Biomarkers, stable carbon isotope, and trace element distribution of source rocks in the Orange Basin, South Africa: implications for paleoenvironmental reconstruction, provenance, and tectonic setting

Abstract

Aptian to Campanian sediments from the Western offshore to Central Orange Basin were studied by integrating molecular geochemistry, inorganic and isotopic studies to recognize their geochemical characteristics via the reconstruction of the Orange basin’s paleoweathering, paleosalinity, paleovegetation, paleoclimate, and tectonic records. Molecular analyses of both aliphatic and aromatic compounds reveal an input dominantly from a marine source. The source rocks accumulated in a reduced, anoxic, saline water column. Based on various biomarker proxies and vitrinite reflectance data, some samples are thermally mature to produce petroleum, while others are not. According to the V/Ni ratio, samples from the Orange Basin in South Africa are mainly anoxic, with only a few samples ranging from suboxic to anoxic. This is congruent with biomarker and isotope analyses that further indicate the presence of marine-derived source rocks with some terrestrial remains generating hydrocarbons. The investigated sediments are made up of intermediate igneous rocks that have undergone moderate chemical weathering. Geochemical figures on tectonic setting discriminant function diagrams revealed a continental rift of passive margin settings. As a result, the extrapolated crustal processes are directly analogous to the genesis and evolution of the Orange Basin, demonstrating Gondwana’s breaking up and the opening of the Atlantic Ocean Margin.

Shale-gas potential from Cretaceous succession in South Africa’s Orange Basin: Insights from integrated geochemical evaluations

“…It is an essential geochemical criterion for determining the probability of a source rock's maturity to the point where it can transform kerogen to produce petroleum (Jarvie et al 2007;Welte and Tissot 1984). There are two primary approaches to this Yelwa et al 2022). The term "paleo-geothermometer" refers to VRo, which is the most widely used technique for determining the optimal paleotemperature and timeframe at any point in geological history (Barker and Elders 1981;Mukhopadhyay 1994) through microscopic examination of kerogens or whole mounts, with a photomultiplier recording the particle's reflectivity.…”

“…Earlier work in the region was not confined to geochemical, geophysical, and sedimentological analyses. Some noble geochemical research was undertaken (Van der Spuy 2003;Hirsch et al 2010;Alexander Hartwig, Anka, and di Primio 2012;Adekola, Akinlua, and Mangelsdorf 2012;Akinlua et al 2015;Yelwa et al 2022), though few studies highlighted the geochemical properties of Orange Basin shales.…”