Basin Evolution and Geochemical Modeling
Tectonic evolution of the Palaeozoic of North Africa
Postdoctoral Research by Dr Gianluca Badalini
Background:
As part of the work undertaken by NARG a synthesis of the recent published research on the structural evolution of the area was undertaken. The most significant tectonic events that affect the region during the Palaeozoic are listed below and their effect on the petroleum systems briefly discussed.
Post Pan-African and Pre-Hercynian:
The collision of a number of micro-continents during the Pan-African orogeny resulted in the formation of a large stable cratonic block that formed the continental margin of Gondwana and Tethys. The Pre-Cambrian basement and underlying structural lineations have had a significant control on the subsequent Post Pan-African and Pre-Hercynian history of North Africa, which has been characterised by extension, transpression, transtension, and intraplate processes. During the Cambrian, North Africa and Arabia were affected by extension and intra-cratonic subsidence that resulted in the deposition of sediments within basins across Morocco, Algeria, Libya, Oman and Saudi Arabia.
An Infra-Cambrian extensional phase is recognised, which was then followed by local transpressional and transtensional reactivation. The deposition and present-day distribution of the late Ordovician reservoir intervals and early Silurian source (the Tanezzuft Shales) are strongly influenced by Cambro-Ordovician tectonism. In Libya and Algeria Lüning et al. (2001) have demonstrated that the deposition of basal Silurian, organic-rich 'hot shales' is restricted to palaeotopographic depressions.
The Ordovician drifting of Baltica and Avalonia from Gondwana was completed during the Silurian, with the two continents being accreted to Laurentia (Stampfli, 1996). This resulted in deformation, uplift, and local erosion (Aliev et al., 1971; Boote et al., 1998). A regionally significant unconformity can be recognised in the late Silurian, followed by a series of progradational clastics that form important reservoirs.
Devonian
By the Middle to Late Devonian, initial collision of Laurussia and Gondwana resulted in further modification of pre-existing structures (Boote et al., 1998). In western Morocco, extension / transtension and crustal thinning at this time created a series of turbiditic pull-apart basins (Piqué et al., 1993).
Carboniferous
Pre-Hercynian intra-Carboniferous deformation in central North Africa has been identified within the Murzuq Basin but not, as yet, in the Algerian basins (Lüning pers. comm.). Glacio-eustatic sea level changes gained in importance during the early Carboniferous (Ziegler, 1989) yet many major intra-early Carboniferous sequence boundaries, evident, for instance, on the North American craton and on the Moscow Platform, cannot be correlated with each other. This suggests that they are not of global eustatic origin but more probably related to intra-plate deformation (Ziegler, 1989).
Hercynian Orogeny
The Hercynian Orogeny represents an intense period of uplift and erosion caused by the closure of the Proto-Tethyan Ocean and the Late Carboniferous collision between Africa and Laurussia. The intensity of deformation decreases from west to east, with folding, thrusting and strike-slip faults in northwest Africa, folds and faulted folds in central North Africa and gentle folds, disconformities and low-angle unconformities in central east Africa. The present-day maturity levels of the main Palaeozoic hydrocarbon source rocks decrease eastwards across North Africa in parallel with the decrease in the intensity of the Hercynian deformation (MacGregor, 1996).
The Hercynian Orogeny had a significant control on hydrocarbon maturity, charge and trap configuration in North Africa. Many Pre-Hercynian structures were modified by Hercynian uplift and erosion, destroying many potential hydrocarbon traps (for example in the Reggane and Tindouf Basins of Algeria). Remaining prospectivity often relies on assessing potential for secondary migration and trap development. Effective Pre-Hercynian traps still exist in areas less affected by inversion. The Hercynian tectonic activity also formed broad regional arches, which acted as the focus for hydrocarbon migration from huge areas, resulting in a number of supergiant oil and gas fields, such as the Algerian Hassi Messaoud structure. These structures were large enough to preserve their integrity through two subsequent phases of uplift (Boote et al., 1998).
Figure 1: A summary chart of the main paleozoic tectono-sedimentary events of North Africa. Click for enlarged version
Permian - Jurassic
The initial phase of the post-Hercynian break-up of the Pangaea supercontinent spanned Late Permian to Middle Jurassic times and culminated in the development of a new divergent/transform plate boundary between Gondwana and Laurussia (Ziegler, 1988; Lambiase, 1989; Stampfli et al., 1991; Guiraud and Bellion, 1996). Repeated phases of rifting and basin inversion have affected the North African margin from the Permian until present day.
The main Palaeozoic tectono-sedimentary events are summarised in figure 1.
Key references:
Aliev, M., Aït Lasoussine, N., Avrov, V., Aleksine, G., Barouline, G., Lakovlev, B., Korj, M., Kouvykine, V., Mazanov, V., Medvedev, E., Mkrtchiane, O., Moustafinov, R., Oriev, L., Oroudjeva, D., Oulmi, M. and Saïd, A., 1971. Geological structure and estimation of oil and gas in the Sahara in Algeria. Altamira-Rotopress, S.A., 265p
Boote, D.R., Clark-Lowes, D.D. and Traut, M.W., 1998. Palaeozoic petroleum systems of North Africa. In: MacGregor, D.S., Moody, R.T.J. and Clark-Lowes, D.D. (Eds.), Petroleum Geology of North Africa. Geological Society Special Publication, 132, 7-68.
Guiraud, R. and Bellion, Y., 1996. Late Carboniferous to Recent geodynamic evolution of the West Gondwanian cratonic Tethyan margins. In: Nairn, A.E.M., Dercourt, J. and Wrielynck, B. (Eds.), The ocean basin and margins, Vol. 8, The Tethys Ocean. Plenum Press, 101-124.
Lambiase, J.J., 1989. The framework of African rifting during the Phanerozoic. Jour. African Earth Sciences, 8, 183-190.
Lüning, S., Craig, J., Loydell, D. K., Storch, P., Fitches, B., 2001, Lower Silurian "hot shales" in North Africa and Arabia; regional distribution and depositional model, Earth-Science Reviews, 49, 121-200.
MacGregor, D.S., 1996. The hydrocarbon systems of North Africa. Marine and Petroleum Geology, 13, 329-340.
Piqué, A.B., G., Boullin, J-P., Chalouan, A., Hoepffner, C., 1993. Southern margin of the Variscan belt: the north-western Gondwana mobile zone (eastern Morocco and Northern Algeria). Geol. Rundsch., 82, 432-439.
Stampfli, G., 1996. The intra-Alpine terrain: a Palaeotethyan remnant in the Alpine Variscides. Eclogae Geologicae Helveticae, 89, 13-42.
Stampfli, G., Marcoux, J. and Baud, A., 1991. Tethyan margin in space and time. Palaeogeography, Palaeoclimatology, Palaeoecology, 87, 373-409.
Ziegler, P.A., 1988. Evolution of the Arctic-North Atlantic and Western Tethys. AAPG Memoir 43, 198p.