Sedimentary characteristics and seismic geomorphology of the upper third member of Eocene Dongying Formation in double slope systems of Laoyemiao transverse anticline, Nanpu Sag, Bohai Bay Basin, China

The upper third member of Eocene Dongying Formation (Ed(3)(u)) is considered to be one of the most important hydrocarbon-rich intervals in Nanpu Sag, Bohai Bay Basin, China. Through integrated analysis of the drilling cores, wire-line logs, and high-quality three-dimensional (3-D) seismic data, the sedimentary characteristics of Ed(3)(u) were analyzed by utilizing the seismic geomorphology approach in the double slope systems of Laoyemiao transverse anticline, Nanpu Sag. The Ed(3)(u), a third-order sequence, can be subdivided into three fourth-order sequences (from base to top as SS1, SS2, and SS3). The fan delta front and sublacustrine fan have been identified from well-based analysis of the sedimentary facies and core examinations reveal that the sublacustrine fan deposits were dominated by six types of resedimented lithofacies, namely, slides, slumps, sandy debris flows, muddy debris flows, turbidity currents, and hyperpycnal flows. Stratal slices were obtained from 3-D single-frequency seismic volumes to map the sediment dispersal characteristics and document the evolution of the depositional systems constrained by the double slope break zones of Laoyemiao transverse anticline. The unique structures of the double slope break zones and the variation in the lake level associated with the system tracts of the third-order sequence jointly control the dispersal patterns of the sand bodies and the spatial-temporal evolution of the depositional systems. Based on the above analysis, the depositional evolution model of Ed(3)(u) was established. The proposed facies-controlled model and sediment transport model can be used to accurately predict the favorable reservoir sandstones.