Abstract:

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.

Key words: shale gas, gas forming process, veins, fluid evolution, fluid inclusion, Niutitang Formation, southeastern Chongqing area