Abstract: Coupling of Secondary Cracking and Retention Phenomena During Petroleum Generation and Expulsion: Application to the Gomo Mb. Source Rocks, Recôncavo Basin, Brazil

Penteado, Henrique L. B. - Petrobras/Cenpes; Françoise Behar and Jean-Luc Rudkiewicz - IFP

The Recôncavo Basin is a Late Jurassic-Early Cretaceous asymmetric rift in Northeastern Brazil. The syn-rift lacustrine shales of the Gomo Mb. (Candeias Fm., Neocomian) have been recognized as the most important source rocks, containing type I kerogen.

Aiming to understand the compositional changes during petroleum generation and expulsion, Gomo Mb. samples from wells throughout the basin, with varying organic richness and maturity, were analyzed. Absolute mass balances of compound classes for each source facies were obtained by integration of Rock-Eval and preparative pyrolyses, whole rock and kerogen extraction, and liquid and gas chromatographic data.

With increasing maturity, saturates increase in total C₁₅₊ extracts (whole rock + kerogen extracts), whereas aromatics and NSO's were almost constant, indicating that partial secondary cracking of the latter occurred within source rocks before expulsion. Using transformation ratios (TR) from natural series of hydrogen indices, the compositional changes of C₁₅₊ extracts with depth were modeled with a kinetic scheme for aromatics and NSO cracking. The amounts of modeled and measured C₁₅₊ extracts were compared to estimate the depth and amount of expelled petroleum according to source rock richness. Petroleum expulsion was assessed to be significant for TR above 65%, which corresponds to an estimated saturation threshold of 10% for expulsion. These values provide important constraints for numerical basin modeling studies. The composition of mature rock extracts closely resembles that of reservoired oils. However, lower contents of NSO's in oils indicate a partial retention of heavy compounds during primary migration, with oil cracking playing a minor role.

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