Hege M. Nordgård Bolås, Christian Hermanrud, and Gunn M. G. Teige
Statoil ASA, Trondheim, Norway
A caprock's capacity to seal hydrocarbons depends heavily on the diameters of the smallest pore throats that the leaking hydrocarbons must penetrate. Measurements of such pore throat sizes are often not available. However, the presence of reservoir overpressures testifies to small pore throats in the seals, and thereby holds indirect information on seal capacity.
We suggest a stepwise procedure that enables quantification of top seal capacities of overpressured traps based on subsurface pressure information. This procedure starts with the estimation of caprock permeabilities, which are consistent with the observed overpressure gradient across the top seal. Knowledge of burial histories is essential for these estimations. Relationships between pore throat sizes and permeabilities from laboratory experiments are then applied to estimate critical pore throat diameters in the caprocks. These critical pore throat diameters, combined with information of the physical properties of the pore fluids, are then used to calculate membrane seal capacities of the caprocks.
Estimates of top seal capacities based on this procedure are rather sensitive to the vertical fluid flow velocity, and they are also to some extent sensitive to inaccuracies of the pore throat/permeability relationship. Despite these uncertainties, our procedure demonstrated that the mere presence of reservoir overpressures testifies to sufficient membrane seal capacity of the caprocks for most geological histories. Exempt from this statement are basins with rapid sediment compaction in the recent past.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005