Resumo

OGALA, Jude E  e  AKAEGBOBI, Mike I. Using aromatic biological markers as a tool for assessing thermal maturity of source rocks in the Campano-Maastrichtian Mamu Formation, southeastern Nigeria. Earth Sci. Res. J. [online]. 2014, vol.18, n.1, pp.51-62. ISSN 1794-6190.  https://doi.org/10.15446/esrj.v18n1.34799.

The concentration and distribution of aromatic biomarkers in coals and shales from five boreholes penetrating the Maastrichtian Mamu Formation of the Anambra Basin, southeastern Nigeria, were investigated by gas chromatography-mass spectrometry to assess the thermal maturity and organic matter input. The study focused on the variations of the relative abundances of naphthalenes, phenanthrenes, and monaromatic and triaromatic steroids identified on the mass fragmentograms. Trimethylnaphthalene (TMN) is the most abundant member of the naphthalene family while methylphenanthrene (MP) is the most abundant phenanthrene family member. The total of phenanthrenes and their isomers was greater than that of naphthalenes. The distribution of these aromatic hydrocarbons and their akyl derivatives was strongly controlled by a selective expulsion mechanism and thermal maturation of organic matter. The low dibenzothiophene/phenanthrene (DBT/PHEN) ratios (0.01-0.06), as well as the enhanced concentrations of 1,2,5-TMN relative to 1,2,7- TMN, indicates organic matter derived mainly from higher plants, and the extract ternary plot of C₂₇, C₂₈ and C₂₉ monoaromatic steroids suggests a Type III and mixed Type II/III kerogen. The calculated mean vitrinite reflectance (%Rm), determined from the distributions of the isomers of methyldibenzothiophene ratio (MDR) in the rock extracts, ranged from 0.51 to 1.43. These maturity values indicate that the coal and shale extracts are marginally mature for hydrocarbon generation.

Palavras-chave : Aromatic biomarkers; Maturity; Mamu Formation; Anambra Basin.

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