Hydrocarbon Phase State Evolution and Accumulation Process of Ultradeep Permian Reservoirs in Shawan Sag, Junggar Basin, NW China

A large amount of oil and gas has been discovered in the ultradeep reservoirs in the Shawan Sag, Junggar Basin. However, the hydrocarbon phases in ultradeep reservoirs are complex, and the controlling factors and evolution have not been studied, which are important for exploration and development. This study determined the hydrocarbon phase states of the reservoirs in the Upper Wuerhe Formation (P3w) of Well Zheng 10 in the Shawan Sag via hydrocarbon components and a pressure–volume–temperature (PVT) phase diagram. We used 1D basin modeling to simulate the thermal maturity of the Lower Wuerhe (P2w) source rock, components of generated hydrocarbons, reservoir temperature and pressure, and hydrocarbon phase. The results show that the hydrocarbons in the P3w reservoir are secondary condensate gas. The source rock began to enter the threshold of hydrocarbon generation at the end of the Late Triassic, and the thermal maturity (Easy Ro%) of the P2w source rock in the P3w reservoir fetch area is approximately 1.5% at present. The reservoir experienced multiple periods of hydrocarbon charging and phase evolution. During the Late Triassic to the early Late Jurassic and the late Early Cretaceous to the Late Cretaceous, the hydrocarbons in the P3w reservoir were liquid. Since the Miocene, a large amount of gas has migrated to the P3w reservoir, leading to gas invasion, which is the key to the formation of a condensate gas reservoir. The hydrocarbons changed from liquid to condensate gas due to the increasing gas–oil ratio and reservoir temperature. This study provides a quantitative method to reconstruct the phase evolution process and establishes an accumulation model of condensate gas reservoirs.