Sedimentary Characteristics And Logging Evaluation Method For Measuring The TOC In Shale Of The Wufeng-longmaxi Formation,Zhaotong Region Of Yunnan Province

The shale of Wufeng-Longmaxi formations in Sichuan Basin is the important formations for shale gas exploration in south of China.However,the research of sedimentary characteristics of shale still be weak,lack of practical classification schemes,and lack of appropriate logging evaluation method for measuring the TOC in shale of south of China.In this paper,selecting the shale of Wufeng-Longmaxi formations in south of Sichuan Basin as the research object,an intensive study of shale is conducted,on the basis of core observation,thin section identification,scanning electron microscopic analyses and a variety of laboratory datas.Based on the control action of the changes in sea-level,research found that the shale of Wufeng-Longmaxi formations were controlled by the four change of eustatic cycle.Wufeng Formation was divided into 1 piece of third classic sequences.And Longmaxi formation was divided into 3 pieces of third class sequences.On the basis of core observation,thin section identification,scanning electron microscopic analyses and a variety of laboratory datas,the sequence boundary and tracts were recognized.Finally,sequence stratigraphy frame was established.On the basis of core observation,thin section identification,scanning electron microscopic analyses and a variety of laboratory datas,the petrologic features and mineral chemistry of shale were analyzed.Connecting with the predecessors research results and referring to the above analysis,the shale of Wufeng-Longmaxi Formation in the study area is divided into five types,which are silty shale,calcareous shale,laminated shale,massive shale and argillaceous siltstone.Then,sedimentary characteristics and genesis of shale were discussed.According to the analysis of single phase and section phase,the plane phases were established.According to the analysis of sequence stratigraphy and sedimentary characteristics of shale,the logging evaluation method for measuring the TOC in shale of the Wufeng-Longmaxi formation was finished.Though the analysis and comparison of the traditional calculating method of the TOC(the method of △log R and improved method of △log R),this study pointed out their deficiency and then make corrections as necessary.Finally,a perfect logging evaluation method for measuring the TOC in shale was found.It was the method of “LSF”: L represents log,S represents sedimentation,and F represents fit.The correlation coefficient between the calculated TOC and the tested TOC of “LSF” method almost reached 0.8-0.9.The calculation results show that the good correlation between lithologies and sedimentary structures zones and TOC of different types of shale,the shale reservoirs can be graded according to 4 types of shale.The new method is more efficient,faster,and has higher vertical resolution than △log R method and improved △log R method.It is especially effective under conditions of insufficient data at the early stage of shale gas exploration of the Longmaxi Formation,Silurian,south of Sichuan Basin.