Organic geochemical signals of Paleozoic rocks in the southern Tethys, Siwa basin, Egypt: Implications for source rock characterization and petroleum system

Abstract

Paleozoic sedimentary succession research is valuable for better understanding the global petroleum province and future hydrocarbon reserves. Paleozoic rocks are the prime source of the enormous hydrocarbon fields in North Africa. Despite the Western Desert of Egypt being part of the North African Platforms and having similar depositional, tectonic, and diagenetic constraints, the hydrocarbon system for Paleozoic rocks in the Siwa Basin is indeed not characterized. Forty-five Paleozoic samples were collected from the Siwa Basin to evaluate the source rock characteristics using Rock-Eval pyrolysis, vitrinite reflectance, stable carbon isotopes, biomarkers, headspace gas, and palynofacies analyses. Additionally, the burial and thermal histories of these rocks were investigated by 1D basin modeling. The Paleozoic succession is represented by the Silurian (Basur Formation), Devonian (Zeitoun and Desouqy formations), and Carboniferous (Dhiffah Formation). The routine bulk geochemical analysis of the studied samples indicates poor to good source rock potential. The studied samples contain mostly kerogen type III with some kerogen type II, whereas the Carboniferous samples contain mostly kerogen type IV. The organic matter of the Devonian samples is suggested to have originated from marine and terrigenous sources, with a dominant upward terrigenous source. The lower part of the Devonian is deposited in more reduced marine conditions than the upper part. The organic matter is found to be mature, as indicated by Rr%, Tmax, and biomarker data. The composition of collected headspace gases in Paleozoic rocks suggests their thermogenic origin. Moreover, the similarity of the molecular composition between Paleozoic source rocks and crude oil in reservoir rocks supports the same hydrocarbon sources. Accordingly, the petroleum system in the Siwa Basin is based on active Paleozoic source rocks that charge and accumulate hydrocarbons in the sand reservoirs of the Desouqy Formation. The Paleozoic section entered the oil window in the Early Cretaceous (∼101 myr) based on its burial history. Thus, this study gives improved insights into the petroleum system in the Siwa Basin. Additionally, the Siwa Basin's lesser thickness and burial depth in comparison to other North African basins most likely contributed to its poor petroleum potential.