Geochemical characteristics of fluid in Carboniferous shale gas reservoir of the eastern Qaidam Basin

The thin interlayer of silty-fine sandstone is mostly developed in Carboniferous shale gas reservoirs of the eastern Qaidam Basin. After generation, shale gas will gather in the adjacent sandstone interlayer. Thus, the fluid inclusions in the diagenetic minerals with different stages of sandstone interlayer will become the “living fossil” for studying the fluid formation and evolution process of shale gas reservoir. In this paper, the generation and preservation conditions of shale gas in the study area were systematically investigated in terms of the geochemical characteristics of shale, the microscopic polarization/ fluorescence characteristics of reservoir, as well as the temperature and composition characteristics of fluid inclusion. The research results indicate that Carboniferous mud shale in the study area has high organic abundance, dominated by the organic matter of Type II2-III, and is generally in the mature-highly mature development stage. There exist two stages of hydrocarbon charging processes in shale gas reservoir. Stage 1 occurred at the end of Early Permian period, in which the hydrocarbon compositions had low maturity, consisting of heavy oil inclusions distributed inside the annular quartz-grain overgrowth edge in sandstone interlayer and residual brown-beige thin-oil asphalt in current reservoirs. Stage 2 occurs from Early Neogene till now, in which the condensate and natural gas inclusions are distributed zonally along the micro-cracks of sandstone interlayer posterior to the quartz grain digenesis period, considered as the hydrocarbon main body of current shale gas reservoirs. In partial sandstone interlayer at the east and northeast edges of the basin, gas inclusions have primary compositions such as CH4+CO2 and CO2+CH4+N2, belonging to metamorphic fluid under magma hydrothermalism. The Carboniferous shale in the central study area has high organic carbon content with moderate maturity (Ro mean of 1.0%±) and good hydrocarbon preservation conditions, where movable light oil and gas exist at present, indicating favorable exploration prospects for shale gas. © 2017, Editorial Office of ACTA PETROLEI SINICA. All right reserved.