Lithofacies and Pore Structures of the Permian Qixia Dolostone Reservoirs (Central Sichuan Basin, China): Implication of Hydrothermal Dolomitization on Reservoir Quality

The Permian Qixia dolostone in the Central Sichuan Basin is a significant hydrocarbon reservoir of hydrothermal origin, linked to the Emeishan Large Igneous Province and structurally controlled by E–W strike–slip faults. However, how this process controls reservoir quality remains poorly understood. To address this, we integrate core observation, thin-section petrography, XRD analysis, thickness mapping, MICP, and μ-CT to characterize the lithofacies and pore structures of the Qixia Formation in the study area. Six lithofacies are recognized, including mudstone (F1), wackestone (F2), packstone (F3), grainstone (F4), rudstone (F5), and dolostone (F6), and F6 is further divided into three subtypes (F6-1, F6-2, F6-3). Dolostones exhibit superior reservoir quality relative to limestones, and among the dolostone, reservoir quality improves progressively from F6-1 to F6-3 with increasing crystal size and dolomite content. Dolostone distribution is spatially tied to E–W strike–slip faults, and its formation age coincides with documented fault activity, implicating these faults as the primary fluid conduits. Quantitative pore structure analyses further indicates that dolomitization enhanced permeability by enlarging pore–throat radii and improving macropore connectivity, with associated dissolution contributing additional secondary porosity.