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to ultradeep-buried volcaniclastic sandstone at depths of 5000–8000 m is an important hydrocarbon reservoir in the central Junggar Basin, northwestern China. This study investigates diagenetic evolution and deciphers the formation mechanisms of anomalously high reservoir quality in deep-buried environments. Based on petrographic and geochemical evidences, extensive alterations of volcanic detritus lead to complicated diagenetic pathways. During eogenesis, grain-coating chlorite, first-stage laumontite (60–70 °C) and subordinate quartz were precipitated through hydration of volcanic glass. More complicated diagenetic products occurred during mesogenesis and contained (1) dissolution of volcanic rock fragments and precipitation of second-stage laumontite (110–120 °C); (2) dissolution of laumontite cements at elevated partial pressures of CO 2 (PCO 2) followed by calcite cementation (δ13C PDB from −16.51‰ to −7.30‰ and δ18O PDB from −13.67‰ to −9.23‰); and (3) quartz cements and albitization. Integration of depositional lithofacies, grain-coating chlorite as well as laumontite alteration, exerts significant impacts on formation of anomalously high reservoir quality in the deeply buried volcaniclastic sandstones. Medium-to coarse-grained sandstones are relatively well-sorted and could maintain high reservoir porosity owing to the development of grain-coating chlorite. Mechanic compaction is commonly effective during eogenetic regimes when the sandstones are lack of ample mineral cementation. The origin of anomalously high porosity zone largely depends on the types and abundances of mineral alteration in the deep buried regimes. Moderate abundances of grain-coating chlorite and laumontite cementation directly determine the formation of anomalously high primary porosity zones and secondary zones, respectively. • Anomalously high reservoir quality is developed in deep-buried volcaniclastic sandstones. • Diagenetic evolution pathway is strongly affected by alterations of volcanic detritus. • Formation mechanisms contain: 1) depositional lithofacies; 2) grain-coating chlorite; 3) laumontite cementation. • Different types and abundances of mineral alterations lead to different origins of anomalously high porosity zones. [ABSTRACT FROM AUTHOR]

Facies identification is a fundamental issue on evaluation and prediction for siliciclastic sediments. The purpose of this paper is to identify sedimentary-diagenetic facies and investigate the influence of sedimentary-diagenetic facies on reservoir quality in coarse-grained deposits from the Eocene Es4 interval, Dongying Depression, Bohai Bay Basin. Based on detailed core observation, thin section petrography combined with conventional wireline logs and core measurement analysis, 12 types of sedimentary- diagenetic facies were divided according to their lithofacies, diagenetic products and related contributions to reservoir quality. The identification and spatial distribution of these sedimentary-diagenetic facies can be determined based on the Bayes discriminant analysis and related cross-plot analysis. The variations of porosity and permeability with burial depth were precisely determined with ±2% for porosity and ±1 × 10–3 μm2 for permeability. The spatial distribution of reservoir porosity and permeability can be predicted quantitatively: high zones of porosity and permeability mainly distributed in the central sections of thick-bedded sandbody, ranging from 6% to greater than 12% for reservoir porosity and from 1 × 10–3 μm2 to greater than 6 × 10–3 μm2; relatively low zones of porosity (less than 6%) and permeability (less than 1 × 10–3 μm2) mainly occur in the bottom and the margin of the thick-bedded sandbody. [ABSTRACT FROM AUTHOR]