--> --> Geochemical and Palynological Studies of Some Maastrichtian Source Rock Intervals (Patti and Gombe Formations) in Nigeria: Implications for Hydrocarbon Prospectivity

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Geochemical and Palynological Studies of Some Maastrichtian Source Rock Intervals (Patti and Gombe Formations) in Nigeria: Implications for Hydrocarbon Prospectivity

Abstract

The Maastrichtian Patti and Gombe Formations are located in Bida and Gongola Basins respectively. These two inland basins form part of the targets currently being tested for hydrocarbon prospectivity in Nigeria. Road cuttings and core samples of coal, coaly shale and shale from the formations were analysed using standard organic geochemical and palynological techniques to unravel their organic matter quantity, quality, palynofacies, expulsion efficiency and thermal history. The age of the sediments were also constrained from the palynological data. Palynological data suggest a Maastrichtian age and influence of freshwater swamps for the sediments based on the microfloral assemblage like Retidiporites magdalenensis, Echitriporites trianguliformis, Buttinia andreevi and Botrycoccus braunii. The organic geochemical results show relatively high TOC for the Patti shales (0.79-12.9 wt.%) and Gombe coals (38.8-61.2 wt.%) implying moderate to high concentrations of organic matter. Hydrocarbon source potential range from 0.19 - 0.70 mgHC/g rock except for a certain interval with high yield (30.23 mgHC/g rock) in the Patti shales. The Gombe coals have source potential ranging from 32.77 - 69.38 mgHC/g rock. Generally the samples show low HI except one of the Patti shale samples with HI of 230 mgHC/g.TOC (thought to be formed under reducing condition) and one of the Gombe coal samples with HI of 170mgHC/g.TOC. In spite of the favourable organic parameters, the thermal maturity is low with vitrinite reflectance and Tmax ranging between 0.41-0.52 % and 413-431 oC respectively. Biomarker analysis of the hydrocarbon extracts show abundance of odd number alkanes C27 - C33, low sterane/ hopane ratio (0.06 - 0.25). Pr/Ph in the samples are greater than 2. We conclude from the study that the Maastrichtian source rocks were sourced terrestrially under a prevailing oxic condition and dominated by Type III organic matter. Type II organic matter with oil and gas potential may be possible in the Patti Formation in Agbaja area of Bida Basin. Thermal maturity and conversion ratio were low and not much hydrocarbon could have been generated from the source rocks.