SEQUENCE STRATIGRAPHY OF NEOGENE STRATA IN OFFSHORE NIGERIA

Abstract

Abstract The Dahomey Basin, located along the western portion of the Nigerian continental margin, is underlain by continental and transitional crust. It exhibits stable to moderately unstable progradation and is characterized by a well-developed shelf-slope break. Systems tracts are similar to those described for other stable progradational margins. In contrast, the Niger Delta Complex, located off the central and eastern portions of the Nigerian coast, exhibits highly unstable progradation. The shelf-slope break is commonly not well-developed. Instead, a ramp, characterized by relatively uniform dip, is present. Large growth faults cut the ramp and define the shelf margin. Systems tracts are similar to those in deep-water Neogene strata of the offshore Gulf of Mexico. The basin floor fan and slope fan depositional systems comprise the lower lowstand systems tract. Basin-floor fans in both areas are defined by a prominent reflection along their upper boundary. This reflection downlaps along the sequence boundary or abuts against the down thrown side of a growth fault surface. The slope fan contains channel-fill complexes characterized by chaotic bedding and less common large channel-fills, which exhibit concave-upward reflections. Most slope-f an strata exhibit discontinuous lo semi-continuous subparallel reflections. In the Dahomey Basin, slope fan deposits commonly pinch out at or near the shelf margin. In contrast, contemporaneous shallow-water facies are developed in the Niger Delta Complex. Deposition occurred from multiple point sources. Large amplitude anomalies in the upper lowstand (prograding wedge) suggest thick sheet sands are present in the Niger Delta Complex. These sands occur in both shallow-water (interpreted as delta front facies) and deep-water environments (interpreted as shingled turbidites). Water depths are constrained in areas in which well control is present by benthic foraminifera. Transgressive and highstand systems tracts are commonly very thin. Different exploration strategies are appropriate for each systems tract off both the western and central portions of the Nigerian Coast. The best plays off the central portion of the Nigerian coast are probably structural traps and combination structural and stratigraphic traps (this includes updip pinchouts), whereas hydrocarbons may occur in smaller pure structural to pure stratigraphic traps along the west coast of Nigeria.