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The U.S. Geological Survey (USGS) assessed the undiscovered, technically recoverable conventional oil and gas in six upper Paleozoic total petroleum systems (TPSs) of the Williston Basin Province in North Dakota, Montana and South Dakota, defining eight assessment units (AUs) and assessing each for oil, gas and natural gas liquids.
The petroleum systems
The systems were defined mainly from the geochemistry of the oils. Each TPS holds oil isolated from the others', but most production lies near major fault systems and regional structures — which suggests oils from several sources may be mixed in many reservoirs, as along the Cedar Creek anticline.
- Winnipegosis TPS. Oil from organic-rich basin shales migrated into carbonate reefs and platforms — platform-interior patch reefs, platform margins, slopes and pinnacle reefs — in the Winnipegosis Carbonate Reservoirs AU, sealed by the evaporites of the overlying Prairie Formation. Most production is near structures such as the faulted Nesson anticline.
- Duperow TPS. Oil possibly from Duperow Formation and adjacent shales migrated into carbonate platform, margin, slope and pinnacle-reef reservoirs in the Dawson Bay, Duperow and Birdbear Formations: the Duperow Reservoirs AU and Birdbear Reservoirs AU. Most production follows regional fault trends.
- Bakken TPS. Oil from organic-rich shales of the upper and lower Bakken members migrated into the siliciclastic middle Bakken and into carbonates at the top of the underlying Three Forks Formation: the Middle Bakken Conventional Reservoirs AU and Three Forks Conventional Reservoirs AU. In North Dakota much Bakken oil stays in the middle Bakken as a continuous (unconventional) accumulation, but a large volume has migrated north into conventional accumulations in southern Saskatchewan.
- Madison Composite TPS. Oil from Madison Group basin shales migrated into Lodgepole Formation, Mission Canyon Limestone and Charles Formation carbonates, in stacked carbonate–evaporite cycles: the Madison Group Reservoirs AU.
- Pennsylvanian Tyler TPS. A shift from carbonates to mostly siliciclastic rocks. Organic-rich Tyler Formation shales, deposited in a restricted sea linked to the Panthalassa Ocean by the east–west Big Snowy trough, charged fluvial-deltaic to shelf sandstones: the Tyler Conventional Reservoirs AU.
- Cedar Creek Paleozoic Composite TPS. Oil and gas from all the Paleozoic systems. The Cedar Creek structure blocked fluids moving southwest through the basin and became a focus for mixing oils from many source rocks: the Cedar Creek Conventional Reservoirs AU.
Figure 1. The four assessment units in the Winnipegosis, Duperow and Cedar Creek Paleozoic Composite TPSs. Base map from the U.S. Department of the Interior, National Park Service. Credit: U.S. Geological Survey.
Figure 2. The four assessment units in the Bakken, Madison Composite and Pennsylvanian TPSs. Base map from the U.S. Department of the Interior, National Park Service. Credit: U.S. Geological Survey.
Figure 3. Stratigraphy and total petroleum systems in the upper Paleozoic strata of the Williston Basin Province. Credit: U.S. Geological Survey.
Results
The fully risked mean totals are 134 million barrels of oil (F95–F5 range 68 to 217), 81 billion cubic feet of gas (42 to 128) and 6 million barrels of natural gas liquids (3 to 9). F95 is a 95-percent chance of at least the amount shown.
| Assessment unit | AU probability | Oil, mean (MMBO) | Gas, mean (BCFG) | NGL, mean (MMBNGL) |
|---|---|---|---|---|
| Winnipegosis Carbonate Reservoirs | 0.9 | 6 | 3 | 0 |
| Duperow Reservoirs | 0.9 | 3 | 3 | 0 |
| Birdbear Reservoirs | 1.0 | 7 | 7 | 1 |
| Middle Bakken Conventional Reservoirs | 1.0 | 8 | 6 | 0 |
| Three Forks Conventional Reservoirs | 0.9 | 3 | 2 | 0 |
| Madison Group Reservoirs | 1.0 | 69 | 48 | 4 |
| Tyler Conventional Reservoirs | 1.0 | 36 | 11 | 1 |
| Cedar Creek Conventional Reservoirs | 0.9 | 2 | 1 | 0 |
| Total | 134 | 81 | 6 |
All are oil accumulations. MMBO, million barrels of oil; BCFG, billion cubic feet of gas; MMBNGL, million barrels of natural gas liquids.
Table 2. Results for the eight conventional assessment units (fully risked estimates). Credit: U.S. Geological Survey.
Sources
- Schenk, C.J., Mercier, T.J., Woodall, C.A., Ellis, G.S., Finn, T.M., Le, P.A., Marra, K.R., Leathers-Miller, H.M., and Drake, R.M., II, 2021, Assessment of undiscovered conventional oil and gas resources of upper Paleozoic strata in the Williston Basin Province, 2020: U.S. Geological Survey Fact Sheet 2021–3023. https://doi.org/10.3133/fs20213023
- The fact sheet draws on Osadetz and Snowden (1995), Jarvie (2001), Lillis (2012), Nesheim (2019), Anna (2013), Gaswirth and others (2013), Pollastro and others (2013) and other studies.
- The maps, stratigraphic chart and results table are taken from the fact sheet's PDF.
- Rewritten in hubnx's own words.
Licencia: CC0 1.0 (dominio público) · Adaptado de pubs.usgs.gov
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