Integrated Biostratigraphic and Geochemical Studies as a Supplementary Tool for the Stratigraphic Review of EZ and BZ Oil Fields in the Western Niger Delta

Uzoamaka O. Anyaeji and I. M. Akaegbobi
University of Ibadan, Ibadan, Nigeria

This project was designed using biostratigraphy and chemostratigraphy (which involves the use of inorganic geochemical data to characterize and correlate strata) to establish correlation between EZ and BZ wells in adjacent fields. Major and trace elements were determined by ICP/MS using Lithium Borate Fusion method and are reported as weight percent for SiO₂, Al₂O₃, Fe₂O₃, FeO, MgO, CaO, Na₂0, K₂0, TiO₂, P₂O₅, MnO and Cr₂O₃ and ppm for Ba, Cu, Zn, Ni, Co, Sr, Zr, Y, Nb, Scand Ta. The reservoir sandstones are composed of 70-90% quartz, 4-15% feldspar and 3-13% clay matrix, with minor mica, bioclasts, carbonaceous debris and glauconite. Grain size varies from fine to coarse and pebbly, implying the presence of sands of varying textural and compositional maturities. Silica, K-feldspars; and carbonate constitute the cement. The depositional environments of reservoir sand bodies are deduced from lithologic characteristics and diagnostic log patterns and are interpreted as fluvial/deltaic distributary channel. The investigated sections of the wells have been dated late Eocene using benthonic foraminifera and palynomorphs. The well logs and biostratigraphic data were used to identify sequence boundaries (SB) and maximum flooding surface (MFS), which were then integrated with seismic interpretation. Paleoecology is inferred from biodata and is interpreted as inner to middle neritic. The geochemical data allows the sequences in the wells to be subdivided into small chemostratigraphic units, which are not obvious from the lithology, thus demonstrating the usefulness of geochemistry in stratigraphic studies.