--> Abstract: Early Cretaceous Rift Source Rocks from Eastern Brazilian Margin: Isotopic and Molecular Correlation, by F. T. T. Gonçalves and E. S. T. Frota; #90933 (1998).

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Abstract: Early Cretaceous Rift Source Rocks from Eastern Brazilian Margin: Isotopic and Molecular Correlation

Gonçalves, F. T. T. and Frota, E. S. T. - Petrobras/Cenpes

The study area comprises Sergipe-Alagoas, Camamu, Almada, Jequitinhonha and Cumuruxatiba basins, located in the eastern Brazilian continental platform (Fig. 1). These basins are part of the rift system formed in the Early Cretaceous, which evolved to a passive margin during the separation of South America and Africa.

Previous studies have shown that the rift stage source rocks were deposited in a lacustrine environment. In this study a detailed investigation revealed variations in bulk data (TOC, HI), isotopic ratios (d13C and d18O), and biomarker parameters. However, whereas single well profiles show marked variations, their correlation over large distances revealed similarities, which point out to a regional nature for the paleolimnological events in these rift basins.

The integration of geological and geochemical data allowed the identification of two main phases of rift lakes evolution:

(a) Berriasian to Hauterivian: Fault-bounded deep meromitic lake with fresh to brackish water. Organic preservation was greatly enhanced by the stable water column stratification and bottom anoxia. The strong C12 isotopic enrichment reflects intense recycling of carbon through the degradation of organic matter by anaerobic bacteria.

(b) Barremian: Broad and shallower (oligomitic?) lake with fresh to saline water. The strong C12 isotopic depletions is interpreted as the result of the enhancement of primary productivity due to increased input and recycling of nutrients favored by the lake morphology and a humid climate.

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