Description Of Thin Interbedded Reservoir And Oil Identification Through AVO Analysis

The sedimentary types of the Neogene in the eastern coastal area of China are braided river and meandering river.The meandering river can be divided into two types: "shale pack sand" and "sand-shale interbed",forming tectonic lithologic reservoirs.At present,the reservoir prediction technology is still immature for the "sand-shale interbed" meandering river reservoir which is the main reservoir type of the coastal area.The variation of lithology and thickness of strata are relatively large and the thickness of these strata is far thinner than the vertical resolution of conventional seismic exploration.How to determine the spatial distribution and the properties of interbedded reservoirs has become an urgent problem to be solved.In this paper,the Chengdao Oilfield in Dongying is taken as an example,and the reservoir prediction and fluid identification are carried out.Chengdao Oilfield is the first large oil field in extremely shallow sea area in China.The upper Guantao Formation is fluvial face and it is enrichment in oil and gas.The formation is divided into five sets of sand groups,the thrid and the forth sand groups are the main oil reservoir,but the tuning effect of thin interbedded sand and shale layers will blur its real AVO characteristics,lead to the wrong result of AVO analysis.At the same time,when the reservoir contains different fluids,the AVO characteristics will change too.All of these factors will lead to the qualitative prediction of reservoirs become inaccurate.Based on the geological sedimentary facies and reservoir geologic model,different thin-interbedded models were established.The forward modelling analysis were carries out to research the influence of the existence of reservoirs on AVO records.At last,an AVO attribute that coupled amplitude and phase information was put forward to improve the prediction accuracy of thin interbedded reservoirs.Geophysical inversion technology is an important way to predict reservoir and AVO inversion is the most widely used technology.It is found that the shale content in the study area has little effect on the p-velocity and s-velocity but has a great influence on the density,which provides a good foundation for the identification of reservoir by the density parameter.The angle of incidence of prestack seismic data in the work area is large,which is helpful to the stability and reliability of density inversion.Based on the premise that the actual characteristics of the work area are consistent with the assumptions,the density information is extracted by AVO inversion based on Fatti approximation.The main hydrocarbon in this research area is heavy oil and the differences of physical properties between sand and shale are small,so the heavy oil can not be accurately indicated by p-velocity,s-velocity and density through conventional AVO inversion.Based on the logging curve and petrophysical analysis,a sensitive hydrocarbon indicator was constructed and we used the extended elastic impedance inversion technique to predict this indicator.This method has a good effect in this work area.