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Similarities and Differences Between the Late Cretaceous Fan Complex and the Overlying Neogene Amazon Cone in the Foz do Amazonas Basin, Northern Brazil – Implications for Future Hydrocarbon Exploration


The proximity of the Foz do Amazonas basin to the 2011 Zaedyus (700 MMbbl) and 2016 Liza discoveries (800 MMboe) in the French Guiana-Guyana-Suriname basins, indicate the presence of a working petroleum system based on Cenomanian-Turonian age source rock charging Late Cretaceous deepwater fan reservoirs. We conducted seismic facies analysis and geomorphology on the Late Cretaceous (Limoeiro Formation) and the overlying Neogene Amazon Cone using 21,369 km of 2D depth-converted seismic tied to 3 exploration wells, in order to: 1) seismically distinguish mud-prone vs. sand-prone deposits; 2) understand depositional controls on stratigraphic traps; and 3) determine their reservoir quality for potential hydrocarbon accumulations. The Late Cretaceous deepwater systems of the Foz do Amazonas basin, deposited at ~40 km from the paleoshelf break, are characterized by: 1) upper slope fans interpreted as sand-prone, channel-levee complexes expressed as strong, discontinuous reflectors with moderate amplitude and concave shape, and 2) basin floors fans seen as high-amplitude, continuous reflection packages (HARPs) of sand sheets with little to no structural deformation. These large fan complexes (~800 km2) were sourced by mature sands derived from hills located close to the paleoshoreline, and deposited in conditions of: 1) ~3.19° gradient and ~95 km-long slope; 2) rapid separation of the South Atlantic to an open marine environment; 3) strong turbidity currents; and 4) steepening of the rifted margin. In contrast, the Neogene Amazon Cone is a cone-shaped, mud-rich, tide-dominated, shelf-edge delta characterized by stacked, muddy, mass transport and channel-levee complexes, expressed seismically as low amplitude packages of dull-chaotic seismic reflections. These vast, overpressured deposits (~50,000 km2) were sourced from weathered, clay-rich soils with high suspended load derived from the Andes, and deposited in conditions of: 1) ~0.44-1.61° gradient and ~170 km-long slope; 2) periodical failure of the prodelta; 3) longshore turbidity currents; 4) gravity tectonics; and 5) storm-surge resuspension. We conclude that more favorable conditions for sandy deepwater fan deposition occurred in the Late Cretaceous with higher relief, shorter slope length, sand-dominated rivers, and turbidity currents. These factors account for better reservoir quality and trap preservation with multiple seals/trapping points in contrast to the mud-prone, present-day Amazon Cone.