Pore Structure Characteristics Of Shale Reservoirs And Their Effects On Oil-bearing Properties In The Zhanhua Sag

As an important unconventional oil resource,shale oil is different from conventional oil resources in accumulation mechanism and distribution regularity.Since shale oil occurs in micro-nano pores and fractures,researches on pore structure characteristics of shale reservoirs are important to understand the accumulation mechanism of shale oil.The lower third member of the Shahejie?Es₃^?? formation in the Zhanhua Sag,Bohai Bay Basin belongs to typical continental shale,which provides a good condition for researches about the effect of pore structure on shale oil.Therefore,this paper took the Es₃^? shale in the Zhanhua Sag as the object of study.Several methods,including total organic carbon?TOC? analysis,X-ray diffraction?XRD? analysis,thin section observation,field emission-scanning electron microscopy?FE-SEM?,CO₂ adsorption,N₂ adsorption,high-pressure mercury injection?MIP?,Nano-CT,quantitative evaluations of minerals by scanning electron microscopy?QEMSCAN?,spontaneous imbibition,Rock-Eval pyrolysis,soxhlet extraction and nuclear magnetic resonance?NMR? analysis,were used to character the pore structure of the Es₃^? shale.We analyzed main factors affecting the pore structure of shale reservoirs,determined the pore size range for shale oil enrichment,and clarified the effect of pore structure on shale oil accumulation.The Es₃^? shale predominantly consists of calcite,quartz,and clay.With increasing depth,the calcite content gradually increases,whereas the clay and quartz contents decrease.Horizontal beddings are universally developed,appearring as the interactive distribution of light and dark lamellar layers.The dark lamellar layers mainly consist of clay and siliceous minerals,whereas the light lamellar layers are mostly composed of calcareous minerals.Organic matter is rich and belongs to Type I and II.The Es₃^? shale possesses a moderate thermal maturity and is primarily in the stage of thermal catalysis.Four types of shale lithofacies associations,organic-rich massive mixed shale?ORMMS?,organic-rich bedded mixed—calcareous shale?ORBMS—ORBCS?,organic-rich laminated calcareous shale?ORLCS?,and organic-fair laminated calcareous shale?OFLCS?are developed from top to bottom in the Es₃^? shale.The Es₃^? shale possesses a complicated pore structure and developes a variety of reservoir spaces,such as organic matter-hosted pores,intergranular pores,intraparticle pores,and micro-fractures.Pores of various scales are developed,especially macropores.Macropores make a great contribution to the total pore volume,whereas micropores provide the major specific surface area.The Es₃^? shale is dominated by slit-shaped and ink-bottle-shaped pores.The pore structure characteristics of different shale lithofacies are quiet different.Pores in the ORMMS are poorly developed,with the majority of micropores.Conversely,pores in the ORLCS are well developed,especially macropores.In addition,ORLCS with a good connectivity possesses the largest pore volume and specific surface area.The pore structure of the Es₃^? shale is influenced by mineral composition,lamellar layers,and organic geochemical characteristics.Mineral composition and thermal maturity primarily affect micropores and mesopores.The high contents of caly and siliceous minerals are propitious to micropores,whereas the high content of calcareous minerals is conducive to mesopores.In addition,the thermal evolution of organic matter is favourable to the development of mesopores.Besides mineral composition and organic geochemical characteristics,macropores are seriously affected by lamellar layers.Only when the carbonate minerals content is much higher and simultaneously the contents of clay and siliceous minerals reach certain amounts will lamellar layers be well-developed,which leads to shale reservoirs present the best storage and percolation properties.Most residual hydrocarbon occurs primarily in the free-state form in pores with diameters of 10?2000 nm.Pores with diameters of 10?2000 nm are regarded as“oil-enriched pores”,which are effective pores for shale oil exploration.A small amount of residual hydrocarbon occurs primarily in the adsorbed-state form in pores with diameters smaller than 10 nm,which could not be exploited at the present stage.Pores with diameters smaller than 10 nm are regarded as“oil-ineffective pores”.As the minimum pore size allowing fluid expulsion is 2000 nm,pores with diameters larger than 2000 nm are considered as“oil-percolated pores”.Shale oil accumulation is the result of hydrocarbon generation capacity and reservoir property.The lower parts of ORBMS—ORBCS and OFLCS achieve the best matching on both hydrocarbon generation capacity and reservoir property,which are the most favourable sections for shale oil accumulation.