Sedimentological and tectono-stratigraphic characterisation of a shallow-marine reservoir, ‘Dona’ field, offshore Niger Delta

Abstract

The ‘Dona’ field is located in the shallow offshore Coastal Swamp depobelt, western Niger Delta. The field contains multiple, stacked shallow-marine reservoir intervals of Miocene to early Pliocene age in the Agbada Formation. The area surrounding the field is characterised by a series of synthetic, listric normal faults that strike north-northwest to south-southeast and dip southwest. These faults show stratigraphic thickening in their hangingwalls, indicating growth, and are associated with the development of rollover anticlines, which define the trap configuration of the ‘Dona’ field. Spatio-temporal variations in stratigraphic expansion indicate that growth faulting started in more landward (northeasterly) locations and migrated progressively basinward (southwestward). These variations are consistent with growth faulting due to gravity-induced shale diapirism, potentially driven by overall progradation of the Niger Delta.Core and wireline-logs from a representative reservoir interval contain a facies assemblage and stratigraphic architecture developed under a mixed-influence depositional process regime, which was dominated by wave processes but influenced by tidal processes. Wave-dominated shoreface deposits occur in a series of coarsening- and shallowing-upward parasequences that are laterally continuous over the reservoir but are locally erosionally truncated by fining-upward tidal channel-fill deposits. The mixed-influence process regime may reflect spatial variations in the dominance of wave, tide and fluvial processes, as in the modern Niger Delta, and/or temporal variations between a regressive, wave-dominated regime and a transgressive, tide-dominated regime. Sedimentological heterogeneities are present across a range of scales, and their distribution reflects the mixed-influence depositional process regime.