Logging Characterization Method Of Continental Shale Oil Reservoir Quality Variation In Junggar Basin

With the transfer of oil exploration from conventional to unconventional reservoirs,the study of continental shale oil reservoirs has become a hot spot of common concern for geologists and geophysicists.Continental shale reservoirs are characterized by diverse mineral compositions,complex sedimentary structures and strong heterogeneity,which make the comprehensive characterization of reservoir quality difficult.In this dissertation,the Fengcheng Formation shale reservoir in Mahu Sag is taken as the research object,and the core,thin section,XRD,conventional logs,Litho Scanner logs and electrical image logs data are integrated.A lithofacies classification scheme according to"composition and structure"is established,and continuous quantitative characterization of mineral composition,sedimentary structure and lithofacies are completed.The pore structure division scheme of the shale reservoir in the study area was established by extracting the sensitive parameters of the core nuclear magnetic resonance experiment.Then the calculation of the physical property parameters of the reservoir and the division of the pore structure with the nuclear magnetic resonance logs were completed.The qualitative identification of natural fractures and the quantitative calculation of fracture parameters are carried out with the calibration of image logs.Next,two-dimensional nuclear magnetic resonance logs and histogram statistical methods are used to clarify the relationship between the difference in reservoir quality and mineral composition,sedimentary structure,lithofacies and pore structure.Finally,based on the results of logging calculation and geological control factors,a classification scheme for the quality variation of continental shale oil reservoirs is established,and a set of logging evaluation methods for the quality variation of shale oil reservoirs in the Fengcheng Formation of the Mahu Sag is formed.Extrapolation to wells has clarified the spatial distribution of"sweet spots"in shale reservoirs.The shale reservoirs of the Fengcheng Formation in Mahu Sag have diverse mineral compositions and are characterized by mixed components,and are mainly composed of terrigenous clastic minerals（quartz,plagioclase and potassium feldspar）,endogenous chemical sediments（dolomite,calcite and alkaline mineral）.Therefore,the shale in the Fengcheng Formation of the study area can be divided into 5 types including felsic shale,dolomitic shale,mixed shale,calcareous shale and clay shale according to the mineral composition.The sedimentary structures can be divided into 3 types of massive mud shale,laminated mud shale and thin layered mud shale according to the scale.There are 15lithofacies types including dolomitic shale,laminated dolomitic shale and massive clay shale.For the composition,the conventional logs combined with Litho Scanner logs can be used to calculate and complete the continuous identification of the composition type.On the other hand,for the sedimentary structure,an dynamic image slab identification pattern is established to complete the vertical continuous identification of the sedimentary structure type.Finally,the vertical continuous division of lithofacies types can be completed by coupling the identification results of mineral components and the identification results of sedimentary structures.The calculation of the total porosity is completed through the nuclear magnetic resonance logging T₂ spectrum,and the 21ms is determined as the cut-off value between the movable fluid and the bound fluid through the production test data and the core NMR experiment.Then the movable porosity calculation is completed,which is consistent with the experimental results.The comparison and verification show that the total porosity and movable porosity calculated by nuclear magnetic resonance logging have a high degree of agreement,and the log T₂ geometric mean and movable porosity are used to complete the continuous division of the reservoir pore structure.Combined with the method of image logs,a fracture identification chart is established.By comparing with the production data of the perforation position,it is believed that high conductivity fractures are an important condition for high reservoir production.On this basis,the quantitative calculation of high conductive fractures parameters is completed.The quality of shale reservoirs in the Fengcheng Formation of the Mahu Sag is affected by mineral composition,sedimentary structure,lithofacies,pore structure,and the type and development of natural fractures.Through two-dimensional nuclear magnetic resonance and histogram statistics,it is believed that the thick layers of thin-layered felsic shale and laminar dolomitic shale are closely related to Type I reservoirs with better pore structure,and are more likely to be stored in the reservoir.The later production process produces natural fractures.On the contrary,the intervals where massive mixed mud shale and massive clay mud shale are located are mostly Type III reservoirs with poor pore structure,and at the same time,they are not easy to be fractured due to the low content of brittle minerals and become low production intervals.A logging characterization method suitable for the reservoir quality variation of continental shale oil reservoirs in the Junggar Basin is established by using the reservoir parameters calculated from the logging data and the main controlling factors of the reservoir quality.The distribution of“sweet spots”are determined by extrapolation,which will point out the direction for the future exploration and development of shale oil reservoirs.