Using the offshore Pelotas Basin, Brazil, as an example, we present a methodology by which p etrophysically-derived Vclay and capillary displacement pressure data, in conjunction with interval isochore maps, can be used quantitatively to evaluate regional, siliciclastic present-day and palaeotop-sea/ effectiveness and relative risk. This method has broad application to frontier and maturing exploration areas where data limitations preclude more sophisticated seismically-derived velocity-based evaluations. As much of the Pelotas Basin is deemed to be gas prone, top-seal effectiveness for normal density (0.1 to 0.2 g/cc) dry gas was assessed quantitatively by establishing relationships between density-log -derived hydrocarbon column height and overburden thickness using a most likely Vclay content. With constant May, column height increases with increasing overburden due to a compaction-driven decrease in mudrock porosity, accompanied by a decrease in permeability and pore-throat diameter. Using these relationships, interval isochore maps (overburden thickness maps) can be transformed into hydrocarbon column-height maps to define spatial variation in top-seal effectiveness, expressed in metres of contained gas column. Laboratory and previously-published model data show that clay content and porosity (ultimately pore throat diameter) are the dominant controls on siliciclastic mudrock permeability, hence top-seal potential. Mudrock porosity is driven dominantly by burial-induced compaction; clay content is dependent on both depositional and diagenetic processes. Overburden can be determined with a reasonable degree of certainty from seismic and well data, whereas regional variations in clay content can, at best, only be estimated from depositional models. Therefore, for a given overburden, it is uncertainty in clay content that comprises the greatest risk in regional siliciclastic top-seal analysis. For this reason, we relate siliciclastic top-seal risk to clay content, when overburden/ mudrock-porosity relationships, fluid densities and requisite column heights are known.