Imprints Of Magmatism on Source Rocks of the Yola Arm, Upper Benue Trough: Insights From Biomarker Analysis and Petrography.

Abstract

This study assesses the thermal maturity of source rocks affected by magmatism in the Yola Arm of the Upper Benue Trough, Nigeria. The objective was to evaluate the thermal maturity of source rocks exposed around volcanic bodies using biomarker and petrographic analyses and to determine whether magmatic heating produced thermal alterations that overprint normal burial diagenesis. Five outcrop samples representing four stratigraphic intervals associated with volcanic rocks were analyzed using petrographic techniques and Gas Chromatography–Mass Spectrometry (GC–MS). Biomarker-derived thermal maturity parameters, including hopane C₃₁ 22S/(22S+22R), sterane C₂₉ 20S/(20S+20R), and trisnorhopane Ts/Tm ratios, indicate that the source rocks are thermally mature and generally within early- to peak-oil-window conditions, except for the degraded sample. Petrographic observations further reveal progressive thermal maturation of the organic-rich shales. Progressive darkening, smearing, and partial coalescence of amorphous organic matter (AOM) into discontinuous bitumen-rich lenses constitute textural evidence of kerogen breakdown and bituminization during catagenesis. Spatially localized fabric disruption, bitumen enrichment, and microfracturing observed in some samples provide evidence of short-duration, high-temperature conductive heating events associated with nearby magmatic activity. The combined biomarker and petrographic evidence demonstrates that burial history alone is insufficient to account for the observed variations in source-rock thermal maturity across the Yola Arm. Consequently, the distribution of intrasedimentary magmatic bodies and the spatial extent of their associated thermal aureoles should be incorporated into petroleum-system models to better constrain source-rock maturation patterns and hydrocarbon-generation histories within the basin.