Study On The Heterogeneity Of The Microfabric Of The Longmaxi Formation Shale In Southern Sichuan

With the continuous growth demand and decreasing production of oil and gas,tight sandstone and shale gas,as two important unconventional natural gas resources,have successively become the main sources for production growth of natural gas.A series of new theories and technologies formed during the exploration and development of tight-sandstone gas reservoir have been provided and applied to the establishment of research methods and indicator system for shale-gas reservoir.Compared with other unconventional reservoirs,shale-gas reservoir is characterized by its basic geological features,including unitary lithology,few typical subdivision indexes for sub-micro facies,abundant clay minerals and organic matter.There also exist engineering issues,for example,complex drilling conditions,high collapse pressure,instability of borehole wall and differentiation of gas-well production capability.The micro-fabric features,such as the change of relative content between brittle minerals and clay minerals in shale,control rock mechanics properties,borehole-wall instability mechanisms,and reservoir reformation effects.Based on shale gas reservoirs,pore structural characteristics,organic geochemical indicators and petrophysical properties are mostly used in the evaluation of shale gas and dessert optimization,while researches on the micro-fabric heterogeneity of shale which can influence reservoir quality and productivity were relatively lacking.Based on drilling data from Z101 well in Fushun-Yongchuan block,W206 well in Neijiang-Dazu block and N210 in Luxian-Changning block,this paper focused on the heterogeneity of micro fabrics in shale-gas reservoir from the Lower Silurian Longmaxi Formation in South Sichuan Basin.By means of identification of thin sections,X-ray diffraction,scanning electron microscopy and other experimental methods,the heterogeneity of mineral composition,pore structure and composition of organic matters in shale were studied.Combined with relevant data and experimental methods,property parameters and heterogeneity of the shale reservoir were quantitatively characterized.Through mutual inversion between these key parameters and micro-fabric features,micro-fabric heterogeneity characteristics of shale reservoir were determined,and qualitative or quantitative attribute parameters that can reflect micro-fabric features of shale were established to evaluate the heterogeneity of shale 's micro fabrics.The achievements and understandings are listed as follows:(1)The content of brittle minerals in Longmaxi shale Formation in South Sichuan Basin is relatively high,ranging from 42.0%-90.0%.At the microscopic scale,the heterogeneity of mineral components in shale is generally reflected by the differences in size,sortability,roundness and spatial arrangement of particles.The content of quartz and clay minerals in bedding structures is different,the higher content of quartz,the more development of bedding structures.(2)Intergranular pores,intraparticle pores and organic pores are observed in Longmaxi shale,and the heterogeneity of pore types is associated with mineral components.Intergranular pores are common between detrital and clay minerals,intraparticle pores are often found in detrital minerals,while organic pores are only present in organic matters.The intraparticle pores in shale reservoir are dominated by intercrystalline pores of clay minerals,with pore size distribution ranging from 20 to 50 nm.Organic pores show double-peak distribution characteristic,with pore size mostly distributing between 10 and 50 nm or over 100 nm.With the increase of organic matter content,the distribution of pore size is more uniform,and the heterogeneity of pore structure is enhanced.(3)The surface to volume ratio of Longmaxi shale averages from 8.0-25.0m2/g.The surface to volume ratio is positively correlated with the content of TOC and quartz,while shows negative correlation with clay minerals.The correlation between quartz and surface to volume ratio is different when quartz have different origins.The correlation between brittle,clay minerals and porosity is poor.(4)The kerogen type in Longmaxi shale is mainly sapropel type(type ?)or mixed type(type?),and organic carbon content is 1.0%-7.0%.The distribution of mineral grains at parallel-bedding direction is disordered,and organic matters are mostly randomly distributed in the form of sheet.While the grains at vertical-bedding direction show good directionality,flaky clay and organic matters distribute in the form of strip,usually parallel to the bedding surface.(5)By means of X-ray diffraction,the directionality(R)of Longmaxi shale was evaluated to good orientation,with R-value 47.0%-81.0%.The anisotropy exists in the wave-velocity ratio of parallel lamina at different angles,and the degree of anisotropy gradually decreases with the increase of the ratio in vertical-bedding direction.The wave velocity is positively correlated with the content of quartz,and negatively correlated with the content of clay minerals.Young's modulus at vertical-plan direction is positively correlated with the content of quartz while negatively correlated with the content of clay minerals.There is no obvious correlation between Young's modulus,Poisson's ratio and porosity,TOC in both horizontal and vertical directions.