Reservoir Quality of Tight Sandstones in the He‐8 Member, Southern Ordos Basin: Coupled Control of Detrital Composition and Diagenetic Modification

ABSTRACT

Reservoir quality of tight sandstones is intrinsically linked to detrital compositions and diagenetic modifications. The Middle Permian Lower Shihezi Formation (He‐8) represents a critical target for tight gas exploration in the Ordos Basin; however, the integrated impact of original mineralogy and diagenesis on its reservoir properties has not been fully elucidated. This study quantitatively characterizes the diagenesis‐porosity evolution of the Permian tight sandstones with a uniform provenance in the southern Ordos Basin. A multidisciplinary approach was employed, integrating thin‐section petrography, scanning electron microscopy (SEM), X‐ray diffraction (XRD) and cathodoluminescence (CL), complemented by fluid inclusion microthermometry and petrophysical measurements. The sandstones comprise quartzarenite, sublitharenite and litharenite and have porosities of 2.6%–12.3% and permeabilities of 0.04–1.54 mD. Using the Ehrenberg approach, cumulative compactional porosity losses were estimated at 19.25%, 23.20% and 22.53% for quartzarenite, sublitharenite and litharenite, respectively, whereas cementational losses were 9.15%, 5.06% and 6.24%. Dissolution increased porosity by only 2.38%–3.15%. The results indicate that despite reaching the same diagenetic stage, sandstones with different detrital compositions underwent distinct diagenetic processes and porosity evolution trajectories. During eodiagenesis, sublitharenite and litharenite experienced greater porosity loss than quartzarenite due to mechanical compaction. Conversely, quartzarenite exhibited more significant porosity reduction during mesodiagenesis, driven by extensive quartz overgrowths, carbonate cementation and clay mineral authigenesis. These findings underscore that detrital composition is a fundamental determinant of tight sandstone reservoir quality, a factor that warrants increased attention in diagenetic modelling and reservoir prediction.