A major challenge when exploring for hydrocarbons in frontier areas is a lack of data coverage. Data may be restricted to regional-scale 2D seismic lines, from which assumptions of the 3D geometric configuration are drawn. Understanding the limitations and uncertainties when extrapolating 2D data into 3D space is crucial when assessing the requirements for acquiring additional data such as 3D seismic or exploration wells and of assigning geologically reasonable uncertainty ranges. The onshore Gulf of Corinth Rift provides an excellent analog for rift-scale structural uncertainty in the context of hydrocarbon exploration. We have used seismic forward modeling to explore this area of uncertainty. Synthetic seismic sections have been generated across the rift based upon fault geometries mapped in the field. Comparisons that we made of these sections with the mapped geometries allowed quantification of uncertainties encountered when extrapolating 2D data into three dimensions. We have determined how potential column heights may be severely over and underestimated due to trap integrity, spill point depth, and fault seal ambiguities directly related to fault geometric uncertainty. In addition, fault geometries and linkages also controlled the location of hanging wall synrift reservoirs. Hence, gross reservoir volumes and sediment facies distributions were also significantly influenced by how fault geometries were extrapolated along-strike from 2D to 3D.