Integrated Organic Geochemical, Biomarker and Carbon Isotope Evaluation of Source-Rock Affinities in the Anambra Basin, Nigeria

Abstract

The Campanian-Maastrichtian succession of the Anambra Basin contains organic-rich lithologies with uncertain contributions to the petroleum system.  This study integrates Rock-Eval pyrolysis, TOC, vitrinite reflectance, biomarker geochemistry, and compound-specific carbon isotope analyses of saturated and aromatic hydrocarbon fractions to assess the hydrocarbon generation potential and source-rock affinities of the Nkporo Shale and Mamu Formation.  Twenty-eight rock samples (eleven Nkporo shales, nine Mamu shales, eight Mamu coals) were analyzed, with a representative subset (six Nkporo shales, six Mamu shales, four Mamu coals, and six crude oils) subjected to detailed biomarker and isotope analyses.  The Nkporo Shale is a fair to good source rock (average TOC = 1.66 wt%) with Type III to mixed Type II/III kerogen, while the Mamu Formation is heterogeneous with organic-rich shales (TOC up to 7.93 wt%).  Thermal maturity parameters (Tmax = 417–443°C, Ro = 0.22–1.51%) indicate immature to peak oil-window conditions.  Biomarker distributions distinguish the two units: the Nkporo Shale exhibits C₂₀–C₂₃ n-alkanes, moderate Pr/Ph ratios (1.78–2.61), and elevated C₂₈ steranes, indicating mixed marine-terrestrial organic matter deposited under mildly oxic conditions.  In contrast, the Mamu Formation shows C₂₇–C₃₉ n-alkanes and elevated Pr/Ph, CPI, and TAR values, reflecting higher-plant input in continental swamp to coastal plain environments.  Principal Component Analysis and Hierarchical Cluster Analysis (PC1 = 37.5%, PC2 = 14.8%; cophenetic correlation = 0.772) corroborate two distinct organic facies and place the ANAR crude oil within the mixed marine-terrestrial cluster containing both Nkporo and Mamu shales.  While biomarker, isotopic, and multivariate evidence points to closer affinity between the ANAR oil and the Mamu Formation, a subordinate Nkporo contribution cannot be excluded.  The Mamu Formation is identified as a significant liquid hydrocarbon source, though quantitative mixing models are needed to precisely partition relative source contributions.