Evolution of nanopore structure in lacustrine organic-rich shales during thermal maturation from hydrous pyrolysis, Minhe Basin, Northwest China

Abstract

The nanometer-scaled pore systems of gas shale reservoirs have a prominent contribution for gas storage. To obtain information about the characteristics of the nanopore structure within lacustrine organic-rich shales during their thermal evolution, artificial shale samples with different thermal maturities were obtained from a hydrous pyrolysis experiment. Nitrogen adsorption, field emission scanning electron microscopy, and porosity tests were used to investigate the characteristic pore structures of lacustrine shales with different thermal maturities from the Minhe Basin. The results show that the total organic carbon content decreased from 41.89% (unheated) to 27.7% (370℃) and that organic pores, intragranular pores of pyrite, and intergranular pores of clay minerals began to form with an increase in the simulated temperature and pressure. The porosity increased from 3.57% (unheated) to 26.09% (350℃) and then decreased to 20% (370℃) on the whole. The pore sizes were distributed from 1.7 to 500 nm, and the average pore diameter first showed a decreasing trend and then an increasing trend. The cumulative pore volume and cumulative specific surface area both presented a slowly increasing trend from an unheated status to 325℃, exhibited a rapid increase at 350℃, and then showed a slow increase at 370℃. This study could provide a reference for the exploration of shale gas in lacustrine shales with different thermal maturities.