Reservoir Characteristics And Diagenetic Evolution Of Wufeng-Longmaxi Formation Shale In The Southern Sichuan Basin

To understand reservoir characteristics and diagenetic evolution of Wufeng-Longmaxi Formation marine shale in the Southern Sichuan Basin,through the experiments and tests of core analysis,thin section observation,field emission scanning electron microscopy analysis of argon ion polishing,X ray diffraction analysis of whole rock and clay,high pressure mercury injection,nitrogen and carbon dioxide isothermal adsorption,the shale reservoir is delicately depicted in six aspects:the TOC content,vitrinite reflectance,mineral composition,lithofacies type,porosity and permeability characteristics and micro-reservoir spatial structure characteristics.The pore structure characteristics,diagenesis type,diagenesis stage and pore evolution model of shale reservoir are expounded in detail.The shale of Wufeng-Longmaxi Formation is rich in clay shale facies and mixed shale facies.The total content of quartz and clay minerals is81.44%,and the average content of carbonate minerals?Calcite and Dolomite?is 18.56%,and potassium feldspar,plagioclase and pyrite are few.The clay minerals are mainly Illite/Montmorillonite mixed layer+Illite+Chlorite with no Montmorillonite or Kaolinite,and the ratio of illite to chlorite is 5%?10%,which is at the late diagenetic stage.The TOC content of shale is 1.5%?3.0%,so organic matter content is higher,and the thermal evolution is at the over-mature stage?Ro=2.35%?3.68%?.The porosity of shale ranges from 0.38%to 9.55%,and the average of permeability is 0.1163×10^-3?m².Based on the DFT theoretical model and statistical method,the pore structure parameters of shale are calculated.It can be seen that micropore and mesopore contribute a lot to the pore volume and specific surface area of Wufeng-Longmaxi Formation shale.The total pore volume is 0.001 cm³/g?0.03 cm³/g,and the total specific surface area is between 5 m²/g?32 m²/g.Whole pore size characterization of micro-pore structure of shale shows that mesopore and macropore mainly provide pore volume,and micropore and mesopore have relatively great contribution to surface area.High pressure mercury injection experiments show that macropore with pore diameter of 200 nm?10?m contributes greatly to pore volume.Nitrogen adsorption reflects that specific surface area is mainly provided by micropore with pore size of 1.5 nm?1.6 nm.Micropore volume and specific surface area can be obtained by using carbon dioxide adsorption method,which are mainly provided by the pore size in the range of 0.33 nm?0.40 nm,0.43 nm?0.65 nm and 0.72 nm?0.86 nm.Because of the high degree of organic matter evolution of Wufeng-Longmaxi Formation shale in southern Sichuan Basin,it is impossible to study the diagenesis and evolution process.In this thises,marine shale samples with low maturityof Zhaojiashan Xiamaling Formation in Xiahuayuan area of Zhangjiakou,Hebei Province are selected as experiment subjects,and 10 temperature points?original 300??330??350??380??410??425??450??480?and 510??were designed to carry out the hydrocarbon generation simulation experiment of organic matter.The liquid hydrocarbon and gas hydrocarbon products during the evolution of organic matter were collected,and the retained solid residues were analyzed by X ray whole rock and clay mineral content analysis,argon ion polishing-field emission scanning electron microscopy observation,argon gas and carbon dioxide isothermal adsorption,and the pore structure characteristics and diagenetic evolution process of shale in each evolution stage was analyzed.With the thermal evolution of organic matter and the increase of temperature and pressure,the diagenetic liquids change from weak alkali,weak acidity,weak alkali,weak acidity,weak acidity to weak alkalinity.The contents of sodium,potassium,calcium and magnesium ions also vary with the change of environment.Gas products are mainly composed of inorganic non-hydrocarbons and organic hydrocarbons,and the content of non-hydrocarbon gases such as N2 andis higher,while hydrocarbon gases were accumulated gradually with the evolution process.The dry coefficientincreases with the increase of temperature,indicating that oil and gas are affected by temperature and pressure in the process of formation.It is concluded that shale undergoes four stages of hydrocarbon generation evolution?diagenesis?:Betumin generation?rapid compaction?stage,hydrocarbon generation?weak dissolution-compaction?stage,wet gas generation?dissolution and cementation?stage,and dry gas?weak cementation and metasomatism?stage.Pore evolution can be divided into early primary pore development and rapid disappearance stage,and middle and late secondary pore development and organic pore evolution stage.The synergistic effect of clay mineral transformation and organic matter hydrocarbon generation evolution provides a good reservoir space for shale gas enrichment.