Crude oil cracking under geological conditions: A case study of the Ediacaran reservoir, central Sichuan Basin, China

Deep strata are extremely rich in hydrocarbon resources, and trap oil has undergone a long-term and multistage geological evolution that results in thermal cracking. However, compared with controlled experimental conditions, studies on the thermal cracking process of trapped oil under subsurface geological conditions are relatively scarce. Therefore, the major aim of this study is to reconstruct the oil reservoir cracking process based on an evolutionary study of a typical Ediacaran gas reservoir in China. By evaluating the detailed reservoir petrology, natural gas composition, isotopes, residual solid bitumen (SB) characteristics, fluid inclusion analysis, and in situ U-Pb dating of dolomite, this study combined an oil cracking kinetic model with actual geological elements and evolution to recover the four stages of trap oil cracking. Mutual verification of the forward model and inversion demonstrated that a suitable oil cracking kinetic model can be extrapolated to geological conditions. With an increase in the thermal evolution of the reservoir, the most unstable component in the oil first cracks, and a small amount of gaseous products are preferentially dissolved in the liquid oil. The major components of petroleum undergo thermal cracking and conversion to produce short -chain liquid hydrocarbons and wet gases. A large amount of wet gas generates an abnormally high fluid pressure. Wet gas usually escapes from potential channels (i.e., caprock microfractures and unconformity surfaces), thus dynamically maintaining the energy balance of the trap system and weakening the impact of high pressure on oil cracking. The wet gas products precipitate asphaltene in the oil, thus adjusting the composition and properties of the trap oil. The escape of the initial wet gas products, accompanied by a thermochemical sulfate reduction (TSR) reaction with residual oil, C2H6, and other wet gases as reactants, led to the formation of CH4-rich natural gas with a high drying coefficient in the trap of the highly over -mature evolution stage. Precipitated asphalt and the asphalt directly produced by oil cracking, coke rapidly under high temperatures and pressures to form SB with abnormally high reflectivity. Under geological conditions, oil cracking behavior is generally controlled by a combination of temperature history, tectonic history, fluid pressure background, trap preservation conditions, secondary alteration (e.g., TSR), and oil type, among which the reservoir temperature history remains the most important. Other factors should not be underestimated in a specific region. This study revealed a pathway model for the thermal cracking of trapped oil under real geological conditions, providing references for similar studies and other pathway models in different regions worldwide.