E.V. Santos Neto¹, Eugenio P. Vasconcelos², João D. Souto Filho², Diógenes C. Oliveira², R.P. Bedregal³, Carla V. Araujo¹, Valéria C. Cerqueira³
(1) Cidade Universitária, Rio de Janeiro, Brazil
(2) E&P-RNCE, Natal (RN), Brazil
(3) Gorceix Foundation, Rio de Janeiro, Brazil

Abstract: Alagamar-Açu Petroleum System (!) in the Potiguar Basin, Brazil

Data from geochemical analyses in oil, gas, and rock samples integrated with geochemical modeling, and geological and seismic data allowed a better understanding of the elements and processes of the Alagamar-Açu petroleum system (!) in the Potiguar Basin, northeastern Brazil.

Comparing with other oils in the Potiguar Basin, the diagnostic characteristics of oil from the Alagamar-Açu petroleum system (!) are: n-alkane maximaC₁₈-C₂₀, low pristane/phytane (<1.0) and hopanes/steranes (<2.0) ratios, high gammacerane (60) and bisnorhopane (10) indices, sulfur contents 0.3%, ¹³ C -26.5‰, and presence of C₃₀ steranes.

Carbon isotopic (C₁-C₄) and chemical compositions of gases are typical of hydrocarbons generated from the peak to late stages of the “oil window”. The more mature gases, which have shown characteristics of secondary cracking, are concentrated offshore.

Pods of generation are located offshore and are represented by thermally mature marine-evaporitic shales, and lacustrine shales. Both the generative sequences belong to the Alagamar Formation. Hydrocarbons are trapped mainly in fluvial sandstones of the Açu Formation, and secondarily in deltaic sandstones of the same Alagamar Formation. Seals are provided by shales interbedded in reservoirs. Normal faults and regional unconformities are interpreted as the effective migration pathways.

Geochemical modeling have suggested a relatively late generation. Secondary migration modeling have shown a distribution of oil throughout the carrier horizons and reservoirs that is compatible with the present geological model.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana