Applied sequence stratigraphy and heterogeneity assessment of the Eocene clastics in the Niger Delta Basin, Nigeria

The discovery of oil in the inland basins of Nigeria has prompted the search for optimal reservoir rocks in the outcropping units of the Niger Delta Basin for potential hydrocarbon prospectivity and utilization. The petrophysical properties of clastics in the Nsugbe-Umunya area was, therefore, evaluated in a sequence stratigraphic framework in order to comprehend reservoir quality and distribution. The analysis integrated lithofacies, stacking patterns, and biofacies data to interpret depositional environment, which formed the basis for sequence stratigraphic interpretation. Lithofacies analysis identified distinct fluvial signatures of the—floodplain/overbank processes as well as and marginal-shallow marine processes. Sequence stratigraphic application delineated two systems tracts: the Highstand Systems Tract (HST), truncated by a subaerial unconformity (Sequence Boundary; SB), and the Transgressive Systems Tract (TST). Petrophysical properties of the identified systems tracts were evaluated using the Porosity–Permeability (P-K) plots and Flow Zone Indicator (FZI) analysis, utilizing the Dykstra-Parson Coefficient model. The P-K plots revealed that HST sands exhibited smaller grain sizes and pore spaces. The HST sands also demonstrated high interconnectivity amongst seven (7) flow units according to SML and FZI plots, thus yielding a negligible reservoir heterogeneity index (VK) of 0.6. Conversely, the TST sands showed less connectivity and greater heterogeneity, with VK = 0.8, with 13 and 5 flow units according to SML and FZI plots, respectively. These findings in the TST sands suggest the presence of materials capable of inhibiting fluid flow. In conclusion, sands within the HST displayed better reservoir qualities due to superior grain size, interconnectivity and homogeneity, emphasizing their better prospects as potential reservoir units.