Pore structure characterization for the Longmaxi and Niutitang shales in the Upper Yangtze Platform, South China: Evidence from focused ion beam He ion microscopy, nano-computerized tomography and gas adsorption analysis

摘要

Pore structure is of great importance in the occurrence of shale gas. In this study, pores in shale samples from the Lower Silurian Longmaxi and Lower Cambrian Niutitang shales in Chongqing, in the Upper Yangtze Platform, South China, were examined using focused ion beam-He ion microscopy (FIB-HIM), nano-computerized tomography (nano-CT), and gas adsorption analysis. The results show that most pores in measured samples of the Niutitarig shale are mineral matrix pores and have poor connectivity, including both intraparticle and interparticle pores. Organic matter pores in measured samples of the Niutitang shale are rare and have small diameters. In measured samples of the Longmaxi shale, OM pores-mostly macropores and large mesopores-are the most abundant. The nano-CT results revealed that the abundance of OM pores led to good pore connectivity in the Longmaxi shale samples, whereas the prevalence of mineral matrix pores resulted in poor connectivity in the Niutitang shale samples. N-2 and CO2 adsorption analyses showed that the mean value of micropore-mesopore volumes in the Niutitang and Longmaxi shales samples are 0.0332 ml/g and 0.0261 ml/g, respectively. The mean values of micropores-mesopores surface area in the Niutitang and Longmaxi shales samples are 33.84 m(2)/g and 32.46 m(2)/g, respectively. Thus, there is little difference between measured samples of the Longmaxi and Niutitang shales in terms of volume and surface area of micropores-mesopores. The equal-vitrinite reflectance (equal-Ro) values of measured samples of the Longmaxi shale are lower than those of the Niutitang shale samples. Over-maturity (equal-Ro>3%) resulted in a reduction of the number of pores in organic matter, small pore size, and poor connectivity in measured samples of the Niutitang shale. Relatively low maturity (2%