Differential diagenetic and densification processes of low-permeability and tight sandstones: A case study of the Jurassic Shaximiao Formation in the central and western Sichuan Basin, China

Abstract

Tight sandstone reservoirs of low permeability are of both scientific and economic significance, but their origin and formation process remain not well understood.. In this paper, we present a comprehensive study of the tight sandstone reservoir of the Middle Jurassic Shaximiao Formation distributed in central and western Sichuan Basin, by integrating scanning electron microscopy, X-ray diffraction, high-pressure mercury injection, and nuclear magnetic resonance. The aim of this study was to investigate the differential diagenetic sequences constrained by quantifying the provenance of the sandstones and densification processes. Overall, the Shaximiao Formation sandstones have low porosity of 0.46 %–18.76 % (averaging 10.17 %) and low permeability ranging from 0.0003 to 632.42 × 10⁻³ μm² (averaging 3.67 × 10⁻³ μm²), with 73.72 % being classified as tight reservoirs. The sandstone samples from the western Sichuan Basin commonly display tubular or needle-shaped throats with less developed microfractures, consequently having weaker seepage capacity compared to the varieties in the central Sichuan Basin that are characterized by abundant lamellar and necked throats. The results suggest that the slightly lower contents of rigid particles and higher contents of fragments and argillaceous matrix facilitated continuous compaction and multitype cementation processes. Meanwhile, strong compaction of the rocks with cements of carbonate, siliceous materials, and laumontite being the cement lead to porosity reduction of 29.8 %, 12.27 %, 13.34 %, and 12.33 %, respectively. In contrast, chlorite coating and dissolution events have crucial roles in preserving pores and enhancing permeability.