AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
The Gambian Basin: Play Analysis and Hydrocarbon Potential
(1) WesternGeco, Schlumberger, Gatwick, United Kingdom.
The Gambia Basin is a poorly explored basin located in the North Western Atlantic margin of Africa. Several discoveries offshore West Africa have given impetus to pursue new hydrocarbon opportunities in the margin. Despite this, exploration is limited to one well, Jammah-1 drilled by Chevron Overseas in 1979; a 3D time seismic volume shot by WesternGeco, covering 500Km2, in 2003 and few 2D seismic surveys.
The analysis of the available seismic data and the examination of seismic attributes, together with maturity modelling made possible the description and evaluation of the frontier basin. Play potential of the basin is limited to the major seismic sequences of the Middle to Upper Cretaceous, Upper Cretaceous, Lower Tertiary and Upper Tertiary to Holocene.
Evaluation of the Turonian-Cenomanian source rock suggests that maturity starts at 2400m subsea deep offshore Gambia; in addition younger source rocks are possibly located within the Oil Window. Generation possibly began in the Oligo-Miocene. Significant play types are present in the basin; stratigraphic pinch-out and onlaping events represent the main seismic evidences that characterize the Middle to Upper Cretaceous plays. Moreover, Lower Tertiary plays include pinch-out and updiping depositional lobes and inter-channel deposits also proven in the seismic data.
Major reservoir facies offshore Gambia include deep water clastic systems, which are recognized by thorough description of contorted and discontinuous highly reflective but disorganised events, that may indicate high energy episodes that could be delineated as turbidite beds. In addition, high amplitude contrast reflectors in the lower Tertiary sequence could be associated to high energy processes depicted by well organized channels and lobes geometries. Potential Seals consist of intra-formational and/or hemipelagic regional marine shales. Migration is believed to occur directly and through porous sequences and/or fractures.
The investigation suggests that the Gambia Basin bears hydrocarbon potential based on a basic prospectivity assessment and that all conditions for generation and accumulation are present deep offshore.