Synergistic Diagenetic Evolution in Tight Sandstone-Shale Assemblage Within Lacustrine-Delta System: A Case Study in the Members 7-8 of the Yanchang Formation, Western Zhidan Area, Ordos Basin, China

Synergistic diagenetic evolution of sandstones and shales significantly impacts the quality of associated tight oil and shale oil reservoirs. Using integrated petrographic (thin sections, fluorescence thin sections, scanning electron microscopy with energy dispersive spectroscopy), mineralogical (X-ray diffraction), geochemical (stable carbon–oxygen isotopes, electron microprobe), organic petrologic, and petrophysical analyses, combined with basin burial and thermal history reconstruction, this study investigates the mechanisms and processes of synergistic diagenesis in the tight sandstone-shale assemblages of the 7th and 8th Members of the Yanchang Formation (Middle-Late Triassic) in the western Zhidan area, Ordos Basin, China. Controlled by basin evolution, the interbedded sandstones and shales, under shared burial-thermal conditions, exhibit strong synergy in four coupled processes: compaction, clay mineral evolution, shale fluid expulsion coupled with sandstone carbonate cementation, and shale hydrocarbon expulsion coupled with sandstone secondary porosity generation. This “fluid supply-response modification” relationship strongly influences diagenetic pathways and reservoir space evolution in sandstones, leading to variable reservoir quality among different sandstone-shale assemblages. Thicker-bedded sandstones interbedded with thinner-bedded shales represent potential targets for high-quality tight sandstone reservoirs. These findings provide a possible theoretical and methodological basis for identifying high-quality tight sandstone reservoirs in lacustrine-deltaic sandstone-shale assemblages.