Seismic Stratigraphy and Fluid Flow of the Late Cenozoic, Deepwater Niger Delta

Abstract

Utilizing a 738 km2 3D seismic data, wireline logs from four wells and available chronostratigraphic information, from 800-1600 m water-depth, western Niger Delta slope, and the methodologies of seismic stratigraphy, seismic geomorphology, and well-log lithological and sequence startigraphic interpretations, we reveal detail stratigraphy, plumbing system and fluid flow features of the Early Miocene to Recent interval on the western Niger Delta slope.Ten mapped seismic surfaces, consisting of seven maximum flooding surfaces and three erosion surfaces, define the stratigraphic framework for the Early Miocene to Recent interval, and broadly divides it into three sub-structural zones; an underlying Miocene compressional zone characterized by thrust-cored folds and associated inter-anticline depocentres; a Pliocene interval with evidence of slope failure and extensional normal faulting; and an unfolded overlying Quaternary zone characterized by extensional normal faults, a NE-SW canyon system as well as vertically and subvertically aggraded channel-levee complexes. Lithological calibration show that the studied interval is mainly characterised by relatively thick successions of hemipelagic marine shales, with interbeds of relatively thin turbidite sands. Gamma-ray log based stratigraphic analysis shows a long term stratigraphic trend of aggradation-progradtion-retrogradition-progradation for the studied interval.Mapped extensional normal faults showed a dominant NE-SW trend, with secondary E-W and NW-SE trends. Evidence of fluid flow such as seafloor pockmarks, seafloor mounds, paleo-pockmarks and gas hydrate were identified. A fluid plumbing system consisting mainly of stratigraphically superimpose turbiditic channel and channel-fan systems as well as faults has been documented for the study area.15 seafloor mounds and 18 shallow buried mounds have been documented. Both mounds are internally characterised by low amplitude, chaotic seismic facies, which is indicative of lack of internal structures. The seafloor mounds, with basal width of 125-1030 m, height of 11-75 m, are mainly cone-shaped, directly overlie vertical zones of acoustic distortion, and are characterised by lower seismic amplitude anomaly than the background seabed sediments. The buried mounds, up 115-680 m wide at base, 12-80 m thick, are capped by conical reflections marked by high amplitude anomalies; and directly overlie vertical zones of acoustic distortion. Most of the mounds occur over subsurface structural highs, particularly in the north-eastern area, and others occur over an intra-slope depocentre.The presence of gas-hydrate rich sediments, 225-272 m thick, in the south-western corner of the study area, has been identified from the occurrence of a bottom simulating reflector (BSR). A BSR-derived shallow geothermal gradient of 4.5-5.2 oC/ 100 m has been estimated for the study area.Canyon confined pockmarks, subdivided into 'canyon-margin' pockmarks and 'intra-canyon' pockmarks, spatially and stratigraphically associated with stratigraphic discontinuities created by erosion cuts of a NE-SW trending submarine canyon and associated channel complexes, active since early Quaternary, have been documented for the first time on western Niger Delta slope. A circular mega-pockmark, up to 1000 m wide and 50 m deep, above a subsurface structural high was also documented.This study gives significant insights into the Early-Miocene to Recent tectono-stratigraphic framework and fluid plumbing system of the western Niger Delta slope; it reveals the occurrence of potential geohazards in the area. These could prove crucial in exploration and production activities in the area and similar basins.

Date of Award	1 Aug 2015
Original language	English
Awarding Institution	The University of Manchester
Supervisor	David Hodgetts (Supervisor) & Mads Huuse (Supervisor)