The U.S. Geological Survey (USGS), a federal mapping agency, has of late been propounding the difference between "resources" and "reserves." It says that although the two terms are used interchangeably, the distinction is simple: Reserves are a subset of resources. Coal resources, as an example, include those in-place tonnage estimates determined by summing the volumes for identified and undiscovered deposits of coal, whereas coal reserves are those resources considered "economically producible" at the time of classification, even though extraction facilities are not in place and operative.
That distinction becomes especially important in light of new assessments from separate groups that claim coal reserves in the U.S. have been wildly overestimated and gas reserves underestimated. And it throws into question the Energy Information Administration’s (EIA’s) assessments, which have long been a yardstick for comparable estimates.
In 2007, the EIA said the U.S. had a "demonstrated reserve base" of nearly 500 billion metric tons of coal, and it regarded 267 million metric tons — enough for 240 years — of that as "economically recoverable." But an extensive USGS analysis of Wyoming’s Gillette coal field — the nation’s largest and most prolific — released this June determined that of 182 billion metric tons of resources in place, less than 9.16 billion (or 6%) were found to be recoverable under "current technological and economic circumstances." This compares with an earlier assessment from 2002 by the USGS in which 20.87 billion metric tons were estimated to be recoverable. The USGS engineers, geologists, and economists explain the discrepancy is a result of using an "improved methodology," which incorporates a new dataset with 10 times as many data points as were used in previous assessments.
In June, meanwhile, the Potential Gas Committee (PGC), a group of industry, government, and academic volunteers, said in a study that U.S. natural gas reserves were likely 1,836 trillion cubic feet. This assessment is up 39% — the highest increase on record — from the group’s estimate of 1,321 trillion cubic feet two years ago. "New and advanced exploration, well drilling and completion technologies are allowing us increasingly better access to domestic gas resources — especially unconventional gas — which, not all that long ago, were considered impractical or uneconomical to pursue," said John Curtis, a committee member and professor of geology at the Colorado School of Mines.
The increase has been tagged to a reevaluation of shale in the Appalachian Basin and in the midcontinent, Gulf Coast, and Rocky Mountain areas. When the PGC’s results are combined with the DOE’s latest available determination of proven gas reserves (238 trillion cubic feet as of 2007), the report says that the U.S. has a total available future supply of 2,074 trillion cubic feet. That’s an increase of 542 trillion cubic feet over the previous evaluation. Curtis cautioned, however, that the current assessment assumes "neither a time schedule nor a specific market price for the discovery and production of future gas supply."
"Estimates of the Potential Gas Committee are ‘base-line estimates’ in that they attempt to provide a reasonable appraisal of what we consider to be the ‘technically recoverable’ gas resource potential of the United States," he explained.
—Sonal Patel is POWER‘s senior writer.