Journal de géologie et géophysique
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Basement Depth and Sedimentary Velocity Structure in Gongola Basin

Epuh EE and Joshua EO

Basement depth in the Gongola basin is found to be much deeper than previously supposed. Gravity modelling of the upper Benue Trough, Nigeria revealed thick sedimentation with maximum values within the range 5.2 km-7.0 km. This is in contrast to the average value of 5.0 km suggested by earlier studies. Gravity modelling across the basin was carried out to determine the basement depth using the second vertical derivative as input anomaly profile. The seismic modelling process in this research involves the determination of the distribution of seismic velocity using the: depth-normalized velocity iteration technique, check shot and sonic log curves. The integrated depth algorithm (IDA) iterative process was adopted in the determination of the interval and depth normalized interval velocities to adequately address the depth conversion for the determination of the lithology of the basin. The localized nature of the interpreted velocity data were extrapolated away from and interpolated between acquisition location using the areal coverage provided by gravity and seismic data. The interpretation involves the integration of the seismic reflection profiles, well logs and potential field data to establish a model of the sedimentary thicknesses and seismic velocities throughout the basin. The Basement depth on the north east of the basin is 7.0 km, southeast is 5.2 km whilst the northwest and southwest ranges between 0.5 km and 1.0 km respectively. The basement depths obtained from the gravity model was compared with that obtained from reflection seismic observation from the study area and the relative error percent were 1.37% and 0.46% respectively. The Precambrian basement depth normalized interval velocities ranges between 6.2 km/s and 6.4 km/s respectively. The qualitative interpretation of the second vertical derivative shows that the rift architecture/geometry is controlled by high angle faults and extended graben structures that form the major depocenters that are predominant in the upper Benue rift system. The grabens, half grabens, faults and deep sedimentation interpreted from the seismic reflection data are hydrocarbon related structural features. Consequently, the potentially hydrocarbon (gas) rich Yolde/Bima (Cenomanian-Albian) stratigraphic formations at depth between 2.1 km and 2.7 km and the depth normalized interval velocity varies from 2.9 km/s to 3.3 km/s and it occurs within the southeast zone of the basin.