Formation mechanisms of anomalously high reservoir quality in deep-buried volcaniclastic sandstones, central Junggar basin, northwestern China

Abstract

The deepto 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 CO2 (PCO2) followed by calcite cementation (δ13CPDB from −16.51‰ to −7.30‰ and δ18OPDB 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.