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The U.S. Geological Survey estimates that the Mississippian Barnett Shale of north-central Texas holds a mean of 53 trillion cubic feet of undiscovered, technically recoverable shale gas, 172 million barrels of shale oil and 176 million barrels of natural gas liquids. The estimate comes from a geology-based assessment of the shale's continuous (unconventional) resources in the Bend Arch–Fort Worth Basin Province.
Why a new assessment
The USGS last assessed the Barnett in 2003, when development relied only on vertical wells. Since then horizontal drilling has taken over: more than 16,000 horizontal and 4,700 vertical wells have been drilled into the shale since its development began, and more than 15 trillion cubic feet of gas and 59 million barrels of oil have been produced from it since the 2003 assessment.
Figure 1: The Bend Arch–Fort Worth Basin Province (blue) and the three Barnett Shale assessment units. USGS; base map from the National Park Service.
The geology
The Barnett was laid down in the Late Mississippian, in a narrow, restricted seaway, as the basin first formed. It is mostly siliceous and calcareous mudstone, with 2 to 6 weight percent total organic carbon, and is thickest, more than 1,000 feet, in the deepest part of the basin, southwest of the Muenster arch. Where the Forestburg limestone is present it splits the shale into upper and lower units. The Viola-Simpson Group below and the Marble Falls Formation above may act as barriers to fracturing where they occur, though drilling goes on beyond them.
The Barnett is both the main source rock and a reservoir in the basin; oil and gas generated in it have also migrated into conventional reservoirs above, together forming the Barnett-Paleozoic Total Petroleum System. Production depends on thermal maturity. This assessment measured it by the hydrogen index (HI): below 100 means gas generation (mostly secondary gas below 50), above 100 mainly oil.
Three assessment units
| Unit | Where | Assessed? |
|---|---|---|
| Barnett Continuous Gas | the eastern shale, where HI is below 100; its "sweet spot" is where HI is below 50 | yes |
| Barnett Mixed Continuous Gas and Oil | next to it, where HI is above 100 and the shale is more than 100 feet thick | yes |
| Western Barnett Continuous Oil | to the west, where the shale thins below 100 feet, out to its western edge | no: too little drilling |
Key inputs (calculated means): the gas unit covers a potential production area of 6,473,000 acres, 73% untested, with wells in sweet spots expected to recover 2.034 billion cubic feet each and elsewhere 0.956; the mixed unit covers 2,019,000 acres, 84% untested, with 60% of untested wells expected to be oil wells.
The estimates
Mean undiscovered resources:
| Unit and accumulation | Oil (million barrels) | Gas (trillion cubic feet) | Gas liquids (million barrels) |
|---|---|---|---|
| Barnett Continuous Gas | 48.3 | 61 | |
| Mixed Gas and Oil: gas | 3.3 | 73 | |
| Mixed Gas and Oil: oil | 172 | 1.4 | 42 |
| Total | 172 | 53 | 176 |
The range is wide: for gas, from 39,718 billion cubic feet (a 95% chance of at least that much) to 69,157 billion (a 5% chance).
Sources
- Kristen R. Marra, Ronald R. Charpentier, Christopher J. Schenk and others, Assessment of undiscovered shale gas and shale oil resources in the Mississippian Barnett Shale, Bend Arch–Fort Worth Basin Province, North-Central Texas, U.S. Geological Survey Fact Sheet 2015–3078. https://pubs.usgs.gov/publication/fs20153078
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