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

Abstract

The upper fourth sub-member and lower to middle third sub-member of the Shahejie Formation (Es4U and Es3M−L) are the main source rocks in the Bohai Bay Basin. The oil generation of two immature shale samples from Es3M−L and Es4U 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 (Es3M−L and Es4U) 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 Es4U are much lower than that from Es3M−L. The two sample share the same organic matter type and predominant activation energy (52 kcal/mol), but sample from Es3M−L exhibits low average activation energy (Ea) and more proportions in low Ea fractions than that from Es4U 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 Es4U is higher than that of Es3M−L, TRs of Es4U are lower than that of Es3M−L because the maturity gap between the two layers is too narrow to compensate the kinetic disadvantage of Es4U. In deep area, Es4U exhibits much higher TRs than Es3M−L as the maturity gap is significant enough to compensate the kinetic disadvantage of Es4U. The oil generation of Es4U in deep area was from 42 Ma to 32.8 Ma, while Es3M−L is still in the progress of oil generation.