1. . 
Ratio of the volume percent oil to the volume percent brine in an oil mud, in which each is expressed as a percent of the total liquid in the mud. OBR is determined by retort analyses of oil and water content, but the brine content is calculated from the water content by using the chloride and calcium titration data. For example, if a mud contains 60 vol% oil and 20 vol% brine, the oil percentage is [60/(60 + 20)] 100 = 75%, and the brine percentage is [20/(60 +20)] = 25%. That OBR is written as 75/25.
Alternate Form: oil/brine ratio