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.
Highlights
During the last thirty years, the South African government has been motivated to explore the Orange Basin due to the discovery of valuable petroleum on the southern Atlantic margins (Jungslager 1999; Hartwig et al 2012; Adekola et al 2012)
The composition and long-term preservation of organic matter deposited in the marine domain throughout the Orange Basin is related to deposition system stacking and, as a result, the depositional systems tracts in which it was deposited (Adekola et al 2012)
ICV versus CIA (Fig. 10) demonstrates that all the original matter analyzed shales plotted above the Post Archean Australia Shales (PAAS) ICV index of about 0.85, indicating immaturity in the composition as well as relatively mild weathering of original source materials (Hernández-Hinojosa et al 2018)
Summary
During the last thirty years, the South African government has been motivated to explore the Orange Basin due to the discovery of valuable petroleum on the southern Atlantic margins (Jungslager 1999; Hartwig et al 2012; Adekola et al 2012). The integration of organic and inorganic geochemistry provides a multiscale perspective that allows researchers to answer some questions such as origin of organic matter, genetic linkage within the source rocks, deposition, thermal maturity, sediment age, type and quality of source rocks, mineral content, and chemical composition of the vast, unexplored Orange Basin post-rift sequence. In this context, molecular geochemistry, inorganic methods and isotopic studies were integrated to recognize their geochemical characteristics through the reconstruction of the Orange basin’s paleovegetation and paleoweathering. Trace elements were examined using the Agilent Technologies 7500S-ICP-MS
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