Hydrocarbon source rock evaluation of the Lower Cretaceous system in the Baibei Depression, Erlian Basin

The K(1)s, K(1)d, K(1)t, and K(1)a Formations are potential source rock intervals for hydrocarbon formation, all of which are part of the Lower Cretaceous system in the Baibei Depression in the Erlian Basin in China. However, no well has found oil flow because the hydrocarbon-generating potential of the source rocks has not been comprehensively evaluated. Based on organic geochemical and petrological analyses, all the source rocks possess highly variable total organic carbon and S-1 + S-2 contents. Total organic carbon and S-1 + S-2 contents indicate that the K(1)a(2) Formation through the K(1)d(1) Formation are source rocks that have fair to good generative potential and the K(1)d(2) Formation through the K(1)s Formation are source rocks that have good to very good generative potential. The organic matter in the K(1)a(2) Formation is dominated by Type I and II kerogen; thus, it is considered to be oil prone based on H/C versus O/C plots. Most of the analyzed samples were deposited in reducing environments and sourced from marine algae; thus, they are oil prone. However, only two source rock intervals were thermally mature with vitrinite reflectance values in the required range. Hydrocarbon-generating histories show that the K(1)t and K(1)a(2) intervals began to generate hydrocarbons during the depositional period of the K(1)d(2) and K(1)d(3) Formations, respectively, and stopped generating hydrocarbons at the end of the depositional period of the late Cretaceous. Therefore, the main stage of hydrocarbon migration and accumulation was between the depositional period of the K(1)d(2) and K(1)s Formations, and the critical moment was the depositional period of the late K(1)s Formation. The generation conversion efficiency reached approximately 55% in the K(1)a(2) Formation and 18% in the K(1)t Formation at the end of the Cretaceous sedimentary stage. In general, the effective oil traps are those reservoirs that are near the active source rock in the generating sags in the Baibei Depression.