Characterizations of pore, mineral and petrographic properties of marine shale using multiple techniques and their implications on gas storage capability for Sichuan Longmaxi gas shale field in China

Pore, mineral and petrographic properties of shale conjunctionally determine the gas storage and transport properties of gas shale reservoirs. To investigate how these characteristics and pore structure influence the methane adsorption capability of shale formation, a total of forty-nine over-matured shale outcrop samples, thirty samples from Upper Longmaxi Formation and nineteen samples from Lower Longmaxi Formation from southern Sichuan Basin in China, were collected. Multiple techniques, including geochemical and mineralogical measurements, field emission-scanning electron microscopy (FE-SEM), mercury intrusion porosimetry (MIP), low-pressure CO2 and N-2 adsorption and high-pressure methane adsorption, were employed to characterize the geo-properties, pore structure and their impacts on methane adsorption capacity under different temperatures. Geochemical and mineralogical results show that both Upper and Lower Longmaxi shales are over-matured which have oil-prone type I kerogen. Based on the FE-SEM observation, most of the organic matter (OM) pores in the tested Longmaxi shales are spongy OM pores due to the over-maturity stage with R-o between 2.36% and 3.15%. In general, Lower Longmaxi shales are rich in micropores with larger pore volume and specific surface area (SSA) compared to those of the Upper ones. A combination of CO2/N-2 adsorption and MIP analyses on the pore structure show a whole-aperture pore size distribution (PSD) ranging from 0.4 nm to 10 mu m. The methane adsorption capacity increases with increasing TOC for Lower and Upper Longmaxi shales, respectively, while increases with increasing SSA for combined Lower and Upper ones. It is noted that, the numerous OM pores in Lower Longmaxi shale are the primary contribution to a better pore network and greater methane adsorption capacities compared to that of the Upper Longmaxi shale, but it all depends on the surface area. Shale samples with higher surface area have higher methane adsorption capacity no matter which Longmaxi shale Formations. In addition, temperature has an apparently negative effect on methane adsorption capacity for both Upper and Lower Longmaxi shales, where temperature is more sensitive to the shale sample with higher TOC content.