Benthic foraminiferal morphogroups from the Oligocene – Early Pliocene, deep offshore southwestern Niger Delta

Abstract

Morphogroup analysis of the Oligocene – Early Pliocene benthic foraminiferal assemblages of the deep offshore southwestern Niger Delta has been carried out from the study of four wells. The wells yielded highly abundant and diverse assemblages of both benthic and planktic foraminifera species.  The benthic species were categorized into eighteen morphogroups based on their general test outline, chamber arrangement and mode of coiling. The morphogroups are the agglutinated tubular, unilocular (MG A); globular, single chambered (MG B1); coiled, flattened (MG B2); flattened or lenticular, planispiral and globular (MG B3); elongate, tapered (MG C1); milioline with agglutinated coat (MG C2); low trochospiral (MG D). The   calcareous morphogroups are the flattened elongate (MG F); tapering or flaring, circular apertural view (MG G); cylindrical forms with circular apertural view (MG H); lenticular, biconvex, sharply angled tests (MG I); planoconvex, trochospiral forms (MG J); overall spherical or pyriform tests (MG K); trochospiral, broadly rounded periphery (MG L); flattened ovoid, biconvex, carinate periphery (MG M); trochospiral, biconvex tests (MG N); rounded planispiral compact tests (MG P) and flattened, elliptical, milioline tests (MG Q). Benthic foraminiferal species were also categorised based on their (supposedly) life position within the sediments as either epifaunal or infaunal. The epifaunal taxa belong to MGs B1, B2, B3, D, J, L, N and Q while the infaunal ones are species assigned to MGs A, C1, C2, F, G, H, K, M and P. Samples with relatively high abundances of infaunal morphogroups have been interpreted as rich in organic matter preserved under dysoxic condition ideal for the formation of hydrocarbon source rocks in the Niger Delta while high epifaunal abundances suggest oxygen rich, organic matter deficient sediments. The deep offshore southwestern Niger Delta consists of two major depositional facies based on interpretation from morphogroup distribution pattern, these are the (i) slope submarine fans and (ii) muddy submarine fans.