Abstract: Mud filtrate intrusion is weak in the extra-low permeability reservoirs Chang3 and Chang 4+5 in the BB Oil Filed,the Ordos Basin.The ordered arrangements of ML and DIL-MSFL which intuitively indicate hydrocarbon reservoir and water zone disappear.Micro-fractures in reservoir lead to the irregular enlargement of holes;hence the oil/gas composition has a smaller part in logging response.The difference is small between the productive low permeability reservoirs and the inefficient ones.Establishing the lowest limit of effective thickness,taking the formation resistivity as the geologic background,analyzing the core electrical characteristics and the relevant statistic criterion,the figure and effective oil/gas composition which affect the parameter variation in extra-low permeability reservoirs are identified by taking full advantage of logging data.Synthetically comparing the logging,geologic and oil-testing data,studying the ration statistics,deduction method and figure affection to log curves of calcic,argillan interbeds in reservoirs with different lithologic characteristics,with the latitude,difference and characteristics of ML log curves,the extra-low permeability reservoirs are finely evaluated and classified.The log interpretation and potential range prediction accuracy have been improved,providing dependable criteria for reservoir enlargement and outcome improvement in extra-low permeability oil fields.