Sedimentary System And Favorable Reservoir Forecast Of Upper Eocene In Xihu Sag,the East China Sea Shelf Basin

In the oil&gas exploration and development history of China's Top 10 largest basins,the East China Sea shelf basin production remains no breakthrough,which is unmatchable to its large scale of 26.7×10~4km~2 basin area.Xihu Depression lies in the middle of the East China Sea shelf basin with the largest area,highest exploration rate and most oil and gas production within the basin.During recent years,due to the unclear distribution of the sedimentary system and uncertainty about the distribution of reservoir properties,drilling and exploration into the targeted Upper Eocene formation of Xihu Sag failed to satisfy the expected results,which is difficult to make a major breakthrough.Therefore,study into sedimentary system distribution and favorable reservoir distribution area,in order to make a breakthrough of current oil and gas exploration.On the basis of core,logging and seismic data analysis,with the reference to previous studies,this paper established the depositional model of Upper Eocene(P1+P2),identified the upper Eocene strata grid,also clarified the sedimentary evolution characteristics of each sequence and favorable reservoir distribution.This paper has made major achievements as follows:1)Based on the Upper Eocene paleogeomorphology recovery and heavy mineral analysis,Pinghu group of Xihu Depression was mainly influenced by two sources,where the western sea reefs is the main source,followed by the eastern part of the Diaoyu Islands uplift zone.Source types are mainly volcanic.2)Fluvial–deltaic sedimentary mode is established for West lake depression based on detailed description of nearly 150m Upper Eocene core and previous research on marine–continental transition facies.3)High resolution of layer structures of West lake Upper Eocene depression is established using sequence stratigraphy theory after determining layer signatures from core,well logging and seismic sequences.4)Depending on the situation of a lack of drilling wells and 3D seismic covered,sedimentary distribution of West lake Upper Eocene depression can be shown by combining well logging and seismic data,and special distribution of sand ratio.5)Several locations show‘sweet spot'characters according to Upper Eocene formation properties of low porosity,low permeability and homogeneous.Then using optimized evaluation parameters,grey correlation theory and seismic property simulation,as well as multi-parameter balanced quantitative evaluation method,potential distribution of each layers of Pinghu Formation is estimated.