Research On The Characteristics Of Organic-rich Shale Reservoirs In The Longmaxi Formation In The Southern Sichuan Basin

The heterogeneity of shale reservoirs is becoming more and more obvious for the development of deep buried shale gas exploration in the south Sichuan Basin.This paper takes the organic-rich shale of the Longmaxi formation in south Sichuan Basin as the research object,through core observation,thin sections,SEM analysis,nuclear magnetic resonance(NMR)analysis,combining with total organic carbon(TOC)test,kerogen microscopy,vitrinite reflectance test,porosity test,X-diffraction fluorescence test and gas content test,respectively,the organic geochemistry,the characteristics of mineral composition,shale reserviors space,physical property and gas content are studied,and then the control factors of shale reservoirs are analyzed.The results are as follows:(1)According to lithology,electricity and the paleontology,the organic-rich matter shale section at the bottom of the Formation is divided into 4 small layers:1,2,3,4 small layer.Vertically,the 1 and 3 layer are in relatively high GR values,2 and 4 small layer are in relatively low GR values.Laterally,the small layers are gradually thickened away from the paleo-uplift area.(2)According to the experiment of kerogen microscopy,it is concluded that the main organic types of the shale are?-?-type kerogen.Because the depth of each block is different,the reflectivity of the equivalent vitrinite is difference,the depth of the Longmaxi shale is relatively shallow in the area of Changning and Weiyuan,the reflectivity of the equivalent Ro is in the 1.2%?2.0%,the organic matter is in the high mature stage.In the west of Chongqing,the buried depth is deep,the equivalent vitrinite is generally more than 2%and organic matter is in the mature stage,reflecting the different degrees of transformation of diagenesis.(3)Under the influence of sedimentary environment,the TOC,mineral composition and porosity of each small layer are different.Vertically,1 and 3 small layer are mainly anaerobic environment,the reduction is stronger,the organic matter is advantageous to preserve,TOC is relatively high.2 and 4 small layer mainly are the poor oxygen environment,the reduction is relatively weak,and the organic matter preservation condition is relatively poor,resulting the TOC relatively low.Regionally,the shale is formed under the regional sedimentary background of the "three-lung clamp and one concave",and the TOC is higher in the west of Chongqing,but the degree of thermal evolution affects the porosity of the organic matter.In the mineral composition,the shale mineral composition has certain regularity in the vertical direction:the clay mineral content increases gradually from the bottom to the top,the quartz content gradually reduces from the bottom to the top,the quartz ingredient is mainly the biological source,which has a certain contribution to the organic matter.The quartz and the TOC show a certain positive correlation.Regionally,the content of clay and carbonate decreased gradually away from the paleo uplift area,while the content of silica was gradually increased.(4)on the porosity characteristics,the vertical change trend of porosity is similar to TOC,which shows that the pore of the shale has a great contribution to porosity.The porosity is higher in the area of Changning and Weiyuan,which is influenced by diagenesis.(5)in the characteristics of reservoir space,there are many types of fracture,mainly structural and interlayer joints,most of which are fully filled by calcite.Micro-cracks are mainly clay-mineral gap,rigid particles edge seam,asphalt contraction seam.Affected by deposition and diagenesis,1 and 3 small layers are dominated by organic matter,while 2 and 4 small layers are inorganic pores.Regionally,the pore development degree in changning and Weiyuan areas is higher than that in West Chongqing.(6)in the gas-bearing characteristics,vertically,the total-gas content gradually decreased upward.Regionally,total gas content gradually increased away from the paleo-uplift area,which is related to the material source.In a comprehensive view,the shale reservoir is in a relatively stable zone,the pressure coefficient is 1.2?2.2,the whole is in a high-pressure state,and the preservation of shale gas plays a protective role,the development of shale reservoir is mainly affected by sedimentation and diagenesis.The variation of the sedimentary environment in each small layer leads to the vertical difference of the shale reservoirs.The different depth of the buried depth and the different degree of diagenesis,result in the difference of shale reservoir in the plane.