Abstract: Petroleum Systems of the Mundau Sub-Basin, Ceará Basin, Northeastern Brazil

Freitas, L.C. da S. - Petrobrás/Cenpes and V. P. C. Pimentel - Petrobrás/E&P

The main objective of this study is to apply the petroleum system concept focusing on the geological, geochemical, geophysical and biochronostratigrafic data gathered along many years of petroleum investigation at the Mundau sub-basin, Ceará basin, located in the continental platform of the Brazilian equatorial margin, a rift basin formed during the separation of the African and South American continents in the Early Cretaceous.

Based on the petroleum system assumptions, it was possible to characterize, two petroleum systems; 1) the Mundau-Paracuru (!) involving oils sourced from organic-rich extracts of the Mundau Fm. (1) (Aptian) deposited under a brackish to freshwater lacustrine paleoenvironment, and with the following geochemical characteristics: Pristane/Phytane ratio>1; d¹³C isotopic values around -27.0?. (lighter) (2); predominance of odd paraffins over even ones along the n-alkanes distribution, dominance of alkanes compounds of high molecular weight (>C₂₃) low amounts of gammacerane (2); and 2) the Paracuru-Ubarana (!) comprehending oils sourced from the Paracuru Fm.(1) (Aptian to Early Albian) deposited under a transitional phase (Gulf) with marine evaporitic influence and the following geochemical features: Pristane/Phytane ration < 1; d¹³C isotopic values around -26.0?. (heavier); predominance of even paraffins over odd ones along the n-alkanes distribution, dominance of alkanes compounds of low molecular weight (C₁₅ to C₁₇) and high amounts of gammacerane (2).

Fluvial, fandeltaic, deltaic and turbiditic sandstones are the reservoirs, and interbedded shales provide sealing for traps. The intense tectonism that occurred during the rift evolution formed faults and unconformities that controlled the traps. Faults are inferred as important pathways for secondary migration of petroleum.

Thermal maturity modeling of Mundau Fm. has shown that relatively rapid subsidence and intense heat flow during the rift evolution provided conditions for organic rich shales to reach the oil window during the Upper Cretaceous (at approximately 86m.y.) and oil peak generation in the Tertiary (around 30m.y.). While the modeling of the Paracuru Fm. showed that the sediments deposited along the plataform reached only the very beginning of the oil window (Ro%=0.5-0.55), what resulted on the generation and accumulation of oils of low thermal evolution. But, it achieved excellent maturation conditions beyond the shelf edge boundaries during the Tertiary (around 15m.y.) that provided most of the high thermally evolved oil reservoired in the sub-basin.

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