Foraminiferal Biostratigraphy of Well Z, OPL 310, Offshore Dahomey Basin, South-Western Nigeria

Abstract

The sedimentary succession penetrated by the Well Z, OPL 310 Dahomey basin, South western Nigeria has been investigated for their microfaunal content. A total of fifty ditch cutting samples from the well were processed and analysed for foraminifera following the standard procedures. The result of the analysis revealed moderately rich and diverse macrofauna totalling a hundred and seven (107) species out of which sixty-five (65) are benthonic and forty-two (42) are planktonic foraminifera species. The sedimentological analysis of the samples revealed two (2) sedimentary sequences: a basal sandstone unit characteristically milky white, coarse to pebbly, sub-angular and poorly sorted with alternation of marine shale overlain by a dark grey, non-fissile shale wih glauconite pellets and mica flakes alternating with sandstone which shows that sediments belong to Oshosun and Afowo Formation respectively. Based on First Downhole Occurrences (FDO) and Last Downhole Occurrences (LDO) of the foraminiferal index forms, four planktonic foraminiferal zones and One benthonic zone corresponding to the P14, P18-P21, N9-N14, N16 and N17 of Blow (1969, 1979) were proposed for the well. The planktonic zones are Globorotalia opima opima Interval zone, Orbulina universa - Globorotalia mayeri Concurrent-range Zone, Globorotalia acostaensis Interval range Zone, Globorotalia plesiotumida Taxon range Zone and the benthonic foraminiferal zone proposed is Uvigerina hourqi Taxon range zone. These zones were used to delineate the sedimentary succession from Middle Eocene to Late Miocene age.