Full-scale pore structure and its controlling factors of the Wufeng-Longmaxi shale, southern Sichuan Basin, China: Implications for pore evolution of highly overmature marine shale

Abstract

Shale gas primarily exists in nanopores in shale as different occurrence phases. Studies on pore structure characterization and its controlling factors are the key to understand shale gas occurrence and gas accumulation mechanism. Mercury intrusion capillary pressure (MICP), low-pressure N2 and CO2 adsorption were applied to quantify pore structure of all sizes for Wufeng-Longmaxi shale from southern Sichuan Basin, China. These shales are currently highly overmature, and their total organic carbon (TOC) contents range from 0.04% to 5.19%. Quartz content is positively correlated with TOC content in the clay-poor shales, whereas it exhibits no correlation with TOC in the clay-rich shales. Different relationships are likely due to difference in quartz origins, since the clay-poor shales contain more biogenic quartz than the clay-rich shales. Furthermore, multimodal pore size distributions were observed. Total pore volume, ranging from 1.41 to 3.03 cm3/100 g, is mainly provided by pores less than 10 nm in diameter, whereas most of the specific surface area, ranging from 19.55 to 36.64 m2/g, is provided by pores less than 2 nm in diameter. Organic matter contributes mostly to micro- and mesopore development. Clay minerals, mainly illite, also make a contribution to pore structure, especially in the organic-lean shales. The porosity development of micro- and mesopores is also controlled by thermal evolution of organic matter. Micro- and mesopores were developed at the immature to early mature stages due to pore rearrangement, expulsion of liquid hydrocarbon, and dissolution of unstable minerals. The following decrease during the late mature stage was mainly caused by oil and bitumen infill. Both micro- and mesopores were reopened at the postmature stage, which was driven by the secondary cracking of residual oil and bitumen. The final significant decline of micro- and mesopore volumes at the overmature stage was caused by persistent compaction and OM carbonization.