Biomarker Distribution and Implications for Organic Matter Input and Depositional Environment of Shales from the Utapate Oilfield, Southeastern Niger Delta Basin, Nigeria

Abstract

Biomarker geochemistry provides a powerful means of reconstructing organic matter provenance, depositional environment, and thermal maturity of sedimentary source rocks. This study investigates the distribution of saturated and aromatic biomarkers in two shale samples from the Utapate Oilfield, Southeastern Niger Delta Basin, Nigeria, using gas chromatography–mass spectrometry (GC–MS). Diagnostic biomarker parameters derived from hopanes, steranes, tricyclic terpanes, oleanane, gammacerane, methylphenanthrenes, dibenzothiophenes, and triaromatic steranes were employed to evaluate organic matter input, paleoenvironmental conditions, and hydrocarbon generation potential. Results show that the shale extracts are dominated by saturated hydrocarbons (≈61–62%), with moderate aromatic fractions and very low asphaltene contents, indicating good oil quality and negligible biodegradation. Sterane distributions characterized by dominant C₂₉ steranes (≈38%), high oleanane/hopane ratios (>1.0), and supporting aromatic biomarker ratios (high P/DBT and low DBT/C₄N) indicate mixed marine–terrestrial organic matter input and Type II/III kerogen. Tricyclic terpane and hopane parameters, low gammacerane indices, and high diasterane/sterane ratios suggest deposition in a siliciclastic delta-front to prodelta environment under predominantly oxic to suboxic conditions, with episodic marine incursions. Thermal maturity indicators from both saturate and aromatic biomarkers (Ts/Tm, sterane isomerization ratios, MPI, F1, and F2) consistently place the shales within the main oil-generation window. The strong similarity in biomarker signatures between SEM-4 and UDY-5 well samples support a common source facies and robust oil–source rock correlation.