Pore And Pressure Evolutions Of Longmaxi Formation Shale And Transformation Of Shale Gas Occurrence In Southern Sichuan Basin,China

The methane stored in the pore space of shale is dominated by free gas and adsorbed gas,which are controlled by organic matter content,porosity,and formation temperature and pressure.The marine shale formation in the Longmaxi Formation in the Southern Sichuan Basin was anciently deposited and experienced complex and composite tectonic movements,which arose changes in the geothermal condition,fluid pressure,and pore characteristics of shale formation,therefore the shale gas occurrence varies as well.In the paper,by conducting field section observation,log analysis,geochemical analysis,experiment series related with pore characterization,analysis of inclusions hosted in vein filling of shale interval,gas adsorption and the likes,the occurrence state of shale gas was studied,revealing the pore evolution law of shale,restoring paleopressure of shale formation,elucidating the variations of contents of free gas and adsorbed gas in geological history,thereby to elucidating the control of geothermal and fluid pressure evolution to gas occurrence states.The amount of organic pore and inorganic pore for shale reservoir was quantitatively analyzed by conducting NMR experiments of shale samples saturated by water and kerosene.According to the characteristics of hydrophilic pore and oleophilic pore corresponding to NMR T₂ spectrum of samples saturated with water and kerosene,coupled with wettability differences of organic pore and inorganic pore,the ratio between the two for shale can be determined.By analyzing shales in different maturities,it is discovered that with the intensifying of diagenesis,inorganic pore is rapidly decreased while organic pore is propagated and predominates.The porosity provided by per unit of organic matter(wt.%)is increased from 1.2%?1.5%(vol.)in low-mature shale to 1.8%?2.5%(vol.)in high-mature shale,and the organic porosity is firstly increased and then decreased with the increase of maturity,with a maximum reached at Ro=2.6%nearly.The fractures and fillings were well developed in the shale interval,five types of fractures can be identified,Type I,Type II₁,Type II₂,Type III and IV.By core observation and petrographic analysis of fracture vein fillings,Type I and IV fractures are filled by quartz,calcite,and pyrite by sedimentation.The vein filling of Type II₁fracture is mainly characterized fibrous calcite accompanied by solid bitumen.The Type II₂ fracture is mainly filled by calcite,with local siliceous veins which are possibly the invasions of siliceous hydrothermal fluid.Type III fracture shows low angle and is filled by calcite or mixed vein fillings.Based on experiment series for inclusion analysis,the inclusions hosted in the veins of the Longmaxi Formation shale include vapor-liquid two-phase type(CH₄-Na Cl-H₂O system)and vapor phase type(pure CH₄ system and CH₄-CO₂ system).The salinity of saline inclusions is 5.2%?15.3%(wt.Na Cl),showing high salinity.The homogenization temperatures of saline inclusions hosted in the vein fillings of Type II₁,II₂,and III fractures are 173.5°C?185.3°C,133.6°C?145.2°C,and113.6°C?136.8°C.The pressure corresponding to 140°C in the shale formation in the Weiyuan area was 62.2 MPa.In the Changning area,that corresponding to 140°C and180°C in the shale formation were 74.3 MPa and 104.1 MPa,respectively.The pressure modeling was conducted under the constraints of known pressure data points to elucidate pressure evolution characteristics of shale formation.Based on the fact that gas adsorption is mainly controlled by organic-hosted nanopores and free gas is mainly subject to porosity and temperature-pressure conditions in shale,new models for calculating free gas and adsorbed gas were established that the evolution of gas occurrence under varying geological conditions can be quantitatively simulated.The shale formations in the Changning and Weiyuan Areas were continuously and moderately uplifted in relatively late stage,however,that in the Pengshui Area was continuously and rapidly uplifted.The differences in structural evolution in the two areas result in great differences in the evolution of gas occurrence states in the Longmaxi Formation.