Identification And Evaluation Of Source Rock And Reservoir In Tight Oil From The 7th Member Of Yanchang Formation In The Center Of Lake Basin,Ordos Basin

Although the global energy structure continues to shift to low-carbon,but the oil and gas energy still remain to be the main energy sources,which will not be changed for a long time.Due to the large amount of tight oil resources in China,the effective development of tight oil is the best way to reduce external dependence of crude oil.Tight oil from the 7th member of Yanchang formation in Ordos Basin,as a representative of continental tight oil in China,has been developed on a large scale after more than 10 years of exploration.However,there are still some problems in lithology interpretation,source rock identification,and comprehensive evaluation.In order to comprehensively evaluate and predict the"geological dessert area"of tight oil in the center of the lake basin,the evaluation was carried out mainly from four aspects:provenance and sedimentary microfacies,hydrocarbon source rocks,reservoirs,and fractures.Based on the existing foundation of the related work such as provenance analysis in the Ordos Basin,this study found that the light and heavy minerals of clastic rocks,the major elements of sandstone,trace and rare earth elements of mudstone have the following distribution rules in the study area.The light minerals in northeast are characterized by high feldspar content and low lithic content,while the heavy minerals are dominated by zircon and garnet.The major elements are characterized by high sodium content,low magnesium content,high aluminum content and low phosphorus content.The samples from the western are rich in lithic and quartz,and the heavy minerals are mainly zircon and white titanium.In the south of study area,the quartz content is high,followed by feldspar and lithic.The heavy minerals are mainly zircon,garnet,chalazite and opalite.Calcium and magnesium are more abundant than those in the west.The rare earth elements of mudstone have the general characteristics of light rare earth enrichment and heavy rare earth deficit,and are closely related to yinshan magmatic rock,southern metamorphic rock and Qinling granite gneiss.The result shows that the main provenance came from the northeast,west,and south,where the provenance effect was weak in the northeast and west,but strong in the south.The deep lake deposits are gravity flow deposits dominated by turbidite fans,which can be divided into two sedimentary subfacies,the sliding turbidite fan and slope turbidite fan according to the development position and characteristics of the sublacustrine fanLithology identification is very important in source rock and reservoir evaluation.In view o the complexity and diversity of provenances of the mixed source area in the center of the lake basin,which caused the content of skeleton mineral to be variable and the different lithologies overlap in logging response.In addition,the logging background value caused by the common development of hydrocarbon source rocks leads to difficulties in lithologic identification.In this study,the following methods were used to effectively classify lithology.1)Sandstone is divided into interbedded sandstone and concealed sandstone according to whether the sandstone reservoir is interbedded with the hydrocarbon source rock.For the above two types of sandstones,the natural gamma,density,and compensated neutron curve overlap diagram can be used to realize lithologic identification in?The multi-curve overlap diagram can well correspond to the lithologic interface.2)In order to identify lithology more easily and quickly,the density and neutron curve normalized superposition method can be used to effectively identify sandstone,siltstone,mudstone,and hydrocarbon source rock.3)The identification range of various lithology can be established by using the cross plot of multi-curve sensitive parameters:density and natural gamma,compensated neutron and natural gamma,and density-neutron normalized difference and natural gamma.4)The regression relationship between the normalized difference of density and neutron curve and the total clay mineral content of core X diffraction was built to establish the clay content calculation model.According to the model,the clay content curve can be calculated,which provides evidence for lithologic identification.The TOC(Total Organic Carbon)content of source rocks in the center of the lake basin is high.The best source rocks(TOC>2%)and good source rock(TOC>1%)are well developed.Kerogen type is given priority to with?and?₁,which has great potential for hydrocarbon generation.The logging curves of hydrocarbon source rocks are characterized by high natural gamma,high photoelectric absorption cross section index,high resistivity,high acoustic time,high neutron,and low density.The?log R method has been used for qualitative identification and quantitative evaluation of the hydrocarbon source rocks in this study.The source rocks are identified qualitatively by using the overlapping curves of resistivity and acoustic time.The quantitative interpretation model of source rock TOC is established by using TOC and?log R regression formula.The model is verified by the organic carbon analysis data of H317well source rock,and the correlation between the model and the measured TOC is well.The reservoir evaluation is carried out from two aspects of physical property and oil-bearing characteristics.It shows that the single porosity logging interpretation model has poor effectiveness in the calculation of porosity.In this study,three parameters of density,acoustic time difference,and volume photoelectric absorption cross-section index were selected to calculate porosity by multiple regression.The absolute error is less than 1%.After removing the data points of low porosity and high permeability in fracture development sites,the permeability interpretation model for the study area is established according to the correlation between porosity and permeability(correlation coefficient 0.79).In order to divide effective reservoirs,the lower limit of reservoir physical property was determined as 4%porosity and permeability as 0.03×10^-3?m² by a comprehensive empirical method and productivity prediction method.The overlap diagram of density and resistivity curves can be used to display oil and water layers quickly and visually.The cross plot of density and resistivity can be used to distinguish the oil layer,oil and water layer,water layer,and dry layer.Because the additional conductivity of clay minerals and other minerals is relatively weak,the oil saturation logging interpretation model can be established by using the Simandoux formula.The calculated results of oil saturation are in good agreement with the sealed coring data.Based on identification and evaluation of source rocks and reservoirs,the classification basis are classified into three categories.Combining production data,four typical sources-reservoir collocation relationships were divided they are namely"high quality source rock and high quality reservoir"collocation,"high quality source rock and medium-poor quality reservoir"collocation,"medium-poor quality source rock and high quality reservoir"collocation,and"medium-poor quality source rock and medium-poor quality reservoir"collocation.Combined with the degree of fracture release and development,the grade A,B,and C"geological dessert area"were predicted.Among them,the grade A"dessert area"is located in the Longdong area,including southeast of Huanxian county,southwest of Huachi County,Qingcheng-Heshui area,and the Southern part of the study area,Shigu-Ning County.The predicted results are consistent with the practice of tight oil exploration in the Ordos Basin.Grade A"dessert area"can be regarded as the focus of the next exploration and development in the southwest of the Ordos Basin.