Pore structure characterization and controlling factors of tight sandstone reservoirs, Shihezi Formation in the Eastern Linxing Area, Ordos Basin, China

Abstract

Continuous development of tight sandstone gas (TSG) worldwide has not only increased the quantity of natural gas resources, but also changed the way hydrocarbon exploration and exploitation are carried out. TSG reservoirs can form in various sedimentary environments. After the later diagenetic transformation, the TSG reservoir exhibits strong heterogeneity. The Shihezi Fm. in the Eastern Linxing Area of the eastern Ordos Basin is characterized by pronounced heterogeneity across stratigraphic intervals. This study determined the internal structure and its principal controlling factors through an integrated analysis of regional structural data, sedimentary facies, cathodoluminescence petrography, scanning electron microscopy petrography, X-ray diffraction, nuclear magnetic resonance, mercury intrusion porosimetry (MIP), and physical property measurements. The Shihezi Fm. represents a terrestrial shallow-water deltaic system. The lower strata are dominated by lithic feldspathic sandstone and feldspathic–lithic quartz sandstone, whereas the Upper strata consist mainly of lithic feldspathic sandstone. The TSG reservoirs display generally low porosity and permeability, with pore–fracture systems comprising intergranular pores, dissolution pores, and fractures. MIP curve analysis distinguishes three reservoir categories: Categories II and III are predominantly developed in the Lower Shihezi Fm., while Categories I, II, and III occur in the Upper Shihezi Fm. The Upper Shihezi Fm exhibits superior quality. The key controlling factors include sedimentary microfacies, mineral composition, and diagenetic processes. Specifically, coarse- to medium-grained sandstones deposited in distributary and subaqueous distributary channel microfacies, coupled with selective dissolution and cementation, significantly enhance reservoir quality. Controlled by the dissolution of feldspar and rock fragments, as well as the pore-lining of clay minerals, the porosity and internal structure of coarse sandstone are superior to those of fine sandstone. These findings provide a robust theoretical framework for optimizing exploration and development strategies in the TSG reservoir.