The VLA-6/9/21 Area, located in the Bloque I of Maraven, S.A., in northern Lake Maracaibo, has produced approximately 500 MMSTB (million stock-tank barrels) of oil from approximately 220 wells in the Lower Misoa Formation (C Members) since the mid-1950's. This field remains in the middle stages of primary recovery, however, and still contains a considerable recoverable oil resource of 634 MMSTB.

At the level of the Lower Eocene Misoa Formation, the VLA-6/9/ 21 Area occupies a structurally high area between two left-lateral strike-slip faults with dip-slip displacement. These faults are the Icotea Fault, which is part of a positive flower structure, and the Eastern Boundary Fault. A westward-dipping reverse fault, the Attic Fault, merges with the Icotea Fault below the Misoa Formation. Although the Attic Fault results in as much as 175 ft (53.4 m) of repeat section in the deeper C7 Member of the Misoa Formation, it has less than 25 ft of displacement (7.6 m) in the shallow C3 and C4 Members. This study interprets the Eastern Fault as a westward-dipping reverse fault with an antithetic, eastward-dipping normal fault. The Eastern Fault results in as much as 100 ft (30.5 m) of repeat section in the C7 Member, whereas the antithetic normal fault has 300 to 600 ft (91.5 to 182.9 m) of missing section.

The portion of the VLA-6/9/21 Area between the two left-lateral faults is divided into multiple structural compartments by numerous primarily northwest-trending normal faults. We interpret these normal faults as extensional fractures that formed during transpression. Some of these faults are inferred to be fluid-flow barriers because of abrupt changes in water-cut values on opposite sides of many of these faults. Furthermore, many examples exist where a water-injection well and a nearby producer are on opposite sides of an intervening fault. Commonly in such cases, the expected pressure increase in the producing well never occurs, indicating that the fault is sealing.

This study recognizes at least eight large-scale (150- to 700-ft [45.7- to 213.4-m]) unconformity-bounded sequences in the C7 to C2 Members. These sequences combine different facies assemblages and sandstone-body geometries from multiple smaller, upward-coarsening intervals (parasequences). To resolve sandstone-body architecture at a reservoir scale, however, we correlated 33 thinner genetic stratigraphic units (submembers). Maps of individual genetic stratigraphic units in the VLA-6/9/21 Area are inferred to depict reservoir geometry more accurately because they are composed of small-scale depositional cycles (parasequences) that have fewer flow units. These units, identified in wireline logs as low-resistivity, high-gamma-ray shale beds traceable across much of the field and typically upward coarsening, are capped by marine flooding surfaces.

Future development strategies in the VLA-6/9/21 Area should include an aggressive campaign of recompletions and infill wells strategically targeted to produce bypassed oil. Additional water-injection wells can provide downdip pressure support to improve oil recovery in poorly swept areas. Location of these producing and injection wells should favorably align fluid withdrawals in order to take advantage of the complex reservoir sandstone-body architecture and structural compartmentalization.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah