Paleoenvironmental significance of trace fossils from mixed tide- and river-influenced marginal-marine settings, Cretaceous U and M2 Sandstone members, Napo Formation, Oriente Basin of Ecuador

The Upper Cretaceous M2 and U Sandstone members of the Napo Formation are prolific hydrocarbon producers in the Oriente Basin of eastern Ecuador. To understand the depositional origin of these reservoirs, an integrated sedimentologic, sequence-stratigraphic, and ichnologic study, using 490 ft (~149 m) of conventional core from six wells of oil fields located in the eastern part of the basin, was performed. Sedimentary facies, stratal stacking pattern, discontinuity surfaces, and trace fossils were documented. Nine lithofacies and two depositional sequences in each member were identified. Three main broad environments, fluvial, estuarine and deltaic, have been interpreted. Trace fossils are very rare in the fluvial deposits and comprise facies-crossing ichnotaxa. The trace-fossil association of estuarine deposits is of low diversity containing ichnotaxa indicative of marine influence (e.g. Asterosoma, Teichichnus). In comparison, deltaic deposits display higher ichnodiversity, and are more intensely bioturbated, locally including some ichnotaxa (e.g. Zoophycos) that suggest the system may have experienced fully marine conditions sporadically. Controlling factors are typically expressed differently in the various subenvironments as a result of the interplay of tides and river discharge. The most important stress factors were substrate type and consistency, episodic sedimentation, high energy, water turbidity, and changes in salinity. Our integrated approach may prompt similar studies in other hydrocarbon-bearing units in Ecuador.