STRATIGRAPHIC CONTROLS ON PALAEOZOIC PETROLEUM SYSTEMS, GHADAMES BASIN, LIBYA

ABSTRACT

The distribution of oil- and gasfields in the Libyan Ghadames Basin is strongly influenced by the stratigraphic architecture of the Silurian-Devonian succession. This interval encompasses part of two (second-order) super-sequences separated by the Caledonian unconformity: (i) a late Ashgillian-Silurian super-sequence, comprising the Melez Chograne-Memouniat periglacial lowstand, the Tanezzuft transgressive to early highstand shales and the Acacus Formation late highstand systems tracts; and (ii) the lower part of a composite Devonian-Carboniferous super-sequence terminated by the Hercynian unconformity, which includes the lowstand Tadrart fluvial-shallow marine sands, the Ouan Kasa (transgressive) and the Aouinet Ouinene transgressive to early highstand systems tracts of Devonian age.

Hydrocarbon accumulations in Acacus, Tadrart and later Devonian reservoirs have been charged from a basal Silurian (Tanezzuft Formation) "hot" radioactive shale source rock. The Acacus and Tadrart sandstones provided regional migration conduits sealed by intra-Acacus, intra-Devonian (Emghaet Formation) and Permian (Bir Al Jaja Formation) shales. Regional stratigraphic continuity of these migration conduits permitted the development of low to moderate impedance petroleum systems dominated by lateral migration.

Basin reconstructions and burial history modelling suggest three expulsion peaks from the basal Tanezzuft Formation source rock, preceding major periods of Hercynian, Austrian and Alpine (mid-late Tertiary) uplift and exhumation. Hydrocarbons trapped prior to the Hercynian event were dispersed during later periods of structural deformation. Some pre-Austrian accumulations may have survived locally in more robust traps. However, most if not all of the oil- and gasfields discovered in the Late Silurian - Devonian succession were probably charged during the Late Cretaceous—Early Tertiary, only to suffer partial dispersal during later uplift, tilting and flushing. Three petroleum systems can be distinguished within the basin: (i) a Tanezzuft -Acacus system to the north; (ii) a Tanezzuft - Tadrart system to the south; and (iii) a Tanezzuft -Acacus/Tadrart(+) system in the centre, with some leakage into overlying Devonian and Carboniferous sandstones locally.