G. H. Lee*#, M. A. Eissa**, C. L. Decker**1, J. P. Castagna**,

D. J. O’Meara** and H. D. Marín***

*Dept of Environmental Exploration Engineering, Pukyong National University,

Busan 608-737, Korea.

** Institute for Exploration and Development Geosciences, University of Oklahoma

Norman, Oklahoma 73019, USA.

1Current address: Gas Technology Institute, Houston, Texas 77090, U.S.A.

*** Tecpetrol, Buenos Aires, Argentina.

# Corresponding Author, email:

The principal reservoirs in the Bermejo field in the NW Oriente Basin of Ecuador are the Cretaceous Hollín Formation and the basal sandstone in the Tena Formation. Analysis of 3D seismic and wireline log data suggests that much of the Hollín Formation consists of stacked fluvial channel deposits. The overlying Napo Formation is characterized by the excellent correlation of log signatures, continuous seismic reflections, and the absence of thick sandstone layers (except for the coarsening-upward Napo "T" sand), suggesting deposition from uniform processes, possibly in a shelf environment. The upper Napo and lower Tena Formations are interpreted as a series of transgressive deposits, punctuated by the basal Tena sand. Truncation in the Hollín Formation is better observed on east-west seismic lines than on north-south seismic lines, suggesting a north-south -trending fluvial system. This, together with the northward thinning of the top Hollín shale suggesting an east-west-trending palaeo-shorezone, further predicts north-south-oriented reservoir distribution. The palaeogeography of the Oriente Basin was probably more complex than that portrayed in previous depositional models which depict a north-south-oriented palaeo-shorezone and sediment supply from the east. Detailed depth structure maps reveal structural closures that have not been tested.

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