The U.S. Geological Survey (USGS) raised eyebrows last week when it released its latest estimate of the amount of “undiscovered technically recoverable” natural gas in the Marcellus Shale formation, a rock formation that reaches through New York, Pennsylvania, Ohio, Maryland, and West Virginia. The estimated volume, around 84 trillion cubic feet (TCF), is 80 percent smaller than an estimate published earlier this year by the Energy Information Administration, an agency within the U.S. Department of Energy.

This discrepancy has added to the growing debate over just how much natural gas there is in the Marcellus Shale, possibly the largest shale gas deposit in the United States. Over the past decade, technological advances have made extracting shale gas much cheaper, and estimates of usable natural gas resources in the United States have skyrocketed. To many people, shale gas represents a crucial component to the country’s energy future, since it could reduce reliance on foreign fuel, and because burning natural gas produces less carbon dioxide than burning coal.

The difference in estimates is at least partly attributable to the fact that the USGS and Energy Information Administration employed different methodologies, and data sets, in reaching their respective conclusions. In practical terms,­ however,­ it may not matter which estimate is more correct, since it appears there are more than enough recoverable resources in the Marcellus to meet foreseeable demand.

The new USGS number is just the latest in a series of wildly varying estimates of the amount of gas that drillers could eventually draw from the formation, which lies beneath large sections of New York, Pennsylvania, Ohio, and West Virginia. The last time the USGS published an estimate, in 2002, it calculated that the volume of recoverable shale gas was only two TCF. Since then, vast improvements in horizontal drilling technology, 3-D seismic imaging, and hydraulic fracturing methods have led to much larger estimates. In 2009, Terry Engelder, a professor of geosciences at Pennsylvania State University, estimated the recoverable supply of gas to be as large as 489 TCF. That same year, the Potential Gas Committee, a nonprofit, volunteer group of academics and gas industry experts estimated that 227 TCF of recoverable gas resources­ existed in the Appalachian Basin—a geological region that extends from New York to Alabama, much of which is taken up by the Marcellus.

The up-and-down nature of these assessments hints at the huge amount of uncertainty involved. “This is an incredibly new, nascent resource,” says Francis O’Sullivan, a research engineer for the MIT Energy Initiative who analyzed supply issues for a study of the future of natural gas released by the initiative in June. There is not yet enough data to definitively forecast the performance of shale gas deposits over the next 20 to 30 years, he says.

Different groups assessing the deposits are liable to begin with very differing assumptions, says O’Sullivan. The extent to which this may have led to the discrepancy between this year’s estimates is not clear, since the details of the respective methodologies have not been made public. Differences between the data sets the organizations worked from are also unclear.

Regardless of which estimate is more correct, maintains O’Sullivan, there is a lot of recoverable gas in the Marcellus Shale formation, which, he estimates, could at its peak contribute 10 to 15 percent of the gas produced in the U.S. “What that means is that whether or not you have 200 TCF, 400 TCF, or 80 TCF of recoverable resources is quite moot at this point, and will be moot for 20 years.” Further, during that period, technology will continue to evolve, changing the definition of “technically recoverable” along the way. “The 20-year time horizon is a very long time in the context of technology in the gas space, and in the context of broader energy technology,” says O’Sullivan.