The Sacha field is a prolific producer of hydrocarbons from the Cretaceous Hollin and Napo formations in the Oriente basin, Ecuador. To understand the depositional origin of these reservoirs, we did a detailed sedimentological study using 516 ft (157 m) of conventional core from seven wells. This study reveals seven lithofacies: (1) cross-bedded sandstone with erosional base (fluvial channels), (2) heterolithic facies with erosive-based, cross-bedded sandstone (tidal channels), (3) heterolithic facies with cross-bedded sandstone showing full-vortex structures, crinkled laminae, 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 (subtidal mud 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 clay layers showing cyclicity (muddy rhythmites), crinkled laminae, and deepening-upward (i.e., transgressive) successions, we interpret the cored intervals of the Hollin and Napo formations to represent tide-dominated estuarine facies. We propose four stages of deposition for the Hollin Formation (oldest to youngest) following the regional uplift and erosion of the Misahualli volcanics: (1) the first stage (during deposition of the lower Hollin) represents minor fluvial channels (low-sinuosity streams) and common tide-dominated estuary, (2) the second stage (during deposition of the lower and upper Hollin) represents a well-developed tide-dominated estuary, (3) the third stage (during deposition of the upper Hollin) represents drowning of a tide-dominated estuary, and (4) the final stage (during deposition of the upper Hollin) represents well-developed shelf environments in the Sacha field area. During Napo "T" and "U" deposition, stages two, three, and four were repeated.
Previous interpretations that the Hollin and Napo formations represent fluvio-deltaic environments are not supported by this study. A tide-dominated estuarine setting is proposed instead. An important aspect of our work is that tidal sand bars interpreted in the Sacha area are predicted to trend east-west, paralleling the direction of sediment transport. In contrast, the conventional fluvio-deltaic model would predict north-south-trending distributary mouth bars with an easterly sediment source. Outcrop, core, seismic, or well data do not corroborate an incised valley-fill model that was applied to the Hollin and Napo formations by other workers. Estuarine facies are quite complex, as this study shows, and may not always fit into a general incised valley-fill model.