Recovery of the Erosion Thickness and Characterization of the Paleogeomorphology in the Southern Lishui Sag, East China Sea Shelf Basin

The restoration of the denudation thickness is the basis for quantitatively analyzing the basin burial history, thermal evolution history and hydrocarbon accumulation conditions. The well data is not sufficient to do this in the southern Lishui Sag with only 5 wells, so the new, high-quality three-dimensional (3D) seismic data is essential. The methods employed to determine the eroded thicknesses of key unconformities in the study area are the mudstone acoustic time difference (AC) method, vitrinite reflectance (Ro%) method and stratigraphic trend comparison method. On the basis of the structural evolution, the restoration of the paleogeomorphology is carried out. The results show that the Wenzhou formation (E(2)w) in the entire study area has been all denuded. The denudation thickness is in the range of 0-550 m, and decreases gradually from west, thicker than 200 m, to east. The erosion of the upper Lingfeng formation (E(1)1) are mainly occurred in the N-P-1 structural trap (N-P-1), the N-P-2 structural trap (N-P-2) and the N-P-3 structural trap (N-P-3). Among them, the eroded area in N-P-1 (near well S-1) and N-P-2 is 200-230km(2), and the denudation thickness is 0-600m, which increases from south to north, even up to 200-400m in the region around well S-1. In N-P-3, the extent of denudation is 30-50 km(2), and the denudation thickness is 0-400m. In the early stage of the upper E(1)1, one north-south strike slip fault began to develop in the southern study area. Affected by this fault, the southern Lingfeng bulge was offset, forming a gorge, and the eastern and western sides of the concave belt connected to each other. On the basis of the paleo-geomorphological characteristics, it is shown that the sedimentary center of the west subsag began to migrate to the south.