3D Palaeotopography Recovery Of Pinghu Structural Belt In Xihu Depression

Paleotopography is the basis of depositional development. The restruction of paleotopography not only restores the original structural form, but also reveals the developmental characteristics and space relationship between provenance and depositional system. This research is based on the data of seismic-log, using seismic-logging-geology comprehensive interpretation technique to study the tectonic evolution of Pinghu structural belt. Besides, backstripping technology for restoring basin palaeotopography is used, aiming to providing support for the resconstruction of Xihu Depression deep sedimentary environment. Previous studies have shown that Xihu Depression mainly experienced Yuquan, Huagang and Longjing movements. These three tectonic inversion movements respectively represent depression, reversion and settlement period which lead to forming T30, T20, T10 regional unconformities.In this paper, the target area is north Pinghu structural belt which has 3d seismic data. Building strata structure framework model and analyzing four unconformity surface from Pinghu T32-T30 to Huagang T21-T20, through the analysis of isochronous surface, residual strata thickness and fault system spread. Using stratigraphic correlation method, acoustic method and wave equation method to calculate erosion thickness of these four unconformity surface and draw pictures for erosion thickness distribution. Based on lithological data, this paper recoveries the original thickness of residual stratigraphic thickness, calculates throw distances based on seismic profiles, calculates settlements of four unconformity utilizing the principle of balanced cross section. Using ancient residues stratigraphic thickness, strata thickness erosion and sedimentation volume data as inputs, this paper obtains a three-dimensional topographic map of Pinghu ancient terrain.The results show that T30, T21, T20 unconformities which were formed in the three te ctonic inversion movements of Pinghu structural zone distribute over the whole studie d area. The denudation of T30 reaches a maximum of 500m, while the maximum den udation of T32, T21, T20 is about 300m. As a whole,the denuded amount of slopes dis tributes along structural trend, showing that the center of basin is thick and the edge a nd near the middle of the basin are thin.The highest denudation is Baoyunting anticlin al belt,and this result is consistent with the geological understanding;compared with other regions of Xihu depression, ersion thickness of Pinghu tectonic is smaller, and unconformity is slight angular mostly, reverse movement has less impact on the slope, so Pinghu structural belt is a better exploration area, T30 has the worst hydrocarbon trap accumulation condition in these 4 unconformities.3D terrain palaeotopography recovery results matches the trends of residual strata, showes that the range of each secondary depression became smaller from rifting period to depression period. In late Eocene, the northern and southern lowlands who are separated by Baoyunting fault accepted a large sediments resulting terrain slowed. Laiheting-Kongqueting-Ningbo14 high terrain which inherited from Eocene to Oligocene, resulting in a high erosion thickness. When getting to the end of the Pliocene, the entire north Pinghu were fully filled with sediments, the whole terrain is gentle, the high and the low-lying terrain are no longer highlighted.