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Seal capacity is the amount of hydrocarbon column height a particular lithology can hold back. Most analyses of seal capacity are madc with the assumption that the petroleum system has reached equilibrium, 1.e. all generation, migration, structural adjustments etc. have ceased, and that either the structural or capillary spill point has been reached. In such static systems, determination of seal capacity is simply a calculation of the relationship between the buoyancy pressure of the hydrocarbon column and the capillary properties of the (up dip, lateral or bottom) seals. However, in many young or rejuvenated petroleum systems, active generation and migration may still be occurring. In these dynamic systems the present hydrocarbon column height is a function of not only the seal capacity, but also of the difference between the rate of influx of hydrocarbon through a feeder or carrier bed, and the rate of outflow (leakage) of hydrocarbon through the seal. This rate difference, or ",lag", is controlled by the relative permeability contrasts between the carrier bed and the seal. Such mechanisms may be responsible for some of the stacked hydrocarbon pools observed within many Indonesian fields.An example of such a dynamic system is the Pagerungan gas field of the East Java Sea, Indonesia. The reservoir at Pagerungan is the Mid to Late Eocene Ngimbang Clastics Formation. Prior to this study there was considerable uncertainty as to which formation was the seal. Therefore, seal capacities were determined for various formations using mercury injection capillary pressure (MICP) analyses. These analyses indicate that the Late Eocene Ngimbang Shale is the best top seal over the Pagerungan field, with seal capacity of approximately 700 ft. of reservoir gas. However, the field contains an observed 1200 ft. gas accumulation. Gas ",chimneys", are seen on seismic sections above most of the structural culminations of the field, and shallower reservoirs on the structure contain gas of the same composition as the main reservoir zone. In addition, rocks which coRtain up to 10% residual (trapped) gas extend down to 250 feet below the present field Free Water Level (FWL), to approximately the same depth as the structural spill point.The interpretation of these observations is that gas is actively migrating, at geological rates, through the Pagerungan system. A pre-existing deeper FWL existed below the one found today, and gas is presently leaking through the Ngimbang Shale. This leakage has resulted in the charging of shallower reservoirs up-structure in Pagerungan, and the creation of ",waste zones", in the poorer reservoir quality rocks. Presumably, this ",tertiary migration", process will continue until the column height equilibrates to capillary top seal constraints. Exploration implications are that though the units between the producing zones and the main sealing interval are waste zones, these rocks are charged and could be economic if reservoir quality rock were to be encountered.