The Permian hybrid petroleum system in the northwest margin of the Junggar Basin, northwest China

Abstract

In this paper, we define the concept of a hybrid petroleum system for the Permian petroleum system in the northwest margin of the Junggar Basin, by documenting the complexity in its formation and evolution. It is termed hybrid because there are three source rock sequences, seven reservoir-caprock combinations, and three petroleum events, and multiple source rocks source the same reservoir and the same source rocks also feed multiple reservoirs. The Mahu sag centre is the petroleum source area with three sets of source rocks: dark shale of the Lower Permian Jiamuhe Formation, black shale of the Lower Permian Fengcheng Formation, and black shale of the Middle Permian Lower Wuerhe Formation. The reservoir rocks include volcanic and clastic rocks of Upper Carboniferous to Lower Cretaceous age, mainly characterized by alluvial conglomerate and sandstone. There are three sets of ultimate caprocks, among which the mudstone of the Upper Triassic Baijiantan Formation is the most important due to its wide distribution throughout almost the whole northwest margin and greatest thickness (up to 300 m), whereas the Lower Jurassic and Lower Cretaceous mudstone acts as an effective seal. Oil and gas were generated mainly during the Middle-Late Permian, Late Triassic, and Middle Jurassic-Early Cretaceous. The reconstruction of the oil-filling history shows that the three source rocks charged reservoirs mainly in three geologic intervals, with common mixing of oils from different sources and/or with different maturities. These three hydrocarbon charges coincide with the three main stages of petroleum generation, related to the Hercynian, Indosinian, and Yanshannian orogenies. During these three petroleum events, oil and gas migrated along the migration framework controlled by faults, unconformities, and good permeability reservoir rocks; the faults are mainly well-developed and episodically active thrust faults and secondary faults. As a result, the Permian petroleum system in the northwest margin of the Junggar Basin is hybrid.