Mineral and pore structure characteristics of gas shale in Longmaxi formation: a case study of Jiaoshiba gas field in the southern Sichuan Basin, China

In this paper, the mineral and pore structure characteristics of Longmaxi shale gas reservoir have been studied with a series of parallel experiments on core samples drilled from Jiaoshiba gas field, southern Sichuan Basin, China, including X-ray diffraction (XRD) tests, total organic carbon (TOC) analysis, field emission scanning electron microscopy (FE-SEM), low temperature nitrogen adsorption/desorption (LTNA), and nuclear magnetic resonance (NMR). The relations among TOC, mineralogical compositions and pore structure parameters, and comparison of pore size distribution (PSD) of shales obtained by LTNA and NMR are discussed. The results show that the Longmaxi shales are rich in organic matter, with an average content of 2.73%. The mineralogical compositions are dominated by quartz and clay minerals, with an average content of 43.72 and 36.32%, respectively. The TOC has strong positive correlations to the quartz and pyrite, but negative correlations to clay mineral. There are three types of pores, including organic pores, inorganic pores (inter-crystalline pores and intergranular pores), and microfractures (diagenetic shrinkage joints and organic evolution abnormal-pressure fracture), which can be seen from FE-SEM, with pore sizes following the order of organic pores <inorganic pores < micro-fractures. The organic matter and quartz have a positive influence on the specific surface (ranging from 7.27 to 14.705 m2/g) and total pore volume (ranging from 0.004 to 0.02 cm3/g), whereas clay minerals have a negative effect on the specific surface and total pore volume. Compared with LTNA, the NMR can better reveal pores, indicating pore size ranging from smaller than 1 nm to several tens of micrometers. The PSDs acquired from NMR present tri-modal distribution when TOC content is higher than 2.5% and clay mineral content lower than 40%. On the contrary, the PSDs present bi-modal distribution when TOC content is lower than 2.5% and clay mineral content higher than 40%.