Reservoir Characteristics Of Continental Shale And Its Influences On Gas-bearing Properties

Continental shale gas resources in China occupy a large proportion of total geological gas resources.However,compared with marine shale,the research on continental shale is weak because the reservoir characteristics of continental shale are more complicated owing to its diverse sedimentary environments.The Changling Fault Depression of Songliao Basin has been proved to develop large-scale high-quality source rock.And the Shahezi Formation have great potential for shale gas exploration.Therefore,taking Shahezi shale as an example,applying multidisciplinary theory and various experimental methods,this paper comprehensively analyzes the reservoir characteristics of continental shale and its effect on gas-bearing properties.The pore structure characteristics affect the shale storage capacity.The continental shale in Changling Fault Depression has more complex pore structure characteristics than the marine shale.Through geochemical and petrologic analyses,field emission-scanning electron microscopy observation,gas adsorption experiments,high pressure mercury injection,QEMSCAN and spontaneous imbibition,the reservoir characteristics and its controlling factors are found out.The results show the pore volume and specific surface area of shahezi formation are similar to those of marine shale,and the mesopore contributes the most to the pore volume,followed by the macropore.Micropore and mesopore contribute more than 99%of the specific surface area.The slit-like pores,intergranular pores and intragranular pores in clay minerals are abundant,which provide most micropores,mesopores and macropores.Organic matter has a certain contribution to the micropores and mesopores.The maturity mainly increases the development of inorganic pores by promoting diagenetic transformation and dissolution of siliceous minerals and carbonate minerals.High content of calcite greatly improves the storage space of shale reservoir.From laminated shale,bedded shale to massive shale,connectivity is successively worse;According to the parameters of pore volume and specific surface area,mixed shale has the best reservoir capacity,followed by siliceous shale and argillaceous shale.Organic pores of continental shale in fault depression show a relatively strong heterogeneity.Through location observation by field emission-scanning electron microscope and fluorescence microscope,energy spectrum analysis,solid bitumen reflectance measurement,kerogen isolation,gas adsorption experiment,combining with Image J software,organic pore are qualitatively and quantitatively characterized.The results show the organic pores developed within organic-clay complex are the most developed.Isolated flaky solid bitumen have different porosity development degree.Pores are present in telinite,fusinite,semi-fusinite and funginite.These differences in pore development can be attributed to four factors:the nature of organic matter itself,the primary organic pore residue,individual differential evolution of solid bitumen and clay minerals catalysis.Kerogen accounts for more than 97%of the total organic matter,and is the main contributor of organic pores.The contribution of kerogen to shale pore volume is only second to clay minerals.The pore volume and specific surface area of isolated kerogen are averagely 0.0128m L/g and 42.60m~2/g,respectively.The pores in the range of 10?200nm are the main contributor of kerogen pore volume,and 0.3?1.0nm is the main contributor of kerogen specific surface area.With the increase of maturity,the porosity of solid bitumen increases first and then decreases,the porosity of vitrinite and inertinite decreases rapidly and then slightly increases.The total organic pore decreases first,and then slightly increase and finally decreases.Shale reservoir characteristics obviously control the gas content.Through high-temperature methane adsorption experiments and well logging interpretation,the influence of reservoir characteristics on gas-bearing properties is analyzed.The results show that the total gas content of the Shahezi shale is up to 8.09cm~3/g,with an average of 1.88cm~3/g.Vertically,the adsorbed gas content in the argillaceous shale interval is high,distributed at 0.297?7.208cm~3/g,with an average of 1.623cm~3/g,which accounts for 77.15%of the total gas content;the free gas content in the siliceous shale,mixed shale interval is high,distributed at 0.297?2.128cm~3/g,averaging 0.989cm~3/g,accounting for48.36?97.44%,with an average of 58.36%.The main factor affecting the adsorption gas content under the reservoir conditions is still organic matter.The laminated shale has high TOC value and can provide specific surface area for adsorbed gas.The main factors affecting the free gas content are siliceous minerals and carbonate minerals.The intergranular pores and intragranular dissolution pores provide storage space for free gas,and the stratified siliceous shale is beneficial to the occurrence of free gas.Comprehensively considering the total gas content,organic-rich bedded siliceous shale,organic-rich bedded mixed shale,and organic-rich laminated argillaceous shale are the most favorable reservoir lithofacies.