Study On Fractal Structure And Reservoir Characteristics Of Shale Pore From The Deep Wufeng Formation To Longmaxi Formation In South Sichuan

The deep Wufeng Formation-Longmaxi Formation marine organic rich shale in southern Sichuan has a wide distribution area and abundant shale gas resources,making it an important shale gas exploration and development layer series in southern China.However,due to the complex formation,tectonic evolution,and sedimentary differentiation of the Sichuan Basin,the strong heterogeneity of the deep shale reservoirs in the Wufeng Formation-Longmaxi Formation has led to significant differences between different blocks,which has brought great difficulties and challenges to exploration and development.Aiming at the current situation of strong heterogeneity and significant differences in pore characteristics of deep shale in the Wufeng Formation-Longmaxi Formation in the study area,this paper takes the shale in the first and second members of the Wufeng Formation-Longmaxi Formation in Luzhou and Dazu areas in southern Sichuan as the main research object.Based on the analysis of the sedimentary characteristics of this set of shale,using N₂ adsorption,CO₂ adsorption,high-pressure mercury intrusion,nuclear magnetic resonance Various testing techniques such as FE-SEM are used to study the lithofacies and pore structure characteristics of deep shale gas reservoirs.The full pore size characterization and fractal characteristic calculation of shale have been carried out.According to the characteristics of shale mineral composition,the shale from the Wufeng Formation-Longmaxi Formation is divided into 8 types of shale rock,including biological siliceous shale,siliceous shale,silty shale,clay-siliceous mixed shale,calcareous siliceous mixed shale,clay-calceous mixed shale,clayey shale,and calcareous shale,using the three-terminal mineral classification method.According to rock type,TOC,brittle mineral content,and gas content characteristics,the study area is divided into dominant and non dominant lithofacies.The dominant lithofacies of Class I are:biogenic siliceous shale facies and siliceous shale facies;Class II non dominant lithofacies are:silty shale facies,calcareous siliceous mixed shale;Class III non dominant lithofacies include clayey siliceous mixed shale and clayey calcareous mixed shale.In terms of pore structure analysis,through mercury intrusion,low-temperature gas adsorption,FE-SEM,and nuclear magnetic resonance analysis of the pore structure of Luzhou and Dazu,the shale in the study area has extremely developed organic pores,which are the main pore type of reservoir space.The abundance of organic matter and the content of brittle minerals are the main controlling factors for pore development.The pore volume and specific surface area of shale are mainly contributed by micropores and mesopores,while the gas bearing capacity of deep shale is mainly controlled by the development of mesopores.In terms of fractal dimension analysis,the box dimension of FE-SEM and the FHH fractal dimension of nitrogen adsorption are jointly characterized.As the fractal dimension gradually increases,the complexity of pore structure increases,and the pore structure of dominant shale is relatively simpler.Fractal dimensions D₁ and D₂ are positively correlated with shale specific surface area,pore volume,TOC,and quartz mineral content,while carbonate minerals are negatively correlated.If D₁ and D₂ are too high,the more developed the pore structure will be,but it will also lead to the more complex pore structure,which is not conducive to later development.The Wufeng Formation-Longmaxi Formation shale section with medium to high fractal dimensions D1 and D2 will be a favorable exploration direction for deep shale gas.In terms of reservoir heterogeneity analysis,heterogeneity evaluation is conducted based on TOC,brittle mineral content,porosity,gas content,pore volume,and fractal dimension.The heterogeneity of the reservoir is mainly controlled by lithofacies,which is relatively weak among the dominant shales.However,regardless of the overall vertical depth of the deep shale in the Wufeng Formation-Longmaxi Formation,or the horizontal comparison between Luzhou and Dazu,the reservoir heterogeneity is strong.The strong heterogeneity leads to strong regionality in different blocks,and also brings great difficulties and challenges to exploration and development.