Oil generation, retention and expulsion processes of the Lucaogou shale in the Junggar Basin: Constraints from improved swelling experiment and basin modelling

The solubility parameters of generated oil are determined by the relative ratios of different oil fractions and vary significantly at different maturation stages. However, previous swelling experiments based on single solvent could only represent the retention behavior for one of oil fractions, which has influence on the accurate evaluation of oil-retention capacity on residual kerogen. In this study, the kerogen of the Lower Permian Lucaogou (P₁l) shale from the Junggar Basin was used to conduct the closed-system pyrolysis experiments and the kinetic parameters were obtained for the evaluation of oil-generation process. The swelling experiments were also performed to evaluate oil-retention amount on residual kerogen. Unlike previous swelling experiments, we chose the mixture of ethyl acetate and iso-octane as swelling solvents, and adjusted the relative ratios to bring the solubility parameter of each mixture close to the generated oil for each pyrolysis temperature as possible. The kinetic model oil generation-retention-expulsion (GRE model) was established based on the results of pyrolysis and swelling experiments. This kinetic model showed that the generated oil of the Lucaogou shale started to expel from the shale when the thermal maturity reached 0.92 %. Furthermore, the GRE model was applied to recover oil generation, retention and expulsion processes of the shale by PetroMod software. The results showed that the generated oil started to expel from the shale at the Late Jurassic (approximately 150 Ma). The current masses of generated, retained and expelled oil of the shale are 12.3, 6.3 and 6.0 × 10⁶ tons/km², respectively.