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3D shallow water seismic survey planning to deliver sub-salt imaging in South GabonNormal access

Authors: Paolo Esestime, Laura Arti, Milos Cvetkovic, Karyna Rodriguez and Neil Hodgson
Journal name: First Break
Issue: Vol 35, No 12, December 2017 pp. 77 - 81
Language: English
Info: Article, PDF ( 604.8Kb )
Price: € 30

Summary:
The shallow offshore in south Gabon has been explored for more than 50 years, highlighting the potential for oil discoveries.However, exploration of the sub-salt syn-rift section requires an advanced support that only modern, regionally consistent 3D seismic data can provide. Such support was the objective of a large-scale multi-client 3D seismic programme, representing a collaboration between Spectrum Geo Ltd and the Gabonese Hydrocarbon Authorities (Direction Generale des Hydrocarbures (DHG)), which commenced in December 2016. This survey acquired 11,500 km2 of 3D seismic data in the ‘Gryphon Area’ – the shallow offshore of Mayumba and Sette Cama, west of the Olowi Field, in a range of water depths from 20 to 1000 m (Figure 1). Advanced and accurate modelling allowed for the survey specifications to be tailored to the geological setting and the exploration targets. The previous geological and geophysical exploration was taken as background information to develop the new subsurface imaging. For many years the accepted exploration target in South Gabon has been the high-quality transgressive Gamba Sand-stone of Lower Aptian age, located just below the Ezanga Salt Fm. As the base of the salt is separated from the near ubiquitous Gamba Sandstone Fm. by thin shale (Vembo shale member) the main challenge for seismic acquisition therefore has been the accurate imaging of the base salt, and more importantly the accurate depth imaging of this unit. The overburden is complicated by heterogeneous and highly variable velocities in the post-salt carbonate of Madiela Fm., complex geometries of top salt and variable compositions and velocities within the salt. In south-west Congo (Brazzaville), a number of discoveries have been made over the past 4-5 years, in a new intra syn-rift play. This play targets the Djeno Fm., equivalent in age to the lower syn-rift Kissenda Fm. of South Gabon. This play has rarely been targeted in Gabon owing to the poor imaging of the pre-salt section on legacy 2D and 3D data. This article describes the methodology applied to the pre-acquisition study, which guided the acquisition operation and gave a preliminary insight of the challenges to be faced during the processing.


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