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