Using the offshore Pelotas basin, Brazil, as anexample, we present a methodology by whichpetrophysically-derived Vclay and capillary displacement pressure data, in conjunction with interval isochore maps, can be used to evaluate regional, siliciclastic present-day and paleo- top-seal 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 Vclay, 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 meters of contained gas column. Laboratory and model data (Aplin, Lang and Katsube, 2000) 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 variation 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.


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