Marlan W. Downey
The role of faulting in hydrocarbon entrapment is examined through the circumstances by which faulting aids or impairs entrapment. The major significance of a fault is that it rejuxtaposes lithologies at different structural attitudes than before faulting. The fault plane itself has little effect on migrating hydrocarbons except in very specific circumstances: (1) very shallow tensional faults may be expected to allow leakage along the fault plane, (2) fault gouge, created by fault action and incorporating large amounts of permeable material, may allow leakage along the fault plane, and (3) faulting within geopressuring may be expected to leak.
Where faults juxtapose permeable beds against adjacent impermeable layers, faulting serves to seal an accumulation. If faulting affects an interbedded series of reservoirs and seals, multiple subparallel faulting greatly increases the probability that any individual reservoir may be sealed laterally. As a generalization, if an interbedded series of reservoirs and seals contains 25% reservoirs, a fault offset provides a 50% chance that an individual reservoir might be sealed laterally across the fault. A second paralleling fault would increase the probability of a sealed reservoir to 75% (50% + 50% × 0.5 = 25%, plus the existing 50%).
AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.