Minable Oil Shale in the Mahogany Zone of Colorado and Utah

Stripping ratio (colors), Mahogany zone thickness (gray contours), and areas yielding at least 15 (red line) and 25 (shaded, Piceance Basin) gallons per ton. USGS.

The resource

The Green River Formation of the Piceance Basin (Colorado) and Uinta Basin (Utah) holds an estimated 2.845 trillion barrels of oil in place in oil shale. That is not what can be recovered: much of it is too low-grade or in beds too thin to develop.

The oil shale interval is divided into 17 alternating "rich" and "lean" zones. The Mahogany zone — laid down when ancient Lake Uinta was at its largest, near the top of the Parachute Creek Member — is the richest and most widespread, and the target of most extraction projects.

This fact sheet sorts the Mahogany zone's resource by the properties that matter for mining:

Grade is the oil a ton of rock yields in a Fischer assay: 100 grams of crushed shale heated at 12 °C a minute to 500 °C and held there, with the oil and water condensed and weighed. Stripping ratio is the overburden's thickness divided by the zone's.

Piceance Basin Mahogany zone oil in place by (A) grade, (B) zone thickness, (C) overburden thickness and (D) stripping ratio; bars show amounts, curves cumulative totals, dashed lines medians. USGS.

The same for the Uinta Basin. USGS.

The key result

• Piceance: 32.3% of the Mahogany zone's oil — 61.8 billion barrels — is in shale of at least 25 gallons per ton, under 1,000 ft or less of overburden, with a stripping ratio below 10.
• Uinta: 14.0% — 29.9 billion barrels — meets the same overburden and stripping limits, but only at the lower 15 gallons per ton. No Uinta resource meets the 25-gallon cutoff together with these limits.

Billions of barrels in place.

Where: in the Piceance Basin, 15-gallon shale with low stripping ratios (<5) and the thinnest overburden lies along the western and southern margins; much of the basin center is rich and thick (>100 ft) but under 1,000 ft or more of cover. In the Uinta Basin, minable shale lies mainly in the east, near the Colorado line, where large areas average about 15 gallons per ton, about 100 ft thick, with stripping ratios under 10.

Why mining matters

The only proven large-scale approach so far is to mine the shale and process it at the surface — burning it for electricity or retorting it to make shale oil by heating kerogen to 350–500 °C or more. Heating can also be done underground (in situ), but above-ground retorting uses established technology and requires mining first.

Scale and impacts

• Supplying the United States' roughly 19 million barrels of oil a day would take mining and processing about 32 million tons of 25-gallon shale every day.
• A western industry of about one million barrels a day — a scale projected as reachable within a few decades — would mean mining about 1.7 million tons and disposing of about 1.4 million tons of spent shale daily.
• Expected impacts of mining and surface retorting: large land disturbance, especially from surface mines; disposal of huge volumes of spent shale; possible leaching of pollutants into surface and groundwater; heavy water use — 2 to 5 barrels of water per barrel of oil; and air pollution.

Sources

Based on Justin E. Birdwell, Tracey J. Mercier, Ronald C. Johnson and Michael E. Brownfield, "In-place oil shale resources of the Mahogany zone sorted by grade, overburden thickness, and stripping ratio, Green River Formation, Piceance Basin, Colorado, and Uinta Basin, Utah," U.S. Geological Survey Fact Sheet 2015–3005 (2015); a work of the United States government in the public domain. The histograms, map and table are taken from the fact sheet's PDF.