Pore evolution in siliceous shales and its influence on shale gas-bearing capacity in eastern Sichuan-western Hubei, China

Abstract

The Upper Permian siliceous shale reservoir, which was less studied before, is considered to be a successor target of marine Wufeng-Longmaxi shale gas in Sichuan Basin. To deeply explore pore characteristics, evolution process, formation mechanism and their influences on shale gas capacity, we performed a comprehensive multiscale characterization on siliceous shales with different maturities from Dalong and Wujiaping Formations using low-pressure N2 adsorption (N2GA), mercury injection pressure (MIP), field emission scanning electron microscopy (FE-SEM), and methane sorption capacity. Results indicate that shale pore development, especially OM pore development, and physical property are strictly controlled by thermal maturation levels. OM pores are seldom in low-mature shales but are well-developed in high-to over-mature shales. Pore structures in shales with different maturities are determined by different factors. The high TOC content is conducive to developing OM micropores for high-to over-mature shales. High content of brittle minerals is conducive to developing interparticle (interP) pores and dissolved mesopores at high-mature stage. In terms of the data and published articles, we established a general pore evolution model in siliceous shales to illustrate shale pore development process and formation mechanism. Similar to OM pore development, methane sorption capacity is controlled firstly by thermal maturation and secondly by TOC content. It's pointed out that over-mature shales have smaller-sized OM pores compared with high-mature shales, and therefore they have higher specific surface areas and methane sorption capacities normalized to TOC. This indicates that siliceous shales with extremely high maturity still have good shale gas potential, which further expand shale gas exploration in over-mature shale gas reservoir, as well as provide exploration suggestion for the siliceous shale gas reservoirs in south China.