Oil pyrolysis with carbonate minerals: Implications for the thermal stability of deep crude oil

Abstract

Gold-tube experiments on oil, oil and dolomite, and oil and calcite were conducted to analyse the thermal stability of liquid hydrocarbons in carbonate reservoirs. The pyrolysates were geochemically characterized by gas chromatography and gas chromatography-isotope ratio mass spectrometer. The experimental results show that temperature is the main controlling factor; however, the influence of inorganic minerals cannot be ignored during oil cracking. Dolomite and calcite have been observed to inhibit the generation of methane to some extent, wherein the methane yields were reduced by 42.73 and 33.86 ml/g, respectively. Moreover, carbonate minerals increase the average activation energy of C1-5 generation, leading to an increase in the thermal stability of crude oil. Carbon isotope fractionation of gaseous hydrocarbons during crude oil cracking showed that carbonate minerals promoted methane carbon isotope rollover at EasyRo <1.75%. The extrapolation of kinetic parameters at a constant heating rate of 2 °C/Ma indicated that carbonate minerals will increase the liquid hydrocarbon preservation threshold temperature by 2–10 °C at the same conversion of C1-5. The calculated maximum depth corresponding to crude oil in carbonate reservoirs maintaining the liquid phase (51% oil cracking) can reach 10,000 m. Therefore, the presence of carbonate minerals needs to be considered during the geochemical evaluation indicators to analyse the potential of gas generation during crude oil cracking, the evolution of carbon isotope composition, and the prediction of hydrocarbon phase behaviour.