Quantitative Characterization and Differences of the Pore Structure in Lacustrine Siliceous Shale and Argillaceous Shale: a Case Study of the Upper Triassic Yanchang Formation Shales in the Southern Ordos Basin, China

Shale oil and gas have attracted more and more attention around the world. It is particularly important to quantitatively characterize the complex pore structure of shales. Comparison of pore structure differences between different types of shales is beneficial to the exploration and development of shale oil and gas. Using the lacustrine siliceous shale (SS) and argillaceous shale (AS) of the Upper Triassic Yanchang Formation in the Ordos Basin as a case study, multiple experimental methods were introduced to quantitatively characterize full-range pore size distribution (PSD) combined with the fractal dimension theory. The experimental data of carbon dioxide adsorption, nitrogen adsorption, and high-pressure mercury injection were integrated to obtain the characterization of full-range PSD and fractal dimensions D1 to D6. The micropore surface area and volume, average mesoporous pore size, and macropore volume of SS are smaller than those of AS. On the contrary, the surface areas of mesopores and macropores, average macropore radius, and mesoporous volume of SS are slightly larger than those of AS. Commonly, the larger the pores are, the rougher the pore surfaces and the more complex the pore structures in the range of mesopores (2-50 nm) become. For macropores (pore diameter > 0.05 mu m), smaller (pore diameter 0.05-1 mu m) and larger (pore diameter > 17 mu m) pores have rougher pore surfaces and more complex pore structures than medium pores (1-17 mu m). The pore surface of SS is rougher, and the pore structure is more complex than those of AS. The interpenetrating contact relationship between quartz and clay minerals makes the pore structure more complex and reduces the porosity and permeability of AS, while the dissolution of feldspar reduces the complexity of the pore structure and improves the petrophysical properties, especially for SS. Average mesopore diameter, macropore surface area, and fractal dimension D3 and D4 can be used as reliable indexes to evaluate petrophysical properties of the shale reservoir.