Pore Structure And Gas Migration Characteristics Of The Tight Sandstone Of The Third And Fourth Members Of The Xujiahe Formation In Yuanba Area

The tight sandstones of the third and fourth members of the Xujiahe Formation from the Yuanba area in the northeast portion of the Sichuan Basin were investigated in this research.Using thin sections,scanning electron microscopy,high-pressure mercury intrusion,nuclear magnetic resonance,it is possible to determine the parameters of the pore structure qualitatively and quantitatively.The relationship between diagenesis and pore structure is quantitatively analyzed using cathodoluminescence and electron probe,and then the fundamental controlling factors of reservoir pore structure development are thoroughly investigated.Step-by-step pressurized gas drive water physical simulation-NMR experiments and theoretical analysis based on dominating microscopic transport channels disclose gas transport characteristics under distinct pore structures.The study’s results indicate that the lithology of the fourth member of the Xujiahe Formation is mostly rock chip sandstone and feldspar sandstone,which are mostly discovered eastern part of the Yuanba area.The lithology of the third member of the Xujiahe Formation is mostly rock chip sandstone,which is mostly discovered in the western part of the Yuanba area.The reservoirs are classified into four types based on pore-throat development characteristics:intergranular pore-internal pore-dominated,intergranular pore-internal pore-dominated,intergranular pore-dominated,intergranular pore-dominated,and fracture-dominated,with the main pore-throat channel combinations of the first three types being "intergranular pore-internal pore-curved lamellar throat" and "intergranular pore-curved lamellar throat",while the fracturedominated reservoirs mainly develop microfractures.The main pore radius of the four types of reservoirs are between 20-300 nm,8-200 nm,40-300 nm,and 10-100 nm in order,and the proportions of dominant connected pores are 33.5 percent,10.4 percent,51.9 percent,and 4.6percent,respectively,according to the results of joint pore radius characterization and quantitative classification of connected pores.The inhomogeneity of the reservoir is classified as intergranular pore-dominated,intergranular pore-internal pore-dominated,intergranular pore-internal pore-dominated,and fracture-dominated using high pressure mercury intrusion piezometric mercury and NMR fractal analysis.According to the findings,the generation of reservoir pore structure in the study area is controlled by sedimentation,diagenesis,and hydrocarbon generation.Intergranular poreinterstitial pore-dominated and intergranular pore-interstitial pore-dominated reservoirs have high brittle mineral content,long sediment transport distances,high compositional and structural maturity,and relatively good pore structure,with intergranular pore-interstitial poredominated reservoirs having early formation of chlorite,which inhibits the cementation of sand.In intergranular pore-dominated and fracture-dominated reservoirs,the high content of plastic minerals and the close transport distance of sediments result in low compositional maturity,low structural maturity,and relatively poor pore structure,but the high formation pressure caused by the generation of hydrocarbons in fracture-dominated reservoirs results in the formation of microfractures,which improve pore structure.The early granular cement was dissolved by organic acids created by hydrocarbon production,resulting in severe compaction and hydrocarbon occupancy,which prevented the transformation of clay minerals.Large throats and high coordination numbers define inter-grain and intra-grain poredominated reservoirs,and the primary microscopic migration routes are "inter-grain and intragrain pores and curved sheet-like throats," with viscous flow-slip flow as the gas flow mechanism.Small throats and a low coordination number of pore throats describe intergranular pore-internal pore-dominated and intergranular pore-dominated reservoirs,and the dominating microscopic migration channel is "intergranular pore-curved sheet-like throat," with slip flow as the gas flow mechanism.The viscous flow mechanism dominates the fracture-dominated reservoir gas flow process.The broad throat,low roughness,low tortuosity,and high driving pressure were found to be suitable for gas migration in the comparison analysis.