Quantitative reconstruction of phase states and evolution of condensate and gas in the western Kelasu Thrust Belt, Kuqa Depression, Tarim Basin.

Various oil-gas reservoirs have been discovered in the Lower Cretaceous Beshijiqike Formation in the western Kelasu Thrust Belt, Kuqa Depression, Tarim Basin. However, uncertain evolutionary characteristics, physical properties, and formation process for diverse phase states hinder further petroleum exploration and development. In this study, we reconstructed models of phase state evolution of six wells integrating PVT simulation and basin modeling. The results indicated that the temperature and pressure of reservoirs increased slowly before the Neogene, and then rapidly reached present value. The density and viscosity of the liquid phase decreased gradually, whereas that of the gas phase to the contrary. The reservoir fluids of all wells except for well DB-302 experienced phase differentiation, changing from gas-liquid coexistence phase (GLCP) to liquid or condensate phase. Three evolutional pathways for the reservoir fluids represent three distinct scenarios: (1) Transition from GLCP to liquid phase, resulting from the reservoir formation mechanism by mainly early oil charge with no or minor late gas charge. (2) Transition from GLCP to condensate phase, resulting from the reservoir formation mechanism by early oil charge and subsequent gas charge. (3) Continuous maintenance in the gas phase, resulting from the reservoir formation mechanism by a large amount of late gas charge with no or very small amount of early oil charge. We proposed a pattern of two early stages of oil-filling followed by a later gas-charging stage, and suggested faults and covariations of trap formation timing with main hydrocarbon generation period are the main factors controlling phase states of the study area. Our results are significant for predicting fluid phase state of the study area.