Illizi basin

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The Illizi basin is a sedimentary rock basin that occupies an area in Eastern Algeria while also leaking into a portion of Western Libya. Formation of the basin commenced in the Paleozoic and early-Mesozoic Eras, making it relatively mature. The basin is bound to the West by the Amguid Ridge and to the East by the Tihemboka Ridge and covers an area of about 44,900 square miles while having an average depth of roughly 5,000 feet.

Map depicting the geologic location of the Illizi basin relative to other provinces and basins surrounding it. (Boote et al., 1998; Klett, 2000)

Exploration in the Illizi basin began in the early 1900s and has been found to contain 5,000 million barrels of oil along with North of 37 trillion cubic feet of gas. Furthermore, the basin is situated on a basement high which causes its shale formations to occur at a shallower depth than in comparison to the nearby Ghadames basin.

Geologic Setting

Located South of the Ghadames basin, the Illizi basin is separated from said basin by a hinge line in the slope of basement rocks, which also controls generation, migration, and accumulation between the two basins. Furthermore, as previously mentioned, it is bound to the West by the Amguid Ridge, to the East by the Tihemboka Ridge, and to the South by the Hoggar Massif. Due to being located in Northern Africa, the Illizi basin permeability is highly variable which forces geologists to be extremely thorough in the prediction of increased permeability and porosity areas in order to fully take advantage of the petroleum systems in the basin.

Reservoir Properties

Rendering showcasing the permeability levels of wells located within the Illizi basin in relation to depth.

Within the Illizi basin, several reservoirs are present but only one is assessed to be prospective which is known as the Silurian Tannezuft Shale. With a TOC ranging from 2% to 10%, including an average of 5.7%, the Silurian Tannezuft Shale falls into the wet gas and condensate window. Along with the Tannezuft Shale reservoir, the Illizi Basin also contains Ordovician Sandstones which contain large-scale hydrocarbon accumulations. Although an important reservoir, variability of reservoir quality in the Ordovician Sandstone unit is exceedingly variable.

The premier reservoir quality has been found to be contained in post-glacial shallow marine sandstones along with glaciomarine fan sandstones. Coarse-grained jet-efflux deposits and laterally extensive high-density turbidite flows with low mud content are the prime reservoir properties found in the Illizi basin. On the other hand, the poorest reservoir quality has been linked to muddy debris flow deposits. As previously mentioned, the Silurian Tannezuft Shale makes up roughly 9,840 square miles in prospective area which contains 51 Bcf per square mile of wet shale gas along with 6 million barrels per square mile of shale oil and condensate.

Petroleum System

Seal

Hydrocarbons in the Illizi basin are predominantly stored in the Cambro–Ordovician, Silurian–Devonian and Triassic clastic reservoirs which are sealed by mostly shale rock formations. More specifically, the Tanzuft "hot" shale primarily overlies the Cambro-Ordovician and acts as an effective seal for aforementioned reservoir due to its low permeability level and formation around the reservoir. A majority of the seals found in the Illizi basin range from Ordovician to Carboniferous age while Silurian reservoirs in the Illizi basin generally lack seals in most cases due to erosion over time.

Source Rock

The geologic evolution of the Illizi basin is a complex one that in turn generated two main types of source rocks; both varying in geological age known as the Silurian and Frasnian Hot Shales. The Silurian Hot Shale is viewed as the most prolific source rock within the basin, as it contains a TOC of up to 17 percent and fits a Type I-II source rock on the Van Krevelen Diagram, characterizing its kerogen as very oil-prone to oil-prone. As for the Frasnian Hot Shale, its TOC is usually capped a few percentages lower than the Silurian, at around 14 percent while also constituting a Type I-II source rock on the Van Krevelen Diagram.

In the central portion of the Illizi basin, accumulation of gas is found due to the proximity of the source rock in the area, while in the North Eastern portion of the basin the source rock has been viewed to be seperated from the reservoir rock system. Furthermore, the Silurian source rocks previously mentioned are buried at a depth of roughly 1.5 kilometers, while the Devonian-age source rocks are positioned at roughly one kilometer below the surface.

Trap

Graphic denoting the petroleum systems within the Illizi basin, including the type of source rocks responsible for hydrdocarbon accumulation.

In order for commercial amounts of hydrocarbons to accumulate in the subsurface, the relative timing of trap formations in sedimentary basins like the Illizi is crucial. The Illizi basin has undergone a series of complex uplift and burial phases throughout its formation history which in turn created the traps currently present in the basin. The large, low-relief structural trap that characterizes the hydrocarbon accumulation in the Illizi basin today was formed as a result of northward tilting of the basin during Eocene uplift of the Hoggar massif.

In the Illizi basin, its anticlinal traps are characterized by and associated with reverse faults and setbacks. These formations are critical in the development of proper traps that proficiently hold in the accumulations of hydrocarbons so that they may not escape upward and/or laterally. These anticlines developed on low-dip reverse faults in the Illizi basin are known as Sbaa Bowl in some areas. Furthermore, the basin contains traps of different aspects such as stratigraphic traps in the Devonian erosion bevels which are created when the barrier or seal is formed during changes in rock type during the deposition of reservoir beds. Hydrodynamic traps are also present in the Illizi basin, specifically in the Tin Fouyé deposit which is located in the Northwestern portion of the basin. Hydrodynamic traps occur when hydrocarbons are trapped due to a flowing aquifer and as a result are migrated from the source rock to the reservoir rock by water-saturated areas.

Geologic Risks/Uncertainties

The Illizi basin poses several risks and obstacles in order to create a good picture of what type of hydrocarbons are present and where they may be located. Basins in Northern Africa often present geologists with tedious explorations for a multitude of reasons. A majority of basins situated in North Africa, inlcuding the Illizi basin, possess highly variable reservoir quality levels which in turn forces exploration for hydrocarbon accumulation to be extremely comprehensive. In order to find areas of increased permeability and porosity, the basin must be thoroughly mapped and examined or a severe economic loss could arise.

Future Petroleum Potential

Exploration in the Illizi basin commenced in the 1950s when large commercial sized hydrocarbon accumulations were discovered in its systems. Since then, hundreds upon hundreds of drilling explorations have taken place in the basin which have led to its exhumation over time. As a basin becomes exhumed, the ability of its reservoirs to hold in hydrocarbons becomes increasingly less effective; in turn decreasing the available hydrocarbons for production.

In general, if a basin is exhumed to the extent of one like the Illizi basin, the probability of seals and traps confining the hydrocarbons is reduced drastically. Porosity, water saturation levels and net-to-gross ratio are all adversely affected as well which lead to a decreased level of recovery factors. A few occurrences can cause a basin like the Illizi to become exhumed, with erosion being one of the main factors. When seals are eroded over time, there is no longer a formation of rock to hold the hydrocarbons in place, essentially reducing the overall available and potential reservoir resources.

Because the basin is exhumed, the future potential for hydrocarbon extraction is dwindling. The Illizi basin has likely seen its most productive days and while there is still hydrocarbon potential in select areas of the basin, erosion of seals and traps will likely drive it towards total exhaustion. Although the basin is eroding away through time, over 5,000 million barrels of oil and 37 trillion cubic feet of gas has been found throughout the basin since production commenced.

Additional Information

Reservoir Quality in Exhumed Basins

Oil and Gas Exploration in Algeria

Formation of Non-Structural Traps in the Illizi Basin

Depositional Environments in the Illizi Basin

Structural Evolution of the Illizi Basin

References

Kara L. English, Jonathan Redfern, Dermot V. Corcoran, Joseph M. English, Rachida Yahia Cherif; Constraining burial history and petroleum charge in exhumed basins: New insights from the Illizi Basin, Algeria. AAPG Bulletin 2016;; 100 (4): 623–655. https://pubs.geoscienceworld.org/aapgbull/article-abstract/100/4/623/31346/constraining-burial-history-and-petroleum-charge. Accessed November 1, 2022.

The Energy Consulting Group, Algerian Oil and Gas, Exploration and Production Industry, http://energy-cg.com/OPEC/Algeria/Algeria_OilGas_Industry.html. Accessed October 31, 2022.

Golovanov, Dmitry Y., and Valery A. Rusacov. "Conditions of Forming and Geological Specifics of Non-structural Traps in the Ordovician Deposits of the Illizi Basin (The Algerian Sahara)." Paper presented at the SPE Russian Oil and Gas Technical Conference and Exhibition, Moscow, Russia, October 2008. https://onepetro.org/SPERPTC/proceedings-abstract/08ROGC/All-08ROGC/SPE-117405-MS/145265. Accessed November 1, 2022.

S. Galeazzi, O. Point, N. Haddadi, J. Mather, D. Druesne, Regional geology and petroleum systems of the Illizi–Berkine area of the Algerian Saharan Platform. https://www.sciencedirect.com/science/article/pii/S0264817209000464#sec5. Accessed October 30, 2022.

Jobst Wendt, Bernd Kaufmann, Zdzislaw Belka, Devonian stratigraphy and depositional environments in the southern Illizi Basin (Algerian Sahara), Journal of African Earth. https://www.sciencedirect.com/science/article/pii/S1464343X09000417. Accessed November 1, 2022.

Gauthier, F J, Boudjema, A, and Lounis, R. The structural evolution of the Ghadames and Illizi basins during the Paleozoic, Mesozoic and Cenozoic: Petroleum implications. United States: N. p., 1995. Web. https://www.osti.gov/biblio/127415. Accessed October 31, 2022.

Galeazzi, S. & Point, O. & Haddadi, N. & Mather, Julian & Druesne, D.. (2012). The Illizi and Berkine Basins in Southern Algeria. 10.1016/B978-0-444-56357-6.00018-4. https://www.researchgate.net/publication/285191275_The_Illizi_and_Berkine_Basins_in_Southern_Algeria. Accessed November 1, 2022.

T.R. Klett, (2000). Total Petroleum Systems of the Illizi Province, Algeria and Libya—Tanezzuft-Illizi. https://pubs.usgs.gov/bul/b2202-a/b2202aso.pdf. Accessed October 30, 2022.