Pore characteristics and formation mechanism of high-maturity organic-rich shale in Lower Cambrian Jiumenchong Formation, southern Guizhou

In order to investigate pore characteristics and formation mechanism in the high-maturity organic-rich shale of Lower Cambrian Jiumenchong Formation in southern Guizhou, the pore structure, pore type and storage properties are well studied through the rock thin section, total rock X-ray diffraction, low-temperature nitrogen adsorption, high pressure mercury injection-adsorption test, helium porosity test, argon ion polishing-scanning electron microscope, thermal evolution and pore evolution history reconstruction, and based on the diagenesis and compaction as well as thermal evolution process, a microscopic pore formation and evolution model of high-maturity organic-rich shale are established. The result shows that the high-maturity organic-rich shale of Jiumenchong Formation has the average total specific surface area of 12.66 m2/g and the total pore volume of 11.54 × 10−3 cm3/g, and the total specific surface area have a positive correlation with total pore volume; the total specific surface area and the total pore volume are slightly lower compared with the Lower Silurian shale. The pores are dominated by micropores and mesopores, while macropores are very rare. The pores of the organic-rich shale mainly are organic pores with small diameter usually less than 30 nm, and the pore boundary form is irregular, the inorganic mineral pores are not developed. Compared with the Silurian shale, the shale has poorer reservoir property, the average porosity is only 2.80%; the horizontal permeability is 1-3 times of the vertical permeability, indicating the horizontal lamellations are not developed. The formation and evolution of pores in high-maturity organic-rich shale is jointly influenced by the evolutionary process of intergranular pores of inorganic mineral under the control of the diagenesis and compaction, the organic pore formation process in the hydrocarbon generation-oil formation-oil and gas transformation sequence under the control of thermal evolution, and the natural gas loss-supply equilibration process under the condition of later pore preservation.