BIOMARKER EVALUATION OF THE PETROLEUM SOURCE ROCK POTENTIAL OF THE AHOKO SHALE (PATTI FORMATION) IN THE SOUTHERN BIDA BASIN, NORTH CENTRAL NIGERIA

The petroleum source potential of Ahoko shale unit of the Maastrichtian Patti Formation was assessed by biomarker analysis with the aim of assessing the provenance and maturity of the organic matter and hence the hydrocarbon-generative potential. Six outcrop samples were analysed by the use of Soxhlet extractor and a Varian 3400 GC. Collection and procession of the gas chromatogram data for alkane compounds was done with Atlas software via a chromatographic server. Eighty three percent (83%) of the results show pristane/phytane ratios >1.0 which indicate that the organic matter is predominantly of terrestrial origin and deposited in sub-oxic to oxic and non-hypersaline depositional environments. The n-alkane distributions from the gas chromatogram shows obvious odd-over-even predominance which point to inputs of terrestrial higher plants organic matter and also an indication of the immature to early mature stage of the organic matter from the Ahoko shale. Eighty three percent (83%) of the results show Odd-Even-Ratio (OER) <1.0 and seventeen percent (17%) OER >1.0. Also, eighty percent (80%) of the results show Carbon Preference Index (CPI) <1.0, ten percent (10%) >1.0 and ten percent equivalent to 1.0. Both values of OER and CPI are generally below 1.0, which means that, the organic matter of Ahoko shale are thermally immature. Due to the results of pristane/phytane ratios and the n-alkane distributions from gas chromatogram, the hydrocarbon potential of Ahoko shale is believed to be gas prone and may have started generating gas in the deeply buried sections.