By Chidozie Izuchukwu Princeton Dim
Detailed studies have been carried out within the Middle to Late Miocene sequence of the Niger Delta basin using well logs, biostratigraphic, paleobathymetric, hydrocarbon type/occurrence and seismic data integrated with sequence stratigraphic tool. This involved interpretation aimed at understanding the sequence stratigraphic framework and structural styles to better unravel possible hydrocarbon leads and prospects at the intermediate and deeper intervals of the Eastern Coastal Swamp depobelt of the onshore Niger Delta basin. Facies interpretations reveal the occurrence of four lithofacies units; namely (a) sandstones facies, (b) shaly sandstone facies, (c) mud-rock facies, and (d) heterolithic facies. Well log sequence stratigraphic interpretation and correlation across these fields indicates that ten major stratigraphic bounding surfaces, five each of sequence boundaries (SB) with ages ranging from 13.1 Ma through 8.5 Ma and maximum flooding surfaces (MFS) with ages between 12.8 Ma and 7.4 Ma respectively, were identified and correlated. Four depositional sequences were delineated and stratigraphic flattening at various ages (MFSs) indicates that there is a shift of the depo-center from north to south. Stacking patterns (progradational, retrogradational and aggradational) were delineated and interpreted as lowstand systems tract (LST), highstand systems tract (HST) and transgressive systems tract (TST) using their bounding surfaces. The alternation of the LST, HST and TST offers good reservoir sands and source/seal shales for hydrocarbon accumulation. Paleobathymetric maps show generally that sediments were deposited within neritic through bathyal environments at different times, aligning with the progradational pattern of deposition of the Niger Delta. The environment of deposition spans through inner mid-shelf, shelf margin and slope margin constituting incised valley and channel-filled deposits. Horizon and faults that were mapped and interpreted in seismic volume shows that the stratigraphy of the area were greatly influenced by structures as sediment deposits thicken down-dip of the down-thrown section of the bounding faults. Structural interpretations reveal the occurrence of back to back (horst block-trapezoid zone), collapse crest structures, simple/faulted rollovers, regional foot walls/hanging walls and sub-detachment structures dominating within the mainly extensional zone and these constitute the major hydrocarbon traps in the area. The distribution of hydrocarbon types is such that gas is concentrated at the proximal end (northern section), oil and gas at the central part while oil predominates at the distal end (towards the southern part). Generally, structural and stratigraphic framework has shown that indeed there exist zones at intermediate and deeper intervals with booming amplitudes and well-developed trapping mechanisms that have not been drilled. These form possible hydrocarbon leads that should be subjected to further revalidation. Hence, an evaluation of lithofacies, depositional environment, stratigraphy, structures, reservoirs, seals, hydrocarbon types and distribution is critical in hydrocarbon prospectivity at intermediate and deeper depths across the eastern Coastal Swamp depobelt of the Niger Delta.