Application Of Nuclear Magnetic Resonance In Genesis Analysis And Recognition Of Low Permeability And Low Contrast Oil Layers

In the exploration and development of major basins,more and more low-contrast oil layers have been discovered,which is also one of the hot spots in exploration and development in recent years.However,the appearance of this type of oil layer makes the characteristics of oil and water layers in logging response unclear.Using conventional logging data to identify reservoir fluids has a low coincidence rate and the identification effect is not ideal.Therefore,it is very necessary to study the cause and identification method of low-contrast oil layer,and it has important practical significance.This paper takes the Chang 8 low-contrast oil layer in Pengyang area of the Ordos Basin as the research object.Based on the cause of the low-contrast oil layer,the genetic analysis of the low-contrast oil layer and the corresponding reservoir fluid identification method research and reservoir saturation evaluation are carried out.First,a series of rock physics experiments based on nuclear magnetic resonance were designed.Based on the analysis of the experimental results,the origin of Chang 8 low-contrast oil layer in Pengyang area was clarified.Then combined with the stratigraphic characteristics of the study area,a new fluid identification factor-the product of reservoir resistivity and reservoir quality factor(RQI/FZI),effective source rock thickness,and ratio of reservoir to effective source rock(H/h),is proposed.Using this parameter can accurately identify the nature of the fluid.In addition,based on the optimized Archie formula and the dielectric water saturation model modeled on Archie,the reservoirs in the study area were evaluated for more accurate water saturation.Based on the evaluation results,a fluid identification method with overlapping water saturation and movable water saturation was established.The main conclusions and understandings are as follows:(1)High bound water saturation is the main reason for the low resistivity of the oil layer in the study area.The quality of the reservoir and the degree of filling affect its accumulation.(2)The fluid identification limit of the plate is clearly defined.It is considered that when the recognition factor is greater than 3,it is a hydrocarbon layer,otherwise it is a water layer.When the water saturation is less than 58% and the movable water saturation is less than 6.5%,it is an oil and gas layer,otherwise it is a water layer.The application found that the two recognition plates have good practicability in the study area,and the coincidence rate of the plate is over 85%.(3)Based on the core dielectric experiment,a model for calculating water saturation using dielectric logging is established.The biggest advantage of this method is that its calculation result is not affected by the formation water salinity.(4)Using the results of core nuclear magnetic resonance experiments,a variable T2 cutoff value based on reservoir classification and a calculation method for bound water saturation are proposed.The correlation between nuclear magnetic binding water saturation and comprehensive physical property index is established.Using this model,the binding water saturation can be calculated in wells without nuclear magnetic data.