Oil generation of lacustrine Type II shale: A case study of the Shahejie Formation, Laizhou Bay Sag, Bohai Bay Basin (China)

The upper fourth sub-member and lower to middle third sub-member of the Shahejie Formation (Es-4(U) and Es-3(M-L)) are the main source rocks in the Bohai Bay Basin. The oil generation of two immature shale samples from Es-3(M-L) and Es-4(U) of the Laizhou Bay Sag were simulated by gold-tube pyrolysis experiment. Total organic carbon, Rock-Eval pyrolysis, maceral analysis and flash pyrolysis-GC-MS were employed to determine the bulk geochemistry and chemical composition. The primary goal of this study was to probe the kinetics and evolution of oil (C14+) generation from the main source rock layers (Es-3(M-L) and Es-4(U)) in the Laizhou Bay Sag. The results show that the two shale samples are Type II1 kerogen, mainly composed by lacustrine microalgaes. While the proportion of long-chain aliphatics (C20+) in sample from Es-4(U) are much lower than that from Es-3(M-L). The two sample share the same organic matter type and predominant activation energy (52 kcal/mol), but sample from Es-3(M-L) exhibits low average activation energy (Ea) and more proportions in low Ea fractions than that from Es-4(U) due to the differences in long-chain aliphatics. The kinetic differences lead to the different variations of transformation ratio (TR) in different areas of the Laizhou Bay Sag. In slope area, TRs of the source rocks are in low values. Although the maturity of Es-4(U) is higher than that of Es-3(M-L), TRs of Es-4(U) are lower than that of Es-3(M-L) because the maturity gap between the two layers is too narrow to compensate the kinetic disadvantage of Es-4(U). In deep area, Es-4(U) exhibits much higher TRs than Es-3(M-L) as the maturity gap is significant enough to compensate the kinetic disadvantage of Es-4(U). The oil generation of Es-4(U) in deep area was from 42 Ma to 32.8 Ma, while Es-3(M-L) is still in the progress of oil generation.