Organic matter/clay mineral intergranular pores in the Lower Cambrian Lujiaping Shale in the north-eastern part of the upper Yangtze area, China: A possible microscopic mechanism for gas preservation

Abstract

Gas shales with a high gas content were encountered in the Lower Cambrian Lujiaping Formation in the northeast part of the upper Yangtze area, China. In this area, the tectonic condition is complex and the organic matter is over-matured, so conventional oil and gas exploration have not previously been considered. During a recent shale gas exploration, canister-desorbed gas contents of more than 1 m3/t were detected from 490 to 825 m depth, with a maximum value of 3.18 m3/t at 762 m. Hence to understand the mechanism of preservation of the gas is important. The gas shales have a total organic carbon (TOC) content of between 0.44% and 6.91%, and vitrinite-like macerals reflectance (Ro) between 3.3% and 4.3% with type II kerogen. The porosity of 22 gas shale samples ranges between 0.4% and 2.29%, and their permeability is between 6.9 × 10− 5 and 3.8 × 10− 1 mD. To investigate the shale's pore structure and its relation to gas storage behavior, we studied the microscopic mechanism of gas preservation using a combination of pore characterization techniques, including focused ion beam-scanning electron microscope (FIB-SEM) imaging, and comparison of gas sorption and free gas storage capacity. Folding, thrusting and subsequent rapid uplifting movements occurred after most of the dry gas had been originally generated, and the present gas is a mixture originated from the cracking of liquid hydrocarbons at high maturity and kerogen pyrolysis in the early hydrocarbon-generation phase. SEM observations, combined with low temperature gas adsorption and mercury injection capillary pressure analyses, show that pores within the organic matter or the mineral particles in the Lujiaping Shale are rare, while the nanometer-sized pores (1–4 nm) between organic matter and clay mineral particles, especially in cleavage domains, are greatly developed. Thus the gas in these shales is mainly adsorbed gas due to the limited free gas storage space. This is unlike most other shale gas reservoirs, pores within the organic matter and pores between or within mineral particles are dominant. Moreover, the desorbed gas content correlates well with TOC content and the microporous and mesoporous specific surface area determined by low temperature CO2 and N2, respectively, suggesting that the nanometer-sized pores associated with the organic matter make up the fundamental storage space for this shale. The calculations based on FIB-SEM observation showed that the nanometer-sized intergranular pores have large specific surface areas, with their small pore throat diameters resulting in high capillary pressure. These characteristics have contributed to the preservation of shale gas in this complex tectonic area.