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SEQUENCE STRATIGRAPHY AND DEPOSITIONAL ENVIRONMENT OF THE NIGER DELTA BASIN IN DELTA STATE
CHAPTER ONE
INTRODUCTION
1.1 Introduction
The Niger Delta Basin is one of the world’s biggest and most productive hydrocarbon-bearing sedimentary basins and is one of the world’s five leading oil and gas provinces in terms of proven reserves and historical production. The Niger Delta, located in the Gulf of Guinea on the west coast of Africa, is a result of the progradation of clastic sediments from the West African craton onto the passive margin created after the South Atlantic Ocean opened during the Cretaceous period (Doust & Omatsola, 1990). The basin covers more than 300,000 km², including onshore Bayelsa, Delta, Rivers, Imo, Cross River, and Akwa Ibom states, deep-water Gulf of Guinea, and has a fill of more than 12 km of sediment.
Sequence stratigraphy is a field of sedimentary geology that studies the divisions of geologic successions into genetically related units (sequences) that are separated by unconformities and their conformable correlatives, formed through the interaction of relative sea level change, sediment supply and tectonics (Vail et al., 1977; Posamentier & Allen, 1999). The use of sequence stratigraphy in exploring the Niger Delta Basin has led to a better understanding of the spatial and temporal distribution of reservoir sands, source rocks, and seals, and has provided a robust conceptual framework for exploration and field development (Ola et al., 2021). The system tracts lowstand, transgressive and highstand of a depositional sequence are linked to specific depositional settings and have specific reservoir properties that control the storage and production of hydrocarbons.
Delta State, in the western region of the Niger Delta, hosts large onshore regions underlain by the Agbada Formation, which is the most important reservoir of Niger Delta, and parts of the coastal swamp and marine subprovinces. The geology of Delta State in the Niger Delta Basin involves a thick stack of Miocene to Recent deltaic rocks consisting of paralic sands, shales and interbedded units that form the progradational wedge of the Niger Delta. Knowledge of the sequence stratigraphy and depositional environment of these sediment packages are crucial for reservoir prediction, correlation of productive layers across wells, and to identify untested zones that may contain hydrocarbons (Oluwajana et al., 2024).
Some recent studies have shown the value of incorporating 3D seismic data, well logs, and biostratigraphic data to build detailed sequence stratigraphic models of the reservoirs in the Niger Delta (Oluwajana et al., 2024; Alao et al., 2021). The integrated workflows allow the identification of maximum flooding surfaces, sequence boundaries, and systems tract boundaries that provide important stratigraphic markers for correlation and facies prediction. Palynology and foraminiferal biostratigraphy are essential to provide age constraints and palaeobathymetric interpretations for calibrating the sequence stratigraphic framework.
1.2 Statement of the Problem
Despite a long history of hydrocarbon exploration and development in Delta State and, in general, the western Niger Delta, there remains considerable uncertainty in the sequence stratigraphic framework of onshore Agbada Formation reservoirs. These include the lateral discontinuity of sandstone bodies caused by syndepositional faulting, the geometry of growth faults and the related rollover anticlines (fault-related fold), and variability in facies architecture, due to the interaction between fluvial-deltaic and marine processes. These pose problems in well targeting,under-development of reservoirs and incorrect correlation of reservoir sands.
In addition, the onshore Niger Delta has undergone extensive exploration for more than 60 years, and the majority of the classical structural traps have been tested. Stratigraphic traps (pinch-outs, channel sands, and shoreface sands) will hold the key to future exploration success in the Niger Delta, which requires a thorough understanding of depositional environments and sequence stratigraphic framework. The lack of a detailed sequence stratigraphic model for onshore Delta State is a major constraint on the development of stratigraphic trap plays in this part of the Niger Delta.
1.3 Objectives of the Study
General Objective
The general objective of this study is to establish the sequence stratigraphic framework and characterise the depositional environment of the Niger Delta Basin in the onshore areas of Delta State, using integrated well log, seismic, and biostratigraphic data.
Specific Objectives
The specific objectives of this study are:
(i) Describe the lithostratigraphic units and identify key stratigraphic surfaces — including sequence boundaries, transgressive surfaces, and maximum flooding surfaces — from well log analysis and seismic interpretation.
(ii) Interpret the depositional environment of identified lithofacies and facies associations using log motifs, core descriptions, and biostratigraphic data.
(iii) Construct a sequence stratigraphic framework showing the organisation of depositional sequences and systems tracts within the study area.
(iv) Correlate identified sequences across wells using log and seismic data.
(v) Assess the implications of the sequence stratigraphic framework for hydrocarbon reservoir distribution and trap geometry in the study area.
1.4 Research Questions
The following research questions guide the study:
- What are the key sequence stratigraphic surfaces and systems tracts present in the Agbada Formation of onshore Delta State?
- What depositional environments are represented by the lithofacies identified from well log and seismic data in the study area?
- How do identified depositional sequences correlate across available wells within the study area?
- What are the implications of the sequence stratigraphic framework for the distribution of reservoir-quality sands in the study area?
- To what extent do growth faults influence the geometry and lateral continuity of depositional systems tracts in the western Niger Delta?
1.5 Research Hypotheses
H₀₁: There is no significant lateral variation in the thickness of individual systems tracts across the study area, indicating uniform depositional conditions.
H₁₁: There is significant lateral variation in the thickness of systems tracts across the study area, reflecting the influence of growth faulting and differential subsidence on accommodation space.
H₀₂: Lowstand systems tract deposits in the study area do not exhibit significantly higher net-to-gross sand ratios than highstand systems tract deposits.
H₁₂: Lowstand systems tract deposits exhibit significantly higher net-to-gross sand ratios than highstand systems tract deposits, making them more prospective as hydrocarbon reservoirs.
1.6 Significance of the Study
This study is of direct significance to petroleum exploration and production companies operating in the western Niger Delta, providing a scientific framework for improving reservoir prediction and reducing drilling risk in Delta State. The sequence stratigraphic model will help identify untested stratigraphic trap plays, which are increasingly important as conventional structural traps become depleted. The Nigerian National Petroleum Company Limited (NNPCL) and its joint venture partners will benefit from improved subsurface characterisation.
The study also contributes to the academic understanding of deltaic stratigraphy and basin evolution, and will serve as a reference for postgraduate researchers in petroleum geoscience in Nigeria. The depositional environment interpretations will enrich understanding of the Cenozoic history of the Niger Delta and the Gulf of Guinea passive margin, with broader implications for palaeoclimate reconstruction.
1.7 Scope of the Study
The study covers the onshore and shallow offshore portions of Delta State within the Niger Delta Basin. Primary data sources include 3D seismic volumes, wireline well logs, and biostratigraphic reports from selected wells. The stratigraphic interval of interest spans from the lower Miocene to the Pliocene, corresponding to the main productive interval of the Agbada Formation. The study does not extend to the deep offshore sub-province of the Niger Delta.
1.8 Limitations of the Study
Access to subsurface data in the Niger Delta is often restricted by the proprietary nature of exploration datasets held by oil companies. The study is therefore limited to datasets that can be accessed through academic agreements or publicly available sources, which may not provide full spatial coverage of the study area. The quality and vintage of legacy seismic data in some parts of the onshore may limit the resolution of stratigraphic interpretations.
Biostratigraphic data availability is variable across wells, and the absence of such data in some wells may limit the precision of systems tract age assignments. Additionally, the absence of core samples from key stratigraphic intervals means that depositional environment interpretations rely primarily on log motifs and seismic facies, which carry inherent uncertainty. These limitations are mitigated by cross-validation between multiple data types wherever possible.
1.9 Definition of Terms
Sequence stratigraphy: A branch of geology that studies rock successions and their interpretation in terms of cyclic variations in relative sea level, organising strata into genetically related packages called depositional sequences.
Systems tract: A linkage of contemporaneous depositional systems forming the subdivision of a depositional sequence, classified as lowstand, transgressive, or highstand based on relative sea-level position.
Maximum flooding surface (MFS): A key stratigraphic surface that represents the time of maximum marine transgression, typically characterised by fine-grained sediments and high gamma ray values on well logs.
Agbada Formation: The principal lithostratigraphic unit of the Niger Delta, consisting of paralic interbedded sands and shales deposited in deltaic to shallow marine environments, forming the main petroleum reservoir of the basin.
Growth fault: A syn-sedimentary normal fault that was active during deposition, resulting in thicker sediment accumulation on the downthrown side; common in the Niger Delta and associated with rollover anticline traps.
Net-to-gross ratio: The ratio of net reservoir (clean sand) thickness to gross interval thickness in a well, used as a measure of reservoir quality and lateral sand connectivity.
References
Alao, D. A., Adenle, A. R., & Omosanya, K. O. (2021). Sequence stratigraphic analysis and hydrocarbon prospectivity of AMO Field, deep offshore Niger Delta, Nigeria. Journal of Natural Gas Geoscience, 6(4), 231–249.
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.
Obaje, N. G. (2009). Geology and mineral resources of Nigeria. Springer.
Ola, S., Ohiochioya, J., & Alabere, O. (2021). Sequence stratigraphy and petrophysical analysis of Aje field, offshore Nigeria. International Journal of Earth Sciences Knowledge and Applications, 3(1), 1–15.
Oluwajana, O. A., Salako, A. O., Osotuyi, A. G., & Adepelumi, A. A. (2024). High resolution 3-D seismic and sequence stratigraphy for reservoir prediction in Stephi field, offshore Niger Delta, Nigeria. Scientific Reports, 14, 1–18.
Posamentier, H. W., & Allen, G. P. (1999). Siliciclastic sequence stratigraphy: concepts and applications. Society for Sedimentary Geology.
Vail, P. R., Mitchum, R. M., & Thompson, S. (1977). Seismic stratigraphy and global changes of sea level. In C. E. Payton (Ed.), Seismic Stratigraphy — Applications to Hydrocarbon Exploration (AAPG Memoir 26, pp. 53–62). AAPG.