Abstract

The GALO basin evolution model described in Makhous et al. (1997) is applied to evaluate hydrocarbon generation and migration histories in several Saharan basins. Three basins, the Oued el-Mya, Ghadames, and Illizi, are located in the central and eastern parts of the Saharan platform and are investigated in detail. The Ahnet, Mouydir, Timimoun, Reggane, and other basins located in the southern and western parts of the platform are also studied. The modeling results, combined with geochemical data, are used in a synthesis of the regional framework. The thermal gradients in the Ghadames and Illizi basins are greater than those in the Oued el-Mya basin. This difference is attributed to differences in sedimentation and subsidence rates, to less Hercynian erosion, and to fewer occurrences of evaporates in the Akfadou region of the Ghadames basin and in the Mereksen region of the Illizi basin. In the southern and western parts of the Illizi province, the major subsidence occurred before Hercynian uplift. Very moderate Hercynian uplift in the Ghadames and Illizi basins did not involve a significant decrease of temperatures, and organic matter maturation continued, but at slower rates. As a result, the realization of hydrocarbon potential appears to be higher than would be expected. Favorable traps are located near subsided areas where the source shales (particularly the Devonian) were not subjected to uplift and erosion. In this respect, the eastern Sahara including the Ghadames and Illizi basins is a favored province. Analysis of the distribution of present-day temperatures and paleotemperatures in the Paleozoic sediments of the Triassic province (Oued el-Mya, Ghadames, Trias, and north Illizi basins), combined with effective source rock occurrences, shows that favorable conditions for hydrocarbon generation during the Paleozoic occurred essentially in the southern and southwestern parts of the province. Hydrocarbon generation during the Mesozoic took place mainly in the central and northern parts of this province; however, some of the hydrocarbon accumulations in the northern and northeastern parts of Oued el-Mya basin and in the northern and western Ghadames basin were subsequently destroyed by the Hercynian orogeny. In the Illizi basin, Ordovician and Devonian structures are the most likely traps for hydrocarbons generated during the Paleozoic, whereas in the Ghadames and Oued el-Mya basins, structures formed prior to the Late Cretaceous are preferentially favorable for oil accumulation. Gas accumulations are found in traps formed in the Late Cretaceous or later, when source shales were buried to 3.3-3.5 km and more.

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