Diagenesis and fluid inclusion records of the oil preservation window in a deep carbonate reservoir

Determining the maximum temperature or burial depth where liquid hydrocarbon is preserved (i.e., oil preservation window) is a critical scientific problem. However, large discrepancy remains in the documented oil preservation window (e.g., 150–175 °C versus >240 °C), significantly influencing our understanding of the distribution of oil in deep earth. In this study, complete diagenesis and fluid inclusion records of oil charging and cracking were discovered in a Permian carbonate reservoir from the eastern Sichuan Basin (South China). Reconstructions of the diagenesis and petroleum evolution history reveal dynamic temperature-pressure controls on oil stability. Specifically, oil can be cracked to form methane-dominated gas after heating at ≤192 °C (σ = 4) and in a hydrostatic pressure regime for ∼20 m.y. Extreme overpressure formed postdating oil cracking due to intense calcite cementation, which resulted in isolated system and favored generation and accumulation of abnormally high pressure (up to 199 MPa) hydrocarbon fluids. The new oil preservation window can be applied to normal pressure or weak overpressure reservoirs, predicting the distribution of liquid hydrocarbon in deep earth.