Diagenetic differences controlled by sediment composition and their implications for reservoir quality in the deep-buried Permian Urho Formation, Fukang Sag, Junggar basin

Deeply buried coarse clastic reservoirs of the Permian Upper Urho Formation in the Fukang Sag, Junggar Basin, exhibit pronounced reservoir-quality heterogeneity in pore systems and lithofacies architecture. However, how provenance-controlled sediment composition regulates mineral transformation and divergent diagenetic pathways, thereby controlling reservoir quality, remains unclear. This study integrates petrographic observations, XRD, SEM, MIP, EPMA, and bulk- and micro-XRF analyses to clarify the diagenetic evolution and the factors controlling reservoir quality in these deposits. The results indicate distinct provenance-controlled diagenetic pathways in two reservoir domains: (1) Reservoirs affected by the Xiquan Uplift Provenance, which contain intermediate-mafic volcanic materials, underwent stronger chemical weathering, enhanced mechanical compaction, and widespread illite-smectite mixed-layer mineral (I/S) cementation, leading to pore systems mainly composed of intercrystalline pores within clay minerals; (2) Reservoirs sourced from the Eastern Provenance, characterized by intermediate-acidic volcanic inputs, exhibit higher textural maturity and better fluid mobility, which favored the formation of early diagenetic authigenic chlorite coatings, the preservation of residual intergranular pores, and the dissolution of laumontite and feldspar; and (3) The alteration of the tuffaceous matrix differed according to diagenetic fluid chemistry. Micro-XRF elemental maps, EPMA oxide compositions, and mineral assemblage trends indicate that K-rich conditions promoted I/S enrichment, whereas Na-rich conditions favored laumontite formation followed by selective dissolution. These contrasting diagenetic pathways exert a first-order control on reservoir-quality heterogeneity. High-quality reservoirs are mainly developed in low-matrix, grain-supported thick sand bodies and sandstone interbeds within conglomerate successions, indicating that facies type, matrix content, and framework support are key screening criteria for reservoir prediction in deeply buried coarse clastic systems.