Logging Evaluation Of Hua Gang Formation Low Permeability Reservoir In Xi Hu Sag,East China Sea Basin

The east China sea basin is one of the research focus of oil and gas research in recent years.There are gas reservoirs in this area.The logging evaluation of reservoirs is an important method in looking for“sweet spot”in tight sand reservoirs.The N gas field is located in the central inversion belt of the Xi Hu depression,and the main layer is the large set of sandstone reservoirs Hua Gang formation,which is typical low porosity and tight sandstone reservoirs.Research area is at sea.Well point is relatively sparse,and its research level is low.It has experienced a complicated diagenesis and formed the complex reservoir characteristics and pore structure types.It has brought great challenges to the exquisite evaluation of logging reservoir and it has influenced the further development of gas reservoir.According to the above problem,this paper is from the aspect of the deposit,geology.On the basis of analyzing the“four property"relationships of reservoir combined with core analysis data,lithology was identified.With the analyzing of NMR analysis and mercury injection data,we analyzed the relationship between physical properties and the micro and macro parameters and then we classified pore structure with the parameters which has a closest relationship with permeability.And based on the classification of pore structure the porosity-permeability model was established which has improved the interpretation precision.This paper studied the relationships between reservoir"four property",and analyzed the reservoir physical property,lithology and electric property and hydrocarbon content.The relationship between the physical property is evidently affected by lithology.Reservoir gas saturation and its correlation with physical properties is obvious,and the better the physical property is,much more reduced the water saturation is.Based on the study on the relationship between the"four property",Bayes'discriminant for lithology identification is conducted.The optimization of curves results into the use of three porosity curve,GR,deep resistivity curve for the identification which are all related to the lithology in the research area.Research area has 5 typical lithology?powder sand,medium sand to fine sandstone,fine sandstone,sandstone and coarse sandstone,mud gravel sandstone?,and each one is related to a discriminant function.On the intersection graph of discriminant function,the differences between different lithology is obvious,and the identification accuracy is more than 90%.Based on geological data such as sedimentation,diagenesis,mercury injection curves and the analysis of the nuclear magnetic,the analysis of the relationship between macro and micro pore structure parameters and reservoir physical property resulted into the main parameters affecting the reservoir permeability which is the average radius of pore throat.Data configuration method is used to divide the pore structure into four categories.The permeability boundary value varies in physical property,mercury injection parameters of various pore structure types.According to the nuclear magnetic log data,the pore structure types of the whole well section is established through the relationship between the logarithm mean of T2 and the pore throat radius.According to the classification of pore structure,different types of pore structure physical model are established,and compared with traditional porosity-permeability model,found that the elevation is obvious with the interpretation accuracy which has effectively solved the permeability model calculation accuracy in tight sand reservoir.The methods of qualitative identification of fluid properties of low permeability reservoirs are mainly the resistivity chart method,the three-porosity overlapping method and the calculation of elastic parameters by using the compressional and shear velocity ratio.Based on the characteristics of the gas layer in the research area,the identification accuracy of the fluid property is high by the elastic parameters.On the basis of identifying gas,water,dry layers,two methods are used to calculate gas saturation.One is using the method of elastic parameters which analyzed the elastic parameters of the rocks filled with liquid,and deduced the equation to calculate the gas saturation of gas.At the same time,the gas saturation is calculated by using the J function method of Leverett,and the calculation results are also consistent with the gas test.Finally,the low limit of effective reservoir study has been carried on.It is concluded that for conventional low permeability reservoir,the low limit of permeability is 0.25×10^-3?m²,low limit of porosity is 7%,and the lower limit of resistivity is a function of porosity,so it can be calculated according to the corresponding values of porosity.For different fluid properties,the corresponding saturation limit value was established,and the lower limit of the effective reservoir was determined by comprehensive analysis.