Print this pageNew Answers to Ancient Questions from the Lower Cretaceous Petroleum Systems of Onshore Alagoas Sub-Basin
Sergipe-Alagoas Basin, Brazil, is considered to be in an advanced exploration stage although many questions concerning the interplay between its diverse petroleum system elements remain unanswered. Some of these responses can be useful not only for the possibility of opening new frontiers within an already explored area, but in the understanding of pre-salt petroleum systems that can be extended to all eastern Brazilian marginal basins. Thus, a complete 3D petroleum system modeling effort was carried out to assess oil and gas generation and migration processes, basin temperature and pressure evolution. For this it was critical to integrate the available geophysical, geochemical and geological data so the model could be calibrated with real data instead of just assuming default library values. Three petroleum systems are active in Sergipe-Alagoas basin, but Alagoas onshore accumulations are charged for only two of them: the Neocomian-Barremian Barra de Itúba/Coqueiro Seco (!) and the Aptian Ibura/Maceió (!). Even though it is considered that most of the onshore petroleum is generated from the distal offshore part of the basin, 3D modeling has proved that major depocenters associated with Alagoas Low source rocks attain an early oil to dry gas window stage (for the Neocomian-Barremian system), and is responsible for most of the Alagoas basin in place oil. Main generation peak for the Neocomian-Barremian source rocks is about 120 to 130My, whilst the Aptian source rocks (immature in most of the studied area), ranges from 100 to 120 My. Major pointed leads are distributed along the siliciclastic reservoirs of Serraria, Barra de Itiúba, Penedo and Coqueiro Seco Fms, being the southwestern area the one with larger amount of oil and gas shallow accumulations. The 3D model also presupposes the possibility of reservoir charge from short-distance migration. Such an assumption is able to efficiently explain most of the onshore accumulations rather than long-distance migration through homogeneous carrier beds where the interbeded rocks of rift sequences are a prevailing restriction. Therefore, the performed 3D model explains some of the larger accumulations in the basin and predicts new areas to be investigated with a lower exploratory risk.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil