P. B. Flemings, S. J. Haggerty, R. N. Anderson
This prolific Plio-Pleistocene field is bounded by a pressure-sealing fault which also seals large hydrocarbon columns. The fault is a key vertical migration pathway for the produced hydrocarbons. As most of the reservoirs are not filled to synclinal spill point, we investigated two other controls on hydrocarbon column height.
In the first scenario, column height is controlled by effective stress (overburden--pore pressure). If the buoyant stress of the hydrocarbon column exceeds the minimum effective stress, the rock fails, and vertical migration occurs via fracture flow. In the second scenario, column height is controlled by the capillary entry pressure. If the buoyant stress of the hydrocarbon column exceeds the entry pressure of the fault, vertical migration occurs via capillary flow.
We find the largest columns in the zone of maximum effective stress, which lies directly above hard geopressure. Yet, the buoyant stresses of the hydrocarbon columns reach only 60% of the failure stress (the minimum effective stress, constrained by frac completion of wells). Thus, the column heights may be controlled by the capillary entry pressure of the fault zone, which is sufficient to seal the observed columns (measured by mercury injection).
Vertical migration rates, implied by the capillary entry pressures, cannot charge traps in the time since hydrocarbon generation. As effective-stress-controlled fracture flow can explain how faults act as both pathways and seals, further work will constrain the failure-stress and explore whether significant fracture flow can occur at a proportion of the failure stress.
AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994