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Understanding the distribution of oil quality and its impact on the development of deep-water reservoirs is a major challenge in many offshore basins of Brazil. Traditional geochemical approaches have used bulk properties (API gravity, viscosity, and sulfur content) and the biomarker compositions of oils to resolve the effects of source rock facies, thermal maturity, and biodegradation on oil quality in the present-day reservoir. These techniques, however, cannot fully resolve the effects of hydrocarbon charge timing, charge rate, timing of trap formation, and reservoir temperature history on the quality of the oil. In the Roncador and Frade fields, offshore Brazil, lacustrine-derived oils from Upper Cretaceous (Maastrichtian) and lower Tertiary (Oligocene–Miocene) reservoirs have gravities ranging from 14 to 33° API. In Upper Cretaceous (Maastrichtian) reservoirs of the Roncador field, better quality light oil (average, 28° API) occurs in the northeastern part, and mostly heavy oil (average, 17° API) is encountered in the southwestern part. The Frade field to the west of Roncador also contains heavy oil (16–19° API) but in shallower lower Tertiary (Oligocene–Miocene) reservoirs.

Geochemical analyses have identified the depletion of n-alkanes and the presence of 25-norhopanes (demethylated hopanes) in varying proportions in oils from the Frade and Roncador fields of the Campos Basin, offshore Brazil, indicating a complex history of biodegradation and mixing from at least two hydrocarbon charges in the reservoir. This study uses both one-dimensional and multisurface thermal models in the area to help determine charge histories for the source rocks and reservoir temperature histories for the reservoirs. These results are used to evaluate the effects of charge and reservoir temperature histories and biodegradation on the ultimate composition and quality of reservoired oils. An interactive biodegradation tool in Trinity software is used to predict the API gravity, and the results are constrained by the geology and the geochemical composition of the present-day fluids in the reservoir. Several examples of charge rate and timing, trap timing, and temperature history are presented for parts of the Roncador and Frade fields to illustrate the importance of these factors on controlling the quality of oil in the present-day reservoir.

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