Tectono-stratigraphic evolution of the Palaeozoic rocks and the petroleum systems of the Ghadamis Basin, Libya
A synthesis of Palaeozoic petroleum systems within the Libyan part of the Ghadamis Basin is presented, based upon data from over 60 wells and approximately 300km of 2D seismic lines. Cross-sections and maps have been constructed to illustrate the structural and stratigraphic evolution of the basin. The techniques of sequence stratigraphic analysis have been applied to these cross-sections to describe the stratigraphic architecture of the Silurian-Devonian succession. The maps and cross¬sections have been analysed to define the maturation and expulsion history of hydrocarbons from the primary Silurian-aged source rocks. Optimum migration pathways have been modelled through time, assuming simple hydrostatic and lithostatic conditions. The results correlate well with the known stratigraphic and geographical distribution of the existing oilfields in the basin. The distribution of oil and gas fields has been strongly influenced by the stratigraphic architecture of the Silurian-Devonian succession. Basin reconstructions and burial history models have been inade, and three expulsion peaks from the basal Tanezzuft Formation source rock have been identified. These precede major periods of Hercynian, Austrian (late Early Cretaceous) and Alpine (mid-late Tertiary) uplift and exhumation. It is suggested that hydrocarbons generated in the Late Palaeozoic were subsequently dispersed during Hercynian uplift and erosion. Some hydrocarbons expelled prior to the Austrian uplift may still be reservoired locally, but most of the existing fields are believed to have been charged in the Late Cretaceous and Early Tertiary. Hydrocarbon accumulations in the Upper Silurian Acacus Formation, the Lower Devonian Tadrart Formation, 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 encouraged the development of low to moderate impedance petroleum systems dominated by lateral migration. Three discrete petroleum systems have been recognized: • An Acacus- Tanezzuft system to the north in fault and subcrop traps • A central basin, Tanezzuft-sourced system with Upper Silurian, Devonian and Lower Carboniferous reservoirs in low relief, fault-controlled traps • A Tadrart-Tanezzuft system to the south in simple fault traps with an associated reservoir sequence in the Upper Ordovician. This integrated analysis provides the basis for understanding the lower impedance (entrapment efficiency) and smaller size of the oilfields in the Libyan sector of the Ghadamis Basin, compared with those further west in eastern Algeria (Berkine Sub-basin). More importantly, this synthesis should help to improve future exploration efficiency in the Libyan Ghadamis Basin.