Inspection of wireline log responses from 80 wells in the North Sea has demonstrated that porosities of individual formations are reduced with depth, irrespective of present fluid pressure. This statement applies to both the Middle Jurassic intra-reservoir rocks and to cap rocks of Cretaceous and Tertiary age. A corrollary of this result is that all pressure compartments leak or have leaked, and that pressure compartments which are at their maximum burial depth (or temperature?) are currently leaking. The critical issue is thus often not whether a compartment leaks, but where and how it leaks.

Theoretically, leakage from a pressure compartment should most commonly take place downflanks, in the water bearing zone. This is so because fluids don't have to overcome capillary pressures to leave the compartment here, and the permeability of water in a water bearing, water wet cap rock will be significantly higher than the relative permeability of hydrocarbons in a cap rock containing both water and hydrocarbons. However, overpressured reservoirs may leak in abrupt manners through faults or hydraulic fracturing of cap rocks. It is suggested that hydrocarbon leakage from overpressured reservoirs as a rule takes place either through faults or through (micro-)fractures, as opposed to leakage through the pore network in homogeneous cap rocks.

Inspection of faults (or cap rocks) for sealing properties should not only focus on the faults which are in contact with the reservoired hydrocarbons, but should rather encompass all the faults, cap rocks and seat seals of the pressure compartment under investigation, in search for the weakest point. Previous seal analyses have far too often been one-dimensional (cap rock integrity vs. bouyancy) or two-dimensional (inspection of single fault planes), disregarding that leakage, in essence, is detemined by the relative properties of all the boundaries of three-dimensional pressure compartments.

In practise, both identification of pressure compartments and accurate assessments of the sealing properties of all their boundaries are close to impossible. Attribute analysis of three - dimensional seismic can however often reveal where some types of leakage takes place, and should be actively used in hydrocarbon migration analysis.

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