--> Abstract: Gafsa Trough of Central Tunisia: Basin Evolution and Maturation of Hydrocarbons, by S. Schamel, J. K. Reed, M. Traut, and K. Ben Hassine; #91004 (1991)

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Gafsa Trough of Central Tunisia: Basin Evolution and Maturation of Hydrocarbons

SCHAMEL, S., and J. K. REED, Earth Sciences & Resources Institute, University of South Carolina, Columbia, SC, M. TRAUT, Occidental International, Bakersfield, CA, and K. BEN HASSINE, ETAP, Tunis, Tunisia

The Gafsa trough of onshore central Tunisia is one of the more interesting and underexplored features of North Africa. It is a 5-12 km deep, east-west-trending depression along the inner edge of the Tunisian-Libyan shelf margin. The basin has had a long and virtually uninterrupted history of subsidence from the late Paleozoic into the early Cenozoic. A substantial portion of the trough remains at present a broad physiographic depression, the Chotts. Subsidence began in the Late Carboniferous, soon after the close of the Hercynian orogeny, and resulted in deposition of a 3000+ m succession of Permo-Carboniferous carbonates and shale that pinches out southward onto the Saharan Flexure. The tectonic setting for this earliest phase of subsidence is not clear. The main episode of subsidenc , which began in the Middle Triassic, continued through the Jurassic as left-lateral, transtensional rifting along the South Saharan and Maghrebian Shear zones. This event led to the separation of Europe and Africa and the development of the broad, highly differentiated, North African passive margin. The Gafsa trough is one of several deep, linear basins developed on this transtensional shelf margin.

A set of organic maturation maps for onshore central Tunisia depicts the minimum time of entry into the oil and gas generative windows of the two potential source rocks in the region, the Lower Silurian Tannezufft Formation and Middle-Upper Jurassic basinal shales. Maturation modeling suggests that the Lower Silurian source rocks beneath the deeper portions of the Gafsa trough are overmature, even for generation of dry gas. Furthermore, Paleozoic source rocks beneath virtually all of the trough had passed beyond the wet gas generative stage by mid-Cretaceous time. Everywhere north of the Saharan Flexure potential Paleozoic source rocks are highly mature to overmature. The Middle-Upper Jurassic basinal shales in the deeper, central portions of the Gafsa trough entered the oil generativ window as early as mid-Cretaceous time and into the gas generative window in the Late Cretaceous-early Tertiary. These possible source rocks are mature to highly mature beneath nearly all of the basin. The Gafsa trough is a probable gas province, with occurrences of condensate possible.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)