Hydrocarbon flow across faults by capillary leakage revisited

Hydrocarbon fluid pressures can equilibrate across faults provided that the hydrocarbon charge into the reservoir is sufficient to keep the buoyancy f orce in the hydrocarbon column above the capillary entry pressure of the fa ult 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 beco me permeable to hydrocarbons once their capillary entry pressure has been e xceeded. The absence of differences in either pressure or hydrocarbon colum n heights across faults does not, we propose, preclude the presence of a ba rrier to fluid flow. Fluid pressure and hydrocarbon column height differenc es between compartments can be controlled by factors such as capillary entr y pressure in the undeformed reservoir and the amount of hydrocarbons enter ing the reservoir, rather than solely by the capillary entry pressure of th e fault rocks present. Fault seal prediction methodologies that are calibra ted, based on cross-fault differences in hydrocarbon column height or press ure, without considering the total hydrocarbon column height are likely to be unreliable. It is therefore recommended that the sealing capacity of a f ault should be calculated from the difference in pressure between the hydro carbon and pore-water at the position along the fault where leakage is most likely to occur. (C) 2001 Elsevier Science Ltd. All rights reserved.