Pore Structure And Fractal Characteristics Of Wufeng And Lower Member Of Longmaxi Shales In Western Hubei And Eastern Chongqing Regions

The marine organic shales of the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation is widely distributed and rich in shale gas resources in western Hubei and eastern Chongqing regions,which is an important shale gas exploration strata in south China.In this paper,the shale of the Wufeng Formation-lower member of Longmaxi Formation in western Hubei and eastern Chongqing regions is the main research object.Based on the analysis of the tectonic and sedimentary characteristics of the shale strata,the petrology,geochemical characteristics,pore structure,fractal characteristics,wettability and connectivity of different wettability pores were studied by multi-scale testing techniques,observation methods and statistical analysis methods such as gas adsorption,high pressure mercury intrusion?MICP?,low frequency nuclear magnetic resonance?NMR?,field emission scanning electron microscopy?FE-SEM?,etc.Then,the differences in pore structure of different lithofacies from different layers were analyzed,and the pore development mechanism,controlling factors and their effects on shale gas-bearing properties were studied.It is expected to provide geological basis for the evaluation of shale reservoir capacity of Wufeng and lower memerber of Longmaxi Formation in western Hubei and eastern Chongqing regions.The main conclusions of this paper can be summarized as follows:1.According to the XRD diffraction analysis of the Wufeng and lower member of Longmaxi shales,reference to the previous lithofacies division method,Wufeng Formation?layer?1??in the study area is dominated by argillaceous/carbonate mixed siliceous shale lithofacies?S-2?and argillaceous-rich siliceous shale lithofacies?S-3?,the lower part of the lower member of Longmaxi Formation(layers?3?⁵)is dominated by argillaceous-rich siliceous shale lithofacies?S-3?,the upper part of the lower member of Longmaxi Formation(layers?6?⁹)gradually transition from argillaceous-rich siliceous shale lithofacies?S-3?to argillaceous/siliceous mixed shale lithofacies?M-2?and siliceous-rich argillaceous shale lithofacies?CM-1?.2.In view of the complex pore structure characteristics of the Wufeng shale and the lower member of Longmaxi shale,through the low temperature and low pressure CO₂and N₂ adsorption,MICP,NMR,FE-SEM and other complementary experiments and observation methods as well as statistical analysis methods,the pore types and pore structure of shale were characterized,and the difference of pore structure in Wufeng shale,the lower part of the lower member of Longmaxi shale and the upper part of the lower member of Longmaxi shale were clarified.The pore types of the Wufeng and lower member of Longmaxi shales are inorganic pores,organic pores and micro fractures.The FE-SEM observation and statistical analysis of the circularity values show that the organic pores of the Wufeng Formation are mostly polygonal or irregular,while those in Longmaxi Formation are mostly elliptical,nearly circular or bubble-like.Morphology of nitrogen adsorption hysteresis loops,pore size distribution of MICP&NMR and organic pores statistics from FE-SEM images show that the pore diameter of Wufeng Formation is the smallest,that of lower part of the lower member of Longmaxi Formation is the largest,and pore diameter in the upper part of the lower member of Longmaxi Formation is between the two cases above.The statistics of about 136,000 organic pores and organic matter particles in 310 FE-SEM images show that the Wufeng Formation?layer?1??has the largest organic matter surface pore percentage,followed by the lower part of the lower member of Longmaxi Formation(layers?3?⁵),and the upper part of the lower member of Longmaxi Formation(layers?6?⁹)has the smallest organic matter surface pore percentage.Indicating that the organic pores of the Wufeng Formation?layer?1??are relatively most developed,and the organic pores of the upper part of the lower member of Longmaxi Formation(layers?6?⁹)are relatively least developed.Wufeng shale has the largest micropore specific surface area,micropore volume,BET specific surface area,mesopore volume,porosity,BJH total pore volume and total volume of MICP,followed by the lower part of the lower member of Longmaxi Formation and the upper part of the lower member of Longmaxi Formation.3.The fractal dimension can represent the complexity and heterogeneity of porous materials.The value of fractal dimension?D?is between 2 and 3.The higher the fractal dimension value is,the more complex the pore structure will be.Conversely,the pore structure corresponding to the low fractal dimension value tends to be simple.The fractal FHH model is based on gas adsorption isotherm curve.Combined with Kelvin equation,it is concluded that the fractal dimension of pore sizes larger than 50 nm(D_>50)is the highest,followed by the fractal dimension of pore sizes between 4 and 50 nm(D_4-50),and the fractal dimension of pore sizes less than 4 nm(D_<4)is the lowest.The main reason for this phenomenon is that the morphology of small pore is regular and the fractal dimension is low.The edges of large pores are often complicated,and there may be multiple small pore throats connected to form a complex network structure,which increases the fractal dimension value of those pores.The comparison between each layers show that the Wufeng Formation?layer?1??has the highest fractal dimension value,the lower part of the lower member of Longmaxi Formation(layers?3?⁵)is the second,and the upper part of the lower member of Longmaxi Formation(layers?6?⁹)is the lowest.The box-counting method is based on the statistical results of organic pores from FE-SEM images.The fractal dimension of the organic pores in different pore sizes range is obtained by screening the pore size range in FE-SEM images.The results of fractal dimension obtained from box-counting based on FE-SEM images are similar to the results obtained from FHH model based on gas adsorption:the fractal dimension of whole organic pores with full range of sizes(D_wop)is the highest,and followed by the organic macropores(D_op>50)and the organic pores with pore sizes between 4 nm and 50 nm(D_op4-50),and organic pores with pore sizes less than 4 nm(D_op<4)is the lowest.The comparison between each layers also show the same fractal characteristics as calculated by FHH model,it indicates that the organic pores in the shale are very complex and the surface is very rough,the pore structure of the Wufeng Formation?layer?1??is the most complex,followed by the lower part of the lower member of Longmaxi Formation(layers?3?⁵),and the pore structure of the upper part of the lower member of Longmaxi Formation(layers?6?⁹)is relatively simple.4.The factors affecting the pore structure are summarized in three aspects:?1?diagenetic compaction controls the preservation of inorganic intergranular pores in shale,and dissolution controls the formation of secondary pores;the development of organic pores is mainly affected by the properties of organic matter?asphalt or oil?and thermal maturity.?2?According to the correlation between shale pores and TOC contents,micropores and even pores with sizes less than 10 nm are mainly provided by organic pores,and mesopores are composed of both organic pores and inorganic pores.Correlation analysis of fractal dimensions and TOC contents indicates that the complexity of organic pores with sizes less than 4 nm is mainly controlled by organic carbon content.The complexity of organic pores with sizes from 4 to 50 nm is mainly affected by organic carbon content,and is also affected by other factor control.The complexity of organic pores with sizes larger than 50 nm is affected by other factors besides the control of organic carbon content.The pores of the argillaceous-rich siliceous shale lithofacies?S-3?are most developed and the pore sizes are large,and their specific surface area and pore volume are also large.And the inorganic pores of the argillaceous/siliceous mixed shale lithofacies?M-2?and siliceous-rich argillaceous shale lithofacies?CM-1?are relatively developed.?3?Under the tectonic stress of the regional compressive nappe,Wufeng Formation is located at the bottom of the Wufeng-Longmaxi shale weakness zone,and the underlying strata are hard carbonate rocks.The bottom of Wufeng shale is the main slippage surface,therefore,Wufeng shale experienced more tectonic compression,slippage and reconstruction,compared with that of Longmaxi shale.As a result,the organic pores of the Wufeng shale are subject to lateral stress-induced deformation and compression,which may be the one of reasons that organic pores in Wufeng shale?layer?1??are more complex than other overlying layers.5.The shale of the Wufeng Formation and lower member of Longmaxi Formation in western Hubei and eastern Chongqing regions has strongly water-oil-mixture wet and very strongly oil-wet.The upper part of the lower member of Longmaxi Formation(layers?6?⁹)has relatively strong water-wet,while Wufeng Formation?layer?1??and the lower part of the lower member of Longmaxi Formation(layers?3?⁵)are moderately strong.The difference in wettability of shale in Wufeng Formation and lower member of Longmaxi Formation is closely related to the organic carbon contents,pore types and mineral compositions of the shale.Spontaneous imbibition experiments of different fluids show that the hydrophilic pore connectivity of the upper part of the lower member of Longmaxi Formation(layers?6?⁹)is the worst,and the hydrophilicity of the Wufeng Formation?layer?1??is relatively best.The connectivity of the lipophilic pores is better than the connectivity of the hydrophilic pores in shale.