The U.S. Geological Survey (USGS) has reassessed the undiscovered, technically recoverable oil and gas — conventional and continuous (unconventional) — in the Lewis Shale, a Late Cretaceous (Campanian–Maastrichtian) formation in the eastern part of the Southwestern Wyoming Province, in Wyoming and Colorado. It was last assessed in 2005, with one conventional and one continuous assessment unit; this time there are five.
The estimate
| Resource | Mean | Range (F95–F5) |
|---|---|---|
| Oil | 294 million barrels | 83–556 million barrels |
| Gas | 11,207 billion cubic feet | 3,444–19,705 billion cubic feet |
| Natural gas liquids | 338 million barrels | 103–591 million barrels |
These are fully risked estimates. F95 means a 95% chance of at least that much; F5, a 5% chance.
One conventional and two continuous assessment units in the Lewis Shale. USGS
Two continuous assessment units for the Asquith marker. USGS
The rock
The Lewis Shale is part of the Cretaceous–Cenozoic Composite Total Petroleum System, bounded on the north, south and east by uplift and on the west where it thins out. It is made of sandstones, siltstones and mudstones laid down from nearshore to deep sea, and yields plenty of natural gas and some oil from reservoirs of low porosity and permeability. It sits on the marginal-marine Almond Formation (Mesaverde Group) and under the shallow-marine Fox Hills Sandstone, interfingering with both.
It has three members, bottom to top:
- a mudstone-dominated lower member;
- the sandstone-dominated Dad Sandstone Member;
- a silt- and mudstone-dominated upper member.
The top of the lower member is the Asquith marker, an organic-rich black shale that shows up strongly on gamma-ray logs across the region — the Lewis's maximum flooding surface. It is more than 30 feet thick in the east-central part of the system, averages 3.2% organic carbon, and, with coal beds in the Almond Formation below, is likely a secondary source of the Lewis's hydrocarbons. It has seen little exploration as a self-sourced reservoir.
Type log from the Barrel Spring Unit 15-1 well: gamma ray (GR) and deep resistivity (ILD) through the Lewis Shale and associated formations. USGS
The five assessment units
| Unit | Defined by | Typical reservoirs |
|---|---|---|
| Conventional Gas | outer edge: uplift or where the shale pinches out; inner edge: the 0.8% vitrinite reflectance line (the gas is thought to come mainly from Almond Formation coals) | normal pressure, porosity above 10%, permeability above 0.1 millidarcy, distinct gas–water contacts |
| Northern Continuous Gas | uplift to the north; the 0.8% line east and west | overpressured, porosity under 10%, permeability under 0.1 millidarcy, no gas–water contacts; smaller sand bodies (under 100 ft) from shelf, delta front and prodelta |
| Southern Continuous Gas | uplift to the south; the 0.8% line east and west | as above, but larger, stacked sand bodies (over 100 ft) from basin-floor fans |
| Asquith Shale Oil | the 0.6% (oil) and 1.3% (gas) maturity lines and a 10-ft minimum thickness | — |
| Asquith Shale Gas | the 1.3% maturity line or a 10-ft minimum, in a northern (Great Divide Basin) and southern (Washakie Basin) area | — |
The northern and southern gas units are split at an inferred ancient shelf break, where the sand bodies change character and the Asquith marker thins sharply.
Only two wells have targeted the Asquith marker in the oil unit, and none in the gas unit — too little production to estimate recoveries. So the assessment borrowed data from the Mowry Shale in the Powder River Basin, which closely matches the Asquith in organic content, hydrogen and oxygen indices, low carbonate, age and location.
Sources
Based on Hearon J, Schenk C, Finn T, Johnson B, et al., "Assessment of conventional and continuous oil and gas resources in the Lewis Shale in the Southwestern Wyoming Province, Wyoming and Colorado, 2023," U.S. Geological Survey Fact Sheet 2024-3025; a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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