Hydrocarbon Accumulation Model Controlled by Overpressure Evolution in the Kelasu Thrust Belt of the Kuqa Depression, NW China

The Kuqa Depression is the most representative overpressured hydrocarbon accumulation area in western China. The early rapid subsidence since 5 Ma and the strong uplift in the late period have led to a complicated evolution history of the overpressure. Three hydrocarbon charging episodes of Cretaceous in Kuqa Depression were identified based on petrography, microthermometry of fluid inclusions, and fluorescence spectroscopy characteristics. Low-maturity oil, represented by hydrocarbon inclusions with yellow-green-yellow fluorescence, entered the reservoir at circa 5-3 Ma, whereas high-maturity hydrocarbons, as indicated by hydrocarbon inclusions with blue-green-blue fluorescence, charged the reservoir at circa 3-1.2 Ma. Pure gas inclusions with nonfluorescence represented one episode of gas charging from approximately 1.2 Ma to the present. By applying the PVTX simulation method of inclusions, the paleo-pressure evolution of formations was reconstructed, that is, the three-stage model of pressurization-release-pressurization. The coupling relationship between the paleo-pressure evolution of formations and the history of hydrocarbon charging at different stages was reconstructed. This conformed to a compound overpressure-controlled hydrocarbon accumulation model with oil convergence in the early period, oil adjustment along the fracture in the middle period, and rapid gas charging in the late period.