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By the USGS Grand Erg/Ahnet Province Assessment Team: Michael E. Brownfield, Christopher J. Schenk, Tracey J. Mercier, Marilyn E. Tennyson, Cheryl A. Woodall, Thomas M. Finn, Phuong A. Le, Janet K. Pitman, Ronald M. Drake II, Stephanie B. Gaswirth and Heidi M. Leathers-Miller
Using a geology-based method, the U.S. Geological Survey estimated a mean of 30.5 trillion cubic feet of undiscovered, technically recoverable tight gas in the Grand Erg/Ahnet Province of Algeria.
Tight gas, for this assessment: gas in sandstone-type (siliciclastic) reservoirs with permeability generally below 0.1 millidarcy, which need stimulated vertical wells to produce.
The province and its assessment units (AUs). USGS.
How the gas got there
- Early Paleozoic: much of North Africa was a north-facing passive margin, with river and delta systems building northward into the paleo-Tethys Ocean (Boote and others, 1998; Dixon and others, 2010). Ordovician sediments spread a thin cover across it.
- End of the Ordovician ice cap: meltwater cut large, north-flowing channels into those sediments.
- Early Silurian: the sea rose, laying organic-rich source rock in the old channels, later covered by late Silurian sediments.
- Devonian: another rise of the sea laid more organic-rich source rock, buried by Devonian and early Carboniferous sediments.
- Mid-Carboniferous: Hercynian compression broke the margin into a mosaic of uplifts and basins (Eschard and others, 2010).
Gas may have been generated from the Silurian and Devonian shales as early as the lower Carboniferous (Lüning and others, 2000). In the Mesozoic, oil would have cracked to gas (Logan and Duddy, 1998), and some stayed in or moved locally into tight sandstones of the Ordovician, Silurian, Devonian and possibly early Carboniferous.
The assessment covers only the basins left after the Hercynian deformation: the uplifts may have lost their hydrocarbons to exhumation and erosion.
Two petroleum systems, seven units
| Silurian system | Devonian system | |
|---|---|---|
| Organic carbon | up to 10 percent | up to 14 percent |
| Hydrogen index (mg of hydrocarbon per g of organic carbon) | up to 380 | up to 580 |
| Shale thickness | up to 100 meters | up to 200 meters |
| Units | Timimoun, Sbaa, Ahnet (and Mouydir, defined but not assessed) Basin Ordovician Tight Gas AUs | Timimoun, Sbaa, Ahnet Basin Devonian Tight Gas AUs |
The reservoirs' limits were drawn largely from the thermal window for gas. Well drainage areas, success ratios and estimated ultimate recoveries come from U.S. tight-gas analogs.
The estimates
For the six assessed units:
- Gas: mean 30,474 billion cubic feet (BCFG) — about 30.5 trillion cubic feet — with an F95–F5 range of 6,336 to 71,077 BCFG.
- Natural gas liquids: mean 103 million barrels, range 18 to 262.
F95 means a 95-percent chance of at least that much; F5, a 5-percent chance.
| Assessment unit | Gas, F95–F5 (BCFG) | Gas, mean (BCFG) | NGL, mean (million barrels) |
|---|---|---|---|
| Ahnet Basin Ordovician | 1,955–21,930 | 9,427 | 31 |
| Timimoun Basin Ordovician | 1,463–16,519 | 7,058 | 23 |
| Ahnet Basin Devonian | 1,090–12,355 | 5,300 | 18 |
| Timimoun Basin Devonian | 964–10,713 | 4,586 | 17 |
| Sbaa Basin Ordovician | 459–5,093 | 2,190 | 7 |
| Sbaa Basin Devonian | 405–4,467 | 1,913 | 7 |
More: USGS Energy Resources Program.
Sources
Based on Assessment of Tight-Gas Resources in the Grand Erg/Ahnet Province of Algeria, 2019, by the Grand Erg/Ahnet Province Assessment Team, USGS Fact Sheet 2020–3003, U.S. Geological Survey; a work of the United States government in the public domain. The map and the results by assessment unit are taken from the fact sheet's PDF, which the import had left out.
Licenza: CC0 1.0 (pubblico dominio) · Tratto da pubs.usgs.gov
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