Depositional environments and sequence stratigraphy of Tertiary sediments of Oleum Field, Eastern Niger Delta Basin, Nigeria

Abstract

Understanding the stratigraphic framework of Oleum field is critical to hydrocarbon exploration and production success, owing to the alternating nature of organic-rich shale envelopes and porous sand sheets of the field. This research is aimed at studying the sequence stratigraphic framework and depositional environments of sedimentary facies of Oleum field, onshore (swamp) Niger Delta. This aim was achieved using sequence stratigraphic principles based on the integration of biostratigraphic, paleobathymetric, seismic and wireline log data of two wells (i.e. Oleum-001 and Oleum-002). This has led to the delineation of two maximum flooding surfaces (? Ma MFS and 13.6 Ma MFS) at 8182 ft and 9080 ft respectively. Each MFS is associated with depositional sequence and systems tracts. The first depositional sequence consists of two systems tracts; transgressive system tract (TST) and highstand system tract (HST), while the second depositional sequence consists of all the three systems tracts; lowstand system tract (LST, TST and HST). The 13.6 Ma MFS was delineated based on the occurrence of Bolivina 29a / Cassidulina 7 fossils. Additionally, the well logs sequence stratigraphy revealed two sequence boundaries which occur at 7255 ft and 8865 ft of Oleum-001, and 7033 ft and 8770 ft of oleum-002. The lithofacies are predominantly sand and shale displaying different kinds of log motifs. The sedimentological log motif shows a coarsening upward and blocky sands of the HST delineated as the system tract with the best reservoir properties. Fining upward and serrated/saw tooth motif of the TST is delineated as the facies with the best seal properties. The result indicates that the stratigraphic framework of Oleum Field is fluvio-deltaic to marine setting.