Estimation of palaeo-slope and sediment volume of a lacustrine rift basin: A semi-quantitative study on the southern steep slope of the Shijiutuo Uplift, Bohai Offshore Basin, China

Abstract

The sequence architecture and depositional systems of the Palaeogene lacustrine rift succession in the southern steep slope of the Shijiutuo Uplift (SU) in Bohai Offshore Basin (BOB) were investigated using combined 3-D seismic, well log and core data. Four second-order/composite sequences and seven third-order sequences were identified. A detailed analysis revealed fan delta, braid delta and lacustrine depositional systems in the third-order sequences 7 (SQ7). Eleven seismic facies were chosen in the SQ7 to calculate the area and sediment budget. We also calculated the palaeo-slope parameters in the southern steep slope of the SU, including the gradient of basin margin fault slope belt, the shape, average width, average height, width/height ratio and cross sectional area of palaeo-valley. The gradient of the fault slope break belt was inversely proportional to the area and volume of sediments. The higher gradient corresponded to the smaller area and volume of sediments. The width and cross sectional area of palaeo-valleys dominated the volume and facies of sediments. A larger cross sectional area of a palaeo-valley involved much more sediments supply and more favorable conditions for the origin of large-scale deltas. Because of favorable sediment transportation path and existence of large-scale braid delta, the eastern part of the SU formed better quality reservoir than that in the west.