Gas-bearing Main Controlling Factors And Occurrence Mechanism Of Terrestrial Shale

China has abundant shale gas resources and has great exploration potential in continental shale stratum.However,large-scale commercial development has not been achieved so far.The critical reason is that the gas-bearing main controlling factors and its occurrence mechanism of continental shale are still not clearly understood.Aiming at the scientific issues,laboratorial methods of geochemical tests,petrological tests,scanning electron microscope,gas adsorption and methane isothemal adsorption are conducted to study the reservoir characteristics and gas-bearing features in continental shale.Further,the features of pore structure,wettability,pore connectivity and the gas bearing control factors are clarified.Combined with water-equilibrium and extremely low temperature nitrogen adsorption tests,the effects of relative humidity on effective pore structure and reservoir gas-bearing capacity are analyzed.The supercritical adsorption models are used to predict the adsorption phase density,and a new supercritical adsorption model based on pore size and relative humidity is proposed.Finally,the gas-bearing occurrence and transformation mechanisms of adsorbed gas and free gas are clarified,and the gas-bearing matching relationship between gas content and reservoir-forming elements of terrestrial shale are also elucidated.This study is helpful to promote the understanding of the occurrence mechanism and guide the exploration and development of continental shale gas.The average total organic matter content is 1.1%in Northeast Sichuan,which has a good material basis.The organic matter types are II₂ and III,and the gas generating capacity is strong.The vitrinite reflectance R_o is between 1.2%and 2.0%,which belongs to the medium-high maturity stage.Quartz and clay are the main minerals,with an average of 44.6%and 41.2%respectively,which indicates the brittleness is not high.Organic matter pores and organic-clay complexus pores play a positive role in continental shale reservoirs,while inert maceral organic matter contributes little to the improvement of reservoir capacity.Micropores and mesopores contribute most of the specific surface area,while the mesopores and macropores in organic matter provide most of the pore volume.The continental shales show partial hydrophilic and mixed wetting features.The pore connectivity of organic-rich shale is the best lithofacies,while the pore of clay shale is easy to be blocked.The favorable lithofacies are organic-rich siliceous shale and organic-rich mixed shale in Northeast Sichuan.The adsorption capacity of different components of shale are much various:kerogen>Clay>Quartz/feldspar.The total organic matter content,water saturation and pore structure are the gas-bearing main controlling factors of terrestrial shale.Organic-rich shale has the strongest adsorption capacity,with the average maximum adsorption gas capacity is about 2.12 cm³/g.Under the condition of relative humidity 98%,the maximum adsorbed gas capacity of dry shale is 1.83～3.87 times than those wetting shales.Water weakens the shale reservoir capacity by affecting the valid specific surface area and valid pore volume,as a result,the content of adsorption gas and free gas capacity decrease significantly.When the relative humidity is 98%,the valid pore volume and valid specific surface area of the wetting shale can be reduced to 1/2 and1/3,in comparition to dry shale,in which water has a significant negative effect on the valid specific surface area of the pore diameter of 2.5～20nm,and on the valid pore volume of 2.5～80nm.From the perspective of adsorption mechanism,pressure has positive correlation with methane bulk volume and bulk density,while from the perspective of thermodynamics,temperature has critical effect on bulk volume,and little effect on bulk density.Moreover,a new supercritical adsorption model based on pore size and relative humidity is proposed.The new model elucidates the gas-bearing occurrence mechanism of continental shale is the coexistence of micropore filling and monolayer adsorption.The calculation results confirm that the methane adsorption in the micropores are 1～3 layers of the continental shale in the Northeast Sichuan Basin,while the adsorption layer in the mesoporous and macroporous adsorption layer are less than1 layer as monolayer adsorption.Generally,the methane adsorption layer in the full aperture is about 1～2 layers.Based on bulk density and volume,a formula for calculating gas content is proposed.The formula overcomes the problem of repeated calculation of partial volume.From the formula,the adsorption gas ratio of shale samples measured in Northeast Sichuan calculates 72.1%,98.9%and 51.6%,respectively.The adsorption gas ratio is closely related with the shale adsorption capacity,porosity,water saturation,temperature and pressure.Meanwhile,the occurrence and transformation mechanism of adsorbed gas and free gas are mainly affected by temperature and pressure.Based on this,two dynamic occurrence modes of unsaturated adsorption and saturated adsorption for shale gas are proposed.In the burial stage,free gas is converted to adsorbed gas,while in the formation uplift stage,adsorbed gas is converted to free gas.Based on the stratum burial history,the evolution of reservoir space and the evolution of water-bearing characteristics,the favorable matching relationship feature between the gas-bearing property and reservoir forming elements of continental shale is clarifed in Northeast Sichuan Basin.