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Henrique Luiz de Barros Penteado1, Jean-Luc Rudkiewicz2, Françoise Behar2
(1) Petrobras R&D Center, Rio de Janeiro - RJ, Brazil
(2) Institut Français du Pétrole, Rueil Malmaison, France

Abstract: 2-D modelling study in the Recôncavo Basin, NE Brazil : Understanding compositional changes during petroleum generation and migration

The Recôncavo Basin is a NE-trending rift formed during the Early Cretaceous breakup of the South American and African continents. A detailed study of the syn-rift lacustrine source-rock shales of the Gomo Mb. (type I kerogen) showed that petroleum (rock extract) compositions change with increasing maturity. NSO compounds predominate in immature extracts, whereas saturates are the major products at peak oil generation. Such compositional changes have been attributed to partial secondary cracking of NSOs and aromatics in source rocks during maturation and before petroleum expulsion.

The individual role of secondary cracking and retention phenomena for petroleum composition was modelled with IFP’s Temispack software. After calibrating thermal maturity indicators along a cross-section in the basin's Southern Compartment, petroleum generation and migration were modelled with compositional kinetic data derived from open-system pyrolysis. The evolution of extract composition was modelled by assigning specific secondary cracking kinetic parameters to several classes of compounds.

Using the same frequency factor, an activation energy for secondary cracking of NSO's and aromatics slightly higher than that of the main primary cracking reaction allowed a good calibration of extract compositions. When coupling petroleum generation and expulsion, a retention factor of around 50% for NSOs in source rocks accounted well for the composition of reservoired oils in two oilfields: one in pre-rift sandstones (Sergi Fm.; Dom João oilfield) and the other in turbidites within the syn-rift Candeias Fm. (Cexis oilfield). Secondary cracking along migration pathways and after accumulation are not important for changes in oil composition in our section.

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