SOURCE ROCK POTENTIAL AND DEPOSITIONAL ENVIRONMENT OF MIDDLE – UPPER JURASSIC SEDIMENTARY ROCKS, BLUE NILE BASIN, ETHIOPIA

Abstract

This study investigates the hydrocarbon generation potential, kerogen quality, thermal maturity and depositional environment of Middle – Upper Jurassic sedimentary rocks in the Blue Nile Basin, Ethiopia, using organic petrography, Rock-Eval pyrolysis and molecular organic geochemistry. Thirty-seven outcrop samples were analysed for their total organic carbon (TOC) and inorganic carbon (TIC) contents. The samples came from a Toarcian – Bathonian transitional glauconitic shale-mudstone unit, the overlying Upper Bathonian Gohatsion Formation, and the Lower Callovian – Upper Tithonian Antalo Limestone Formation. Thirteen samples with sufficient TOC contents for further analysis of the organic matter, eight from the Antalo Limestone Formation and five from the glauconitic shale-mudstone unit, were selected and analysed using Rock-Eval pyrolysis. Vitrinite reflectance (VR_r) was measured on random particles, and qualitative maceral analysis was performed under normal incident and UV light. Nine samples were selected for molecular organic-geochemical analyses. All the samples originating from the Gohatsion Formation showed TOC values which were too low for further analyses of the organic matter.

The TOC contents of shales and limestones from the Antalo Limestone Formation and and of shales from the glauconitic shale-mudstone unit were 3.43-6.43% (average 4.85%) and 0.76-3.15% (average 1.72%), respectively, and two coaly shale samples from the latter unit have average TOC values of 18.48%. HI values are very high for shales in the Antalo Limestone Formation (average 575 mg HC/g TOC) but lower for the shales in the glauconitic shale-mudstone unit. The vitrinite reflectance of shales from the Antalo Limestone Formation ranged between 0.21% and 0.47%; coaly shales from the glauconitic shale-mudstone unit have VR_r% of between 0.29% and 0.35%. Pr/Ph ratios for samples of the Antalo Limestone Formation shales ranged from 0.8 to 1.1, indicating anoxic to suboxic depositional conditions; while shales in the glauconitic shale-mudstone unit show higher values of up to 4.9.

In terms of organic petrography, the Antalo Limestone Formation samples are dominated by finely dispersed liptinite particles and alginite; the organic material in the glauconitic shale-mudstone unit is of higher land plant origin, with abundant vitrinite and inertinite. Sterane and hopane biomarker ratios suggest an anoxic/suboxic depositional environment for the Antalo Limestone Formation shales and limestones. These values together with Rock-Eval Tmax (average 414 °C), the high ratio of pristane and phytane over the n-alkanes C₁₇ and C₁₈, and hopane biomarker ratios indicate that the Middle – Upper Jurassic succession is of low thermal maturity in the central parts of the Blue Nile Basin. The Antalo Limestone Formation shales have a high petroleum generation potential, making them a viable target for future exploration activities.