Using Iron and Sulphur Ratio Against Biomarkers to Delineate Oil Accumulation Depth in the Niger Delta

Author

Nmerukini C., Abrakasa S., Chiazor F. I., Njoku G. O.


Abstract

 Iron/vanadium (Fe/V) and dibenzothiophene/phenanthrene (DBT/P) ratios are widely used, independently, as proxies for detrital input and reduced-sulphur availability during source-rock deposition. This study brings the two together, alongside a suite of biomarker maturity and source-input ratios, for six Niger Delta oils (Utorogu, Ohaji, Abo, Escravos, Bonga, and Agbada), to test whether their combined signature can help differentiate the relative burial or accumulation position of oils across the basin. Two facies groups emerge consistently across Fe/V, V/Ni, and DBT/P zonation: a low-Fe/V, high-V/Ni group (Utorogu, Ohaji) linked to restricted, reducing, terrestrially influenced settings, and a high-Fe/V, low-V/Ni group (Escravos, Bonga, Agbada) linked to more detrital-rich, sub-oxic marine to outer-delta settings, with Abo occupying an intermediate position. Biomarker maturity indices (S/(S+R) C₃₁ hopane), however, cluster tightly around 0.6–0.7 across all six wells, indicating a broadly uniform thermal history that does not track the Fe/V-DBT/P grouping. This suggests that, in the present dataset, iron and sulphur-proxy variability is controlled more by source facies and depositional redox than by differential burial depth. Because no elemental iron or sulphur measurements and no direct reservoir depth data were available for these samples, the iron and sulphur components of this study are represented by proxies (Fe/V and DBT/P) rather than bulk elemental values, and the conclusions regarding accumulation depth are offered as a testable framework rather than a calibrated depth model. A path toward a fully quantitative version of this approach combining a degree-of-pyritization-type iron–sulphur index with depth-tagged samples  is outlined for future work.


Keywords

Iron/vanadium ratio; dibenzothiophene/phenanthrene; biomarker maturity; degree of pyritization; oil accumulation depth; Niger Delta



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References


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