Pore Structure Characteristics Of Organic-rich Shale In Fuling Area,the Southeastern Sichuan Basin

The Jurassic is one of significant oil and gas exploration strata in Sichuan Basin.Sinopec and CNPC,has obtained a satisfying result of petroleum exploration in this formation.Along with the broad vision and increased investment,shale gas exploration also made a breakthrough,especially in Yuanba and Fuling blocks.Recently,the shale gas development of Daanzhai was a success in Fuling district,and the first domestic demonstration area of shale gas productivity construction was established by Sinopec,showing a good exploration prospect in this suit of strata.In this paper,On the basis of previous achievements,drilling data and exploration result,the study on pore structure characteristics of organic-rich shale of Daanzhai in Fuling area has been carried out,to discuss pore structure characteristics from micro to macro view point.And related geological factors were analyzed,to lay the foundation for future shale gas reservoir evaluation.Based on the sedimentology,petroleum geology,structural geology theory,and summarizing the previous researches,Daanzhai strata in Fuling district is mainly shallow lacustrine-semi lacustrine deposits,located in the center of lake basin.The upper and lower part of Daanzhai is shallow lacustrine deposits,its lithology is mainly brown,gray shelly limestone,the middle part of this formation is mainly organic-rich shale,are high quality soure rocks and good continuity.Faults are developed,and the structure of this study area is a northeast-southwest direction syncline,along the southeastern edge of this area is steep.The well logging identification has been made according to shale electrical characteristics,and the effective thickness of the study area is 30⁵0m based on the divided single shale strata,which is buried moderate,the depth in most area less than2750m.The organic carbon content which is low based on the well logging interpretation results of geochemical parameters,mainly distributed between1.0%¹.8%.While the organic matter maturity is high,mainly distributed in more than 1.4%,the facies of northeastern work area is semi lacustrine deposits,which is relatively high organic carbon content,controlled partly by sedimentary facies on the plane.The shale pore types are divided into organic pore,inorganic pores?including intergranular pore,intragranular pore,mineral crystal pore?,micro-fractures?including grain-edged fracture,schistosity fracture?three categories upon the observation of environment scanning electron microscope?ESEM?.Inorganic pores are the main pore type,and often constitute a combination with micro-fractures,the organic pores are less developed.Nanoporous and microns are distributed in various types of pores.Pore structure characteristics of Daanzhai shale samples has been studied by Using both nitrogen gas adsorption and high pressure mercury intrusion method.The average shale pore specific surface area is 3.549 m²·g^-1,and average total pore volume is 0.010 mL·g^-1 from measured experimental adsorption isotherm data.Pore diameters are mainly distributed in the range of 2¹0nm with large peak width,and poorly sorted.Mesoporous?2⁵0nm?provide the main shale gas reservoir space,and the specific surface area is mainly provided by the pores whose diameter is less than10nm.Pore volume and pore specific surface area does not necessarily linked,when the pore diameter is consistent,pore volume and pore specific surface area showed a positive correlation.Pore volume and pore specific surface area decrease exponentially with the increasing pore diameter?10nm increments statistics?.According to the high-pressure mercury intrusion result,pore throat radius mainly distributed around 4nm and 10?m,its contribution to the permeability almost is zero when less than 4?m.The pore is divided by size into micropores?<2nm?,mesopores?2⁵0nm?,macropores?50nm⁵?m?and supermacropores?>5?m?,based on internal or external pore classification standards,and combination of pore size distribution characteristics.According to the analysis of factors related to pore development,the,main factors affecting the pores are thermal evolution of organic matter,mineral composition and compaction.The organic pores are controlled by thermal evolution of organic matter,the increasing content of clay minerals help to improve the specific surface area of micropores,while the compaction will reduces the porosity.