Three post-rift marine petroleum systems in the Amazon mouth are characterized, with maturation, migration, and Neogene reservoir distribution controlled by rapid deposition related to Andean tectonics. Two of the potential hydrocarbon source intervals (Upper Cretaceous and Paleogene) are present on the shelf and upper rise, but their basinward extension is only inferred (Fig. 1). Extrapolation of potential sources from the shelf into the basin requires an understanding of the controls on distribution and preservation of organic matter. Amazon mouth sources have three main controls: (1) paleogeography, (2) oceanic conditions, and (3) terrestrial river input (Fig. 2).

The Upper Cretaceous (Limoeiro) clastic source rock is well documented on the shelf, where it is of fair to good quality (TOC 1%-5 %, HI 150-500). The source rock quality should improve basinward where terrestrial input and siliciclastic dilution decrease. Source rock thickness may be a risk, but oil-prone, clastic- starved marine shales are predicted in the deep basin. Primary controls on the distribution of the Turonian source rock are productivity and oceanic circulation.

The Paleogene (Amapa) source rock is also occurs on the shelf, but is less oil-prone (TOC 1%-5%, HI 200-350) than the Upper Cretaceous source rock in the area. Based on map distribution and biomarker data, we interpret this source to be limited to a back-reef lagoonal environment and absent seaward of the carbonate shelf edge. The key to better organic enrichment is interpreted to be paleogeography: carbonate highs cause restricted circulation and increase the potential for organic preservation.

Source rock potential is also postulated for the Miocene-Pleistocene (Pirarucu) interval. Tropical Tertiary age depocenters around the world have produced hydrocarbons with distinctly terrestrial signatures, and some of these systems produce large amounts of oil. The Amazon mouth region possesses many attributes which could allow terrestrial sources to be capable of oil generation, but more data are required to test this concept.

Hydrocarbon maturation is driven by rapid Miocene-to-present burial that causes the Limoeiro to be locally overmature. Postulated Pirarucu sources may be mature for oil in some areas. Rapid sedimentation resulted in deep listric faults, shale diapirs and toe-thrust structures that provide abundant vertical migration pathways. The presence of interbedded Tertiary sources and sands would allow for simple migration scenarios.

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Figure 1. Potential source limits for the Amazon Mouth Basin.

Figure 2. Main controls on source development.

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