New insights on Muglad’s Cretaceous source rocks: a paleo-geographic review & organic geochemical characterization, Muglad basin, Sudan

Abstract

Pyrolysis, chromatographic, and isotopic techniques were applied to study a suite of (174) rock cuttings from Muglad’s block 2 & 4, aiming at identifying and fully characterizing the potential source rock/s in the basin. Two source rock units were recognized and described as good-very good for hydrocarbon generation in the study area. These include the middle Early Cretaceous Abu Gabra and the top Late Cretaceous Baraka formations. The former consists of a good-very good organic rich shales (TOC = 1.0-4.0 %, S2 >20 mg HC/g TOC), of mixed kerogens dominated primarily by the oil prone type-I (HI = 600 mg/g) in association with few type-III, and is currently thermally mature for hydrocarbon generation in the basin peripheries and over mature in the centre. The Baraka displays a wide hydrocarbon generative potential ranging from fair to very good (TOC = (0.5-4.0%) and S2 yield of (2.0-20 mg HC/g TOC), of mainly type-II oil and gas prone kerogen (HI = 450 mg/g), and thermally mature for hydrocarbon generation in the basin centre only.Biomarker characteristics of the analyzed samples reveal three main organofacies units; a (I) freshwater oxic to sub-oxic clay-rich Early Cretaceous distal lacustrine, (II) a highly anoxic, and clay-poor Late Cretaceousdistal marine, and a (III) proximal lacustrine and/marine of both E. and L. Cretaceous. Organofacies-I shows relatively high ratios of: [Pr/Ph (1.2-3.02), C26/C25 tricyclic terpanes (1.11-3.12), Ts/Tm (0.95-1.33), gammacerane indices (0.14-0.50), and diasterane/regular steranes] with low: [tri- and tetracyclic terpanes, C17/C25 n-alkanes (1.0-1.5), C29/C30 hopanes (0.55-1.14), and pregnanes & homopregnanes]. Organofacies-II inversely, is characterized by very low [Pr/Ph ratios (0.37-1.0) suggesting extreme anoxic conditions, C26/C25 tricyclic terpanes between (0.50-1.0), Ts/Tm (0.21-0.50), Gammacerane indices (0.05-0.22), and low diasterane/regular steranes (0.34-0.66)] with high: [tricyclic terpanes content dominated by the C23 variety, C24 tetracyclic terpanes, C17/C25 n-alkanes ratios (3.7-5.12), C29/C30 hopanes (0.4-1.11), and abundant pregnanes and homopregnanes]. Organofacies-III is amerely proximal sub-facies of the former two differentiated basically on the basis of the redox conditions of their corresponding depositional environments and amount of terrigenous organic matter supply. The isotopic data is fairly consistent with the biomarkers in delineating the same three organofacies units. We relate the marine input into Muglad basin during the L. Cretaceous to the global L. Cretaceous sea transgressions in Africa and thus foresee a booming new exploration opportunity in the basin.