Quantitative Seismic Interpretation And Inversion Of Rock Physical Parameters In Shale Gas Reservoir

Shale gas,as an important unconventional natural gas resource with widespread distribution and large reserves,has become a significant focus for global oil and gas exploration.Efficient exploration and development of shale gas require identifying favorable reservoirs with high organic matter enrichment,a high brittleness index,and moderately developed fractures.Among these factors,organic matter enrichment serves as the material basis for shale gas formation.The brittleness index of rocks is a crucial parameter that influences the effectiveness of hydraulic fracturing.Additionally,the extent of fracture development influences the formation of fracture networks after hydraulic fracturing in shale gas reservoirs and determines the pore permeability of the reservoirs.Currently,geophysical methods for predicting fractures in shale gas reservoirs are relatively mature.However,establishing effective seismic methods to predict organic matter enrichment in reservoirs remains a significant challenge when evaluating shale gas reservoirs.This is primarily due to the incomplete understanding of the elastic and inelastic seismic response characteristics of shale associated with organic matter enrichment.Furthermore,the valuable kinetic information contained in seismic signals has not been fully explored and utilized.Meanwhile,further investigation is needed to determine the validity of various methods for defining the brittleness index.Therefore,it is necessary to conduct shale gas reservoir rock physical modeling research,develop quantitative seismic interpretation and inversion methods based on clarifying the seismic rock physical response mechanism of shale gas reservoirs,and predict crucial reservoir parameters such as organic matter enrichment and brittleness index of shale gas reservoirs.This will enhance the accuracy of predicting favorable shale gas reservoirs.In this paper,we develop a rock physics model and analyze it for shale gas reservoirs in the study area.We construct effective reservoir parameters to characterize organic matter enrichment and brittleness index.Additionally,we develop quantitative seismic interpretation methods to predict the spatial distribution of favorable shale gas reservoirs based on the developed rock physics model.Furthermore,the organic matter enrichment factor is constructed based on a rock physics model.The organic matter-matrix decoupling AVO（Amplitude Variation with Offset）equation for shale gas reservoirs is introduced.Correspondingly,methods for elastic impedance inversion and seismic wave dispersion attribute inversion are developed.These methods are used for the high-precision prediction of organic matter enrichment in shale gas reservoirs,providing a novel and effective approach to quantitatively characterize shale gas reservoirs using seismic data.The main research content and results are as follows:（1）A rock physical model is established to describe the decoupling relationship between reservoir microphysical characteristics and the elastic and inelastic properties of shale gas reservoirs.Addressing the complex microstructure of shale gas reservoirs,the rock physical model quantitively characterizes the seismic rock physical response mechanism of minerals,kerogen,pores,fluids and other unique storage states and physical characteristics.The rock physical modeling is applied to conduct sensitive parameters analysis to clarify the influence of physical parameters,such as organic matter enrichment and brittleness index,on the elastic and inelastic properties of shale.This provides a theoretical basis for the development of subsequent quantitative seismic interpretation and inversion methods in this paper.（2）Based on the geological and geophysical data of the study area and the rock physical modeling analysis in this paper,we establish effective reservoir parameters KP（Kerogen and Porosity）and QC（Content ratio of Quartz to Clay）to characterize the organic matter enrichment and brittleness index of the reservoir.We develop quantitative seismic interpretation methods based on rock physical templates and apply seismic elasticity inversion results to predict the spatial distribution of organic matter enrichment and brittleness index in the shale gas reservoir simultaneously.This approach improves the reliability of reservoir parameter prediction,and provides an effective method for predicting favorable shale gas reservoirs.Meanwhile,the effective reservoir parameter（KP）that describes organic matter enrichment addresses the regional limitations of conventional methods for Total Organic Carbon（TOC）prediction based on elastic parameters（elastic impedance and density）using empirical relationships.On the other hand,the effective reservoir parameter（QC）that describes the brittleness index provides a more physically meaningful prediction of brittleness index.The reservoir classification study conducted on this basis presents an effective method for predicting favorable shale gas reservoirs.The prediction results provide essential information for the comprehensive investigation of the geological theory related to shale gas formation and storage.（3）Aiming at the unique characteristics of organic matter and fluid storage in shale gas reservoirs,we develop an organic matter-matrix decoupling seismic rock physics model and corresponding AVO equation for shale gas reservoirs.Additionally,the novel AVO equation expands the AVO equation on fluid factors as applied to conventional reservoirs.the factor M_c proposed by the seismic rock physical model can quantify the seismic response characteristics associated with organic matter enrichment.This factor can be used as an effective parameter to characterize shale gas content.Meanwhile,the AVO equation of organic matter enrichment factor M_c provides an effective method to predict organic matter enrichment directly from pre-stack seismic data.The accuracy comparison analysis demonstrates that the novel AVO equation exhibits high accuracy when compared to the exact solution of the reflection coefficient.This study validates the theoretical feasibility of predicting the organic matter enrichment factor（M_c）through pre-stack seismic inversion.Moreover,the proposed AVO equation can be widely applied to the quantitatively characterize unconventional and special reservoirs,such as shale gas,shale oil,tight oil,and natural gas hydrate,where hydrocarbons are characterized by solid-fluid mixing.（4）An elastic impedance inversion method based on the organic matter-matrix decoupling AVO equation is developed to directly predict the organic matter enrichment factor（M_c）using pre-stack seismic data.This method offers an effective approach for predicting gas content in shale reservoirs.The theoretical model test shows that the factor M_c has higher sensitivity to organic matter enrichment compared to elastic parameters such as P-wave impedance and density.Meanwhile,the application of real data demonstrates that the spatial distribution of the predicted organic matter enrichment factor M_c agree with the production status of horizontal wells in shale gas reservoirs in the study area.This agreement can provide a reliable foundation for predicting gas content in shale reserviors.（5）On the basis of considering the inelastic characteristics of shale due to the factors of organic matter and fluid storage,this paper extends the organic matter-matrix decoupling AVO equation to the frequency domain.It establishes a seismic dispersion attribute inversion method that fully utilizes seismic wave dynamics information,such as pre-stack time-frequency attributes,to calculate the dispersion attribute D_Mcassociated with the organic matter enrichment factor.This method provides new solutions for predicting organic matter enrichment and gas content in shale gas reservoirs.The theoretical model test reveals that the dispersion attribute D_Mc exhibits greater sensitivity to organic matter enrichment in shale compared to elastic parameters such as P-wave impedance and density.Additionally,D_Mc demonstrates higher sensitivity and prediction accuracy compared to the conventional P-wave velocity dispersion attribute.The application of the real data reveals that the predictive results of the dispersion attribute D_Mc agree with the production status of horizontal wells,offering a novel and effective parameter for estimating shale gas content.Further comparative analysis of the organic matter enrichment elasticity factor（M_c）and the dispersion attribute(D_Mc)can offer theoretical basis for a more profound understanding of the seismic rock physical response characteristics of shale reservoir gas content.In summary,this paper elucidates the seismic rock physical response characteristics of shale gas reservoirs through rock physical modeling and analysis.It establishes a quantitative seismic interpretation method for shale gas reservoirs and constructs the organic matter-matrix decoupling AVO equation.Additionally,it develops the corresponding elastic impedance inversion method and seismic dispersion attribute inversion method.It provides a theoretical basis and novel effective method for predicting important reservoir parameters in shale gas reservoirs,such as organic matter enrichment and brittleness index.