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SWANSON, DONALD C., Swanson Consulting Services, Houston, TX, and JUDITH JAIMES, CARMEN VALDEZ, BAUSIS PIRELA, FREDRICO PUCHE, DORIS ROJAS, and EDUARDO MOYA, Corpoven, S.A.

ABSTRACT: The Boscan Field -- Venezuela's Giant Stratigraphic Trap With A Complex History

The giant multi-billion barrel Boscan Field lies in the northwest quadrant of the Maracaibo Basin, Venezuela. It is a large stratigraphic trap that has produced more than 800 million barrels of heavy oil since 1946, principally from Eocene fluvial-deltaic clastic reservoirs. These are faulted, folded, and are truncated by a pre-Oligocene unconformity. There is also significant production from Oligocene fluvial sandstones lying upon the paleotopography of the unconformable surface. This pre-Oligocene surface dips south-southwest and completely truncates the underlying Eocene reservoirs.

Cores taken by Corpoven, S.A. show Boscan reservoirs to be mostly fluvial-deltaic with braided stream deposits dominant. In parts of the field these occur in thick stacked valley-fill deposits. Elsewhere, braided stream reservoirs are part of widespread fluvial sheets. Point bar, distributary channel fill, crevasse, backswamp-marsh, and delta front deposits also are common.

Geologic events chronicling the formation of the Boscan Field have been interpreted as follows:

1. Eocene fluvial-deltaic deposits prograded east and southeast across the Boscan Field Area. Their source was generally to the west and the source-delta continuum had a strong northwest-southeast orientation. As deltaic deposits from the Guyana Shield and Perija Mountains built north and east across the Maracaibo Basin, a subsiding area northeast towards the State of Falcon received tremendous quantities of Eocene fine grained organic-rich deposits in pro-delta and deep water environments.

2. After sufficient burial, the Eocene shales in the deep basin began to generate hydrocarbons that began to migrate out of the deep basin towards the west and southwest.

3. In the Boscan Area, Eocene deposits were folded, faulted, and tilted towards the east and southeast. An active fluvial floodplain truncated the stacked northwest-southeast trending fluvial-deltaic deposits resulting in subcrop bands that form the pre-Oligocene unconformity topographic surface.

4. Hydrocarbon generation and migration continues in the deep basin.

5. Oligocene fluvial sandstones were deposited on the underlying unconformity. They often cut into and are in direct contact with truncated Eocene sandstones. This condition, along with numerous faults, allowed some oil to leak up into the Oligocene deposits.

6. By the end of the Oligocene, migration of oil into the gross Boscan trap was well under way. The east-west and northwest southeast trending fluvial conduits carried the oil out of the Eocene basin northwestward until it was blocked by reservoir truncation on the west and northwest sides of the Boscan Field.

7. From the beginning of the Miocene to the present time, regional subsidence of the Maracaibo Basin has caused the Boscan reservoirs to dip towards the southwest. This has caused realignment of fluids in the Boscan reservoir segments. The zero reservoir forming the northeastern edges of the linear trends now also became local updip traps to individual reservoir segments.

The present-day Boscan Field is a composite of stacked, linear, faulted, and truncated reservoir segments. They may be isolated or interactive with other segments, and form a complex "plumbing" tangle.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.