Hydrocarbon flow across faults by capillary leakage revisited

Abstract

Hydrocarbon fluid pressures can equilibrate across faults provided that the hydrocarbon charge into the reservoir is sufficient to keep the buoyancy force in the hydrocarbon column above the capillary entry pressure of the fault rock. A fault surrounded by a complex damage zone does not necessarily have a higher sealing capacity than a single fault since, provided there is sufficient hydrocarbon charge, faults within the damage zone will all become permeable to hydrocarbons once their capillary entry pressure has been exceeded. The absence of differences in either pressure or hydrocarbon column heights across faults does not, we propose, preclude the presence of a barrier to fluid flow. Fluid pressure and hydrocarbon column height differences between compartments can be controlled by factors such as capillary entry pressure in the undeformed reservoir and the amount of hydrocarbons entering the reservoir, rather than solely by the capillary entry pressure of the fault rocks present. Fault seal prediction methodologies that are calibrated, based on cross-fault differences in hydrocarbon column height or pressure, without considering the total hydrocarbon column height are likely to be unreliable. It is therefore recommended that the sealing capacity of a fault should be calculated from the difference in pressure between the hydrocarbon and pore-water at the position along the fault where leakage is most likely to occur.