The Evolution Process Of Pores And The Characteristics Of Residual Hydrocarbon Of Shale In The 7^th Member Of The Triassic Yanchang Formation,Ordos Basin

As the pore structure of shale has a significant influence on the residual capacity for the hydrocarbon,and the residual oil and gas in shale is the material basis of shale oil and gas reservoir,the study on the characteristics of pore and residual hydrocarbon is of great theoretical and practical significance for the shale oil and gas enrichment mehanism.The characteristics of pore types,pore structures,residual hydrocarbon and the influence factores on development of pores and residual capcacity of hydrocarbon were studied in detail through the data of well drilling,well logging,geochemical analysis,SEM,X-Ray Diffraction,nitrogen adsorption,isothermal adsorption.The thick lacustrine shale deposited in the 7^th Member of the Triassic Yanchang Formation has a total area of 6.6x10⁴ km² and an average thickness of 38.6m.Mineral compositions of the shale are mainly brittle minerals and clay minerals.The average content of clay minerals is 43.9%,and illite/smectite mixed layer is the main component of clay minerals.The brittle minerals mainly have quartz,feldspar and pyrite,with an average content 28.8%,12.4%and 8.3%,respectively.The abundance of the organic matters is 5.7%.The type of the organic matters is mainly II₁ and the vitrinite reflectance is between 0.5%and 1.17%.All above provide the basic material condition for shale oil and gas formation.SEM shows that pores of the shale in the 7^th Member are mainly intragranular pores found in feldspar and clay minerals and contain a certain amount of intergranular pores and micro-fractures,while the organic pores are poorly developed.N₂ adsorption experiments proved that the shape of pores in shale are mainly the slit type of parallel plate.The diameter of pores is 2²00nm,and mainly concentrated in around 4nm.The average total pore volume is 0.01012m³/g and the average specific surface area is3.6720m²/g.Transitional pores,accountting for 57.85%of the total pore volume,are the main pores followed by micropores,and mesopores are the least.Micropores,contributing as much as 76.87%of the specific surface area,dominate the specific surface area.Totally,The pores in study area are not developed.The shale in the 7^th Member contains a large amount of residual asphalt.By comparing the pore volume,specific surface area before and after extraction of the residual asphalt indicates residual asphalt in shale resulting in a significantly smaller measure value of pore volume and specific surface area.The pore volume and specific surface area are mainly controlled by inorganic minerals.The content of quartz and feldspar has a significant effect on all kinds of pore volume and specific surface area,but the content of illite/smectite mixed layer mainly affects micropores,transitional pores volume and specific surface area.The organic pores have a small effect on the pore volume and specific surface area.The shale in the 7^th Member is in moderate-mature A stage.The evolution process of pore volume is mainly controlled by inorganic pore volume,and presents decrease at first,then increase,but decrease soon afterwards.The mechanisms affecting the pore volume evolution include the hrdrocarbon generation and diagenesis.Hydrocarbon generation process in the study area can not only generate organic acids that have dissolved minerals,but increase the pressure in shale.Diagenesis in the study area mainly includes compaction and transformation of clay minerals.Combining with the results of other researchers,the pore evolution model of terrestrial facies shale in the study area was established.The shale in the 7^th Member has a high content residual capacity for hydrocarbon.Organic matters,inorganic minerals and pore structure have different mechanism for the residue of liquid hydrocarbon and gaseous hydrocarbon.Organic matters play a significant role in the residue of hydrocarbon and the mechanisms affecting the content of liquid hydrocarbon for organic matters are mainly the hydrocarbon generation and expulsion,while the mechanisms affecting the content of gaseous hydrocarbon for organic matters are the adsorption of the organic matters and the solution of residual asphalt.Inorganic minerals controlling the pore structure of shale are playing an important role for the residue liquid hydrocarbon,but have a poor influence on the adsorption of gaseous hydrocarbon.Pore structure is a key factor that affects liquid hydrocarbon,while has a little influence on gaseous hydrocarbon.