U.S. Geological Survey, National and Global Petroleum Assessment. Fact Sheet 2020–3015, March 2021.
The result: a mean of 135.4 trillion cubic feet of undiscovered, technically recoverable continuous (shale) gas in Devonian–Mississippian shales of the Horn River Basin, Cordova Embayment and Liard Basin, in the Alberta Basin Province of Canada — about 79 percent of it in the Horn River Basin.
Figure 1: the seven assessment units. The Bovie Fault separates the Liard Basin (west) from the Horn River Basin. Base map from the National Park Service. USGS.
How the shales formed
- Middle Devonian to Early Mississippian: carbonate platforms lined North America's western passive margin, beside deep, oxygen-poor basins where organic-rich shales settled.
- Carboniferous to Triassic: terranes colliding with the continent brought a long spell of uplift, erosion or no deposition.
- Late Jurassic to Paleogene: a fold-and-thrust belt and foreland basin buried the shales deep enough to generate oil and gas.
The target shales
Several hundred meters of shale were laid down, but only parts are organic-rich; sequence stratigraphy picks them out.
| Unit | What it is |
|---|---|
| Evie Member (Horn River Formation) | a siliceous, low-clay shale laid across the Keg River carbonate platform as the sea rose — a prime horizontal-drilling target |
| Otter Park Member | organic-lean wedges above and below an organic-rich middle; its fractures reach up into the Muskwa, so the two are drilled together |
| Muskwa Formation | an organic-rich shale that thins landward |
| Exshaw Formation | organic-rich, equivalent to the Bakken; a single drilling target |
| Besa River Formation (Liard Basin) | the equivalent of the whole Evie-to-Exshaw interval |
The Fort Simpson Formation's clinoforms are generally not a source rock, though opinions differ. The shales are varied in rock and structure, which affects fractures — natural and induced — and so production.
Figure 2: how the rock units of the Liard and Horn River basins relate across the Bovie Fault. Not to scale. USGS, after the National Energy Board (2016).
Three petroleum systems, seven units
| Petroleum system | Units | Geologic model |
|---|---|---|
| Middle–Upper Devonian | Horn River Basin Evie; Horn River Basin Muskwa-Otter Park; Cordova Embayment Evie; Cordova Embayment Muskwa-Otter Park | oil and gas from Middle to Late Devonian source rocks; oil cracked to gas under Late Cretaceous–Paleogene burial; some gas kept in the shales. The shales are in the dry-gas window, so usual measures like total organic carbon help little |
| Upper Devonian–Mississippian | Horn River Basin Exshaw; Cordova Embayment Exshaw | gas retained in the shales after generation, migration and deformation |
| Middle Devonian–Mississippian | Liard Basin Besa River | the Liard lies on the downdropped west side of the Bovie Fault, buried several hundred meters deeper; that and high thermal maturity may have reduced recoverable gas |
Well recovery and drainage inputs follow Chen and Hannigan (2016) and Yousefzadeh and others (2016).
Table 1: key inputs for the seven units. USGS.
Results
Table 2: fully risked. F95 means a 95-percent chance of at least that amount; F5, a 5-percent chance.
| Unit | Gas, billion cubic feet: F95 / F50 / F5 (mean) | NGL, million barrels (mean) |
|---|---|---|
| Horn River Basin Evie | 13,680 / 44,144 / 82,957 (45,742) | 107 |
| Horn River Basin Muskwa-Otter Park | 15,286 / 49,592 / 92,273 (51,111) | 119 |
| Cordova Embayment Evie | 2,092 / 7,842 / 17,579 (8,565) | 20 |
| Cordova Embayment Muskwa-Otter Park | 2,906 / 9,519 / 17,855 (9,861) | 23 |
| Horn River Basin Exshaw | 2,302 / 9,042 / 21,991 (10,183) | 34 |
| Cordova Embayment Exshaw (probability 0.9) | 0 / 1,747 / 4,546 (1,941) | 6 |
| Liard Basin Besa River | 1,830 / 7,084 / 17,051 (7,947) | 26 |
| Total | 38,096 / 128,970 / 254,252 (135,350) | 335 (range 83–710) |
About 107,036 billion cubic feet — 79 percent — lies in the Horn River Basin.
Sources
Based on Assessment of Continuous Gas Resources in the Horn River Basin, Cordova Embayment, and Liard Basin, Canada, 2019, USGS Fact Sheet 2020–3015, by the assessment team: Christopher J. Schenk, Tracey J. Mercier, Cheryl A. Woodall, Thomas M. Finn, Phuong A. Le, Michael E. Brownfield, Kristen R. Marra, Stephanie B. Gaswirth, Heidi M. Leathers-Miller and Janet K. Pitman, U.S. Geological Survey; a work of the United States government in the public domain. The import had kept only the map's labels and the reference list; the text, map, cross section and tables are rebuilt from the fact sheet's PDF.
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