247 / 2022-03-29 08:21:44

Multi-scale Quantitative Characterization and Comparison of Pore Structure Heterogeneity in Lacustrine Siliceous Shales and Argillaceous Shales of Chang 7 Formation in the Southern Ordos Basin, China

siliceous shale,argillaceous shale,pore structure,gas adsorption,high-pressure mercury injection,fractal dimension

       As two indispensable parts of the lacustrine shale strata, siliceous shales and argillaceous shales, their pore structure heterogeneity has long been a vital factor affecting the exploration and development of shale oil and gas. In this study, multiple experimental methods (carbon dioxide adsorption, nitrogen adsorption, and high-pressure mercury injection) integrated with fractal theory were used to quantitatively characterize the full-range pore size distribution and compare the pore structure heterogeneity at multiple scales of siliceous shales and argillaceous shales.

       As the main storage space of siliceous shales, the surface area and volume of mesopores are significantly higher than those of argillaceous shales. Although macropores contribute 40% to the pore system of argillaceous shales, the macropore surface area and average size of siliceous shale are larger. In contrast, the surface area and volume of micropores of siliceous shales are lower than those of argillaceous shales. The fractal dimension indicates that the pore surface of siliceous shales is rougher and the pore structure is more complex. The dissolution of feldspar reduces the pore structure complexity and improves petrophysical properties of siliceous shales, while the interpenetrating contact between quartz and clay minerals heightens the pore structure heterogeneity of argillaceous shales. Besides, resemblance exists in the trend of full-range pore size distribution of siliceous shales and argillaceous shales. Larger pores have rougher surfaces and more complex structures in the range of mesopores. Average mesopore diameter, macropore surface area, and fractal dimension D3 and D4 can be used as reliable indexes to evaluate petrophysical properties of shale reservoirs. This study contributes to enrich the research of fine-grained sediments with weighty theoretical significance, and can be used as the theoretical basis to guide the fine exploration and effective development of shale oil and gas.