Frasnian organic-rich shales in North Africa: regional distribution and depositional model
Published:January 01, 2003
S. Lüning, K. Adamson, J. Craig, 2003. "Frasnian organic-rich shales in North Africa: regional distribution and depositional model", Petroleum Geology of Africa: New Themes and Developing Technologies, T. Arthur, D. S. MacGregor, N. R. Cameron
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During the Frasnian, organic-rich shales were deposited across much of North African, most notably in parts of Morocco, Algeria, southern Tunisia, western Libya and the Western Desert of Egypt. They are estimated to be the origin of about 10% of all Palaeozoic-sourced hydrocarbons in North Africa. The depositional, palaeoecological and geochemical characteristics of this black shale unit can be best studied in the eastern Algerian Berkine (i.e. western Ghadames) Basin where the thickest and organically richest ‘hot shales’ occur. In wireline logs, the Frasnian hot shales are marked by high gamma-ray values, often in excess of 300–400 API, which, according to gamma-ray spectrometry, almost exclusively originate from an elevated uranium content. Comparison with total organic carbon (TOC) data shows that the gamma-ray curve can be used as a proxy for the TOC content of the Frasnian shales, with 150 API correlating approximately with TOCs of about 3% in eastern Algeria.
The hot shale unit usually consists of high-frequency, high-amplitude, metre-scale gamma-ray cycles; however, especially in the thicker hot shale units, the lower frequency envelope curve of the high-frequency gamma-ray cycles has a gradual, bell-shaped form. The gradual increase and subsequent decrease in organic richness over time may be interpreted as evidence for a gradual rise and subsequent fall of the oxygen minimum zone (OMZ), with invasion of oxygen-depleted waters onto the North African shelf. The rise of the OMZ may have been triggered by the Early Frasnian transgression, which has been described in detail from Morocco, where it is now well-dated by conodonts and is associated with characteristic black shales and carbonates. Additional high-resolution biostratigraphic data are still needed in order to better correlate the Frasnian hot shales of Algeria, Tunisia and western Libya with other Late Devonian dysaerobic/anaerobic facies in Morocco, western Egypt, Europe, South and North America.
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Petroleum Geology of Africa: New Themes and Developing Technologies
As frontier exploration in Africa moves into ever-deeper water and new basins, there is an increasing need to mitigate the high costs by reducing risk. One way of doing this is by drawing on knowledge from other areas. The book includes regional papers ranging from syntheses on a continental scale to details of the salt tectonics of the West African margin. It also addresses the need to continue using technologies to their limits by including papers on topics as diverse as gravity interpretation and fluid inclusion studies. The book provides essential reading for those requiring further insight into the origins of Africa’s diverse petroleum systems, and will be key to geologists and geophysicists involved in the exploration for and exploitation of oil and gas throughout Africa. Of particular note is the supplementary CD-ROM containing maps of Africa’s petroleum basins classified by age of target reservoir system.