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A schematic model of oil generation, migration, accumulation, and alteration is presented for the Campos basin, a sedimentary province covering an area of nearly 31,000 km2 (12,000 mi2) offshore southeastern Brazil, where an estimated 1 billion m3 (6.3 billion bbl) of oil in place has been discovered since 1974.
Source rocks for this oil belong to the Lower Cretaceous Lagoa Feia Formation; oil generation probably started in the Miocene. At that time, a series of local windows opened in the regional evaporite seal at the top of the Lagoa Feia Formation and focused the upward escape of hydrocarbons, mainly along halokinetic fault surfaces. Reservoirs of Albian, Late Cretaceous, and Tertiary age were charged and their porosities enhanced by natural fracturing, solution, and/or grain rearrangement. Original oil (postulated range of 30°-35° API gravity) underwent differentiation by migration, reflected in relative enrichment of aromatics and of the light 12C stable carbon isotope. Alteration of oil by water occurs if one of the two fluids in contact is allochthonous; bacterial alteration is important in low-temperature regimes. Oil entrapment is helped by hydrodynamic conditions, with the intake area of the Paraiba do Sul river delta supposedly playing an important part.
Lopatin-type plots gave the first clue for establishing this model, which takes into account a large number of facts about the basin, such as well and seismic information, clay diagenesis, water and petroleum geochemistry, pressure data, and their geologic field relationships.
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