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The occurrence of tar mats, their lateral extent and volume in place is important for reservoir assessment since it may provide information to improve reservoir management, help to assess the producible reserves and my help to increase the oil recovery. We discuss here currently available tools to locate tar mats in petroleum reservoirs. Dominantly core based detection methods are discussed and several log based methods will be briefly described. A newly developed core based method and a new log method are introduced: The low cost core density approach is based on routinely determined core petrophysical data (porosity, grain density). Porosity and rock density are usually measured on plugs which have been rinsed using organic solvents. In tar mat intervals the petroleum present in the core plug is frequently incompletely extracted and the petrophysical data result in artificially lower porosities and grain densities. The core density approach uses the difference between the real grain density and the calculated grain density to detect the presence of heavy oil or tar mat. The dipmeter approach makes use of the high vertical resolution resistivity measurements to detect thin tar mats which are characterized by a very high resistivity. Both new tar mat detection methods, i.e., the core based core density approach and the log based dipmeter approach allow tar mats to be readily identified and may serve as exploratory screening tools for more detailed organic geochemical analysis. |
