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Abstract: Tide-Dominated Estuarine Facies in the Hollin and Napo (“T” and “U”) Formations (Cretaceous), Sacha Field, Oriente Basin, Ecuador

SHANMUGAN, G., Mobil Technology Company; M. POFFENBERGER, Mobil New Exploration and Producing Ventures; and J. TORO ALAVA, PETROPRODUCCION


In order to develop a sedimentological model for the Cretaceous Hollin and Napo formations in the Sacha Field, Oriente Basin, Ecuador, a total of 516 ft of conventional core from 7 wells was described. Based on the presence of mud drapes on bedforms, heterolithic facies, double mud layers, bidirectional (i.e., herringbone) cross bedding, thick-thin alternations of sand bundles, and deepening-up (i.e., transgressive) sequences, the cored intervals of the Hollin and Napo formations are interpreted to represent tide-dominated estuarine deposits. Previous interpretations that the Hollin and Napo formations represent deltaic environments is not supported as progradational sequences are absent in the core. An incised valley fill model is also difficult to envision due to the lack of evidence in the outcrop, core, seismic, and stratigraphic analysis.

Based on bed thickness, grain size, depositional matrix, and the nature of lithofacies, four general reservoir facies are suggested: (1) tidal sand bars with excellent reservoir properties (i.e., 100% sand, elongate bar geometry), (2) fluvial and tidal channels with good reservoir properties (i.e., 90-100% sand, lenticular geometry), (3) tidal sand flats with moderate properties (i.e., 50-100% sand, high matrix, sheet geometry), and (4) shelf sands with relatively poor properties (i.e., 80-100% sand with high matrix and glauconite, bar to sheet geometry).

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