Sequence stratigraphy and depositional environments of Middle-Late Miocene sediments in the eastern part of the Coastal Swamp depobelt, Niger Delta Basin, Nigeria

Abstract

Detailed studies on depositional sequence, genetic facies, and depositional environment were done using sequence stratigraphic tool in the paralic sequence of Middle to Late Miocene age in the Coastal Swamp depobelt of the Niger Delta Basin. This was aimed at increasing the resolution for prediction of reservoir, seal, and source rocks in the eastern part of the basin. Integration of well logs and biofacies data with paleobathymetric control allowed regional correlation of major transgressive-regressive events between wells, resulting in the generation of a sequence stratigraphic framework and environment of deposition (EOD). Dominant genetic facies include the following: marine shale, lower shoreface, upper shoreface, channel sands, and channel heterolithics. Three major stacking patterns were delineated and interpreted as Lowstand Systems Tract (LST), Highstand Systems Tract (HST), and Trangressive Systems Tract (TST). Correlation reveals ten major stratigraphic bounding surfaces of various ages (five sequence boundaries and five maximum flooding surfaces) and four depositional sequences. Stratigraphic flattening of events at various ages indicates a shift of the depositional center from north to south, aligning with the progradational pattern of deposition of the Niger Delta. The stratal package also thickens towards the south (basinwards). Generated paleobathymetric maps show generally that sediments were deposited within neritic through bathyal environments at different times. The EOD spans through incised canyons, channels, inner- mid shelf, shelf margin, and slope margin. A combination of the reservoir sands of the LST and HST and the shale units of the TST offers good stratigraphic traps for hydrocarbons and hence should be targeted during exploration.