Whole-aperture characteristics and controlling factors of pore structure in the Chang 7th continental shale of the Upper Triassic Yanchang Formation in the southeastern Ordos Basin, China

Abstract

Quantitative assessment is still lacking on the pore-sizes distribution of micropores and mesopores, and the major controlling factors remain enigmatic for the continental Chang 7th shale reservoirs in southeastern Ordos Basin. In this study, scanning electron microscopy, organic geochemical analysis, low-temperature gas adsorption, high-pressure mercury injection, and X-ray diffraction analysis were conducted on 29 core samples from representative eight wells in the Xiasiwan-Yanchang area, yielding a particular investigation on characteristics and controlling factors of the structure in the Chang 7th shale. The Chang 7th shale is mainly characterized by organic and secondary micropore and mesopore, respectively, ranging the peaks of pore size from 0.35 to 0.65 nm, 0.75 to 1.00 nm, 1.10 to 1.35 nm, and from 4 to 13 nm. Whole-aperture characterization of the pore structure shows that meso- and macropores demonstrate a major contribution to total pore volume (PV), and the surface area is mainly provided by micro- and mesopores with pore radii less than 20 nm. A weak positive variation was identified between the PV of micromesopores and total organic carbon content, implying that micropores are currently in the process of developing. The micromesopore specific surface area and volume, respectively, yield a significant positive and weak variation with the quartz and clay mineral content, suggesting that siliceous minerals are favorable for micromesopore development. The infusion of siliceous biogenic debris and silica-replaced carbonate biogenic debris from the northeastern and southwestern source areas, respectively, is likely responsible for development of biogenic siliceous minerals in the Chang 7th shale. Furthermore, the hollow cavities in siliceous organisms have considerably contributed to the preservation of primary pores owing to their characteristic skeleton-supported framework.