Controls of organic and inorganic compositions on pore structure of lacustrine shales of Chang 7 member from Triassic Yanchang Formation in the Ordos Basin, China

Abstract

Abstract To quantify the contributions of organic and inorganic compositions to pore volumes of Chang 7 shales, sub-critical gas adsorption experiments were performed on a suite of shale and organic matter samples. The parameters of pore structure were then calculated from the gas adsorption data. The grain densities of the studied samples were also measured. Based on the total organic carbon (TOC) contents and pore volumes of shale and organic matter samples, the contributions of organic matter and inorganic compositions to pore volumes were estimated. The results showed that the adsorbed gas volumes, pore volumes, surface areas and pore size distribution lines of organic matter samples were all higher than those of shale samples. It indicated that organic matter was more porous than other fractions in the studied samples. The average contributions of organic matter to the volumes of micropore, mesopore and macropore are 45.98%, 27.20% and 33.27%, respectively. The contributions of organic matter to pore volume increased with the maturity, which can be attributed to the formation of organic matter pores during maturation. The contributions of organic matter to mesopore and macropore volumes correlated positively with TOC content, which suggested that TOC content is another controlling factor of the contributions of organic matter to pore volume.