Geophysical Prediction Of Deep Source Rocks Based On Organic Geochemical Analysis

The Bozhong sag is the largest hydrocarbon-rich sag in the Bohai Sea,and it is also the subsidence center of the Bohai Bay Basin during the Cenozoic period.The source rocks of the Dongying Formation and the Shahejie Formation are deeply buried,most wells are located at structural highs,and there are fewer wells drilled into high-quality source rocks,and it has strong heterogeneity.It is difficult to make a fine evaluation of the source rocks in the study area by only using geochemical analysis test data.Therefore,based on organic geochemical analysis,combined with geophysical logging data and seismic data,this paper analyzes the deep continental source rocks in the southwestern Bozhong Sag.Comprehensive analysis provides a certain basis for the study of deep source rocks in the study area.Four sets of source rocks(E₃d₂^L,E₃d₃,E₃s₁₊₂and E₂s₃)were evaluated using core observation,thin section identification,rock pyrolysis,organic carbon content,chloroform pitch"A",group components,vitrinite reflectance,kerogen microscopic components,elemental analysis,saturated hydrocarbon gas chromatography and chromatography-mass spectrometry,etc.The abundance of organic matter,type of organic matter,maturity of organic matter and the source of parent material and sedimentary environment of the source rock.The results show that the source rock types of the E₃s₁₊₂and E₂s₃Formation are the best,and the organic matter abundance is the highest,which is of high quality.For source rocks,the quality of the source rocks of the E₃d₃is the second,and the quality of the source rocks of the E₃d₂^Lis relatively poor.The source rock of E₃d₂^LFormation has a strong territorial intrusion,which belongs to a freshwater-brake water,weak reduction-weak oxidation sedimentary environment;the source rock of the E₃d₃Formation has a strong territorial intrusion,which belongs to a freshwater-salt water,weakly reducing sedimentary environment;Territorial intrusion of the source rocks in the E₃s₁₊₂Formation are weaker than the Dongying Formation,and belong to a brackish-salt water,weakly reducing sedimentary environment;The source rocks of the E₂s₃Formation are relatively weak in territorial intrusion,belonging to a freshwater-brackish water,weakly reducing sedimentary environment.Using geophysical logging data,based on the classification of sedimentary facies,the longitudinal heterogeneity of the source rocks in the study area was evaluated.TheΔlog R series method,sensitive log multiple regression method and BP neural network method were used to calculate the total organic carbon content（TOC）and the cracked hydrocarbon content（S₂）.The single well prediction is performed,and various methods are compared and optimized,and the organic facies analysis and prediction of source rocks are carried out in combination with organic geochemical data.Using pre-stack 3D seismic data,based on the pre-stack simultaneous inversion method,combined with seismic attributes,3D seismic prediction of the source rock total organic carbon content（TOC）and cracked hydrocarbon content（S₂）are carried out.The results show that the single-well prediction results of the BP neural network method are more accurate;the organic facies of the source rocks of the E₃d₂^LFormation are mainly BC face,C face and CD face of Jones’organic facies classification.The organic facies of the source rock in the E₃d₃,the E₃s₁₊₂and E₂s₃Formation are mainly B face,BC face and C face;the prediction results of the total organic carbon content TOC and the content of cracked hydrocarbons S₂based on seismic multi-attribute analysis correspond well to the single well prediction results,and the plane prediction results It is in good agreement with the sedimentary facies map of the study area.The overall values of TOC and S₂in the E₃s₁₊₂and E₂s₃Formation are relatively high,which are basically consistent with the results of geochemical analysis,which proves the feasibility of the method.