Reservoir Description & Regional Geology of Neocomian to Aptian Carbonates and Contemporaneous Non Carbonate Facies
The prospectivity of the South Atlantic Margin has experienced a major resurgence in recent years with the discovery of giant oil fields in Pre-Salt carbonate facies of Brazil’s Santos, Campos and Espirito Santo Basins and more recently in South Angola on the West African conjugate margin. The Brazilian Pre-Salt discoveries are hosted within laterally extensive, laminated lacustrine/marine-influenced carbonates (microbialites) and associated coquina limestone facies, while initial data from the new Angolan Pre-Salt discoveries suggest a variety of lacustrine facies, including significant fractured, vuggy and permeable crystalline carbonates.
These deepwater Pre-Salt carbonates represent a new play type for the South Atlantic Margin and since little geological data has been released from the new deepwater wells, it seems imperative to ensure maximum utilization of legacy subsurface datasets, in order to fully understand the controls on carbonate development and reservoir quality preservation along the margin. This study proposes to integrate previously evaluated well log and rock data with a comprehensive suite of newly acquired log, cuttings and key core material. This extension to our Pre-Salt, West African Margin Study Report (2006), will provide a detailed and fully integrated regional assessment of the sedimentological and diagenetic characteristics of Pre-Salt carbonates and will also evaluate the complex relationship between tectonic and igneous activity and sedimentation patterns within the syn-rift basins from this region.
The new dataset incorporates key wells from onshore and offshore Angola, including core material from Pre-Salt carbonates in the Benguela Basin, updip from the recent deepwater discoveries and new prospects and also cuttings and log material from onshore wells, which will allow us to evaluate the factors controlling facies development within the anomalous Barremian Salt Basin in Angola. In addition, a new look at key carbonate units from Congo and Gabon, in particular, more detailed evaluation of the carbonate and evaporite facies of the Aptian Vembo Formation will provide valuable insights into the Pre-Salt carbonate potential of the West African margin north of Angola. Finally, the inclusion of five wells from Equatorial Guinea will provide increased knowledge of the thick Aptian to early Albian lacustrine carbonates present in the northern part of the region, and will allow comparison of late-rift sedimentation trends in transform versus rifted segments of the South Atlantic Margin.
STUDY PROGRAM
- Core material for two key wells in Benguela Basin (Denden-1 and Abacaxi-1, Block 9) will be used for detailed core description and advanced petrographic analysis, in order to develop a sedimentological and diagenetic model for Pre-Salt carbonates in South Angola, in the shallow offshore region updip from recent discoveries.
- Newly acquired data will be used to upgrade existing SEDlogs and construction of new SEDlogs for 16 wells.
- Newly acquired cuttings data for Maculungo-1 will be used to evaluate facies composition and sedimentation controls within the anomalous onshore ‘Barremian salt basin’ and will allow investigation of potential presence of onshore algal laminites in South Angola.
- New downhole log data will also be used to perform cyclostratigraphic analysis of new and existing wells in order to provide detailed correlation of Pre-Salt stratigraphic packages across the margin.
- Newly acquired cuttings samples will be used to perform thin section, SEM and XRD analysis of carbonate and evaporite facies and new biostratigraphic analysis of selected shales. Additional petrography will also be performed on volcanic facies in order to fully characterized the range of lavas, tuffs, altered volcanics and volcaniclastics present in the syn-rift succession and to evaluate the relationship between igneous activity and sedimentological and petrographic trends within the sedimentary basin fill succession.
- Review of existing Pre-Salt database in Gabon to further evaluate the Aptian Vembo Member carbonates and evaporites which are coeval with Brazilian and Angolan Pre-Salt carbonate discoveries.
- Integration of all new sedimentological, petrographical and stratigraphic data from new wells with existing Pre-Salt dataset will allow major upgrade of paleogeographic maps across the West African Margin. In particular, the incorporation of Cyclostratigraphic analysis will allow more detailed sub-division of the basin-fill succession and facilitate higher resolution mapping of syn-rift sequences.
- Correlation and mapping of all data on a regional scale will allow evaluation of the different periods of syn-rift carbonate development (Neocomian to Aptian) and specifically will allow detailed interpretation of sedimentation trends during the end-rift transgression and transition into the overlying salt succession.
