John S. Sneider1
(1) Sneider Exploration, Houston, TX
ABSTRACT: Sequence Stratigraphic Controls on Reservoir Distribution in the Eocene "C" Sandstones, VLC-363, Lake Maracaibo, Venezuela
The VLC-363 is a super-giant field located in the southeastern part of Lake Maracaibo. The Eocene "C" sandstones were deposited during an overall transgression that rests unconformably on a regional onlap surface. The "C" sandstones produce over an approximately 1,200 foot interval. High frequency stratigraphic cycles superimposed on the long-term transgression control reservoir properties, reservoir distribution and vertically compartmentalize the field. The lower "C" sandstones are fluvial-deltaic, and the upper "C" sandstones are shallow marine. Reservoir permeability varies from a few millidarcies to approximately a darcy.
The lower fluvial-deltaic interval contains three separate sequences subdivided into seven systems tracts. The best reservoirs and the bulk of the reserves are in the lowstand prograding wedge (LPW) which consists of proximal mouth bars and distributary channels that are generally in good lateral pressure communication. Shales associated with the parasequence flooding surfaces subdivide the LPW and form excellent seals that vertically compartmentalize the systems tract. The transgressive systems tract (TST) deposits vary from fluvial to distal deltaic. Facies control the reservoir quality in the TST; the best reservoir quality is in the fluvial interval that onlaps the regional unconformity at the base of the "C" sandstones. The highstand systems tract is finer grained than the LPW and TST and has lower reservoir quality with fewer reserves.
The upper marine interval consists of two separate sequences with four well-developed systems tracts. Parasequences and parasequence sets subdivide the systems tracts into separate reservoirs. The overall reservoir quality is lower than the fluvial-deltaic interval.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado