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The U.S. Geological Survey estimates that Algeria's Grand Erg/Ahnet Basin Province holds a mean of 80,109 billion cubic feet of gas (80.1 trillion cubic feet) in undiscovered, technically recoverable shale gas, plus 275 million barrels of natural gas liquids. The range is wide: 13,745 to 191,175 billion cubic feet of gas and 39 to 706 million barrels of liquids, from the 95-percent to the 5-percent chance.
The eight shale-gas assessment units in the Grand Erg/Ahnet Basin Province. USGS.
How the gas got there
The province is one of a series of Paleozoic intercratonic basins across much of northern Africa. Its story, in the USGS geologic model:
- Early Paleozoic. Northern Africa is a north-facing passive margin, with Ordovician, Silurian and Devonian sediments building out toward the paleo-Tethys Ocean.
- Ordovician ice age. As the regional ice cap retreats, its meltwater cuts north-trending valleys as much as 200 meters deep.
- Early Silurian. A major rise of the sea fills those valleys with organic-rich shale.
- Late Devonian. A second major transgression lays organic-rich shale across much of the margin.
- Carboniferous. The Hercynian orogeny squeezes and uplifts the region, breaking it into basins — Timimoun, Sbaa, Ahnet, Benoud-Melrhir, Oued Mya, Mouydir — separated by uplifts, and carving the Hercynian unconformity found in every northern African basin. Oil and gas may already be forming in the shales.
- Mesozoic burial. Oil cracks to gas.
The assessment rests on the idea that part of that gas stayed in the shales through uplift, erosion and migration, forming shale-gas accumulations.
The source rocks
| Devonian shales | Silurian shales | |
|---|---|---|
| Total organic carbon | up to 14 weight percent | up to 10 weight percent |
| Hydrogen index | up to 580 mg of hydrocarbon per g of organic carbon | up to 380 |
| Thickness | up to 200 m | up to 100 m |
Silurian shales in the Ahnet Basin are overpressured, but how far overpressure extends in either shale is not known.
Where the gas is estimated to be
The USGS defined a Devonian and a Silurian Total Petroleum System and, within them, eight assessment units. Fully risked mean estimates:
| Assessment unit | Probability | Mean gas (billion cubic feet) | Mean liquids (million barrels) |
|---|---|---|---|
| Timimoun Basin Silurian | 1.0 | 27,438 | 92 |
| Timimoun Basin Devonian | 1.0 | 19,784 | 73 |
| Ahnet Basin Silurian | 1.0 | 9,618 | 32 |
| Oued Mya Basin Silurian | 0.9 | 8,599 | 29 |
| Ahnet Basin Devonian | 1.0 | 5,391 | 18 |
| Benoud-Melrhir Trough Silurian | 0.9 | 5,123 | 17 |
| Sbaa Basin Silurian | 1.0 | 2,220 | 7 |
| Sbaa Basin Devonian | 1.0 | 1,936 | 7 |
| Total | 80,109 | 275 |
The two Timimoun Basin units also have the largest potential production areas in the assessment: most likely 14,927,000 acres (Silurian) and 10,705,000 acres (Devonian).
The assumptions
Every unit is 100% untested, and all eight share the same well assumptions: a drainage area of 80–160 acres (most likely 120), a success ratio of 10–90% (most likely 50%), and an average estimated ultimate recovery of 0.1–1.3 billion cubic feet per well (most likely 0.4; mean 0.447). F95 means a 95-percent chance of at least the amount shown, and the other fractiles are defined the same way.
Sources
Based on "Assessment of Undiscovered Shale-Gas Resources in the Grand Erg/Ahnet Basin Province of Algeria, 2026," U.S. Geological Survey Fact Sheet 2026–3005, by the Grand Erg/Ahnet Basin Province Assessment Team (Michael E. Brownfield, Christopher J. Schenk, Tracey J. Mercier, Marilyn E. Tennyson, Cheryl A. Woodall, Thomas M. Finn, Phuong A. Le, Heidi M. Leathers-Miller, Janet K. Pitman, Ronald M. Drake II and Stephanie B. Gaswirth); a work of the United States government in the public domain.
Licencia: CC0 1.0 (dominio público) · Adaptado de pubs.usgs.gov
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