Evaluation Of Geological Sweet Spots Of Lucaogou Shale Oil Reservoirs In Jimsar Sag

Shale oil is one of the most important unconventional oil and gas resource.Although oil generally exists in shale oil reservoirs within the oil generation window,the commercial development of shale oil must rely on some technologies such as horizontal drilling and largescale fracturing in the interval with better oil-bearing property and mobility.The classification and optimization of sweet spots interval are the basis to ensure the commercial development of shale oil,which is also the core objective of geological sweet spots evaluation.However,for shale oil reservoirs of Lucaogou Formation in Jimsar Sag,the nanoscale pore throat system is dominant and the shale oil is heavy,which leads to the strong heterogeneity of physical properties,oil content and mobility.The determination of an effective grading evaluation related to reservoir physical properties and shale reservoir oil-bearing property are the difficulties.The shale oil occurrence state and mobility in this area also need to be evaluated.This paper takes the shale oil reservoirs of Lucaogou Formation in Jimsar Sag as the research target,firstly evaluates its physical property,oil-bearing property and mobility,and then carries out geological sweet spots evaluation.The microscopic characteristics such as pore types and pore throat structures of the shale oil reservoir were revealed by casting thin sections,field emission scanning electron microscopy,high-pressure mercury injection,NMR and other experiments.According to the fractal theory,the pore throat of the shale oil reservoir was divided according to size,and the reservoir classification related to physical property was established based on the cluster analysis method.The oil-bearing property of the shale oil reservoir was described from macro to micro scale by core oil grades and laser confocal test of fresh samples,and the occurrence state of shale oil was quantitatively characterized by the combination of NMR and centrifugation test.Based on the oil test productivity,oil occurrence state and oil saturation test of the cores,the grading evaluation related to shale oil performance was realized.The shale oil mobility was quantitatively characterized and evaluated by the combination of NMR and displacement test.The classification scheme of shale oil geological sweet spots was established by weight analysis of 3 geological sweet spots evaluation indexes related to physical property,oil-bearing property and mobility through the grey correlation analysis.The results show that: 1)The pores of shale oil reservoirs in this area are mainly composed of residual intergranular pores in micro-nano scale,intergranular dissolved pores,intragranular dissolved pores,and intercrystalline pores,with few organic pores and microfractures.The average pore throat radius(R50)varies from 0.007 μm to 0.39 μm with an average of 0.058 μm.T2 is mainly distributed in the range from 0.1 ms to 50 ms,and the corresponding pore diameter ranges from about 0.01 μm to 0.5 μm.2)Based on the fractal theory,the pore throats of the shale oil reservoirs of Lucaogou Formation are divided into micro pores(with the pore throat radius less than 15 nm),small pores(with the pore-throat radius between 15 nm to 100 nm),medium pores(with the pore-throat radius between 100 to 750 nm)and large pores(with the pore-throat radius greater than 750 nm).By clustering analysis of different scales of pore throat proportion,the shale oil reservoirs are divided into five types,including I-IV reservoir and invalid reservoir,and the boundaries related to physical properties and pore throats are determined.3)Lithology,pore throat size and source-reservoir combination modes control the oil-bearing property of Lucaogou Formation.The dolomitic siltstone,feldspar sandstone and arenaceous dolomite have larger pore throats and better oil-bearing properties,while the interbedding of source-reservoir has the best oil bearing property and the large sections of mudstone has the worst.On the microscopic level,the oil and water distribution shows the characteristics of "oil contained in large pores and water in small pores",in which the light oil mainly exists inside the larger pores and the heavy oil exists inside the smaller pores or on the pore wall in the form of adsorption.4)Based on Langmuir’s theory,the proportion of free oil in shale oil reservoirs ranges from 27.40% to 83.99%,with an average of 54.01%.Pore size and mineral composition both affect the occurrence state of shale oil.The proportion of adsorbed oil was positively correlated with specific surface area,TOC,clay mineral content,dolomite content,R50,and micropore proportion,and it was negatively correlated with the medium-macropores proportion.5)Under the displacement pressure of 10 MPa,the shale oil displacement efficiency in this area ranges from 5.04% to 65.08%,with an average of 35.99%.It is mainly affected by three aspects,including irreducible water,physical property and pore structure,among which R50 presents significant effect.Two inflection points corresponding to20 nm and 70 nm exist in the diagram of R50 and different displacement pressures,which can be used as the boundary of whether shale oil is mobile or not.When R50 less than 20 nm,shale oil mainly occurs as adsorption state,the displacement efficiency under 10 MPa is generally less than 10%,with an average of 7.16%.When R50 ranging from 20 to 70 nm,shale oil occurs in the combination of adsorbed state and free state,and the displacement efficiency under 10 MPa is between 10-40%,with an average of 27.10%.When R50 larger than 70 nm,shale oil mainly occurs in free State,and displacement efficiency under 10 MPa is more than 40%,with an average of 57.02%,and increases rapidly with increasing R50.6)Permeability,oil saturation and movable porosity were selected as the evaluation index of physical property,oiliness,and mobility,respectively,and the weight coefficients related to three indexes were calculated with the grey correlation analysis,which is 0.398: 0.199: 0.403.According to the physical property and oil-bearing property classification,the geological sweet spots are divided into three types,including I-III geological sweet spots Among them,I-II geological sweet spots of upper sweet spot sections is mainly developed near Wells J37,J305 and J10050,while that of lower sweet spot sections is mainly developed near Wells J176,J179 and J10050.From three aspects such as physical property,oiliness,and mobility,the comprehensive evaluation scheme of geological sweet spot was established,which provides technical support for the effective development of Lucaogou shale oil and reducing the geological risk and cost.