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city of a fault is variable, and over a range of textures the capacity to accu-
mulate will be limited so that only some critical vertical dimension of oil or
gas can accumulate.
We do not know what happens when this critical vertical dimension is ex-
ceeded, and the pressure within the petroleum exceeds the capillary displace-
ment pressure. Once a continuous oil phase, for example, exists througha
fault plane, it is possible that the whole accumulation will drain through,
leaving some residual oil in the original reservoir. But if oil migration away
from the fault is intermittent, breaking the oil-phase continuity in the fault
plane, it is possible (as Smith, 1966, suggested) that the reservoir will be
maintained at about its critical vertical dimension, with the surplus passing
through. The existence of several fault traps in one field (e.g., Seria field,
Brunei, shown in Fig. 15-13) suggests that the latter occurs. In either case,nc
generalizations can be made except that in areas rich in fault traps, it is prob-
ably safe to say that all faults are sealing.
Smith reports evidence that depletion of reservoirs on one side of a fault by
production led to the breakthrough of undepleted juxtaposed reservoirs when
the differential pressure across the fault reached 1 or 2 MPa (150-300 psi)

fault plane

Fig. 11-7. Migration through faults must be distinguished from migration in faults.