Sedimentary heterogeneity-constrained differential diagenesis in fluvial tight sandstones: Implications for reservoir quality heterogeneity in sandbody architectural elements

Fluvial tight sandstones, as unconventional reservoirs, feature complex sedimentary, diagenetic, and reservoir quality heterogeneities that fundamentally control their exploration and development. This study focuses on typical fluvial tight sandstones of the Shanxi Formation in northern Sulige Gas Field (Ordos Basin), using core samples, field sections, logging data, and analytical tests to investigate the coupling relationships among sedimentary, diagenetic, and reservoir quality heterogeneities. Results demonstrate that: the target layer in the study area develops six lithofacies, three braided river sandbody architectural elements, two meandering river sandbody architectural elements, and five diagenetic facies. Different architectural elements and their internal lithofacies exhibit differences and coupling relationships in the strengths of sedimentary heterogeneity and diagenetic heterogeneity. Specifically: strong compaction diagenetic facies primarily occurs in siltstone lithofacies of sand-dominated, weakly sedimentary heterogeneous architectural elements; strong dissolution + microfracture development diagenetic facies is mainly distributed in coarser-grained sandstones of sand-dominated architectural elements with low sedimentary heterogeneity; clay mineral intercrystalline pore-dominated diagenetic facies is primarily located near interbeds within all architectural elements; strong carbonate cementation diagenetic facies predominates in thick mudstones at the top/bottom of architectural elements, and at contacts with pure mudstone interbeds; strong clay mineral cementation diagenetic facies concentrates in medium-heterogeneity architectural elements near pure mudstone interbeds and at contacts with silty mudstone interbeds. Sedimentary and diagenetic heterogeneities jointly control reservoir quality distribution in all architectural elements (excluding basal conglomerates): porosity and permeability decrease in positive rhythm as lithofacies grain size fines, while irreducible water saturation increases in inverse rhythm. The findings of this study can provide a robust basis for the exploration and development deployment of fluvial tight sandstone reservoirs.