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%) 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%), 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.
Wang et al. (Tue,) studied this question.