Palaeozoic petroleum systems of North Africa

Abstract

Abstract The Palaeozoic petroleum systems of North Africa contain five large giant (> 1 billion barrels of oil equivalent) and 24 giant (> 250 million barrels of oil equivalent) oil and gas fields with total recoverable reserves discovered to date of more than 46 billion barrels of oil equivalent. This article presents a classification of these petroleum systems based upon their productivity and maturity. Productivity of each system has been estimated from the associated hydrocarbon reserves and maturity from an analysis of their geological history ranging from initial genesis to maturity, destruction and final extinction. Key factors controlling both productivity and maturity include hydrocarbon charge, style of drainage and entrapment, and intensity of post-entrapment tectonic, thermal and hydrodynamic destructive processes. The regionally extensive Lower Silurian Tanezzuft Formation is the origin of 80–90% of Palaeozoic sourced hydrocarbons, with a further 10% from the Upper Devonian Frasnian shales, charging a number of intra-Palaeozoic and basal Triassic reservoirs. Triassic fluvial sands are the most important of these, hosting just over half of the total reserves, while Cambro-Ordovician and Lower Devonian F6 sandstone reservoirs are the second and third most significant, respectively. Three categories of Palaeozoic petroleum systems have been identified: (1) Mesozoic to early Tertiary charged systems with Triassic-Liassic shale and evaporite seals in the Mesozoic sag or ‘Triassic’ Basin of the northern Sahara Platform. These include > 78% of the total discovered reserves, with > 56% in the supergiants, Hassi R’Mel and Hassi Messaoud fields (2) Mesozoic to early Tertiary charged systems with intra-Palaeozoic shale seals in basins of south and east of the Triassic Basin. These include > 18% of the total discovered reserves, mostly in the prolific Illizi Basin. (3) Now largely extinct Palaeozoic charged systems with intra-Palaeozoic seals in basins of southwest Algeria and Morocco with 3% of discovered reserves. The productivity of these systems varies considerably. Hassi R’Mel and Hassi Messaoud are classified as super-productive, located on the crests of broad Palaeozoic arches which encouraged extremely efficient lateral migration focusing, and a very high impedance entrapment style. Other petroleum systems within the Triassic Basin are of high productivity with somewhat less effective migration focusing and impedance characteristics. Because of a regional evaporite seal and minimal late stage modification, these systems are all preserved in a mature phase of evolution. Basins south and east of the Triassic Basin are in various stages of destruction with variable productivities reflecting both less robust seals and post-entrapment modification by Austrian and mid-Tertiary uplift, tilting, remigration, spillage and freshwater flushing. The Illizi Basin is the least affected by these late stage destructive processes with some 15% of total discovered reserves still remaining. The Palaeozoic charged systems of southwest Algeria and Morocco were largely destroyed by Hercynian, Austrian and mid-Tertiary deformation. Only the high relief Hercynian anticlines of the Ahnet-Gourara Basin retained their trapping integrity and still reservoir a very significant amount of gas. Apart from scattered hydrocarbon shows and a few small residual accumulations the other basins in this region now all appear to be extinct.