Research On Saturation Method Of Marine-continent Transitional Shale In Block A,Sichuan Basin

China is very rich in shale gas resources.There is a relatively mature evaluation system for marine shale exploration and development in China.There are also precedents for exploration and development of continental shale.Saturation is one of the important parameters for shale gas reservoir logging evaluation.The Longtan Formation in Block A,Sichuan Basin,is a marine-continent transitional shale with low reservoir resistance due to the influence of lithology,physical properties and maturity of organic matter,therefore,the classical resistivity Archie model and its derived formula have low accuracy in calculating saturation.In order to accurately evaluate the saturation parameters of shale gas reservoirs in the study area,core data such as core diffraction,physical property analysis and organic geochemical characteristics are used.This paper analyzed the correlation between the “six characteristics” of the reservoir and the saturation,summarized the main controlling factors that cause the low resistance of the reservoir,and deduced the resistivity saturation model for the cause of the low resistance,at the same time,the non-electric saturation calculation model was studied based on the “Excavation effect”.The results show that:(1)high clay mineral content,complex pore structure,pyrite,high maturity carbonized shale and coal decrease reservoir resistivity;(2)based on the parallel conductivity theory,this paper analyzes the conductivity of carbonized shale,coal and other minerals,improves the traditional parallel conductivity model,and puts forward an improved parallel conductivity saturation model suitable for the study area;(3)based on the theory of “Excavation effect” of gas-bearing reservoir,a new overlapping parameter ΔDDEN of density-neutron curve is constructed to quantitatively characterize “Excavation effect”,on the basis of analyzing the influence of lithology,physical property and gas-bearing property on parameters,the relationship between curve separation degree and saturation is quantitatively calibrated,and a nonelectric saturation model suitable for the study area is constructed(4)using 15 models including improved conductivity saturation model,non-electrical saturation model and machine learning model to evaluate shale gas reservoir saturation parameters in the study area,the results show that the calculation error of the improved parallel saturation model and the non-electric saturation model is less,and the saturation prediction effect can be effectively improved,at the same time,the computational accuracy of machine learning model can meet the needs of reservoir evaluation when the core data is sufficient.This study provides a new method for the calculation of shale reservoir saturation in the Longtan Formation of marine-terrestrial transitional facies in Block A,Sichuan Basin.The results of practical application show that the calculation accuracy of saturation can meet the requirements of reserves calculation and reservoir evaluation,and provide more accurate and reliable logging interpretation parameters for exploration and development in the study area,it has a certain reference value for saturation logging evaluation of similar low resistivity shale gas reservoirs.