Logging Identification Of Lamina Combination Characteristics Of Fine-grained Sedimentary Rocks In Continental Lake Basin

With the increasing demand of oil and gas resources all over the world,unconventional oil and gas has increasingly become an important alternative source of oil and gas resources.As the source rock and important reservoir of unconventional oil and gas,fine-grained sedimentary rock has gradually become a hot spot and frontier field of sedimentology and petroleum geology in China and foreign countries.As the basic unit of fine-grained rocks,almost all fine-grained sedimentary rocks contain laminae in varying degrees.Lamina combinations are widely developed in fine-grained sedimentary rocks.It determines the reservoir quality and controls the enrichment of oil and gas.Therefore,the study of lamina combination characteristics is very important.It is urgent to establish a logging evaluation method of lamina combination characteristics to realize the evaluation of lamina characteristics in non-cored well sections.The 7^thMember of Yanchang Formation in Ordos Basin is a typical fine-grained sedimentation with great exploration potential,but the research on the correlation between fine-grained sedimentary laminae and reservoir,logging identification and prediction is weak.On the basis of fully absorbing the previous research results on the lamina structure of fine-grained sedimentary rocks,this study comprehensively uses the analysis methods of core observation,thin sections identification,logging data interpretation,reservoir physical property analysis to identify the lamina combination characteristics of the 7^th Member of Yanchang Formation in Ordos Basin.The results show that the lithology of Chang 7 member in the study area is a complex rock combination composed of fine-grained materials mainly including fine sandstone,siltstone,mudstone and shale.According to the core and thin sections microscopic observation of Chang 7 member of Yanchang Formation in Ordos Basin,the stacking relationship of lamina minerals is determined.According to the scale of lamina development,Chang 7member is divided into three types:massive,layered and lamina.The lamina is further divided into binary lamina（felsic+clay,felsic+organic matter,tuffaceous+organic matter,clay+organic matter）,ternary lamina（felsic+clay+organic matter,felsic+tuffaceous+organic matter）,multiple lamina（felsic+organic matter+tuffaceous+clay）,a total of seven categories.Using imag logs,imag logs a slab-like images and imag logs micro button conductivity（resistivity）cruve,the recognition plots of different types and scales of lamina structures of massive,layer and lamina with maximum resolution of 2.5mm are made.Combined with the measured data of physical properties and T2 spectrum of nuclear magnetic resonance logging,it shows that the porosity and permeability of massive siltstone and fine sandstone are good.Through the comprehensive analysis of nuclear magnetic resonance logging,it is pointed out that the massive siltstone and fine sandstone in Chang 7 member of Ordos Basin have good oil and gas potential and have great exploration prospects,followed by layered siltstone and shale,and the massive mudstone is the worst.Among the seven types of shale lamina combinations,felsic+clay binary lamina has the best porosity,permeability and oil and gas potential,while clay+organic binary lamina the worst.The established plots are used to predict and divide the development lamina in single well,and compared with the actual physical property test results and logging data.The predicted results are in good agreement with the reality.Based on the results of coring sampling well sections,the corresponding identification plots of different scales and mineral superimposed lamina logging are established,and the coupling relationship between different lamina and favorable reservoir is clarified.The logging prediction of favorable reservoir in well sections can be realized based on lamina structure.The fine-grained sedimentary rock lamina logging identification method established in this study can guide the further exploration of unconventional oil and gas in the 7^th member of Yanchang Formation in Ordos Basin.