The Microstructure And Pore Development Characteristics Of Graptite In The Shale Gas Reservoir Of Wufeng Formation And Longmaxi Formation In Southern Sichuan

The upper Ordovician Wufeng Formation and The Lower Silurion Longmaxi Formation of the Sichuan Basin has deposited a set of black organic-rich shale rich in graptolite.In this paper,the graptolite in the shale of the Wufeng-Longmaxi Formation in southern Sichuan are taken as the research object.Through the basic data investigation,field profile observation and sampling,and experimental testing,the distribution characteristics and microstructure characteristics of the graptolite in the Wufeng-Longmaxi Formation are investigated.Pore structure characteristics,and significance for shale gas reservoirs,etc.The specific understanding is as follows:The deep-water continental shelf facies of the lower section of Wufeng Formation-Longmaxi Formation are mainly black mud shale,carbonaceous mudstone,siliceous mudstone,calcareous silty mudstone,and are high TOC（>2%）shale sections;the upper section is The sedimentary facies of the shallow water shelf,the lithology is a combination of silty shale,argillaceous siltstone and fine sandstone.The organic matter types of shale are mainly type I and some type II₁.The TOC value of organic matter abundance is mainly concentrated between 0.5%and 7%,with an average value of 2.32%.The organic matter maturity of the lower Longxi Formation is between 1.55%and 5.10%.the average value is 2.68%,which is in an over-mature stage.The mineral composition of shale is quartz and clay minerals.The average quartz content is 58%.The clay minerals are mainly composed of illite and ymen layer.The average content is 14%,and the carbonate content is 2%to 32%.The average content is 12%,less pyrite and feldspar content.In the Wufeng Formation-Longmaxi Formation,4 Graptolite belts and 1 shell facies belt were identified.The preservation methods of penstone mainly include carbonized film flattening preservation,three-dimensional semi-stereomineralization preservation,and pyrite mineralization preservation.Through observation,the penstone is generally incomplete,and the remaining part still retains the residual biological structure.The skeleton of the penstone is composed of two parts,the outer shell and the inner skeleton.The outer shell of the penstone body is often explained by calcite,and some of the stone sees the residual calcite explained by quartz.The endoskeleton is mainly filled with clay,and the organic matter in the cell is developed.The composition of the pencil stone carbon film is mainly C element,which can reach90%to 98%.The outer shell of the pencil stone can be divided into three layers,namely calcite（or pyrite,quartz,etc.）-the second and third layers are clay.Observing the microscopic pores of the penstone,there are inorganic pores,organic pores and micro cracks.Inorganic pores mainly include pyrite dissolution pores,quartz impression pores,penstone shell pores（intergranular pores,clay mineral layer gaps）,organic pores are seen in the organic matter in the cell,the pore size is 4nm～40nm,directional arrangement,many shapes It is round or oval,and the microcracks are mainly developed in the organic matter of penstone and the edge of the outer shell,and the width is 0.2μm～2μm.There is less abundance of graptolite in the strata from the bottom to the top.The abundance of graptolite is positively correlated with the organic carbon content measured in the experiment,which reflects the significant contribution of the remaining organic matter of graptolite.The abundance of penstone is also proportional to the porosity of the rock,reflecting the development of organic pores related to penstone,and becoming an important storage space for shale gas reservoirs.