Accumulation Mechanism And Mobility Evaluation Of Shale Oil In Fengcheng Formation Of Mahu Sag

As an unconventional oil resource,shale oil has great resource potential and is the main replacement field of oil and gas in the future.The enrichment mechanism and mobility of shale oil are the most concerned issues in the exploration and development of shale oil.Mahu Sag in Junggar Basin is a famous hydrocarbon-rich sag,with great shale oil resources potential.However,the exploration and development of shale oil is still in its infancy.Besides,it has the characteristics of special sedimentary environment,complex and diverse lithology,strong heterogeneity,special alkali minerals and special hydrocarbon-generating parent materials.Based on the above description,this paper selected the Fengcheng Formation shale in Mahu Sag,Junggar Basin as the research object.Geological and geochemical analysis methods such as core and thin section observation,X-ray mineral component analysis,rock pyrolysis,total organic carbon analysis,and unconventional reservoir research methods such as scanning electron microscopy,nitrogen adsorption,high-pressure mercury injection,contact angle,and spontaneous imbibition were used.combined with molecular dynamics simulation methods,the hydrocarbon generation capacity,reservoir capacity,shale oil occurrence state and shale oil flow characteristics of Fengcheng Formation shale were investigated.The enrichment mechanism and mode of shale oil in Mahu Sag are discussed,and the mobility of shale oil in the study area is evaluated.The mineral composition of the Fengcheng Formation shale in the Mahu Sag is mainly felsic minerals and carbonate minerals.The content of pyrite is high,the content of clay minerals is generally low,and a variety of alkaline minerals such as reedmergnerite,shortite,leucosphenite,and eitelite are developed.The current TOC content of the Fengcheng Formation is low,mainly distributed in the two ranges of0.3%～0.6%and 0.6%～0.9%.The organic matter type is mainly type II₁,which is a better source rock.The maturity of organic matter R_o is between 0.66%and 0.74%,which is in the oil window.Based on the grain size and mineral composition,lithofacies of Fengcheng Formation shale in the study area are divided into 9 lithofacies types,which are further divided into four main lithofacies combinations:felsic shale+dolomitic felsic shale（LA-1）,mixed shale+dolomitic felsic shale（LA-2）,dolomitic felsic shale+felsic dolomitic shale（LA-3）,siltstone+dolomitic felsic shale（LA-4）.The shale reservoirs of the Fengcheng Formation mainly develop inorganic mineral dissolution pores and microcracks,and the organic matter pores are less developed.Shale mainly develops pores with pore sizes less than 100 nm,and siltstone mainly develops pores with pore size greater than 100 nm.Regardless of shale or siltstone,the development of alkaline minerals can significantly improve the pore structure of the reservoir.The Fengcheng Formation shale is water-wet,and the contact angle of deionized water is less than 75°.The siltstone is oil-wet,and the contact angle of deionized water is greater than 105°.Spontaneous imbibition experiments show that felsic shale,dolomitic felsic shale and dolomitic felsic siltstone have good oil-wet pore connectivity.The influence of organic matter on the pore structure of Fengcheng Formation is not obvious,and carbonate minerals,especially dolomite,have a great influence on the pore structure.The shale oil of Fengcheng Formation in Mahu Sag mainly occurs in inorganic pores and micro-fractures in free and adsorbed states.Oil also exists in pores with pore size less than 5 nm in shale,while oil in siltstone mainly exists in pores with pore size greater than 30 nm.When the pore size is greater than 4nm,free oil begins to appear in the pore,and the thickness of adsorbed oil is 1.92nm.The proportion of adsorbed oil pore volume in shale is significantly higher than that in siltstone.Lamina-type shale oil and interlayer-type shale oil are the two shale oil enrichment modes of Fengcheng Formation in Mahu Sag.The lamina-type shale oil is a source-reservoir integration.The contents of TOC and S₁ are high,and they are consistent in the vertical direction.The reservoir pores are mainly mesopores,and water wetting,and the enrichment mechanism of shale oil is organic matter adsorption,inorganic mineral adsorption and capillary pressure sealing.The interlayer-type shale oil is source-reservoir interlayer,the S₁ of enrichment layer is higher,the TOC of adjacent layer is higher.The reservoir pores are mainly macropores,and oil wetting.The mechanism of shale oil enrichment is mainly the oil charging force provided by hydrocarbon-generating overpressure and the physical sealing effect of upper and lower shaleThe shale oil retention mechanism is mainly capillary pressure sealing and physical property sealing of the upper and lower shales.The lamina-type shale oil is mainly distributed in the semi-deep lake to deep lake facies in the depression area,and is mainly distributed in the lower part of the third member of the Fengcheng Formation in the vertical direction.The interlayer-type shale oil is mainly distributed in the shallow lake to semi-deep lake facies in the slope area,and is mainly distributed in the middle and lower part of the second member of the Fengcheng Formation in the vertical direction.The content and proportion of movable oil in siltstone are much higher than those in shale.Organic matter abundance,mineral composition,pore size,pore morphology,temperature and pressure significantly affect the mobility of shale oil.The lithofacies combination of siltstone+dolomitic felsic shale（LA-4）has the highest movable oil content and high movable pore volume of shale oil,which is the best lithofacies combination of shale oil mobility in Fengcheng Formation of Mahu Sag.According to the content of residual hydrocarbon S₁ and chloroform bitumen“A”,7 shale oil enrichment layers of Fengcheng Formation in Maye 1 well were divided.According to OSI and movable pore volume,4 layers with good shale oil mobility were divided.Finally,5 shale oil sweet spots of Fengcheng Formation in the Maye 1 well were obtained,which were located in the lower part of the third member of the Fengcheng Formation and the middle and lower part of the second member of the Fengcheng Formation.