Divergent Chlorite and Kaolinite Authigenesis and Reservoir Quality Controls: Chang-8 Tight Sandstones, Ordos Basin

The genesis of authigenic clay minerals in tight sandstones fundamentally controls reservoir quality and micro-pore evolution. This study investigates the differential formation mechanisms of authigenic chlorite and kaolinite and their modulating effects on pore systems in the Chang-8 Member tight sandstones, Ordos Basin. Thin-section petrography, X-ray diffraction, scanning electron microscopy, and high-pressure mercury injection were utilized to quantify mineralogical and petrophysical characteristics. Results show chlorite (averaging 4.8%) and kaolinite (averaging 1.6%) are the dominant authigenic clay minerals with distinct spatiotemporal distributions. Chlorite nucleated as pore linings during early diagenesis under alkaline, oligohaline to mesohaline conditions driven by volcanic material hydration. Conversely, kaolinite precipitated as pore-filling during mid-to-late diagenesis (80–120 °C), driven by organic acid pulses from underlying source rocks causing feldspar dissolution. We conclude that early chlorite linings constructively preserve primary porosity by mechanically resisting compaction and chemically inhibiting quartz cementation, despite narrowing pore throats. Meanwhile, kaolinite acts as a pore-type modulator, restructuring macro-pores into micro-intercrystalline pores, which significantly impairs permeability only when its proportion crosses a critical threshold. The diagenetic fluid transition from alkaline to acidic ultimately dictates this mineralogical succession and subsequent reservoir heterogeneity.