ABSTRACT
Closures sealed by basement normal block faults are most commonly formed by a grid of faults with multiple orientations and by a favorable dip of the reservoir strata. Hydrocarbons are trapped updip by structural upturns along the edge of the downdropped blocks or at the upthrown or downthrown edge of blocks where block tilt is away from the sealing faults. Closure parallel with reservoir strike is provided by (1) updip junction of primary oblique or oblique and longitudinal block-bounding faults, (2) internal secondary faults that trend transverse or oblique to the block boundaries, (3) positive bend of reservoir strata, (4) plunge of block-edge folds, and/or (5) stratigraphic terminations of the reservoir.
Fault seal upward along the faults is most likely where a thick section of impervious strata above the reservoir is in contact with itself across the fault or where the fault does not extend upward into shallower permeable zones. Seal across the fault surface is most certain where impermeable rocks are laterally juxtaposed against the reservoir. Impervious material within the fault zone, such as shaly smear gouge, cataclastic gouge, mineral deposits, and asphalt or tar impregnations, can be effective barriers where reservoir facies are juxtaposed across the fault. The traps can form a major hydrocarbon play on the stable shelves of foredeep or foreland basins, as demonstrated by the San Joaquin Valley and eastern Venezuela basins. The traps provide secondary prospects in rift basins, such as the Viking graben and Vienna basin.
