Abstract: Urucutuca-Urucutuca (?): A new Petroleum System in Espirito Santo Basin, Brazil

D?avila, R.S.F; Biassusi, A.S.; Guirro, A.C.; Brandão, J.R. - Petrobras/E&P; Frota, E.S.T. - Petrobras/Cenpes

The evolutionary sequence of the Espirito Santo basin is represented by an initial rift phase constituted of continental deposits with evaporites of Neocomian to Aptian age (Cricaré and Mariricu Formations) followed by a transitional/gulf phase composed by mixed neritic carbonate/siliciclastic deposits of Albian age (Regência and São Mateus Formations), and a drift phase (Cenomanian to present day) characterized by marine deep water deposits with turbidites (Urucutuca Formation), progressively covered by progradational deposits (Rio Doce and Caravelas Formations). Oil and gas accumulations of the onshore area are associated with two main petroleum systems whose source rocks are of lacustrine and marine carbonate origin.

Recent offshore exploration activity east of the Rio Doce delta unveiled geochemistry data allowing the recognition of a new petroleum system: the Urucutuca-Urucutuca (?), whose source-rocks and reservoirs consist of drift phase marine deposits of the Urucutuca Formation. Molecular data from gas, condensate and light oils indicate origin from source rocks deposited in an anaerobic marine siliciclastic environment with deltaic influence, as depicted by the presence of biomarkers like oleanane and diasteranes, diagnostic of land higher plants, and clay in the environment.

Even though the pod of this petroleum system has not yet been drilled, organic immature rock extracts of tertiary age - mainly Paleocene and Eocene marine dark shales of the Urucutuca Formation - reveal distributions of biological markers compatible with those verified for the condensates and light oils. The existence of thicker and mature source rock facies elsewhere in the deep basin is expected, considering the advanced thermal evolution stage of these hydrocarbons (from 30º up to 52° API).

Reservoirs are confined siliciclastic turbidites deposited in troughs created by salt movement during Eocene and Oligocene times. The main reservoir facies is coarse to fine-grained massive sandstone beds, frequently amalgamated. Mud intervals between these beds are rare, allowing good vertical continuity in the reservoir. The absence of a muddy interval on the top of the sands is due to removal by bottom currents action, possibly resulting from tidal currents along the depositional troughs.

Faults associated with halokinesis are possible conduits for hydrocarbon migration. Turbidite systems are sealed by shales of the same stratigraphic level (Urucutuca Formation). Traps can be stratigraphic or mixed, due to thrust fault development, originated by reverse salt-flow. The weight of the Abrolhos volcanic complex (middle Eocene) is the probable cause of thrusting.

Geochemical modeling and geologic data indicate that generation and migration started in the Neogene. The Urucutuca-Urucutuca (?) may be an undercharged petroleum system with dominant lateral migration.

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