Fractured reservoirs around the world were studied to determine how inherent reservoir and fluid properties (such as porosity, permeability, viscosity, mobility ratio, water saturation, wettability, fracture distribution, and drive mechanism vs. reservoir-management strategy) affect ultimate recovery. Fractured reservoirs were divided into four groups. Two groups were of particular interest: one with low matrix porosity and permeability and the other with high matrix porosity and low matrix permeability. Previous fractured-reservoir classifications did not distinguish between these two reservoir types.