Geochemical Characterization of Subsurface Upper Ordovician Glaciogenic Deposits: Implications for Provenance, Tectonic Setting, and Depositional Environments

Ancient glaciogenic deposits contain significant hydrocarbon reservoirs in the Middle East and North Africa. For instance, the Lower Paleozoic glaciogenic deposits of Saudi Arabia are a potential unconventional gas reservoir in the Rub' al Khali Basin. In addition, they contain significant reservoir intervals in other basins. Although the sedimentological and stratigraphical characteristics of these glaciogenic deposits are fairly well understood, their provenance, tectonic setting, and the impact of weathering on the subsurface rocks of these deposits are not well constrained. Therefore, this study used geochemical data to characterize the depositional environments and investigate the provenance and tectonic setting of Hirnantian glaciogenic deposits. Four facies associations (FAs)-namely, fluvial (FA1), glaciolacustrine (FA2), subglacial (FA3) and glaciofluvial (FA4)-from the Upper Ordovician Sarah Formation were selected for this study. It was found that the maturity of the FA1 and FA4 sediments (average SiO2/Al2O3 > 27.1 +/- 5.4) was higher than that of FA2 and FA3 (average SiO2/Al2O3 < 14 +/- 1.8). Furthermore, FA1 and FA4 showed similar geochemical patterns (SiO2 > 90%), although the latter was deposited in a more proximal setting than the former, as interpreted from core samples. In contrast, FA2 and FA3 contain more geochemical variations and greater chemical weathering impacts than FA1 and FA4, as indicated by several chemical indices (e.g., Parker's Weathering Index). The results also indicated that all the studied FAs were deposited in a passive margin and were most probably derived from continental felsic rocks of the Arabian Shield. These findings are not only crucial in predicting the reservoir quality of such deposits, but also contribute to the understanding of the geochemical distributions, associations, affinities and variabilities in Upper Ordovician glaciogenic deposits, and the extent of these deposits on the Arabian Plate, associated with a large ice sheet in northern Gondwana.

Automated summary

This study characterized the depositional environments of Hirnantian glaciogenic deposits using geochemical data, and investigated their provenance and tectonic setting. The research found that the sediments from different facies associations showed varying levels of maturity and weathering impacts, with important implications for reservoir quality prediction and understanding the geochemical distributions in these deposits.