No abstract

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 μm), smaller (pore diameter 0.05–1 μm) and larger (pore diameter > 17 μm) pores have rougher pore surfaces and more complex pore structures than medium pores (1–17 μm). 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.

Redox conditions of lacustrine strata assessed by redox proxies with unified thresholds were conflicted in the Ordos Basin. The internal cross-calibration approach of multiple redox proxies, based on the enrichment sequence model of authigenic redox-sensitive element (RSE) governed by redox potential, has recently been proposed to calibrate redox thresholds in marine depositional systems. However, this assessment model often induces an overlapped threshold between anoxic–ferruginous and anoxic–euxinic conditions. Except for the redox potential, the enrichment of RSE is also controlled by its host phase. Thus, we introduced the enrichment degree and occurrence state model controlled by the host phase content into this approach to identify anoxic–euxinic conditions by an enrichment threshold of Mo (or U). Based on this method including double redox assessment models, we calibrated the thresholds of various redox conditions of the Chang 8–Chang 7 Members in the southern Ordos Basin. Subsequently, the effectiveness of previous bi-element proxies, the evolution of redox conditions, and their influencing factors were analyzed based on the calibrated thresholds. Results show that this method is applicable in the lacustrine strata. The cross plot of DOPT (degree of pyritization based on the total Fe and S content) (or SEF)–MoEF (or UEF) (enrichment factor) is suggested for calibrating thresholds of different redox conditions. Previous bi-element proxies are not suggested to be used again. There is a positive redox evolution sequence from oxic–suboxic (suboxidized) to anoxic–euxinic conditions in Chang 8–Chang 7 Members. Hydrothermal activity and paleoproductivity play different roles in the formation of redox conditions.