Growth Faulting, Rate of Oligocene Sediment Accumulations, and Stratal Geometries in a Greater Ughelli Depobelt Oil Field, Niger Delta Basin, Nigeria

Abstract

To ascertain the rate of sediment accumulations and relationship to growth faulting and the formation of stratal geometries favourable to petroleum accumulation, seismic data interpretations and stratigraphic datum age-depth plot analysis as well as well-log correlations within a sequence stratigraphic framework were conducted. Biostratigraphic data show that the age of sediments penetrated by oil wells from 2280 to 3544 m depth ranged from Early Oligocene to Late Oligocene and deposition took place in inner to middle neritic environments. For 7.8 million years, the rate at which sediments accumulated over the under-compacted Akata Formation shales varied between about 144.34 and 301.54 m/Ma. The high rate of sedimentation triggered the formation of a growth fault and the nucleation and growth of other synthetic faults. The growth faults are characterised by rollover anticline and drag folds consisting of footwall anticline and hanging-wall syncline. The footwall anticlines, like the rollover anticlines, offer drilling targets. Despite strata faulting and associated complexities, principles of sequence stratigraphy was used to successfully correlate well logs across the field in both the strike and dip directions. Four third-order depositional sequences, five genetic sequences, and system tracts were identified. Highstand Systems tract (HST) and Transgressive Systems tract (TST) are well preserved in the four depositional sequences, while LSTs are primarily found in Early Oligocene deeper sequences and in downdip wells, but all generally pinch-out in updip direction. The dip-orientated stratigraphic correlation implies stepwise progradation or down-stepping stratal geometries as a result of the non-uniformed subsiding platform generated by growth faulting.