Seismic Response Characteristics And Reservoir Prediction Method Of Sinian Dengying Formation In Gaoshiti Area Of Central Sichuan Basin

Since the high-production gas was drilled in well GS1 in 2011,Sinian Dengying Formation gas reservoir in Gaoshiti area has experienced exploration and development for nearly 10 years.But there are still some problems,such as unclear response characteristics of high-quality reservoir,mismatch between reservoir prediction results and drilling results.Therefore,a series of studies such as reservoir seismic response characteristics,petrophysical modeling of siliceous dolomite,AVO forward modeling and effective reservoir prediction methods are used to solve these problems.Firstly,a variety of data are comprehensively used to analysis the geological characteristics and control factors of the reservoir in the study area,the paleotopography is restored,and the macro distribution law of the reservoir is discussed on the basis.Secondly,based on the best processing of seismic data,the source and typical seismic characteristics of reservoir are analyzed by forward modeling which is combined with geological characteristics.Thirdly,a new petrophysical model conforming to the characteristics of reservoir is set up,AVO forward modeling is conducted according to the model,and the sensitive parameters and AVO characteristics of siliceous dolomite and reservoir are analyzed.Fourthly,by the analysis of seismic attribute characteristics,pre stack and post stack inversion and other means,the de-siliceous physique statistical inversion method and porosity prediction method based on depth learning are set up to quantitatively predict the reservoir,then,the spatial distribution characteristics of the reservoir are obtained.Finally,combined with reservoir distribution characteristics and influencing factors of seismic response,the causes of reservoir are summarized.The above research shows that:Some reservoir characteristics can be seen intuitively in post stack seismic,which is in the fourth member of Sinian Dengying Formation（Z₂dn₄）,Gaoshiti area.The"identifiable"seismic response can only appear when typical type of reservoir or cave reservoir are developed,and the difference of"identifiable"seismic response mainly comes from the vertical distribution and thickness change of the reservoir.Because the background lithology will affect the seismic response,only using the bright spot in the middle of Z₂dn₄ will fall into the problem of ambiguity.It is necessary to add the"variable"to truly characterize the seismic response of the reservoir.The seismic response of the reservoir at the top of Z₂dn₄ is mainly the weakening or pull-down of the peak,the reservoir in Z₂dn₄² can be identified by the characteristics of bright spots in the middle and wave peaks at the bottom,the main identification basis of reservoir in Z₂dn₄¹ is single bead reflection and pull-down of bottom peak.Based on the actual seismic response characteristics,five reservoir seismic genetic identification models of Z₂dn₄ in Gaoshiti area are set up.According to the characteristics of dolomite reservoir,a new petrophysical model is constructed by comprehensively using solid replacement equation and other methods.The new model can calculate the volume fraction of pores and the equivalent aspect ratio parameters of siliceous minerals under the constraint of P-wave velocity.Compared with the traditional model,the new model describes the structural characteristics of minerals.The shear wave prediction results of the new model in well GS1 are basically consistent with the measured velocity,and various statistical results also show that the results of the new model are better than Xu-Payne model and the complex carbonate model proposed by predecessors.Through this method,the relationship between reservoir,siliceous layer and various seismic inversion attributes can be accurately analyzed by petrophysical analysis,fluid substitution and AVO model analysis.After analyzing the reservoir petrophysical parameters of well logging,P-wave velocity（Vp）and lame constants（λ）is proved to be sensitive to the reservoir.However,more detailed analysis shows that the parameters are still affected by the siliceous layer to a certain extent.Combined with the petrophysical analysis of siliceous layer,it is recommended that the S-wave velocity（Vs）and shear modulus（μ）can be used to distinguish siliceous layer in petrophysical point.Then,the reservoir indicator can be calculated by the relationship between Vp andλ,which can semi-quantitatively characterize the reservoir.AVO forward modeling based on petrophysical model shows that the AVO response types of reservoirs in the study area are mainly class I and class IV.When the advantage（porosity and thickness）of the reservoir increase,the AVO characteristics of peaks or throughs near the top of the reservoir always decrease the P value or increase the G value.When the disadvantage（silicon content）increase,it is characterized by the decrease of G value and class III type.Different caprock conditions directly affect the AVO response type of reservoir in the study area.When the caprock velocity is less than the reservoir,it is class I,and when it is higher,it is class IV.However,the analysis of actual seismic shows that the P and G values calculated from the original seismic amplitude are difficult to classify the reservoir type,and AVO attribute can only qualitatively characterize the reservoir.Through the“de siliceous”geostatistical post stack inversion,the impedance of siliceous dolomite is suppressed,and the accuracy of reservoir identification is improved.It means that low impedance can reflect the reservoir,rather than the common response of reservoir and siliceous layer.The inversion results show that the impedance of Z₂dn₄is significantly lower,and the reservoir characteristics are clearer.The impedance interface in the middle of Z₂dn₄ is also clearer,which is more consistent with well interpretation and more conducive to the analysis of reservoir distribution characteristics.The porosity inversion method based on depth learning is used,porosity is used as the parameter to characterize the reservoir,which combine the characteristics of seismic amplitude,pre stack and post stack inversion results,petrophysical analysis results and other data that can characterize the reservoir.Thus,the porosity volume which can accurately and quantitatively characterize the spatial distribution characteristics of reservoir is obtained.The plane distribution of reservoir thickness extracted from the volume can be in good agreement with well and production data.Compared with the traditional qualitative method（favorable area division）,it can more intuitively show the3D distribution of the reservoir and better guide the reservoir development.Through the research of geological and seismic characteristics of the reservoir,the reservoir genesis of Z₂dn₄ in Gaoshiti area mainly includes structural conditions,facies and karstification.Excellent structural conditions（anticline and boundary fault on the west）not only ensure sufficient gas sources,but also supply good preservation conditions.The algal dolomite widely developed in the hilly beach facies at platform margin is conducive to the preservation of karst fractures and caves after dissolution,which means a good material basis for the formation of high-quality reservoirs.The multi-stage weathering and karstification of Z₂dn₄ are the key factors for the formation of high-quality reservoirs.