Reservoir-forming pattern with multiple sources in second member of Shahejie Formation, Bonan Sag, Jiyang Depression, Bohai Bay Basin

The second member of Shahejie Formation (Es₂) in the Bonan Sag of Jiyang Depression has multiple hydrocarbon sources, both from top and bottom. Drilling, logging, seismic and testing data were applied in the analyses of Es₂ in view of the unclear understanding of multi-source hydrocarbon accumulation models and the low overall study level of Es₂. It was concluded that the hydrocarbon in the Es₂ was sourced from three sets of source rocks in the upper section of the fourth member (Es₄), the third member (Es₃) and the first member (Es₁) of Shahejie Formation. The fans and carbonate rocks at the bottom and the mudstones on the top of Es₂ served as favorable reservoirs and cap rocks, respectively. Faults and unconformities formed a 3D transport system, which had a significant influence on hydrocarbon migration and accumulation. The physical properties of reservoirs restricted hydrocarbon enrichment. Two reservoir-forming patterns, ‘accumulation during the middle stage with two sources’ and ‘accumulation during the late stage with three sources’, were established according to the difference of accumulation stage and source. Lithologic, structural and structural-lithologic traps were formed with single or multiple sources under these two patterns. The major accumulation stages have a trend of being late gradually from bottom to up and from the sag center to the edge.