Relationship Between Silica's Characteristics And Reservoirs Of The Organic-Rich Shale In Wufeng Formation-Longmaxi Formation In Southern Sichuan Basin

In order to clear up the silica's characteristics and the relationship between silica's characteristics and reservoirs in shale,the shale in wufeng-longmaxi formation in Southern Sichuan Basin was selected as the research object.On the basis of field observations,through thin section identification,scanning electron microscope,energy spectrometer analysis,X-ray diffraction for mineral component,geochemical measurements and physical properties analysis,methane isothermal adsorption experiment for the shale samples,it is concluded that the following:The main lithology of the shale of the wufeng-longmaxi formation in southern sichuan is black and gray-black shale,which is rich in biota such as graptolite,radiolarian,sponge spicules and so on.Quartz,clay and carbonate are the primary minerals for the studied shale samples.The kerogen type of the studied shale is type I.The organic carbon content of the samples is high.Organic matters are in the over-matured stage.The shale in the study area is a low-porosity and low-permeability reservoir.The pores mainly include intergranular pores,intra-granular pores,organic matter pores and micro-fractures.The organic matter pores are mainly cavernous organic matter pores.The four are related to each other and together constitute the site of shale gas storage and migration channel.There are four types of quartz in the studied shale.Terrigenous clastic quartz was deposited by transport,with the particle size less than 40 ?m,and the morphology was mostly subround-subangular,with cracks in some quartz particles.There are two kinds of quartz related biological bones.One is the original siliceous organisms,such as radiolarian,sponge spicule,etc.The other is the biological skeleton metasomatism by quartz,and the residual structure can be seen.Under the scanning electron microscope,such quartz is platy and relatively loose and porous in the biological remains.The siliceous sources of authigenic quartz during diagenesis are mainly biogenesis and transformation of clay minerals.Biogenic quartz is mainly deposited irregularly in small particles,most of which are associated with asphalt.Quartz formed by clay mineral transformation is deposited in the form of quartz aggregate.In the fracture,quartz grows along the inner wall of both sides of the fracture and is filled with calcite or clay mineral.The content of shale debris silicon in the wufeng-longmaxi formation in the changning area of south sichuan was 7.7% ? 34.55%,with the mean value of the lower section being 9.55% and the upper section being 30.59%.The content of clay silicon ranged from 4.62% to 20.73%,the mean value of the lower section was 5.73%,and the upper section was 18.35%.The content of biosilicon ranged from 0.24% to 61.23%,with an average value of 47.52% in the lower segment and 4.89% in the upper segment.The content of detrital silicon in the mabian area in the south of sichuan was between 6.5% and 34.97%,with an average value of 21.82% in the rich organic shale and 33.06% in the poor organic shale.The clay silicon content was 3.87%?27.10%,the average value of the organic matter rich shale was 11.57%,and the average value of the organic matter poor shale was 23.33%.The content of biosilicon ranged from 0.96% to 70.98%,which was only enriched in the organic shale.The biosilicon content of the shale in the study area gradually decreases from bottom to top,while the content of clastic silicon and clay silicon gradually increases.The siliceous content of the organic-rich shale section(mainly refers to the bottom of the wufeng formation and longmaxi formation)is mainly of biogenesis,while the siliceous content of the organic-poor shale section is mainly of clastic origin.Biosilicon in the shale of wufeng-longmaxi formation can reflect the degree of organic matter enrichment.The micropores related to silica mainly include clastic quartz grain margin joint,clastic quartz fissure,clastic quartz dissolution pore,authigenic quartz intergranular pore,authigenic quartz internal pore,authigenic quartz stuck grain joint(shrinkage organic matter pore),siliceous biological cavity hole and siliceous filled microfracture.The mathematical correlation between total quartz content and porosity is not obvious,but biosilicon has a good correlation with the surface porosity of organic pores.Biosilicon is also conducive to the development of microfractures.Quartz itself has very low adsorption capacity,but biosilicon can indicate the gas-bearing property of shale.In addition,the pores associated with quartz can serve as storage space for free shale gas.