The three Tyler Formation assessment units in the Williston Basin, North Dakota. U.S. Geological Survey.
The estimate
Using a geology-based method, the U.S. Geological Survey (USGS) estimates a mean of 161 million barrels of oil and 93 billion cubic feet of gas, undiscovered but technically recoverable, in the Tyler Formation of the Williston Basin Province in North Dakota — in both conventional and continuous (unconventional) resources.
The rocks
The Tyler formed in the Pennsylvanian, when glacially driven swings in sea level laid down thin cycles of mudstone and limestone, some of them potential source rocks. The basin's western edge then opened to the Panthalassa Ocean through the Big Snowy Trough, running east–west across Montana; each rise in sea level sent marine water east through the trough into the shallow basin.
Two separate areas of the Tyler hold thermally mature source rock:
| Area | Rocks | Richness |
|---|---|---|
| North (Lower Tyler) | cycles of subtidal shale, grading from organic-rich to organic-lean as bottom waters gained oxygen | up to 4% total organic carbon; hydrogen index up to 400 |
| South (Upper Tyler) | shales and limestones laid down mostly in brackish water | up to 6% total organic carbon; hydrogen index up to 800 |
Oil from the upper Tyler came mostly from Type I organic matter, likely from lake shales.
The assessment units
- Lower Tyler and Upper Tyler Continuous Oil units, bounded by mature shale and overpressure data: oil generated in these source-rock pods, partly kept in the shale after some was expelled.
- Tyler Conventional Reservoirs unit: Tyler-sourced oil that migrated from marine shales into river-delta to shelf sandstones in structural and stratigraphic traps.
Results
| Resource | Mean | F95–F5 |
|---|---|---|
| Oil | 161 million barrels | 41–369 million |
| Gas | 93 billion cubic feet | 21–224 billion |
| Natural gas liquids | 5 million barrels | 1–11 million |
By unit (mean oil, gas): Lower Tyler continuous, 63 million barrels and 63 billion cubic feet; Upper Tyler continuous, 62 and 19; conventional reservoirs, 36 and 11. The two continuous units hold 125 million barrels (78 percent), 82 billion cubic feet (88 percent) and 4 million barrels of liquids (80 percent). Developing the continuous oil would take a mean of about 21,000 wells.
Sources
Based on Assessment of Undiscovered Oil and Gas Resources in the Pennsylvanian Tyler Formation of the Williston Basin Province, North Dakota, 2020, U.S. Geological Survey Fact Sheet 2020–3067, by the USGS Williston Basin Province Assessment Team, USGS Publications Warehouse; a work of the United States government in the public domain. The map and unit results were recovered from the fact sheet's PDF.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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