Pore system and gas adsorption potential of lacustrine Yanchang Mudstone, Ordos Basin, China

Abstract

Understanding the pore system and the geological factors controlling the adsorption of gas in mudstones and shales is a key element in predicting the gas accumulation potential of an unconventional reservoir. A series of integrated methods, including basic geochemistry, X-ray diffraction, scanning electron microscopy, low-pressure gas (CO2 and Ar) physisorption, and high-pressure methane adsorption were performed on a set of mudstones from the Triassic Yanchang Formation in Ordos Basin, to characterize the pore system and gas adsorption capacity and their geological controlling factors. The results show that the studied Yanchang Mudstones are mainly siliceous rocks with high total organic carbon concentration (average of 7.63%) and low to moderate thermal maturity (vitrinite reflectance of 0.55–0.92 %). The microscopic visible pores are mainly intergranular pores between organic matter and rigid mineral grains, while organic matter pores are less well-developed. The pore volume is mainly contained in micropores (diameter < 2 nm) and mesopores (diameter = 2–50 nm), while much of the specific surface area occurs in the micropores, mainly in the organic matter pores. The occurrence of rigid minerals can inhibit the compaction of the intergranular pores. The abundant organic matter can provide potential adsorption sites for gas molecules, while clays and rigid minerals also have a limited positive effect on adsorption capacity. The adsorbed methane is mainly stored in micropores, which accounts for about 55%–90% of the total adsorbed methane content.