The effects of calcite and montmorillonite on oil cracking in confined pyrolysis experiments

Three sets of pyrolysis experiments were performed for oil alone, oil plus montmorillonite and oil plus calcite at two heating rates of 2 °C/h and 20 °C/h in confined systems (gold capsules). The main observations can be listed as follows: (1) the ratios of i-C4/n-C4, i-C5/n-C5 and the amount of butanes (n-butane + i-butane) are significantly higher in the experiment for oil plus montmorillonite than oil alone and oil plus calcite, indicating the acidic catalysis by montmorillonite; (2) at low conversion values (<0.5 for methane generation), the formation rates of methane and total hydrocarbon gases in all the three experiments are very similar, demonstrating that neither montmorillonite nor calcite significantly influence the primary cracking of oil components (C6+) into gaseous hydrocarbons (C1–C5), while at high conversion values (>0.5 for methane generation), the formation rates of methane and the total hydrocarbon gases in the oil plus calcite experiment are relatively lower than the other two experiments, demonstrating that calcite hindered the secondary cracking of wet gases (C2–C5) into methane; (3) both montmorillonite and calcite greatly reduce the carbon isotope fractionation during methane formation from oil cracking, resulting in substantially higher methane δ13C values in the oil plus montmorillonite or calcite experiments than for oil alone. Based on the kinetic parameters determined from the oil cracking experiments, the predicted temperatures and vitrinite reflectance values (% Easy Ro) for the formation of methane and the total gaseous hydrocarbons at 10% conversion are 190–192 °C and 184–187 °C, and 1.90–1.93% and 1.80–1.86%, respectively at the heating rate 1 °C/my, demonstrating that oils are very thermally stable in sedimentary basins.