Seal capacity is the calculated amount of hydrocarbon column height a particular lithology can support. Most analyses of seal capacity are made with the assumption that the petroleum system has reached equilibrium; i.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 seal. 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 the common observation of stacked hydrocarbon pools within Indonesian fields.

An example of such a dynamic systems 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 900 ft. of reservoir gas. However, the field contains an observed 1100 ft. gas accumulation. Gas chimneys are seen on seismic sections through the field, and shallower reservoirs on the same structure contain gas of the same composition as is produced in the main reservoir zone. In addition, rocks which contain up to 10% residual (trapped) gas extend several hundred feet below the interpreted present field Free Water Level (FWL).

Assuming that the analyzed samples are truly representative of the best seals in this field, it is concluded that gas is actively migrating, at geological rates, through the Pagerungan system. An earlier FWL existed below the one found today, and gas is presently leaking through the Ngimbang Shale. This migration has resulted in the charging of shallower reservoirs up-structure in Pagerungan, and the creation of “waste zones” in the poorer reservoir quality rocks between the Ngimbang Clastics Fm. and the Ngimbang Shale Fm. Presumably, this migration process will continue until the column height equilibrates to capillary top seal constraints, or until the column is drawn down by production of hydrocarbons.

The application of seal capacity measurements in prospect and reserves assessments needs to take account of the possible dynamic nature of the petroleum system involved. The Pagerungan field provides an analogue for situations where measured seal capacities may be exceeded and where charging of younger reservoirs through capillary leakage may have occurred.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah