Geology of Tunisia and Adjacent Parts of Algeria and Libya

The southern part of the study area is part of the stable Saharan platform which, since the beginning of the Paleozoic, has been affected only by epeirogenic movements. Pre-Carboniferous Paleozoic strata are largely clastic, both continental and marine. Early invasions of the sea came from the west, but Carboniferous and younger transgressions came from a northerly (Tethyan) direction. Caledonian tectonic movements produced structural alignments and important regressions, but general uplift resulting from the Hercynian orogeny caused the major withdrawal of the sea. The dominant feature of post-Hercynian erosion in Algeria is a great T-shaped anticlinorium which projects into Tunisia. On the east, a projection of the Hercynian Nefusa uplift of Libya extends westward and c nnects with the anticlinorium. Just south of these positive areas is the Erg Oriental basin, which was connected during most of the Paleozoic with the Homra basin of western Libya. The former is bounded in the western part of the study area by the north-northeast-trending Hassi Touareg anticlinorium. Restriction of the western Tethys basin and post-Hercynian subsidence along the margin of the African landmass led to deposition of a thick series of Triassic and Liassic evaporites. Younger strata of the Saharan platform are relatively thin, and although continental sands were important during certain stages, lagoonal dolomite, evaporite, and shale are the dominant rock types. North of the platform was a subsident zone, the Jeffara flexure, where considerably thicker and more marine strata are characteristic. Associated with the flexure and extending beyond it is a complex series of faults. On the north and west, the principal later Mesozoic events consisted largely of epeirogenic uplifts with regression and local erosion, and transgressive downwarps. Frequent and more intense moveme ts characterized the early Cenozoic, and local deformation, marking the onset of Alpine orogeny, began in the Oligocene. Stronger orogenic pulses occurred at the end of the Miocene, but the last and most violent phase of the Tunisian Atlasic orogeny took place during the Pliocene-Pleistocene. In addition to the Saharan platform and Jeffara flexure, Mesozoic-Cenozoic tectonic features include stable shelves of the Pelagian craton in eastern and offshore Tunisia, and of Kasserine Island in central Tunisia. The latter was exposed during latest Cretaceous time and is bounded on the east by a series of north-south-trending faults and Jurassic horsts marking the eastern boundary of the Atlas zones. Between this axis and the Pelagian shelf and between the Saharan platform and Kasserine Island is a series of Cretaceous and Tertiary intracratonic basins, and in northern Tunisia are similar ones containing thick sections of Miocene-Pliocene clastic rocks. In northwestern Tunisia are many Triassic evaporite diapirs, and some thrust faults are present in a zone of very thick Oligocene flysch. Three hydrocarbon-productive trends are present in the study area: (1) Ypresian (early Eocene) carbonate rocks: facies change regionally southwestward from globigerinid mudstone (source beds) to dolomitized nummulitic calcarenite (reservoir) to evaporite (updip seal). Recent discoveries seem to be associated with small structures, but regional extent of the trend and attendant possibilities for stratigraphic entrapment suggest the potential for large reserves. (2) Paleozoic and Triassic sandstones: this trend contains the only proved, major reserves of Tunisia in the Triassic reservoir at El Borma field. Cambrian-Ordovician strata are productive at Rhourde El Baguel in Algeria, and sandstones of the Silurian-Lower Devonian Acacus Formation are capable of production in many Libyan well . Major fields are structurally controlled, but distribution of reservoir rocks is related to Hercynian uplift, which influenced erosion of Cambrian-Ordovician clastic strata and deposition of presalt Triassic detritus. Hydrocarbon source beds for both systems are believed to be Silurian euxinic shales. (3) Cretaceous shoreline: several small oil fields and one gas field currently are productive, mostly from bioclastic carbonate reservoirs of Aptian to early Senonian age. These accumulations are on surface anticlines in a foothills belt which is not deformed so severely as the main Atlas ranges.