The potash prospect area was located near Brazzaville, Congo and was originally mapped in the late 80’s and early 90’s by an oil exploration dynamite seismic reflection survey. Survey objectives were to map the top and base of the evaporite sequence (hosting the potash), identify any slumping or faulting of the same and map the basement contact of the sedimentary package. Of particular concern were the slump structures as they yielded significant gas “blow-outs” when intersected by drilling and thus posed a significant safety hazard for the exploration project. The distribution of these slump structures was not well defined in the original survey data.
GAP proposed the use of a high resolution 2D seismic surveys to meet the survey objectives. Since the site was remote, equipment was flown in using an Ilyushin IL 76 aircraft instead of relying on ground transportation and clearing multiple national boarders (Figure 1). This approach allowed the field crew to be fully operational in the Congo within 6 days. Seismic equipment comprised 2 Mini Vibs, a 200 channel Sercel 388 seismograph, plus seismic cables and geophones with geophone and shot intervals of 8m.
As expected, the high-resolution Mini Vib data was of significantly higher resolution than the original dynamite data (Figure 2). This allowed for the identification of numerous slump structures and faults plus multiple horizontal lithological contacts (Figure 3). The entire evaporite sequence was found to have a median width of ~400m over a range of 350m to 460m. Gridded isopach images of the interpreted evaporite showed the distribution of the slump structures over the prospect area (Figure 4).
Figure 2 : Comparison of original dynamite data and high resolution Mini Vib data.
Figure 3 : Interpreted high resolution seismic section showing multiple faults (pink lines) and a ~500m wide slump structure.
Through the use of high-resolution seismic reflection surveys all mapping objectives were met. Later drilling exploration confirmed that the existence of the imaged slump structures. This provided the client the ability to refine the local resource model and to identify high risk areas associated with the identified slump structures.
Figure 4 : Gridded evaporite isopach image showing the distribution of slump structures (red circles) over the prospect area.
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