Interpretation of Gravity Gradiometry data and integration with PSDM workflow – imaging sub-salt structures in Gabon

FTG data were acquired over an onshore Gabon Block. The objectives of this survey were to delineate accurately the salt structures; to derive a base salt structure map and to map basement structures associated with rifting. The survey area is situated along a clear trend of oil & gas subsalt fields that runs roughly north-south from offshore fields in south Gabon to the Lambaréné horst in the north. As a proven reservoir rock, the primary objective is the Gamba Sandstone. It is overlain by the Gamba Formation Vembo Shale which in turn is overlain by the Ezanga Salt formation. The trap is formed by reactivation of Cocobeach Formation faults due to the sediment load of the overlying Gamba and Ezanga Formations and the Madiela Group limestone and dolomite. The Ezanga salt also provides the top seal. Prior to the FTG survey three wells were drilled to target the Cretaceous Gamba sandstones. Although one of the wells appeared to be gas-bearing, the other two wells were dry. In addition, the significant discrepancies between predicted and actual depths demonstrated the need for new and independent geophysical data that would shed a new light on the “salt problem”. Acquisition of seismic data in this area is difficult given the environment, described as coastal, marshy, tree covered areas with small rolling hills. Processing and depth migrating the 2D seismic data is also difficult for several reasons, including a thick and variable weathering layer, 3D nature of salt structure and uncertainties in velocities. These problems resulted in misinterpretations of the seismic data and significant errors on the predicted depths of formations tops. The excellent quality of the FTG data and the large density contrast between salt and the surrounding sediments (clastics and carbonates) ensured that the workflows employed returned a successful result. Compared to traditional land gravity techniques, airborne FTG data has the advantage of offering fast and complete horizontal coverage, and providing the high resolution and bandwidth that is necessary to image the shallow salt structures with confidence. The workflow involved a back-stripping approach in which the model is constructed from top to bottom. Independent data (seismic and magnetic data) was also used to constrain the results. Following on from this an area of interest was defined to focus the interpretation effort on the most promising targets. In this core area, eleven seismic lines were selected for reprocessing and depth migration. The new PSDM seismic data were then used as constraints, together with well data, to build a model in 3D. Manual 3D forward modelling was used to build detailed surfaces with a maximum control on the geometry.