Gas-in-place and its influence factors of the upper Paleozoic coal-bearing shale in the Qinshui Basin, China

Abstract

Coal-bearing shale shows great potential for unconventional gas resources in China, while its exploration and development have been challenging for a long time. Gas-in-place (GIP) is critical to shale gas evaluation, but the major factors controlling the GIP content of coal-bearing shale remain unclear. To address this issue, the coal-bearing shales of the upper Carboniferous-lower Permian Taiyuan and Shanxi formations in the Zuoquan Block, Qinshui Basin, China, were collected for GIP measurements and an integrated investigation, including organic geochemistry, inorganic mineral compositions, and pore characterizations, was carried out. Our results show that the GIP content of the studied shales displays relatively low values and wide variations, which range from 0.30 to 2.28 m³/t. The GIP is dominated by desorbed gas and residual gas. Total organic carbon (TOC) contents of the studied shales vary from 0.92% to 16.91%, and inorganic minerals are dominated by clays that mainly consist of illite/smectite mixed layer (I/S) and kaolinite. Inorganic pores have been widely observed in the studied shales, while the organic matter-hosted pores are rarely found using SEM observations. Total porosity of the studied shales is primarily contributed by clay minerals, followed by organic matter and quartz. Weak positive relationships between the GIP content and pore structure parameters imply that the adsorption of methane to nanopores is relatively weak, which may be attributed to the hydrophilicity of clay-hosted pores. Moreover, hydrophobic organic pores are not well developed. Positive correlations between the GIP contents and contents of TOC, clays, and the I/S indicate that major factors influencing the GIP contents of the coal-bearing shales are clays (especially I/S) and TOC content. In summary, these findings would be very helpful to reveal the enrichment mechanism of coal-bearing shale gas and provide a scientific basis for the exploration and development of coal-bearing shale gas.

Graphical abstract

Keywords

coal-bearing shale / clay minerals / inorganic pore / gas potential / unconventional gas

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