Tectonic history

The South Atlantic margin developed on Archean stable cratons consisting of hard and resistant rocks and partly on the Neoproterozoic mobile belts composed of less resistant metamorphic rocks.^[4] The Precambrian basement of the Santos Basin is exposed as the Araçuaí Belt along the Brazilian coast, most notably in the inselbergs of Rio de Janeiro, of which Sugarloaf Mountain is the most iconic. The ancient rocks consist of a Neoproterozoic to Cambrian high-grade metamorphic core of granites and gneisses, formed during the collision of Gondwana in the Pan-African-Brasiliano orogeny.^[5] Basalts similar to the Paraná and Etendeka traps, exposed to the west in the Paraná Basin, have been found underlying the Santos Basin.^[6] The Tristan da Cunha hotspot, known as the Tristan hotspot, is considered the driver behind the formation of these flood basalts.^[7]

During the Early Cretaceous, the former continent Gondwana, as southern part of Pangea, starting to break-up, resulting in a sequence of rift basins bordering the present-day South Atlantic. The Pelotas-Namibia spreading commenced in the Hauterivian, around 133 million years ago and reached the Santos Basin to the north in the Barremian. Seafloor spreading continued northwards to the Campos Basin in the Early Albian, at approximately 112 Ma.^{[citation needed]}

Five tectonic stages have been identified in the Brazilian basins:^[8]

1. Pre-rift stage – Jurassic to Valanginian
2. Syn-rift stage – Hauterivian to Late Barremian
3. Sag stage – Late Barremian to Late Aptian
4. Post-rift stage – Early to Middle Albian
5. Drift stage – Late Albian to Holocene

Stratigraphy

Oil reservoirs include formations deposited during the Aptian and pre-Aptian continental rift phase, of post-salt Albian-Cenomanian shallow-water marine carbonates and deepwater sandstones, and in turbidites of the open marine drift phase of Late Cretaceous and early Tertiary ages.^[2]

The Namorado Field "location was selected based on seismic interpretation of a structural high at the top of the Macaé Formation (Albian limestones)" at a depth of about 3 kilometres (9,800 ft),^[2] and the reservoirs are marine turbidite deposits transgressing over the Albian limestone shelf.^[2]

The stratigraphy starts with basalt flows dated at 120 Ma, overlain by the Lagoa Feia Group, consisting of the organic-rich lacustrine "green shales" followed by lacustrine limestones and continental sandstones and conglomerates, transitioning into marine sediments with evaporites, limestones, and limestone altered dolomites.^[2] The shallow marine limestones of the Macaé Formation follow, then the Namorado turbidite sandstones, and finally the Campos Formation, consisting of the turbidite sandstone Carapebus Member and the prograding slope and shelf Ubatuba Formation.^[2]