Objective Carboniferous volcanic reservoirs in the western Well Pen−1 Depression, an important area for oil and gas exploration in Junggar Basin, multiple phases of tectonic activity and secondary hydrocarbon drainage in this area are superimposed on each other. This has led to the complication of the hydrocarbon formation process in the Carboniferous volcanic reservoirs and the unclear understanding of the hydrocarbon formation law. This study aims to accurately determine the stages and timing of hydrocarbon charging through fluid inclusion analysis. Combined with source rock evaluation, the hydrocarbon generation potential and accumulation mechanism are comprehensively analyzed to provide a scientific basis.

Methods Based on the morphology and homogeneous temperature of fluid inclusions in the Carboniferous volcanic reservoir, this study applies fluorescence light microscope observation, fluid inclusion thermometry and basin modeling to constraint the timing of hydrocarbon charging and accumulation.

Results The results show that: ① fluid inclusions within the Carboniferous volcanic reservoir in the western Well Pen−1 Depression comprise mainly of hydrocarbon containing saline inclusions, hydrocarbon containing gas inclusions and gas inclusions. The oil inclusions with bluish green and bluish white colors are usually developed in the cracks of quartz grains and in the fracture−filled calcite. This indicates the charging of middle mature and high mature oils during the geologic history. ② The oil inclusions, gas inclusions, and gas−liquid two−phase brine inclusions associated with hydrocarbon inclusions in the Carboniferous volcanic reservoir present a wide range of homogeneous temperature with multiple peaks, indicating the multiple phases of hydrocarbon charging in the study area. ③ Integrating the modeling results of burial and thermal histories with homogeneous temperatures of brine inclusions associated with hydrocarbon inclusions suggests the existence of two phases of hydrocarbon accumulation in study area. The first phase ranges from the early Late Triassic to the Jurassic and the second phase extends from the Late Cretaceous to the early period of Quaternary.

Conclusions This study provides a theoretical foundation for clarifying the mechanism of forming oil and gas accumulations in the Carboniferous volcanic reservoirs of the western Well Pen−1 Depression, Junggar Basin.