页岩气成气过程的流体包裹体证据--以重庆秀山剖面下寒武统牛蹄塘组为例

Characterization of the shale gas formation process based on fluid inclusion evidence:An example of the Lower Cambrian Niutitang shale formation,Xiushan section,southeastern Chongqing

页岩气的成气过程一直以来难以获得直接的地质证据,而厘清页岩何时生油何时生气以及这些过程对应的温度、成熟度指标对未来的南方页岩气勘探意义重大。脉体中的流体包裹体可以记录地质历史时期流体活动的特征,为恢复流体演化历史提供依据。本次研究选取了重庆秀山剖面下寒武统牛蹄塘组的页岩样品,采用流体包裹体系统分析的技术方法对页岩脉体中捕获的各类包裹体进行研究,共检测到3种类型的烃类包裹体,分别为沥青+气包裹体、沥青包裹体和烃气包裹体。这3种类型的烃类包裹体指示了页岩生气过程中的不同演化阶段。研究结果表明,重庆秀山剖面下寒武统牛蹄塘组页岩气演化可以划分为初次生油阶段、原油裂解为湿气阶段、湿气二次裂解为干气阶段和页岩气散失阶段4个阶段。与酸性流体活动伴随的异常高温对页岩油向页岩气的快速转化起到了关键作用,对牛蹄塘组进一步的页岩气勘探应寻找曾经有过异常高温热流体活动并且在后期构造变形较弱的地区。

Direct geological evidences for the shale gas formation process are scarce.However,a clear understanding of the shale gas formation process in terms of oil/gas generation stages and the relevant temperatures and thermal maturity indicators is essential for shale gas exploration.Fluid inclusions can record the geological information on fluid activities during geological time therefore can provide evidence to reconstructing the fluid evolution history.In this study,shale samples are collected from the Lower Cambrian Liutitang Formation,Xiushan section,southeastern Chongqing to investigate the fluid inclusions entrapped in veins,using integrated fluid inclusion analysis method.Three types of hydrocarbon inclusions are detected,which are bitumen+gas,bitumen and gas inclusions indicating different stages of the shale gas forming process.The shale gas forming process can be divided into four stages:primary oil generation,oil cracking into wet gas,wet gas cracking into dry gas,and shale gas loss,where abnormal high temperatures associated with acid fluids activity play a key role in the process of transforming oil into gas.These findings can be applied to advancing shale gas exploration in areas that have a history of abnormal high temperature associated with thermal fluid activity and experienced relatively weak late-stage tectonic deformation.