114.1 Trillion Cubic Feet of Undiscovered Tight Gas in Algeria, Tunisia and Libya

The U.S. Geological Survey (USGS) has estimated how much tight gas — gas locked in dense sandstone — could still be found and technically recovered in three basins of northern Africa: the Berkine, Illizi and Hamra Basins, which span eastern Algeria, southern Tunisia and western Libya.

Mean estimate: 114.1 trillion cubic feet of gas and 3.8 billion barrels of natural gas liquids.

The five tight-gas assessment units in the Berkine, Illizi and Hamra Basins. Credit: U.S. Geological Survey.

What tight gas is

Tight-gas sandstones typically have a matrix permeability below 0.1 millidarcy and porosity generally under 10 percent, lost through burial compaction and cementation. Such reservoirs:

• often extend across whole regions;
• can be abnormally pressured, either high or low;
• draw dry gas from a nearby source such as shale or coal beds;
• need stimulation, such as hydraulic fracturing, to produce.

Some are hundreds of meters thick and need closely spaced vertical wells; thinner, blanket-like sandstones are best tapped with horizontal wells. Tight gas has been produced in the United States for decades, and U.S. fields served as geologic and production analogs for this study.

How the basins formed

The tight sandstones here are mainly Ordovician and Devonian.

1. Early Paleozoic: northern Africa was a north-facing passive margin on the paleo-Tethys Ocean. From the Cambrian into the Ordovician, rivers flowing north gave way to deltaic and shallow-marine sands, blanketing the margin.
2. End of the Ordovician glaciation: meltwater carved a series of north-trending valleys.
3. Silurian: a major rise in sea level filled those valleys with organic-rich mud. These Silurian shales, lying against the widespread Ordovician sandstones, became the sandstones' local gas source.
4. Late Devonian: another major transgression laid down a second organic-rich shale, close to Devonian river-delta and nearshore marine sandstones.
5. Carboniferous: compression during the Hercynian orogeny formed the Berkine, Illizi and Hamra Basins, separated by uplifts where the Silurian–Devonian source rocks and sandstones were largely eroded away.

Throughout, burial compaction and cementation steadily tightened the sandstones.

Two petroleum systems, five assessment units

The Ordovician units were mapped where thermally mature Silurian source rock and low-permeability sandstone coincide, since gas could only have migrated short distances.

Shared inputs. Every unit was treated as entirely untested, with a unit probability of 1.0, an average drainage area of 80 acres (range 40–120), a success ratio of 50% (range 10–90%), and an average estimated ultimate recovery of 0.440 billion cubic feet of gas per well (range 0.08–1.2, mode 0.4). Mean potential production areas ranged from 928,800 acres (Illizi Devonian) to 14,700,300 acres (Illizi Ordovician).

Results

BCFG = billion cubic feet of gas; NGL = natural gas liquids; MMBNGL = million barrels of NGL. F95 means a 95% chance of at least that amount; F5, a 5% chance. Fully risked estimates.

The assessment team: Michael E. Brownfield, Christopher J. Schenk, Tracey J. Mercier, Thomas M. Finn, Cheryl A. Woodall, Phuong A. Le, Heidi M. Leathers-Miller, Kristen R. Marra, Ronald M. Drake II and Scott A. Kinney. Results are also posted on the USGS Energy Resources Program site.

Sources

Based on Brownfield, M.E., and others, "Assessment of undiscovered tight-gas resources in the Berkine, Illizi, and Hamra Basins of northern Africa, 2026," U.S. Geological Survey Fact Sheet 2026–3020 (doi:10.3133/fs20263020); a work of the United States government in the public domain.