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U.S. Geological Survey, National and Global Petroleum Assessment. Fact Sheet 2017–3003, February 2017.
The result: a mean of 4.5 trillion cubic feet of undiscovered, technically recoverable coalbed gas in the Kalahari Basin Province of Botswana, Zambia and Zimbabwe.
The province
Figure 1: the Kalahari Basin Province and the coalbed gas assessment unit, spanning the Kalahari and Zambezi basins. Base from the National Park Service. USGS.
- Size: about 648,670 square kilometers of complex geology.
- Rocks: up to 1,500 meters of Lower Permian sediments.
- Coal: in the Permian Ecca Group — a coal-bearing zone 500–550 m thick, with 55–125 m of net coal, in beds from 1 m to 30 m thick.
- Exploration: twenty-six coalbed gas wells drilled; several test wells in Botswana and Zimbabwe reported recoverable gas or production.
- Gas content: up to 9.5 cubic meters per ton (334 standard cubic feet), rising with depth.
The system
The USGS defined the Permian Composite Total Petroleum System, with one unit: the Kalahari Coalbed Gas Assessment Unit — thick Ecca coals less than 1,800 m below the surface. Production tests give limited but good evidence the system works.
| Zambezi Basin (Zambia, Zimbabwe) | Kalahari Basin (Botswana) | |
|---|---|---|
| Coal rank | subbituminous to medium-volatile bituminous | high-volatile bituminous |
| Other traits | few dolerite intrusions | vitrinite reflectance 0.5–0.7%; ash averaging 39%; sulfur averaging 0.45%; rank raised by dolerite intrusions |
| Gas | biogenic (low-rank coals) and thermogenic (high-rank) | thermogenic methane |
Key inputs
Table 1. Minimum / mode / maximum (mean). The probability of the unit is 1.0.
| Input | Values |
|---|---|
| Potential production area (acres) | 10,000 / 4,000,000 / 50,000,000 (18,003,333) |
| Drainage area per well (acres) | 40 / 80 / 120 (80) |
| Success ratio (%) | 10 / 20 / 35 (21.7) |
| Average estimated ultimate recovery per well (billion cubic feet) | 0.04 / 0.08 / 0.3 (0.092) |
Results
Table 2: fully risked. F95 means a 95-percent chance of at least that amount; F5, a 5-percent chance.
| Gas, billion cubic feet | F95 | F50 | F5 | Mean |
|---|---|---|---|---|
| Kalahari Coalbed Gas unit | 622 | 3,523 | 11,721 | 4,504 |
No natural gas liquids are expected.
Sources
Based on Assessment of Coalbed Gas Resources of the Kalahari Basin Province of Botswana, Zimbabwe, and Zambia, Africa, 2016, USGS Fact Sheet 2017–3003, by the Kalahari Basin Province Assessment Team: Michael E. Brownfield, Christopher J. Schenk, Timothy R. Klett, Marilyn E. Tennyson, Tracey J. Mercier, Stephanie B. Gaswirth, Kristen R. Marra, Sarah J. Hawkins, Thomas M. Finn, Phuong A. Le and Heidi M. Leathers-Miller, U.S. Geological Survey; a work of the United States government in the public domain. The map and both tables are taken from the fact sheet's PDF, which the import had left out. The text says the inputs follow U.S. coalbed gas analogs; the table's note says shale-gas and shale-oil analogs. Methods: USGS Energy Resources Program.
Licenza: CC0 1.0 (pubblico dominio) · Tratto da pubs.usgs.gov
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