranes,and terpanes have been observed in saturated hydrocarbons from the sampled shales' chloroform extract.This demonstrates that the conversion from liquid hydrocarbons to gaseous hydrocarbons (i.e. cyclo-polycondensation and hydrocracking) has been restricted within the shale strata,and thus the carbon skeleton of typical saturated hydrocarbon biomarkers could preserve at 200℃ to 300℃ for 50 to 180 Ma.What is also shown by the research is that some saturated hydrocarbon biomarkers in over-mature shales have not lost their ability to indicate maturity and biogenesis completely.Judging from the carbon number range and bimodal distributions of n-alkanes,and C tricyclic terpane/C hopance ratios in the sampled shales,the shales in the Shuijingtuo Formation have a higher level of organic maturity and the prokaryote is a major source of their organic matter,while the organic parent substance for Wufeng shales and Longmaxi shales are mainly derived from bacteria and alga.
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