Volcanic reservoirs lacking a paleoweathering crust in Carboniferous deposits of the Junggar Basin, western China: Characteristics, controlling factors and hydrocarbon potential

Abstract

The Junggar Basin is the basin with the richest exploration results of volcanic reservoirs among petroliferous basins in China. The oil test data and production data show there also exhibit good hydrocarbon displays in the Kebai Fault Zone where the paleo-weathering crust is not developed. In this study, we utilized core data and thin sections to investigate volcanic reservoir rocks lacking paleo-crusts of weathering in Carboniferous deposits in the western Junggar Basin, Northwestern China. The studied interval is dominated by seven types of volcanic rocks, two types of metamorphic rocks and three types of sedimentary rocks, which are grouped into six lithofacies. From northeast (NE) to southwest (SW) of study area, the dominant rock type changes from sedimentary rock, to pyroclastic rocks, tuffites, and then to welded pyroclastic rock, lavas, respectively volcanic rocks. And the corresponding dominant lithofacies changes from delta facies, to volcanic-sedimentary facies, explosive facies, and then to sub-volcanic facies, and effusive facies. Fault-induced fractures and dissolved pores are the dominant reservoir space types. Faults and lithology jointly played a significant role in controlling the macro-distribution, physical properties, and hydrocarbon accumulation in volcanic reservoirs. The geological setting controlled the influences of primary and secondary processes. This study also reveals that the volcanic lavas with high-angle fractures of effusive facies located in SW of the study area are favorable lithofacies for hydrocarbon accumulation. This study highlights the differences in volcanic reservoir rocks with and without paleo-crusts of weathering in terms of reservoir spaces, controlling factors and hydrocarbon accumulation. This study also reveals that volcanic reservoir lacking paleo-crust of weathering can have a high hydrocarbon potential.