Regionally extensive organic-rich shales characterize the lower section of the Early Silurian Qusaiba Member, Qalibah Formation. These shales are believed to have acted as the primary Paleozoic petroleum systems source rock in Saudi Arabia. Based on the interpreted paleoenvironmental/chronostratigraphic variability of these organic-rich hot shales (Hayton et al., 2017), significant variation in the organic matter (OM) composition is expected to be present within these source rocks. With notable exceptions including Cole (1994) and Abu-Ali (2005), most of the OM typing work on the Qusaiba Member source rocks has been largely inferred from bulk geochemical analysis. OM typing inferred from bulk geochemistry (e.g. Rock-Eval) is undoubtedly useful, but as it is a weighted average of the mixture of the individual OM components, organic petrography is required to understand the actual OM composition.

Organic petrography conducted on 47 Qusaiba Member core samples from eight wells across the Kingdom identified 5 major groups of OM: (1) aquatic palynomorphs, like acritarchs (strongly fluorescent), (2) terrestrial palynomorphs, like cryptospores (moderately fluorescent), (3) structureless brownish organic matter (BOM), (4) graptolites and chitinozoa (nonfluorescent), and (5) inertinite and black degraded OM (nonfluorescent). Most samples are dominated by partially intensely degraded, nonfluorescent BOM. In one well much of the BOM shows intense fluorescence, indicating significant quantities of hydrogen-rich OM in this well. In all other wells the abundance of hydrogen-rich OM (group 1 and 2) is usually less than 5% each. As expected, given the observed variation in graptolite occurrence on bedding planes in core, group 4 OM abundance is highly variable.

This is the first study aimed at quantifying the variability in the nature of the OM within the Early Silurian source rocks of Saudi Arabia. The findings show that there is significant variability in the nature of the OM, both laterally and chronostratigraphically. Therefore, great care needs to be taken to ensure that geochemical and especially kinetics work on Qusaiba source rocks takes into account this variability, as this will have significant impact on volumetric resource calculations when integrated with basin modeling.

The identification of BOM as the dominant type of OM in the Qusaiba Member means that this organic matter (of hitherto unknown biological affinity) has played a major role in controlling the hydrocarbon generative potential of these source rocks. This highlights the need for further work to understand its primary composition, biological precursor, and hydrocarbon generative potential. Future work will continue to explore for ways to link the organic matter composition to the chronostratigraphy, so that the organic matter composition and correspondingly the hydrocarbon generative potential of the organic-rich shales of the Qusaiba Member can be determined in a predictive manner to support exploration activities.

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