Unraveling the Two-Stage Phase Evolution Mechanism of Deep Lacustrine Type I Kerogen: Integrated Insights from Pyrolysis Kinetics, Isotopic Fingerprinting, and PVT Modeling

Deep and ultradeep petroleum systems exhibit highly complex fluid phase behaviors due to extreme temperature and pressure conditions, particularly in lacustrine source rocks dominated by Type I kerogen, where the coupling between hydrocarbon generation mechanisms and phase evolution is not fully clarified. As a case study exemplified by the Fukang Sag in the Junggar Basin, this research employed low-maturity Type I kerogen and integrated gold-tube pyrolysis, hydrocarbon generation kinetics, carbon isotopic fingerprinting, and PVT phase behavior modeling to unravel the "two-stage" phase evolution mechanism in deep settings. Kinetic analysis revealed a bimodal distribution of activation energies for methane generation: a minor, low-energy peak (∼50-58 kcal/mol) corresponding to primary kerogen cracking and a dominant high-energy peak (61 kcal/mol) aligning with secondary oil cracking. Consequently, we propose an early stage (EasyRo < 1.1%) dominated by oil generation from primary kerogen cracking, and a late stage (EasyRo > 1.1%) governed by gas generation via the secondary cracking of earlier-formed oil. PVT simulations further elucidate the "dual-control" effect of nonhydrocarbon CO₂. During the oil-dominated stage (EasyRo < 1.66%), CO₂ acts as a light component, increasing the saturation pressure and facilitating early oil migration. In the gas-dominated stage (EasyRo > 1.66%), it behaves as an important component relative to methane, significantly elevating the dew-point pressure and broadening the thermodynamic stability window of the condensate gas phase from EasyRo = 2.05-2.95% in a pure hydrocarbon system to 1.66-3.46% in a CO₂-bearing system. Integrated with geological modeling, our results indicate that deep source rocks in the Fukang Sag have generally entered the high to overmature stage. Therefore, the exploration strategy should pivot from traditional "oil-seeking" to "gas-targeting", focusing on structural belts adjacent to hydrocarbon kitchens and connected by deep faults to discover large-scale condensate and wet gas accumulations derived from secondary oil cracking. The potential impact of CO₂ on retrograde condensation risks during development must also be carefully considered.