Turkish Journal of Earth Sciences




The Late Ordovician glacial deposits are widespread and constitute a significant portion of the Early Paleozoic stratigraphic sequence of the cover rocks of the Arabian and African platforms. The glaciation lasted less than one million years between the Late Ashgillian (445 Ma) and Early Silurian (444 Ma) but it had a strong impact on the whole area. It has been proven that the glaciofluvial sandstones form important oil and gas reservoirs in Saudi Arabia and in North African and other Middle Eastern countries, including SE Turkey, but their heterogeneity and internal lithofacies characteristics have not been fully understood yet. In Saudi Arabia the hydrocarbon-producing glacial paleovalley systems forming the Sarah Formation extend 70 km in the W-E direction. In southern Turkey, similar glacial paleovalleys, defined as the Yurteri Formation, extend in a NW-SE direction. It consists of massive, well-sorted, porous reservoir sandstones of good quality and produces significant amounts of oil and sweet gas in SE Turkey. When the glaciation ceased in the Early Silurian time, the ice mass melted and released large volumes of water into the oceans. Returning waters resulted in rapid sea-level rise and onlap relations across the platforms. This widespread marine transgression deposited organic-rich, potential source-rock hot shale facies directly on the good-quality glaciofluvial reservoir sandstones. This publication is based on intensive outcrop and subsurface studies on the core samples of the reservoir sandstones and the source-rock shale facies in every part of Saudi Arabia and SE Turkey. These studies clearly indicated the hydrocarbon potentials of the Late Ordovician glaciation formed on the Gondwana plate. The aim of this paper is to demonstrate the presence of a close genetic relationship between the glaciofluvial reservoir sandstones and overlying hot shale source-rock facies and downward migration of oil and gas into the underlying porous sandstones mainly due to hydrostatic pressure.

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