Lower Silurian 'hot shales' in North Africa and Arabia: regional distribution and depositional model

Lowermost Silurian organic-rich ('hot') shales are the origin of 80-90% of Palaeozoic sourced hydrocarbons in North Africa and also played a major rol e in petroleum generation on the Arabian Peninsula. In most cases, the shal es were deposited directly above upper Ordovician (peri-) glacial sandstone s during the initial early Silurian transgression that was a result of the melting of the late Ordovician icecap. Deposition of the main organic-rich shale unit in the North African/Arabian region was restricted to the earlie st Silurian Rhuddanian stage (acuminatus, atavus and probably early cyphus graptolite biozones), During this short period (1-2 m.y.), a favourable com bination of factors existed which led to the development of exceptionally s trong oxygen-deficiency in the area. In most countries of the study area, t he post-Rhuddanian Silurian shales are organically lean and have not contri buted to petroleum generation. The distribution and thickness of the basal Silurian 'hot' shales have been mapped in detail for the whole North Africa n region, using logs from some 300 exploration wells in Libya, Tunisia, Alg eria and Morocco. In addition, all relevant, accessible published and unpub lished surface and subsurface data of the lower Silurian shales in North Af rica and Arabia have been reviewed, including sedimentological, biostratigr aphic and organic geochemical data. The lowermost Silurian hot shales of no rthern Gondwana are laterally discontinuous and their distribution and thic kness were controlled by the early Silurian palaeorelief which was shaped m ainly by glacial processes of the late Ordovician ice age and by Pan-Africa n and Infracambrian compressional and extensional tectonism, The thickest a nd areally most extensive basal Silurian organic-rich shales in North Afric a occur in Algeria, Tunisia and western Libya, while on the Arabian Peninsu la they are most prolific in Saudi Arabia, Oman, Jordan and Iraq. The hot s hales were not deposited in Egypt, which was a large palaeohigh at that tim e. The depositional model presented may help in better understanding the so urce potential of the basal Silurian shales in less-explored regions of Nor th Africa and Arabia including Morocco, northern Niger and the Kufra Basin in southeast Libya. (C) 2000 Elsevier Science B.V. All rights reserved.