Differences in the Nanopore Structure of Organic-Rich Shales with Distinct Sedimentary Environments and Mineral Compositions

Shale pore structures play an important role in hydrocarbon migration and the long-term gas supply of shale gas wells. Typical marine (Wufeng-Longmaxi), transitional (Dalong), and lacustrine (Dongyuemiao) shale samples in the Yangtze Platform were selected to contrast the pore differences between various shales through N-2 adsorption, field emission scanning electron microscopy, and nano-computed tomography. The highest values of pore volume, specific surface area, and connected pore proportion are recognized in the overmatured marine shales, with an average of 29.27 x 10(-3) cm(3)/g, 21.38 m(2)/g, and 68.2%, respectively. The storage space is closely associated with the abundance of organic matter (OM) pores. Due to the limitation of OM type and thermal maturity, matured nonmarine shales are dominated by pores related to matrix minerals. The pore structure parameters are also relatively poor. The average pore volume and average specific surface area of transitional shales are about 7.94 x 10(-3) cm(3)/g and 2.9 m(2)/g, respectively, while those of clay-rich continental shales are found to be 12.01 x 10(-3) cm(3)/g and 8.1 m(2)/g. Meanwhile, the connected pore proportions of transitional and continental shales decreased to 32.64 and 34.54%, respectively. Nonmarine shales have fewer fractal geometries than marine shales, which is mainly because of low hydrocarbon porosity. The analysis reveals that the differences in the occurrence state and production effect of shale gas are immediately affected by the pore type, size, and connectivity within shale reservoirs.