--> Abstract: Depositional Systems of Lower B Sands in VLC1184 Well, VLC100/949, Block III, Maracaibo Basin, Venezuela, by S. Herrera, H. G. de Villarroel, and M. Taha; #90933 (1998).

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Abstract: Depositional Systems of Lower B Sands in VLC1184 Well, VLC100/949, Block III, Maracaibo Basin, Venezuela

Herrera, S. - PDVSA; H. Gamero de Villarroel - Schlumberger, Venezuela; and M. Taha - Schlumberger, Argentina

The Eocene B Member of Misoa Formation in Block III is subdivided into the Lower B (B-7 and B-6) and the Upper B (B-5 and B-4). A Sedimentological study was performed over the B-6 and B-5 sands in well VLC-1184. The aim of the study was to develop a sedimentological model within a regional sequence stratigraphic framework using 300 ft of core data, 500 ft of borehole images, openhole logs and biostratigraphic data.

B-6 Sandstone: The B-6 interval (Fig. 1), consists of two stacked fining-upwards sequences separated by interbedded fine grained sandstone, siltstone and shale, interpreted as delta plain/tidal flats deposits (Fig 2 a). The B-62 sandstone is 55 ft thick and consists of stacked fining-upward successions, with sharp erosional bases and gradational tops, interpreted as channel-fill sequences. The individual sequences are separated by levee and delta plain/tidal flat deposits. The borehole images shows that the B-62 sand has a highly dipping erosional base with a cemented channel lag, overlain by medium to fine grained sandstone with lateral accretion surfaces dipping towards NW and SE and cross bedding dipping towards the NE and SSW. This interval is interpreted as stacked point bar deposits. The dominant NE-dipping cross bedded sandstones are the result of migration of sand dunes by unidirectional-fluvial currents. The cross laminated sandstones dipping towards the SW reflect the migration of ripples and sand dunes by tidal currents. In summary, B-62 sandstone is interpreted to be deposited by a fluvial channel with a NE-SW orientation that was locally subjected to limited tidal influence.

The overlying B-61 sandstone is 45 ft thick and consists of stacked fining-upwards successions bounded by truncation surfaces and clay drapes. The borehole images and core data show a sharp erosional base, overlain by medium to fine grained sandstone with lateral accretion surfaces dipping to the SE and NW. This suggests that the migratory channel had a NE-SW orientation. The dominant SSW-dipping cross beds reflect the migration of sand dunes by tidal currents. The lesser amount of cross bedded sandstones dipping towards the NE is evidence of fluvial influence. In summary, B-61 sandstone is interpreted to be deposited by a meandering fluvial channel with a NE-SW orientation under a strong tidal influence and a minor fluvial influence

B-5 Sandstone: The B-5 sandstone is 25 ft thick and consists of stacked fining upward successions. The borehole images and core data show a sharp erosional surface with an associated chaotic deposit. It is interpreted as a channel lag, overlain by a medium to fine grained sandstone with lateral accretion surfaces dipping towards the E and SE, and cross bedded sandstone dipping towards the NE and SW (Fig. 2b). Accretion surfaces dipping to the E and SE suggests that the migratory channel has a NNE-SSW orientation and are the result of migration of sand dunes by unidirectional fluvial currents. The minor cross bedded sandstones dipping to the SW, reflect minor tidal influence. B-52 sandstone is interpreted as a migratory fluvial channel with a NNE-SSW orientation and a limited tidal influence.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil