Comparison of organic matter pores of marine and continental facies shale in China: based on Focused Ion Beam Helium Ion Microscopy (FIB-HIM)

The continental shale in the Triassic Yanchang Formation in the Ordos Basin of the central and western China, and the marine shale in the Lower Silurian Longmaxi Formation and the Lower Cambrian Niutitang Formation in South China were chosen for Focused Ion Beam Helium Ion Microscope (FIB-HIM) observation in order to clarify the development characteristics of organic matter pores in marine and continental shale with different maturities. Few organic matter pores developed in the Yanchang shale, and the pore diameter is usually small. Micro-fractures are widespread in the organic matter, and a large amount of interparticle pores were present in the clay minerals of the shale. The Longmaxi shale samples developed a large number of organic matter pores, and there are many of small-diameter pores nested in large-diameter organic matter pores, which increased specific surface area and pore connectivity, and are favorable for the occurrence and migration of hydrocarbon gases in organic matter pores. The Niutitang shale samples contain a large number of interparticle pores and few organic matter pores. Organic matter pores contributed a vast majority of the effective storage space and connectivity of the pore system. The development of organic matter pores was mainly controlled by thermal evolution. Low thermal evolution (0.5% < R_o < 1.5%) can not forma large number of organic matter pores and hydrocarbon gas supply. Thermal evolution that is too high (3.0% < R_o < 4.0%) will lead to organic matter pore disappearance and hydrocarbon gas lost during geological history. The appropriate extent of thermal evolution (1.5% < R_o < 3.0%) ensures that organic matter pores are well developed, and is also important for the storage and seepage capacity for hydrocarbon gases in shale reservoirs. Therefore, for the exploration and development of highly-mature and over-mature marine shale gas in South China, the shale horizons with a moderate thermal evolution degree should be focused on.