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Foz do Amazonas and Pará-Maranhão Ready to Replicate Guyana Success

Abstract

The Foz do Amazonas and Pará-Maranhão Basins in Northern Brazil constitute the natural uninterrupted continuation of the Guyana-Suriname-French Guyana continental margin to the South. They are part of the Brazilian Equatorial Margin that straddles from Foz do Amazonas Basin to the Potiguar Basin in the extreme East. Since the Zaedyus discovery in French Guyana, only 50 km to the North of the Brazilian border, the hopes for further similar discoveries in the Brazilian side were high. The recent extraordinary success of ExxonMobil in Guyana strongly reinforced such belief. The successful petroleum system played in these deep and ultra-deep waters consists of Late Albian to Turonian (maybe Coniacian) anoxic marine shales as source rocks and Cretaceous (even Cenozoic) turbidite fans and channels. The trapping mechanism is invariably stratigraphic in nature, with lateral and updip pinchouts limiting the reservoirs containing hydrocarbons.

The Foz do Amazonas Basin is a large Cretaceous to Cenozoic passive margin basin, in the middle of which the huge Amazon Cone was deposited during the last 10-9 Ma. This huge Neogene pile of mostly muddy sediments may reach 11 km of thickness. It bent the underlying Paleogene and Cretaceous rocks by load flexing. The situation was ideal to force the source rocks gradually into the oil window and then into the gas window. Maturation and hydrocarbon generation is guaranteed. The Cretaceous turbidite reservoirs were already in place when maturation and migration took place. Oil was expelled from underneath the Cone in radial directions upwards towards regional highs. The load-induced flexure downwards created a compensation positive flexure between the Amazon Cone and the Pará-Maranhão Basin. A powerful migration route was established from the Southern margin of the Amazon Cone towards the Pará-Maranhão Basin.

Ultra-deep two-dimensional seismic surveys image in a very clear way the continental and oceanic crusts bent downwards underneath the Amazon Cone and upwards in the Pará-Maranhão Basin. Thick Cretaceous strata are harmonically folded with the Moho discontinuity. These Cretaceous sequences display source-rock seismic facies in the lower part and channels and lobes of turbidites highlighted by bright amplitude anomalies in the upper part. Vertical faults caused by the load flexure link the source rock strata to the Paleogene sequence underneath the Cone and around its margins. The hydrocarbon kitchen is clearly seen below the Amazon Cone. The updip pinchout of turbidites towards the Southeast indicate optimal trapping conditions. Several leads of large areal extent and thickness were mapped. They point towards the replication of the successful history of exploration in the nearby continental margins of the Equatorial Atlantic.