Organic Mesopore and Micropore Structures and Their Effects on Methane Adsorption in Marine Organic-Rich Shales

Abstract

Organic pores in organic-rich shale reservoirs can provide major pore space for adsorbed and free gas storage. In this study, the organic micropore and mesopore structures in organic-rich shale reservoirs and their influence on gas adsorption are investigated by comparing CO2, N2, and methane adsorption isotherms for organic matter-combusted and non-combusted organic-rich shale samples taken from the Wufeng–Longmaxi Formation in the Sichuan Basin. Experimental results show that organic pores in organic-rich shales from the Wufeng–Longmaxi Formation in Sichuan Basin are mainly smaller than 15 nm in diameters. Volumes of organic micropores and mesopores are mainly distributed in the diameter size ranges of 0.3–0.9 and 2–8 nm, respectively. Specific surface areas of organic micropores and mesopores are mainly distributed in the diameter size ranges of 0.3–0.7 and 2–4 nm, respectively. Organic pores dominate in the organic-rich shales from the Wufeng–Longmaxi Formation, contributing 49–77% and 50–84% of the entire micropore and mesopore volumes and specific surface areas, respectively. Organic micropores contribute 42–84% and 44–85% of the entire micropore volume and specific surface area, respectively, and organic mesopores contribute 48–84% and 59–89% of the entire mesopore volume and specific surface area, respectively. Organic pores are dominant pore spaces for adsorbed methane in organic-rich shale reservoirs and contribute 75–87% of the total amount of adsorbed methane in shales from the Wufeng–Longmaxi Formation. Outcomes of this study demonstrate that organic matter combustion is a useful method for evaluating organic pores in organic-rich shales mainly according to following lines of evidence: (1) after combustion, samples have low volumes and specific surface areas for micropores and mesopores as well as extremely low contents of adsorbed methane. This suggests that combustion does not result in the generation of <15 nm inorganic micropores and mesopores in clay minerals to provide pore space for adsorbed methane. (2) The total organic carbon content (TOC) positively correlates with the organic pore volume and specific areas as well as the absolute amount of methane adsorption, suggesting that organic pores determined in this study are mainly controlled by TOC values and combustion caused the removal of organic micropores and mesopores.