Abstract: A method of mapping reservoir fluid in a mature field includes identifying a region of interest within a hydrocarbon field, drilling a plurality of production wells through the region of interest, and collecting mud gas data as each production well is drilled. This mud gas data is used to generate a reservoir fluid property log, such as gas-oil ratio log, along a length of each of the production wells, which is in turn used to generate a refined model based on the 4D seismic data. The refined model permits better reservoir fluid mapping and has an improved vertical resolution at the region of interest than the first model of the field.