Hydrocarbon phase evolution in deep oil and gas reservoirs under the coupling of temperature and pressure: a case study from the Fuman area, Tarim Basin, China

The ultra-deep Ordovician strata in the Fuman area hosts large-scale oil reservoirs. However, the evolution history of hydrocarbon phase under coupled temperature–pressure conditions remains unclear, which limits the understanding of the liquid-phase oil preservation mechanisms. By integrating fluid inclusion petrography, microthermometry, laser confocal microscopy, and PVTsim numerical simulation, the temperature–pressure evolution history of Ordovician reservoir was reconstructed, and the characteristics of hydrocarbon phase evolution were revealed. The results indicate that from the Caledonian to the present, reservoir temperature increased with burial depth. Reservoir pressure rose during hydrocarbon charging and later stabilized at hydrostatic levels, with pressure intensity shifting from weak (early stage) to strong (late stage) during hydrocarbon accumulation. By combining temperature–pressure data with phase envelope curves, it is found that the study area was generally charged with liquid-phase oils in the Caledonian and Hercynian. In the Himalayan, liquid-phase oils were predominantly charged, except in the Manshen and Yuke well blocks, where natural gas charging occurred. Early oil reservoirs remained in the liquid phase throughout the temperature–pressure evolution history and underwent minor thermal cracking during the Himalayan period, leading to increased oil maturity. Low-temperature and high-pressure conditions represent the key preservation mechanism for Ordovician liquid-phase oils in the Fuman area.