Abstract: Albacora Oil Field: an Example of the Lagoa Feia-Carapebus (!) Petroleum System, Campos Basin, Brazil

Appi, Ciro J.; Luiz A. F. Trindade; Felix T. T. Gonçalves; Ricardo P. Bedregal - Petrobras/Cenpes

The Albacora giant oil field is located offshore Campos Basin, at the southern portion of the Brazilian margin and comprises several vertically stacked siliciclastics reservoirs ranging from Albian to Miocene (Fig. 1). Petroleum accumulated in these reservoirs has migrated from pods of active generation comprising thick successions of organic-rich Barremian shales (Lagoa Feia Fm.) deposited under lacustrine conditions during the rift phase of the basin.

Reservoir rocks are represented mostly by thick massive sandstones deposited in deep-water environment. Detailed sequence stratigraphy framework indicates three major reservoir compartments: Albian (Namorado sandstones-Macaé Fm.), Oligocene/Miocene and Miocene (Carapebus Fm.). Geochemical study of more than 50 oil samples indicates that, despite a common source, subtle compositional variations associated with biodegradation and thermal evolution are observed by GC-ratio polar-plots (Figure2) and biomarkers, suggesting three oil groups.

The Cretaceous turbidite sandstones in the Albacora field area (Namorado sandstones) represent a lowstand systems tract of the Albian sequence at the base of the Upper Cretaceous transgressive fine grained megacycle, which act as seal. The oil in this unit is slightly more thermally evolved than the shallower units and the maturity decreases eastward within the Namorado reservoir, suggesting that migration occurred from the fault that bound the field to the west, which is supported by fractionation of nitrogen compounds.

The Tertiary deep water sedimentary section comprises other two distinct major reservoir compartments. The first one includes seven thick sandstone bodies related to third order sequences deposited from Oligocene to the lowermost portion of Lower Miocene time interval. It is filled with oil less biodegraded than that present in the younger compartment. They show a tight cluster, with a few outliers which are associated with confined units. The second one comprises two other thick sandstone bodies and were deposited during the upper portion of Lower Miocene time interval related to the fine-grained sigmoidal prograding system. Miocene reservoired oils are the most biodegraded compared to the others, and in particular, show higher levels of recent biodegradation.

The Southwest-Northeast structure formed by the Albacora field, was affected by fractures and listric faults caused by salt movements that acted as pathways for oil migration from the Barremian source rocks to the younger reservoirs. Geochemical modeling on a cross section through the field allowed a better understanding of the role of faults as migration pathways, which combined with reservoir geochemistry investigation, suggested the direction of reservoir filling.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil