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Acritarchs in the Silurian Qusaiba Shale and Related Biomarkers: Implication for Identifying Paleozoic Hydrocarbon Charge

Abstract

Lower Silurian organic-rich hot shales constitute one of the major Paleozoic source rocks across the Arabian craton and northern Africa. In Saudi Arabia, the Qusaiba Member of the Qalibah Formation is Rhuddanian in age as evident from acritarch and chitinozoan palynomorphs together with graptolite-based dating. Further understanding of the Qusaiba's kerogen macerals and biomarkers becomes crucial in delineating maturation and revealing hydrocarbon charge that will ultimately help identify sweet-spots in unconventional targets within the Qusaiba Shale. In this study we present different acritarch species observed from polished rock in epoxy pellets and examined samples using reflected-light microscopy. Change in the acritarchs' morphology and florescence intensity as a function of thermal maturity is observed and compared with solid bitumen and graptolite reflectance. We examined a total of eight rock samples with maturities ranging from immature into the condensate window. Four of the examined samples matured over geological time, whereas the other four samples were artificially matured in the laboratory using hydrous pyrolysis. Expelled oil from hydrous pyrolysis, together with extracted bitumen, were examined using Gas Chromatography – Flame Ionization Detector (GC-FID) and Gas Chromatography - Mass Spectrometry (GC-MS) techniques to analyze for n-alkanes, isoprenoids and biomarkers. Preliminary results suggest a possible relationship between acritarchs and the presence of methyl sterane biomarkers and their triaromatic homologues, corroborating the results from earlier studies. This relationship provides a useful tool that can be used as diagnostic fingerprint for unraveling hydrocarbon charge contribution from the Paleozoic petroleum system.