High-Resolution Biostratigraphic Characterization of the Early Miocene Agbada Formation: Sequence Stratigraphic Implications for Reservoir Continuity in The Niger Delta Basin

Abstract

The paper represents a detailed biostratigraphic and sedimentological investigation of Well HI-001 in the Niger Delta Basin by use of composite cutting samples comprising of fifteen ditch cuts, which cover the depth ranges of 2780 m to 3980 m. The main agenda was to answer the biochronological age and paleoenvironmental evolution of the stratigraphic section in resolving the existing gap between the site-specific correlation in the Early Miocene depobelts. The characterization of the lithostratinology has defined seven different units that alternate between shaly sands, sandy shales, and separate clay layers that show a typical example of a paralic sequence of the Agbada Formation. Sixty-five (65) species of foraminiferar were identified, including 16 planktons and 49 benthic foraminiferar. Even though the sampling resolution was limited to 30-meter composite intervals, the recovery of the Praeorbulina evolutionary lineage of Praeorbulina curva, Praeorbulina glomerosa, and the First Appearance Datum of Orbulina suturalis clearly places the studied section as either Early or Middle Miocene (Burdigalian or Langhian). This coincides with N8-N9 planktonic zones of Blow (1969), which is the critical period in the region of the accumulation of hydrocarbons. Paleobathymetric assessment of benthic index species Spiroplectammina wrightii, Bulimina buchiana and Lenticulina spp would indicate a deepening-upward succession of Coastal Deltaic, Inner Neritic and Outer Neritic environments. High-energy depositional environment caused by periodic marine incursion is confirmed by the prevalence of the quartz (>80 interpret of the sedimentological record) and the occurrence of authigenic glauconite and shell fragments. A separate Lowland Systems Tract (LST) is defined by coarse grained channel sands, superimposed by a Transgressive Systems Tract (TST) as defined by glauconitic sands with the surface being a Maximum Flooding Surface (MFS) in the calcareous shales of Unit 4. Essentially, the results are closely related to the geographical tendencies in the Greater Ughelli Depobelt regarded as the reflection of the stratigraphic setting that was captured within the Oloibiri and Ughelli fields. This makes Well HI-001 a dependable source of information on the Early Miocene period. The paper determines that although the Unit 1 sands have great reservoir potential, the structural complexities and rate of facies change inherent to this depobelt would necessitate very strict biostratigraphic control that would be useful in predicting the continuity of laterality and integrity of seals.