FAULT SEALS AND PETROLEUM ENTRAPMENT: EXAMPLES FROM SOUTHEAST ASIAN BASINS

Petroleum basins are tectonically formed and structurally deformed sedimentary basins in which faults of various scales occur and play important roles in petroleum migration and entrapment. Fault analyses in petroleum traps, long confined to the geometry of fault closures, have, over the past two decades, focused on fault-rock seals as well. Studies of normal faults in the Miocene-age deltaic sandstones in Sarawak, Malaysia, indicate that shale smear and deformation bands are two major processes operative in fault-rock seals in porous clastic reservoirs at least in relatively shallower basin depths. Algorithms designed for shale smear factor (SSF) and clay content ratio (CCR) were then applied to normal faults in two Miocene clastic fields, the Temana field offshore Sarawak and the Erawan field in the Gulf of Thailand. The results indicate that SSF values lower than 6 and CCR values greater than 30 percent on the fault surface indicate across-fault sealing at sand-sand interfaces and that fault seals appear to be effective enough to support hydrocarbon columns filled down to the structural spillpoints of the reservoirs. Fault-rock permeabilities calculated for these fault seals are less than 0.3 millidarcy. Our studies also show that while fault-rock seals are important features, the traditional concept of shale-sandstone juxtaposition may as well explain a considerable number of fault traps. Moreover, irrespective of what fault sealing process may have been operative, the present stress field acting upon a given fault ultimately determines whether the fault seal has been breached or remained intact. As discoveries of oil from giant anticlinal structures have dwindled, oil reservoirs compartmentalized by faults have drawn increasing attention. However, scientific capability to model and predict the various sealing processes in subsurface fault zones is still in its infancy, thus calling for breakthrough works in this area of research and development.