Campos and Santos Basin

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Fig. 1 - Campos-Santos Basin map[1]

History of the Basin

The Campos basin is considered as the most prolific basin located in Brazil. It has both onshore and offshore production. It is located on the southeastern region of Brazil, and it occupies an area of 115,000 km^2. The first oil field (Garoupa field) was discovery in 1974 on the Albian carbonate reservoirs at a depth of 120 meters (under the water). Therefore, oil production began on August 1977 in the Enchova field at a water depth of 124 meters using a semi-submersible platform.  

Tectonic evolution

Campos basin’s tectonic evolution is related with rifting (Mesozoic) in the South Atlantic which resulted in the breakup of Pangea and the development of several basins. Its origin is assumed to be by domal uplift, erosion and subsequent subsidence. Furthermore, this basin is associated with three stratigraphic sequences with the stages of development described as rift, proto-oceanic, and oceanic (lower and upper part). The Neocomian rift-stage lacustrine deposits is considered the lowest sequence in which refers to the fault-controlled subsidence with occurrence of stretching that preceded the emplacement of oceanic crust, and deposition of basalt dated at 130 Ma. The Aptian proto-oceanic is characterized by a sequence of evaporitic rocks associated with intense sea-water inflows (Lagoa Feia formation). The lower part of the oceanic stage addresses the thick sequence of Albian/Cenomanian limestones with clastic intercalations which grades upwards and basin wards into deep water marls and shales. Also, this section is associated with listric detached normal faults that sole out on the Aptian evaporites (salt). The oceanic upper cretaceous to recent clastic section is characterized by tectonic quiescence and continued subsidence with occurrence of some diastrophic structures. Also, there is a residual salt movement and increase in the intensity in deep water.  

Fig. 2 – The TGS line across the Campos Basin offshore Brazil. From bottom to top. Seismic reflection, and geologic interpretation. (Courtesy: Unternher et all, 2010). [2]
  2. (Unternehr et al, 2010)

Petroleum Geology

Source Rocks

The Campos Basin contains a source rock divided in two parts. One is the Lower Neocomian (Late Cretaceous era - 119 Ma to 144 Ma) having black shales deposited in a lacustrine environment with hyper-saline water, and the other source is the Upper Neocomian containing calcareous black shales and marls deposited in a lacustrine environment with alkaline affinities. Furthermore, these black shales were formed on a anoxic condition allowing them to have a high-quality organic matte composition of TOC up to 9%, and  low Sulphur type I kerogen.

Reservoir Rocks

The reservoirs range from fractured basalts and porous coquinas (bioclastic limestone) in the Lagoa Feia formation, to sandstones in the Campos formation, and limestones and sandstones in Macae formation.  

Traps and Seals

Campos basin shows a great correlation between trapping mechanism and tectonic evolution. The southern part of the basin consists of structural highs in which coquina rocks are affected by normal faults and the stratigraphic control is provided by the pinch out of the reservoir in the direction of the structural high. Also, basalts produce oil due to their open fractures caused by dissolution of calcite. Bonito, Bicudo, and Pampo fields display listric normal faults that created roll-overs and draping on salt. Therefore, in the Lower Tertiary and Eocene sandstones the stratigraphic control is associated with pinch-outs towards local highs and compactional effects associated with draping residual salt domes and porous carenites. Overall, the oil accumulation occurred by deep water fans distributed in the stratigraphic column from the late cretaceous to the late Tertiary. Turbiditic sand bodies of the Middle Eocene acted as hydrocarbon collector system allowing oil migration from the growth fault systems or unconformities.

Fig 3. Hydrocarbon migration pathway model of some oil accumulations observed in the Campos Basin. D: Gas Chromatograms of the Upper Neocomian source rock. B: oil pooled in Upper Neocomian reservoir, associated with migration through direct contact or unconformities and/or with normal faults. C: oil pooled in Albian carbonate reservoir, associated with migration through pre-salt. T: oil pooled in Tertiary turbiditic reservoirs associated with migration through pre-salt. [1]