--> Hydrocarbon prospectivity offshore Angola: Integrating regional gravity and magnetic data with seismic interpretation

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Hydrocarbon prospectivity offshore Angola: Integrating regional gravity and magnetic data with seismic interpretation

Abstract

Exploration and production offshore Angola continues to be successful, with a number of high profile discoveries made over the last decade (Cameia, Azul). In 2011, PGS acquired a 12,700 km regional 2D grid using GeoStreamerĀ® dual-sensor towed streamer technology to provide broadband seismic imaging to address the hydrocarbon potential on a regional scale.

As exploration has matured offshore Angola, new frontiers have begun to open up, especially in the deep-water pre-salt-play, analogous to that seen across the Atlantic in the conjugate margin in Brazil (Lula Field for example). To better delineate this play and gain a greater understanding of the structural development offshore Angola, PGS conducted an integrated study of ship acquired gravity and magnetic data with a regional interpretation project, the results of which are discussed here.

A two stage gravity and magnetics interpretation approach was used. The first was making a regional qualitative interpretation using gridded gravity and magnetic data, utilising PGS data as well as public domain data. The second stage was quantitative, generating and iteratively updating 2D density models until a 2D model was reached whose synthetic response sensibly matched the high resolution data recorded along the seismic lines.

The map interpretation afforded the opportunity to extract regional trends and lineaments. These relate to a combination of basement structure, salt structure and sediment thickness. Additionally, large scale features such as the Continental Oceanic Boundary and large volcanic trends can be identified.

Densities used in the 2D gravity modelling were constrained using a combination of density logs and public domain data. Interpreted seismic horizons were used to constrain the structure of the 2D models and less well defined structures were then altered in order to obtain a good fit between observed and model data. This process afforded the opportunity to test competing seismic interpretations and reduce the interpretation uncertainty. Additionally, some features that may otherwise have been neglected in the interpretation, such as subtle volcanics and additional pre-salt sediments, were included as a result.

In conclusion the research has enhanced the understanding of the distribution of syn-rift depocentres, the location of thick and thin salt and has better defined the deep structural setting across the Kwanza, Benguela and Namibe basins. By understanding the regional variation in deep structure and stratigraphy, explorationists can make accurate regional judgements on where to focus pre-salt exploration efforts.