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PETROPHYSICAL EVALUATION AND RESERVOIR CHARACTERIZATION USING WELL LOGS IN BAYELSA STATE
CHAPTER ONE
INTRODUCTION
1.1 Background to the Study
The West African passive margin, the Niger Delta Basin is the most important petroleum system in Nigeria and one of the most prominent in Africa. Bayelsa State, in the center of the Niger Delta in southern Nigeria, is underlain by some of the greatest accumulations of petroleum-rich sediments in the basin. Ever since crude oil was discovered at Oloibiri in Bayelsa State in 1956 the first commercial oil discovery in Nigeria the state has been in the epicentre of Nigeria’s petroleum production with many commercial oil fields, both onshore and in shallow offshore regions. Notwithstanding years of oil production, there is still so much more that can be done in terms of reservoir optimisation and field discovery, especially in poorly understood areas of the subsurface (Adizua & Uzu, 2024).
Petrophysical evaluation is the quantitative analysis of reservoir rock and fluid properties using wireline well logs, core samples and other types of downhole measurements, aimed at calculating reservoir volumes of in-place hydrocarbons and reservoir productivity. Volume of shale (Vsh), effective porosity (φ), water saturation (Sw), and hydrocarbon saturation (Sh) derived from well logs, as well as net pay thickness, are key petrophysical properties required in reservoir modelling and volumetrics (Schlumberger, 1989). Petrophysical evaluation accuracy affects the accuracy of reserve estimates, which then impact investment strategies for field development and production strategies.
Wireline well logs such as gamma ray, resistivity, neutron, density and sonic logs measure in-situ formation physical and chemical properties and fluids as a function of depth, offering detailed subsurface vertical profiles. Well log interpretation in the Niger Delta, characterised by reservoirs that are generally made up of unconsolidated to weakly consolidated sands alternating with marine and deltaic shales of the Agbada Formation, is based on calibrated results with core information and involves application of suitable shale volume correction techniques to prevent over-estimation of non-reservoir rocks (Harry et al., 2022; Kamayou et al., 2021). In the Niger Delta, the integration of petrophysical interpretations with seismic attribute analysis has been seen to provide significant improvement in reservoir characterisation and identification of prospective reservoir sand bodies (Odoh et al., 2024; Ejepu et al., 2024).
Reservoir characterisation involves more than just petrophysical analysis; it also includes the distribution and connectivity of reservoir units, structural interpretation and reservoir heterogeneity analysis. In Bayelsa State with producing fields that are characterised by complex faulting, lateral facies and multi-layers of reservoir sand bodies, an in-depth reservoir characterisation study using the well log information in the available wells will be necessary for identifying undiscovered hydrocarbon potential, designing infill development wells and predicting production rates (Adizua & Uzu, 2024).
1.2 Statement of the Problem
Despite the extensive history of exploration and production in Bayelsa State, several hydrogeological and petrophysical problems are still not well understood. First, the complex nature of reservoir sands in the Agbada Formation poses serious challenges to petrophysical analysis, especially where the distinction between net and shaly sands is not clear (or where conflicting results are given by various methods of estimating porosity). Second, the estimation of water saturation in the Niger Delta is confounded by the occurrence of fresh formation water in shallow reservoirs, which can lead to false identification of fresh water saturated sands as pay zones using electrical logs.
Third, many producing fields in Bayelsa State are in advanced stages of primary production and operators are looking for more pay zones in order to improve secondary recovery operations. This necessitates a rigorous re-interpretation of available well log data with state-of-the-art petrophysical software and techniques that may not have been considered during the initial well evaluation. Finally, a lack of published systematic petrophysical studies for individual fields in Bayelsa State is reflected in a gap in the technical literature which this study seeks to fill.
1.3 Objectives of the Study
General Objective
The general objective of this study is to evaluate the petrophysical properties of reservoir sands and characterise the hydrocarbon-bearing intervals in selected wells within an oil field in Bayelsa State, using well log analysis and standard petrophysical evaluation techniques.
Specific Objectives
The specific objectives of this study are:
(i) Identify and delineate reservoir and non-reservoir intervals from gamma ray, resistivity, neutron, and density log analysis.
(ii) Compute key petrophysical parameters including volume of shale, porosity, water saturation, bulk volume water, and permeability for the identified reservoir sands.
(iii) Determine the fluid types (oil, gas, or water) present in reservoir intervals using crossplot techniques and formation water resistivity data.
(iv) Correlate reservoir sands across available wells in the field to establish reservoir continuity and assess lateral heterogeneity.
(v) Estimate the net pay thickness and evaluate the hydrocarbon potential of the characterised reservoir intervals.
1.4 Research Questions
The following research questions guide the study:
- What are the petrophysical characteristics (porosity, permeability, water saturation) of the reservoir sands identified in the study wells?
- How do petrophysical properties vary laterally across the study field, and what does this imply for reservoir continuity?
- Which fluid types — oil, gas, or water — are present in the identified reservoir intervals, and how are they distributed?
- To what extent are there bypassed pay zones in the study field that could be targeted by infill drilling?
- What is the estimated net pay thickness and hydrocarbon pore volume in the characterised reservoir intervals?
1.5 Research Hypotheses
H₀₁: There is no significant variation in effective porosity between reservoir sands deposited in different depositional sub-environments (e.g., channel versus mouth bar) within the study field.
H₁₁: Effective porosity varies significantly between reservoir sands deposited in different sub-environments, reflecting the influence of grain size and sorting on pore geometry.
H₀₂: The hydrocarbon saturation of reservoir sands does not decrease significantly with depth below the original oil-water contact in the study wells.
H₁₂: Hydrocarbon saturation decreases significantly with depth below the original oil-water contact, indicating differential depletion and the potential presence of bypassed pay.
1.6 Significance of the Study
This study is of direct relevance to petroleum operators working in Bayelsa State. The petrophysical evaluation results will support better-informed decisions regarding infill drilling, well completion design, and enhanced recovery operations in the study field. Accurate reserve estimates derived from rigorous petrophysical analysis will also be of value to regulators, including the Nigerian Upstream Petroleum Regulatory Commission (NUPRC), in verifying field development plan submissions.
The study contributes methodologically to the field of Niger Delta petrophysics by applying updated evaluation techniques and comparing results with published regional benchmarks. It will serve as a useful reference for petroleum engineering and geoscience students at Nigerian universities and research institutions. The study also indirectly benefits communities in Bayelsa State by contributing to the sustainable and optimised exploitation of the petroleum resources that underpin local and national revenues.
1.7 Scope of the Study
This study covers the petrophysical evaluation of well logs from a minimum of three wells within a selected onshore oil field in Bayelsa State. The reservoir interval of interest is within the Agbada Formation, targeting sand units deposited in fluvio-deltaic and shallow marine environments. The study uses standard wireline log suites including gamma ray, deep resistivity, neutron porosity, density, and caliper logs. It does not extend to seismic attribute analysis or reservoir simulation, which are recommended for future study.
1.8 Limitations of the Study
The study is constrained by the availability and quality of well log data, which is subject to borehole conditions such as washouts and invasion that can affect the reliability of log readings. The absence of whole core or sidewall core data from the study wells means that log-derived petrophysical parameters cannot be directly validated against core measurements, introducing uncertainty in porosity and permeability estimates.
Formation water resistivity data, required for accurate water saturation calculations using the Archie equation, may not be available for all reservoir intervals, necessitating the use of regional averages that may not be representative of the specific field conditions. The proprietary nature of the well log data also means that full disclosure of field location and well identities may not be possible in the published study.
1.9 Definition of Terms
Petrophysics: The study of the physical and chemical properties of rocks and their interaction with fluids (water, oil, and gas), particularly as applied to the evaluation of hydrocarbon reservoirs using well log data.
Porosity: The fraction of the total volume of a rock that consists of void spaces (pores), expressed as a percentage; it determines the storage capacity of a reservoir.
Water saturation (Sw): The fraction of pore volume in a reservoir rock that is occupied by water; the complement (1–Sw) represents hydrocarbon saturation.
Volume of shale (Vsh): The proportion of shale or clay mineral content in a reservoir rock, derived from gamma ray or other logs; used to correct porosity estimates for clay-bound water.
Net pay: The thickness of reservoir rock that contains hydrocarbons above a defined cut-off in porosity, water saturation, and shale volume, and contributes to production.
Gamma ray log: A wireline log that measures the natural radioactivity of formation rocks; high gamma ray values typically indicate shale, while low values indicate clean sand reservoirs.
Resistivity log: A wireline log that measures the electrical resistance of formation fluids and rocks; hydrocarbon-bearing sands exhibit high resistivity relative to water-bearing or shale formations.
References
Adizua, O. F., & Uzu, D. U. (2024). A petrophysical properties evaluation study of a Niger Delta field using well logs and core data. Oriental Journal of Physical Sciences, 9(1).
Doust, H., & Omatsola, M. E. (1990). Niger Delta. In J. D. Edwards & P. A. Santogrossi (Eds.), Divergent/Passive Margin Basins (AAPG Memoir 48, pp. 239–248). American Association of Petroleum Geologists.
Ejepu, S. J., George, N. J., Harry, T. A., & Udo, I. G. (2024). Focused reservoir characterization: analysis of selected sand units using well log and 3-D seismic data in Kukih field, onshore Niger Delta, Nigeria. Scientific Reports, 14, 13763.
Harry, T. A., Etukudo, N. J., & Owoeye, T. A. (2022). Petrophysical analysis of XYZ field, south-east, Niger Delta using well logs. International Journal of Earth Sciences Knowledge and Applications, 4(2), 259–270.
Kamayou, V. M., Ehirim, C. N., & Ikiensikimama, S. S. (2021). Estimating volume of shale in a clastic Niger Delta reservoir from well logs: a comparative study. International Journal of Geosciences, 12, 949–959.
Odoh, B. I., Okeke, H. C., Obasi, R. A., & Onuoha, K. M. (2024). Lithofacies, sequence stratigraphy and geostatistical evaluation of petrophysical parameters in the Tertiary reservoirs in AVA Field, offshore Niger Delta. Journal of Sedimentary Environments.
Schlumberger. (1989). Log interpretation principles/applications. Schlumberger Educational Services.