Nuclear Magnetic Resonance Response Characteristics And Oil Saturation Evaluation Of Lacustrine Shale Oil Reservoir

The lithology,mineral composition and pore structure of the lacustrine shale oil reservoirs with diverse types and wide distribution are extremely complex,and both organic and inorganic pores developed,leading to the complexity of shale oil fluid occurrence and making fluid identification difficult,which poses challenges to the quantitative evaluation of shale oil saturation.Currently,there are few methods for evaluating the oil saturation of shale oil reservoirs,and many studies are still exploratory.Based on nuclear magnetic resonance（NMR）technology,this paper characterizes the fluid occurrence state and evaluates the oil saturation of shale oil reservoirs by evaluating the NMR response characteristics and rules of shale oil.This paper focuses on the nuclear magnetic resonance（NMR）response characteristics and fluid occurrence state of lacustrine shale oil by conducting NMR experiments on shale oil rock samples collected from the Mahu Sag in Xinjiang,combined with scanning electron microscopy,physical property experiments,and XRD whole-rock mineral analysis data.The results and findings are as follows:the shale oil reservoir in the study area is dense,with a complex pore structure that includes organic and inorganic pores.The presence of solid kerogen and bitumen causes a rapid attenuation of NMR signals,and the surface relaxation effect manifested by the short T₂relaxation time is more pronounced.Additionally,the complex lithology,including the interbedding of sand shale,dolomitic shale and mudstone,makes the NMR response characteristics complex,and different types of fluid NMR signals often overlap.The conventional method of using T₂cutoff values is difficult to accurately distinguish between free oil and bound oil,making it difficult to quantitatively evaluate oil saturation.Thus,based on the above experiments,this paper established a method to quantitatively calculate the fluid saturation of different components in shale oil,and proposed a method to divide reservoirs based on lithology for complex lithologies.Using XRD analysis and logging interpretation data,the mixed rock index and mud index are calculated to classify the shale oil reservoirs in the study area into sand shale,dolomitic shale,and mudstone.Based on the classification results of the reservoirs and the NMR response characteristics of the shale oil in the study area,a two-dimensional T₁-T₂NMR fluid identification map is constructed for the shale oil in the study area,which classifies the shale oil into bitumen,clay-bound water,capillary-bound water,bound oil,and movable oil.And on this basis,we propose a NMR two-dimensional T₁-T₂fluid saturation quantitative evaluation model to calculate the saturation of different fluids.The method is applied to the two-dimensional NMR logging fluid saturation evaluation,and it is found that the sand shale of the Mahu Fengcheng Formation shale oil is a dominant reservoir with high movable oil saturation and good development potential.