Controlling Factors And Quantitative Evaluation Of Hydrocarbon Micromigration Of Fengcheng Formation Shale In Mahu Sag

There are abundant terrestrial shale oil resources,which is an important replacement field under the background of the shortage of conventional oil and gas resources.Fengcheng Formation in Mahu Sag,Junggar Basin is an ancient alkali lake high-quality source rock with large buried depth.Currently,it is in the mature to high mature evolution stage and has strong oil generation capacity.Under the background of low maturity of continental shale and poor fluidity of shale oil,it is a favorable target to achieve breakthrough in the exploration and development of continental shale oil.However,the shale of Fengcheng Formation in Mahu Sag is a mixed fine grained deposit with strong heterogeneity,oil content and composition difference,which makes it difficult to predict and evaluate the sweet spot.At present,it has been found that there is obvious micromigration in shale layer,which affects oil and hydrocarbon components of shale.However,the controlling factors of micromigration are not clear,and quantitative evaluation methods of micromigration need to be established urgently.By means of XRD,pyrolysis,high pressure mercury injection,low temperature gas adsorption,Soxhlet extraction,chromatographic mass spectrometry,XRF,scanning electron microscopy,laser confocal,two-dimensional nuclear magnetic resonance and other experimental means,it is confirmed that the Fengcheng Formation has obvious micromigration hydrocarbon.The potential difference method of hydrocarbon generation is used to quantitatively evaluate the micromigration hydrocarbon quantity of Fengcheng Formation shale.Based on the characteristics of lithology,lithofacies,geology,geochemistry and pore structure,the main controlling factors affecting the micromigration phenomenon were identified,and a quantitative evaluation model of shale oil micromigration was established.The results show that the organic matter abundance（TOC）of Fengcheng Formation ranges from 0.14%to 1.85%,with an average value of 0.77%.The organic matter type is mainly kerogen type II.The whole is in the mature-high mature stage,entering the peak of oil generation.Oil saturation index（OSI）ranges from 19.84 to 1825.27 mg/g,with an average value of 268.27 mg/g.Three types of pores are mainly developed:intergranular pores,intergranular dissolution pores and lamellar clay mineral pores.Macropores and mesoporous pores are relatively developed.The correlation of geochemical parameters,fluorescence slices and laser confocal microscope indicate the existence of micromigration in shale.The distribution range of micromigration hydrocarbon（ΔQ）in the shale of Fengcheng Formation is-1746.56-600.54mg/g,with an average value of-128.97mg/g,in which 69%of the samples are charged with foreign migration hydrocarbon,and 31%of the samples are expelled.The difference method of hydrocarbon generation potential was verified by analysis of family components,saturated hydrocarbon chromatographic mass spectrometry and Mn element.ΔQ has a strong correlation with OSI and movable oil content(S₁₁c+S₁₂c),indicating that high oil content is related to the charging of foreign hydrocarbon migration,and establish a quantitative evaluation model of shale movable oil.It is found that TOC,organic matter type（original hydrogen index）,calcite content,feldspar content,total pore volume and capillary force difference（mean pore throat radius）are the controlling factors for the amount of micromigrated hydrocarbon in shale.Combined with numerical simulation software,a quantitative evaluation model of micromigration hydrocarbon is established.The correlation coefficient of this model is 0.72,which can be used for quantitative evaluation of micromigration hydrocarbon in Fengcheng Formation shale.