Two major factors affect exploration efficiency: undiscovered pool or field sizes and the uncertainty of turning a prospect into an economic discovery by exploration drilling. One can neither stop the trend of decreasing pool size with exploration maturity in a play, nor completely eliminate the exploration risk. However, one can reduce exploration risk significantly by gaining a better understanding of the geographic distribution of petroleum accumulations. This paper presents an integrated approach for visualizing exploration risk by uncertainty mapping. Exploration uncertainty is expressed as the probability of petroleum occurrence as estimated from both exploration drilling outcomes and the spatial variation of geological favourability for petroleum accumulation. The Mahalanobis D2 distance, a measure in a multivariate space of the difference between the geological feature of discovered pools and the feature at untested locations, is used to represent the geological favourability. This method is applied to the Middle Devonian Keg River reef play of the Rainbow sub-basin, in the Western Canada Sedimentary Basin. A historical analysis of uncertainty mapping in the Keg River reef play shows that the proposed approach could improve the exploration success rate by up to 20%.