Sedimentology and hydrocarbon potential of the Late Ordovician glacial deposits on the Arabian Platform and southeastern Turkey

Abstract

The Late Ordovician glacial deposits are widespread and constitute a significant portion of the Early Paleozoic stratigraphicsequence of the cover rocks of the Arabian and African platforms. The glaciation lasted less than one million years between the LateAshgillian (445 Ma) and Early Silurian (444 Ma) but it had a strong impact on the whole area. It has been proven that the glaciofluvialsandstones form important oil and gas reservoirs in Saudi Arabia and in North African and other Middle Eastern countries, includingSE Turkey, but their heterogeneity and internal lithofacies characteristics have not been fully understood yet. In Saudi Arabia thehydrocarbon-producing glacial paleovalley systems forming the Sarah Formation extend 70 km in the W-E direction. In southernTurkey, 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 glaciationceased in the Early Silurian time, the ice mass melted and released large volumes of water into the oceans. Returning waters resultedin rapid sea-level rise and onlap relations across the platforms. This widespread marine transgression deposited organic-rich, potentialsource-rock hot shale facies directly on the good-quality glaciofluvial reservoir sandstones. This publication is based on intensiveoutcrop and subsurface studies on the core samples of the reservoir sandstones and the source-rock shale facies in every part of SaudiArabia and SE Turkey. These studies clearly indicated the hydrocarbon potentials of the Late Ordovician glaciation formed on theGondwana plate. The aim of this paper is to demonstrate the presence of a close genetic relationship between the glaciofluvial reservoirsandstones and overlying hot shale source-rock facies and downward migration of oil and gas into the underlying porous sandstonesmainly due to hydrostatic pressure.