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Thermal Cracking of Oil Under Water Pressure Up to 900 Bar at High Thermal Maturities: Implication for Oil Stability in Deep Basin


The influence of temperature on the thermal stability of crude oil in deep basin has received much attention. However, the effect of pressure remains speculative. This study investigates the effect of water pressure up to 900 bar on gas yields and their stable carbon isotopic compositions during thermal cracking of crude oil. Using a 25 mL Hastelloy pressure vessel, a C9+ fraction of saturate-rich Tertiary source rock-derived oil from the South China Sea basin, was pyrolyzed under normal and supercritical water at a range of temperature from 350 to 425 °C for 24 h. In this study, pressure generally retards oil cracking, as evidenced by reduced gas yields, but the trends depend upon the level of thermal evolution. In the early stages of cracking (350 and 373 °C, R0< ∼1.1%), the suppression effect increases with pressure from 200 to 900 bar, but it is most marked between 200 and 470 bar. At the later stages in the wet gas window (390, 405, and 425 °C, R0 >1.3%), pressure still has a strong suppression effect from 200 to 470 bar, which then levels off or is reversed as the pressure is increased further to 750 and 900 bar. Interestingly, the stable carbon isotopic composition of the generated methane becomes enriched in 13C as the pressure increases from 200 to 900 bar. A maximum fractionation effect of ∼3‰ is observed over this pressure range. Due to pressure retardation, the isotopically heaviest methane signature does not coincide with the maximum gas yield, contrary to what might be expected. In contrast, pressure has little effect on wet gases carbon isotope ratios, which show a maximum variation of ∼1‰. The results suggest that the rates of methane-forming reactions affected by pressure control methane carbon isotope fractionation. Based on distinctive carbon isotope patterns of methane and wet gases from pressurized oil cracking, a conceptual model using “natural gas plot” is constructed to identify pressure effect on in situ oil cracking providing other factors excluded. The transition in going from dry conditions to normal and supercritical water does not have a significant effect on oil-cracking reactions as evidenced by gold bag hydrous and anhydrous pyrolysis results at the same temperatures as used in the Hastelloy pressure vessel. The results have important implications for evaluating the thermal stability of oil in deep petroleum reservoirs. The pressure effect on oil cracking should be included in the kinetic models.