SKALOUD, DIETER K. and PHILLIP CASSIDY, Shell Nigeria Expl. and Prod. Co. Ltd., Lagos
Abstract: Exploration of the Bonga and Ngolo Features in Deepwater Nigeria
In the first round of deepwater awards in Nigeria in 1993, two oil prospecting licences (OPL) on the continental slope were granted to Shell Nigeria Exploration and Production Company Ltd. (SNEPCO). The blocks are located to the west (OPL 212) and to the southeast (OPL 219) of the Niger Delta, in 200 to 1400 m of water. The applications for the blocks were based on 2D seismic. Canyon systems, potentially feeding sediments into deeper water, were known to be present. Significant structures were identified, resulting from differential loading on a mobile shale in front of the advancing Niger Delta. The blocks were chosen specifically, however, for the absence of intense shale diapirism and associated small isolated minibasins. No wells had previously been drilled in this deepwater province. Parts of the holding in both blocks have been farmed-out in 1993, to Esso, Elf and Agip, whilst SNEPCO retained the operatorship.
3D seismic surveys were completed in 1994, amounting to 3500 km2 , covering almost the entire area of the blocks (Fig. 1). Substacks of full and far offset data have been interpreted. Window measurements of seismic attributes below the seafloor and volume rendering techniques were used to quickly scan the data volume. Numerous amplitude anomalies were identified, many with an element of structural conformity. By analogy with rock property information from similar continental slope areas, it was assumed that turbiditic sands at prospective depth would be acoustically soft and hence match the character of the anomalous amplitudes observed. Prospects Bonga in OPL 212 and Ngolo in OPL 219 were selected for the first exploration wells, in 1995 and 1997 respectively. Multiple objectives, stacks of amplitude anomalies, were combined in one well bore (Fig. 2). The prospects are not straddling block boundaries. The objective intervals are interpreted as a gradually prograding succession of different turbiditic reservoir deposits, separated by thick layers of hemipelagic mudstones. They are dominated in the shallowest zone by channel-levee complexes, deeper by intermediate mid-slope fans and amalgamated channel systems in erosional scours, and finally by base-of-slope fans. This was related to the outbuilding Niger Delta from northeast to southwest and documented sea level fluctuations.The rise of shale ridges and associated crestal relief faulting may have triggered hydraulic jumps within the turbidite flows, and caused sand deposition on the outboard side of such structural elements.The age of the reservoirs evaluated to date is Mid Miocene to Upper Pliocene. In all cases, the trapping mechanism for hydrocarbons here is predominantly stratigraphic, defined by the depositional distribution of sands. However, the updip seal is generally fault related, reducing hydrocarbon leakage through sands of the channel/feeder system. In turn, the same faults are thought to act as hydrocarbon migration routes.Analysis of oils from the Bonga and Ngolo fields have indicated the source rock to contain marine and terrigenous organic facies, with oil and gas prone kerogen. Based on current knowledge, the distribution of oil and gas in the traps described is more complex in the east.
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