Pore characteristics and controlling factors of the Lower Cambrian Hetang Formation shale in Northeast Jiangxi, China

To identify the microscopic pore characteristics and controlling factors of Hetang Formation Shale in the Lower Yangtze Region, the pore types, pore size distribution characteristics, and controlling factors of the Lower Cambrian Hetang Formation (ЄIh) marine shale in Northeast Jiangxi were analyzed by using low-temperature liquid nitrogen, X-ray diffraction, scanning electron microscope, mercury intrusion porosimetry, isothermal adsorption experiment, and geochemical indicator test system. The research results show that the pore size distribution curve of Hetang Formation Shale is characterized by “two peaks” and dominated by micropore (2 nm) and mesopore (47–82 nm). The hysteresis loop shows that the open parallel-plate pore and slit pores are the main pore types in shales. The pore volume of Hetang Formation Shale is only positively related to total organic carbon, without obvious correlation with mineral composition and thermal evolution degree. The controlling factors of pore structure characteristics of Hetang Formation Shale are rather complicated. Further analysis shows that diagenesis and excessive thermal evolution are the two main controlling factors restricting the microscopic pore characteristics. Due to great burial depth, organic matter generates numerous micropores during pyrolysis and hydrocarbon generation, and clay minerals generate a lot of micropores and mesopores during conversion from montmorillonite to illite. On the other hand, the development of mesopore and macropore is far better than that of nanoscale pore, because rigid quartz mineral is the dominant composition of shale and the strong compaction resistance of quartz can increase macropore volume with the increase of shale burial depth. It can be inferred that Hetang Formation Shale is a relatively ideal horizon for shale gas development, since the proportion of potential free gas is relatively high and induced cracks are prone to be formed, which is conducive to seepage and desorption of shale gas.