Comparison of pore systems of clay-rich and silica-rich gas shales in the lower Silurian Longmaxi formation from the Jiaoshiba area in the eastern Sichuan Basin, China

The Jiaoshiba area in the eastern Sichuan Basin hosts the largest commercial shale gas field in China with an annual production of 1.7 BCF. The gases are primarily produced from the clay-rich siliceous marine shales in the Wufeng and Longmaxi formations. In this paper, sixteen organic-rich shale samples from a variety of lithofacies from the Longmaxi Formation in the Jiaoshiba area were studied to evaluate the pore systems of clay-rich shales by comparison with the silica-rich gas shales. The investigation includes the assessment of porosity and pore size distribution by means of shale bulk and skeletal density measurements, low-pressure N-2/CO2 gas adsorption and mercury intrusion. The shale samples with different lithofacies have porosity values ranging from 3.25% to 5.65% and display similar fractional volumes of micropores, mesopores and macropores, and corresponding specific surface areas. All three pore fractions contribute to the shales' total pore volumes, while the total specific surface areas are mainly contributed to by micropores and mesopores. There is a positive correlation between pore volumes and TOC contents, indicating that the porosities in the shales are dominated by organic matter pores with TOC content being the main parameter controlling pore development. TOC-normalized pore volumes decrease with a further increase in the TOC content from 1.6% to 4.2%. This may be related to the fact that the TOC-rich shales are more susceptive to mechanical compaction. Clay mineral content appears to have little effect on the pore volume or specific surface area development in the shales. Shale samples with clay-rich siliceous shale lithofacies display similar TOC contents, porosities, and fractional volumes of all pore sizes and specific surface areas with those of the silica-rich argillaceous and argillaceous/siliceous mixed lithofacies. This suggests that the marine shales with silica-rich argillaceous and argillaceous/siliceous mixed lithofacies from the Longmaxi Formation in the Jiaoshiba area are capable of generating significant amount of pore spaces for shale gas storage.