--> Quantitative Assessment of Biodegradation in Hydrocarbon Fields of the Cabo Frio Area, Brazil, Bender, Andre A.; Kemna, Hans A.; Mello, Marcio, #90100 (2009)

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Quantitative Assessment of Biodegradation in Hydrocarbon Fields of the Cabo Frio Area, Brazil

Bender, Andre A.1
 Kemna, Hans A.2
 Mello, Marcio1

1HRT & Petroleum, Rio de Janeiro, Brazil.
2
UCON,
Krefeld, Germany.

Oil biodegradation in the Tertiary sequences of the Campos basin is a widespread phenomenon and has important implications with regard to oil quality. The great majority of the oils discovered in the Cabo Frio area are mixtures of oils with different degrees of biodegradation, but all derived from the same Lagoa Feia source rocks. Oil trapped in the Lower Cretaceous reservoirs are rarely biodegraded and present good quality due to higher temperature and depth. In contrast, oils within Upper Cretaceous and Tertiary reservoirs frequently show evidence of biodegradation and present poor quality. The most extensively biodegraded oils are characterized by a depletion of normal and iso-alkanes and the formation of demethylated hopanes (25-norhopanes). The presence of 25-norhopanes in combination with n-alkanes indicates mixing of biodegraded and fresh, non-biodegraded oils.

In order to understand the final composition of the oils in the Cabo Frio area, 3D compositional basin modeling was performed on a geological framework built with interpreted public-domain 2D seismic lines (ANP). The results were calibrated with bottom hole temperature data, maturity data and geochemical data of 23 wells from the HRT database (BrazilGeodata). A migration history was estimated for the area that revealed fill-and-spill scenarios by which hydrocarbons moved from the sub-salt source rocks upward to the Upper Cretaceous and Tertiary reservoirs. Hydrocarbons are estimated to have reached the fields beginning around 90 Ma, but other pulses reached the area between Upper Cretaceous and Eocene times. Some areas were charged even later, in Miocene times. Timing, composition, amount of multiple charges of oil and reservoir temperature histories were modeled using the PetroMod software package the results of which were fed into the biodegradation tool of Schlumberger (BioPetS). This innovative approach provides a consistent view concerning composition, degree of biodegradation, and their distribution in a sedimentary basin.


AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil