Petroleum accumulation systems and oil enrichment patterns of Yanchang Formation in Zhenjing area, southern Ordos Basin

Detailed analysis of the distribution of source rocks, the source-reservoir assemblages, the characteristics and the accumulation mechanisms of oil pools, etc. allow the Yanchang Formation in the Zhenjing area to be divided into near-source-contact type and near-source-leap type petroleum accumulation systems. The near-source-contact type petroleum accumulation system is characterized by lithologic traps, short migration distance, with the pressure difference between source and reservoir serving as major migration force and subtle transportation system in which hydrocarbon enrichment is mainly controlled by high-quality source rocks, favorable sand bodies as well as fractures. The oil enrichment pattern can be described as "source rock-sedimentary microfacies-fracture combined control". The near-source-leap type petroleum system is characterized by structural-lithologic traps, relatively longer migration distance, with the pressure difference between source and reservoir or buoyancy force serving as the major migration driving forces and the faults connecting source rocks serving as the main pathway, in which hydrocarbon enrichment is mainly controlled by the faults connecting source rocks, high quality sand bodies and fractures. The oil enrichment pattern can be summarized as "faults connecting source rock-sedimentary microfacies-fracture combined control".