AAPG Bulletin, V. 84,No. 5 (May 2000), P. 652-682.

Tide-Dominated EstuarineFacies in the Hollin and Napo ("T" and "U") Formations (Cretaceous), SachaField, Oriente Basin, Ecuador ¹

G. Shanmugam,² M. Poffenberger,³and J. Toro Alava⁴

©Copyright 2000. The AmericanAssociation of Petroleum Geologists. All rights reserved.
¹Manuscript received September 5,1997; revised manuscript received August 31, 1998; final acceptance October30, 1999.
²Department of Geology, The Universityof Texas at Arlington, Box 19049, Arlington, Texas 76019; e-mail: [email protected]
³Mobil New Exploration and ProducingVentures, P.O. Box 650232, Dallas, Texas 75265-0232.
⁴Petroproducción, Unit of Researchand Laboratories, Quito, Ecuador.
We thank Manuel Berumen (Mobil) forassistance during core and outcrop examination; Joe Hayden (Mobil) forseismic interpretation; Jorge Montenegro and Carlos Huaman (Petroproducción,Quito, Ecuador) for discussion; R. J. Moiola, J. B. Wagner, M. Berumen,D. W. Kirkland, and P. L. Kirkland for reviewing an earlier version ofthe manuscript; J. E. Krueger for managerial support; and M. K. Lindseyfor drafting. We wish to thank Petroproduccion, Amoco, and Mobil for grantingpermission to publish this paper. We thank Bulletin Associate EditorJ. A. May for his critical comments that considerably improved the manuscript,Bulletin reviewers H. J. White and K. W. Shanley for their helpfulreviews, and AAPG Editor N. F. Hurley for his constructive comments.

ABSTRACT

The Sacha field is a prolific producerof hydrocarbons from the Cretaceous Hollin and Napo formations in the Orientebasin, Ecuador. To understand the depositional origin of these reservoirs,we did a detailed sedimentological study using 516 ft (157 m) of conventionalcore from seven wells. This study reveals seven lithofacies: (1) cross-beddedsandstone with erosional base (fluvial channels), (2) heterolithic facieswith erosive-based, cross-bedded sandstone (tidal channels), (3) heterolithicfacies with cross-bedded sandstone showing full-vortex structures, crinkledlaminae, sandy rhythmites, and double mud layers (tidal sand bars), (4)heterolithic facies with flaser-bedded sandstone (tidal sand flats), (5)muddy rhythmites with silty lenticular beds and double mud layers (subtidalmud flats), (6) bioturbated glauconitic sandstone (sandy shelves), and(7) bioturbated and laminated mudstone (muddy shelves).

Based on the presence of mud drapes on bed forms,heterolithic facies, double mud layers, bidirectional (i.e., herringbone)cross-bedding, sandy rhythmites, thick-thin alternations of silt and claylayers showing cyclicity (muddy rhythmites), crinkled laminae, and deepening-upward(i.e., transgressive) successions, we interpret the cored intervals ofthe Hollin and Napo formations to represent tide-dominated estuarine facies.We propose four stages of deposition for the Hollin Formation (oldest toyoungest) following the regional uplift and erosion of the Misahualli volcanics:(1) the first stage (during deposition of the lower Hollin) representsminor fluvial channels (low-sinuosity streams) and common tide-dominatedestuary, (2) the second stage (during deposition of the lower and upperHollin) represents a well-developed tide-dominated estuary, (3) the thirdstage (during deposition of the upper Hollin) represents drowning of atide-dominated estuary, and (4) the final stage (during deposition of theupper Hollin) represents well-developed shelf environments in the Sachafield area. During Napo "T" and "U" deposition, stages two, three, andfour were repeated.

Previous interpretations that the Hollin and Napoformations represent fluvio-deltaic environments are not supported by thisstudy. A tide-dominated estuarine setting is proposed instead. An importantaspect of our work is that tidal sand bars interpreted in the Sacha areaare predicted to trend east-west, paralleling the direction of sedimenttransport. In contrast, the conventional fluvio-deltaic model would predictnorth-south-trending distributary mouth bars with an easterly sedimentsource. Outcrop, core, seismic, or well data do not corroborate an incisedvalley-fill model that was applied to the Hollin and Napo formations byother workers. Estuarine facies are quite complex, as this study shows,and may not always fit into a general incised valley-fill model.