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Mobile Shale's Evolution and Deformational Patterns in a Neogene Gravity-Driven System, Offshore Western Niger Delta


Detailed analysis of seismic geometries and stratal thicknesses within time-constrained horizons combined with 3-D geomorphological maps provide useful insights into the timing of deformation within the Neogene sediments in the western Niger Delta’s slope. Two seismic geometries (SG ‘A’ and SG ‘B’) characterize the syn-shale kinematic and post-shale kinematic sediments on the slope. SG ‘A’ are characterized by convergent/sub-parallel seismic reflections and occur within the Chattian, Burdigalian and Tortonian respectively. SG ‘B’ are characterized by parallel/sub-parallel seismic reflections and are associated with the Serravallian and Plio-Quaternary sediments. SG ‘A’ are correlated with times when shale was mobilized while SG ‘B’ represent times of relative quiescence. The association between seismic geometries, time and deformation suggest that shale mobilization initiated early in the evolution of the Niger Delta between the Chattian to Burdigalian, then transitioned into inactive period in the Serravallian, subsequently re-activating in the Tortonian then returning to relative quiescence over the Plio-Quaternary intervals. The onset of shale mobility (Late Oligocene/Early Miocene) as shown in this study correspond with the timing of increased sediment flux to the Niger Delta, which is documented in recent studies to have been triggered by the uplift of Hoggar Mountain. Compressional deformations associated with mobile shales and syn-sedimentary collapse resulted in the development of folds and normal faults with opposing trends that support the Niger Delta’s structural variability due to its radial spreading and buckling with divergent orientations. Detailed observation on seismic sections and 3-D geomorphological maps show that hydrocarbon including but not limited to gas has leaked through pockmarks. Thermogenic processes at depths >4 km and differential compaction in buried sediments are the major sources of fluid overpressures, which constitute geohazard to offshore petroleum infrastructure.