Undiscovered Oil and Gas in the Taoudeni Basin, Mali and Mauritania: A USGS Assessment

The Taoudeni Basin Province and its four assessment units (AU). USGS; base map, National Park Service.

Using a geology-based method, the U.S. Geological Survey estimated mean undiscovered, technically recoverable resources in the Taoudeni Basin Province of Mali and Mauritania of:

• 160 million barrels of conventional oil;
• 1,880 billion cubic feet of conventional gas;
• 602 million barrels of shale oil;
• 6,395 billion cubic feet of shale gas.

A frontier province

The province covers about 1.9 million square kilometres, mostly in Mali and Mauritania, and holds as much as 6 kilometres of infra-Cambrian (Neoproterozoic and Early Cambrian) and other Paleozoic sedimentary rock. Only five oil and gas exploration wells were drilled there between 1973 and 2011, so it counts as a frontier province. One of them, Abolag 1, in the southwest, yielded 0.48 million cubic feet of gas a day in 1974, reportedly from an infra-Cambrian carbonate reservoir.

Petroleum systems and assessment units

Oil and gas assessments rest on the petroleum system: source rocks and how they generated and released petroleum, reservoir rocks, and traps. The USGS defined:

• an Infra-Cambrian Composite Total Petroleum System, with a conventional Infra-Cambrian Reservoirs AU and a continuous Infra-Cambrian Shale Gas AU;
• a Silurian-Devonian Composite Total Petroleum System, defined by Silurian and minor Devonian source rocks, with a conventional Lower Paleozoic Reservoirs AU and a continuous Silurian Continuous Oil AU.

Shows and limited well tests in the five wells give some evidence that these systems exist. For continuous resources, source rocks must hold more than 2 percent organic carbon by weight, contain Type I or II organic matter, be thermally mature for oil or gas, include organic-rich shale more than 15 metres thick, and generally lie deeper than 1,500 metres.

Infra-Cambrian source rocks probably began generating petroleum in the Cambrian, and Silurian ones possibly in the late Silurian; both continued into the Carboniferous, until the Hercynian orogeny. Uplift and erosion then stripped at least 2,400 metres of sediment from the centre of the basin, which may have affected whether petroleum stayed in traps or source rocks.

Inputs

Well drainage areas and success ratios come from U.S. shale-oil and shale-gas analogs. Values are minimum / mode / maximum (calculated mean); each unit's probability is 0.9.

Conventional units: the Lower Paleozoic Reservoirs AU was assessed for 1 / 25 / 150 oil fields (mean 29) of 1 / 3 / 150 million barrels (mean 5); the Infra-Cambrian Reservoirs AU for 1 / 50 / 400 gas fields (mean 62) of 6 / 18 / 600 billion cubic feet (mean 27).

Results

Fully risked; F95 is the amount with a 95 percent chance of being at least that large, and F5 and F50 likewise. All liquids in gas accumulations count as natural gas liquids (NGL). Fractiles add up only on the assumption of perfect positive correlation.

Every F95 value is 0.

Sources

• Michael E. Brownfield, Christopher J. Schenk, Timothy R. Klett, Marilyn E. Tennyson, Janet K. Pitman, Stephanie B. Gaswirth, Phuong A. Le, Heidi M. Leathers-Miller, Tracey J. Mercier, Kristen R. Marra and Sarah J. Hawkins, Assessment of Undiscovered Oil and Gas Resources of the Taoudeni Basin Province, Mali and Mauritania, 2015, USGS Fact Sheet 2016–3003, March 2016. https://doi.org/10.3133/fs20163003
• The map and tables are taken from the fact sheet's PDF.