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Campanian Estuarine Reservoirs in the Oriente Basin, Ecuador


Campanian M1 sandstone, one of the major prospective sandstone units in the Oriente Basin, Ecuador, are derived from the eastern uplands of the Brazil Craton and dispersed northwestwards along the axis of the embryonic Andean foreland basin. Previous studies of cores and well logs have suggested a deltaic depositional system followed by the development of extensive tidal estuarine systems that infilled gradually. However, there are local problems caused by unpredictable lateral sand-mud heterogeneity. The current study uses seismic data, 11 cores and more than 300 well logs to investigate the M1 sandstone. The M1 sandstone is always markedly sharp based, averages 25 m in thickness, shows upward increasing marine bioturbation and generally fines upward from coarse to very fine grained sandstone, all in keeping generally with a transgressive origin. In cores, the sandstones are amalgamated medium and coarse grained with prominent cross stratification (dm thick), sometimes clearly bi-directional and contains mud drapes. These suggest strong tidal or fluvial-tidal currents in estuary channels. The fine grained intervals are brackish-water intensely bioturbated and dominated by mud drapes, wavy and flaser bedding suggestive of estuary-margin intertidal flats. Associated overlying coals and coaly shales suggest supratidal conditions as the estuaries infilled. Large-scale lateral sandstone-mudstone heterogeneity has been imaged on seismic amplitude maps where a series of slightly sinuous muddy ‘lineaments’ up to 1 km wide have been mapped named “shale barriers”. Each of them is more than 15 km long and 20 – 30 meters thick. Well logs near the “shale barriers” show these are dominated by mud but contain thin sandstones immediately above the main sandstone unit. Mud thickens and sand thins toward the center of these shale barriers, with an overall decrease in total thickness of mud and sand. A core near the shale barrier shows that the main M1 sandstone is still relatively thick (30 feet), amalgamated and cross bedded. Possible explanations of the muddy ‘barriers’ are:(1) they are tidal channels that are sandy during the transgression but become abandoned and filled with mud as the late-stage estuaries infill, and (2) they are the meandering and low-energy tidal-fluvial channels which develop and are maintained in the proximal areas of the estuary. The first interpretation is favored because it explains the presence of both shale barrier and underlying sandstone.