This text refers to a brief review of activities in the largely un-explored Amazon River Mouth Basin, its seismic and drilling exploration history and its potential hydrocarbon reserves. Since the start of exploration drilling in the past century close to one hundred wells have been drilled in its continental shelf with two minor discoveries deemed non-commercial, and several wells with oil shows. The modern phase of exploration started in 1997 with the opening of acreage for exploration by Petrobras and competition. In the last two decades state-of-the-art geophysical data acquisition and processing defined an abundance of new exploratory plays these compare favorably with the prior definition by vintage seismic data and contributed for a better understanding of the basin petroleum systems.

Introduction

The Amazon region embraces several large sedimentary basins that extend both onshore and offshore. Onshore, the Amazon rifted basin is one of the largest Paleozoic basins on earth and in the offshore the Amazon River Mouth Basin represents its Cretaceous counterpart. The Amazon region onshore is flanked by the Guiana shield and by the Brazilian craton both of Pre-Cambrian age (Almeida et al, 1973). The Amazon River runs along the axis of the basin that is filled mostly with early Paleozoic sediments. In the offshore region, the Amazon River Mouth Basin is part of the widespread Brazilian Equatorial Margin. In the plate tectonics context the region is part of the Transform Equatorial Margin that spreads from the Brazilian Equatorial Coast to the regions offshore the Equatorial West Africa (Bryant et al., 2012). Fracture zones displace the mid-Atlantic ridge crest, these equatorial fractures are amongst the largest in the planet (Figure 1), North Brazilian Ridge which is part of St. Paul Fracture Zone intersects the Brazilian Equatorial Margin (Hayes and Ewing, 1970).

Exploration on the Amazon Region dates back to the early activities of the CNP (Conselho Nacional do Petrõleo). One of the strongest promoters for the search of petroleum was Monteiro Lobato a renowned Brazilian writer and diplomat. Still, detailed mapping of the Amazon Rift started in 1953 shortly after the formation of Petrobras (Petróleo Brasileiro S.A. established by Brazilian Government Law 2004, signed by President Getúlio Vargas. Regional mapping was made in these early exploration programs by geologic field mapping and by geophysical reconnaissance programs of seismic refraction surveying, gravity and magnetics. These pioneering work in the Amazon jungles were conducted by Petrobras geologists with persistent work, for months in a row, surviving with whatever means possible, essentially describing outcrops and measuring dip and strike of formations. Geophysics refraction interpreters mapped the Paleozoic strata based on accurate seismic velocities. Reflection crews contracted by Petrobras (United Geophysical and GSI) carried out the field reflection programs. The very early G & G mapping culminated with the discovery in 1956 in Nova Olinda (Leinz, 1955; Linsser, 1967; Figure 2) in the mid-Amazon Basin. The mapped prospect was a low relief anticlinal structure that was drilled at its top. Oil was found at 2,750 meters depth. However this discovery and subsequent drilling were deemed non-commercial leading Petrobras to start and focus the exploration efforts in the offshore Amazon River Mouth Basin. With the advent of better technology Petrobras exploration on the Amazon onshore were rewarded with significant discoveries, the Juruá Gas Field in 1976 and the Urucú Oil Field these are still the major producers of hydrocarbons in the onshore basin.

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