PSDM imaging of the Petroleum Systems of the South Atlantic

 

speaker Dr. Steve Henry

 

co-authors  Al Danforth, Consultant, Houston TX

Naresh Kumar, PhD, Growth-Energy, Richardson, TX

Peter Nuttall, ION-GX Technology, Houston, TX

Sujata Venkatraman, ION-GX Technology, Houston, TX

 

Recently acquired (2004-10) regional (100’s Km dip oriented), long offset (10 Km), long record length (18 sec.) reflection seismic data (SPANs) has been processed to provide pre-stacked depth migrated (PSDM) images (25-40 Km) on both sides of the South Atlantic, and along the South American equatorial margin. Dip oriented lines from Angola, Congo and Gabon (63 profiles) and Brazil (69 profiles ) have been interpreted, with about 20 horizons (sequence boundaries) correlated and tied to 25 wells in West Africa and more than 75 wells in Brazil. The spacing of the dip line ranges from 10 to ~75 Km (Avg. ~50 Km). Long strike lines (1500-5000 Km) on both margins have an average spacing of about 50-100 Km. These constrain and correlate the major sequences boundaries mapped on the dip lines. This data has provided a better understanding of the existing / producing petroleum systems, extending these systems across the South Atlantic, and identifying new potential petroleum systems.

 

In this presentation we will show seismic data illustrating the pre-salt petroleum system in the Brazilian Santos Basin and the recent (2011) pre-salt discovery on the conjugate margin in the Angolan Kwanza Basin. Analogs for the Equatorial African Jubilee (Sierra Leon) and Venus (Ghana) discoveries in the Early Drift sequences will be shown along the Brazilian Equatorial Margin, along with a recent Early Drift discovery along the NE coast of Brazil. These petroleum systems and their relative timing will be put into context using a new model for the opening of the South Atlantic (Fig. 1). In this model the northward opening consists of a series of stalled extensions with the separation of the cratons taken up in continental strike slip faulting. This model is supported by mapping the age of the first sediments deposited on the newly formed oceanic crust. 

 

Remarkably similar features and the timing of their development are observed on both the West African and South American margins (Fig. 2). Regional mapping (West Africa: 300 x 2000 Km, Brazil: 600 x 3000 Km) of the major features has been critical for understanding the tectono-stratigraphic relationships and have been used to define a model of diachronous rifting. In this model, failed (no ocean crust) episodes of rifting developed during periods of extension, and subsidence, with sag basins forming over the thinned crust. Shown in Figure 2 is the mapped position of sag basins (green) formed over the failed Valanginina rift. These sag basins which only overlie failed rifts, contain rich and laterally extensive source rocks (Brazil: Lagoa Feia, Angola: Organic Bucomazi / Falcao) that drive the petroleum system responsible for the pre-salt discoveries. The successful episode of rifting that separated the continents along this segment of the South Atlantic was in the Barremian (blue dashed line), and the syn-rift for this episode is volcanic with less potential for source rock development.

This regional PSDM data is also ideal for examining the Early Drift and looking for new petroleum systems such as those identified along the African Equatorial margin. As the continents separate, especially along the transform / strike slip segments, the first basins to develop have restricted circulation and therefore have potential for developing source rocks. Recent discoveries along the Brazilian Equatorial and Northeaster margins, illustrates the usefulness of reconstructing conjugate margins and the timing of their openings.