Saturated hydrocarbons (biomarkers) in Permian source rocks from Maniamba Basin, Mozambique (Central Gondwana): A contribution for a better understanding of palaeoenvironments

Abstract

Karoo Palaeozoic sediments from the Maniamba basin, Mozambique were analysed by integrating the molecular composition of biomarkers, inorganic chemistry, and palynofacies proxies to further understand their origin. Unimodal distribution is revealed through gas chromatography-mass spectrometry for n-alkanes coupled with the abundance of compounds of medium and high molecular weight. The dominance of stigmastane over cholestane and ergostane, Terrigenous Aqueous Ratio (TAR>1), and high concentration of C19 and C20 tricyclic terpenes are indicative of terrestrial organic matter dominance. The Carbon Preference Index, the Odd over Even Preference for the distribution of n-alkanes, the low values for the ratio 18α(H)- trisneohopane over 17α(H)-trisnorhopane, the C29 ααα S/(S+R) isomerization ratios in C29 steranes, C30 βα/(βα+αβ), C31 αβ S/(S+R) and C32αβ S/(S+R) isomerization ratios suggest organic matter (OM) with low thermal evolution. These data are supported by Spore Color Index and vitrinite reflectance. The nature of the OM is supported by isoprenoids Pristane/Phytane ratios, palynofacies, and high contribution derived from an ecosystem dominated by ancient Glossopterid and conifer flora. Finally, data from studied proxies indicate weak to strong palaeoweathering, highly proximal, marginal oxic-anoxic Basin characterized by kerogens associated with weakly and strongly oxidized swamps suggesting an existing potential for gas. Lastly, analyzed material strengthen the occurrence of continentalization process across Permian deposits of Gondwana.