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ABSTRACT: Facies Distribution and Reservoir Potential of the Oligocene Sequence, El Furrial-Musipan Giant Field, Venezuela

Andreina Isea, Santosh Ghosh, Bernabe Aguado

The Oligocene strata in eastern Venezuela constitutes an unusually thick (500 m), shaly clastic sequence, in which major oil discoveries have been made in the last four years. Recoverable reserves are on the order of 5 billion bbl of oil.

Sedimentologic analyses of approximately 950 m of core and subsurface data reveal that this sequence was deposited in a high-energy nearshore marine environment with dominant littoral and barrier bars, subordinate sublittoral bars, inlet and estuarine channels, and marsh deposits. Seven lithofacies characterize the system: (1) coarse-grained microconglomeratic sandstone, (2) fine- to medium-grained high-angle cross-stratified sandstone, (3) highly bioturbated sandstones, (4) very fine-grained thinly laminated silty sandstone, (5) bioturbated heterolithic (sand-shale) unit, (6) gray shale, and (7) massive siltstone. In addition, minor coaly layers are also present.

Lithofacies 1 and 2 dominate the reservoir and consist of vertically and laterally continuous amalgamated, coarsening-upward sequences. High porosity and permeability are characteristic in these mature quartzarenitic sandstones. Lithofacies 3 shows relatively less porosity and permeability values due to its higher clay content. Lithofacies 4 and 5 are petrophysically poor and may form local flow barriers due to their silty and argillaceous content. Lithofacies 6 and 7 possibly could represent very local seals.

Lithostratigraphic reservoir-wide correlations show remarkable lateral and vertical continuity of the dominantly west-southwest-east-northeast-trending nearshore bar sand bodies traversed locally by north-south-trending highly sandy inlet and estuarine channel facies.

The hydrocarbon production in this field seems to be mainly facies and structurally controlled. The delineation and mapping of the facies sequences and understanding of the structural details will increase the success of future discoveries in the area.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990