The U.S. Energy Information Administration (EIA) said it estimates that shale formations in the United States contain 483 Tcf of unproved technically recoverable natural gas and 33.2 billion bbl of unproved technically recoverable tight oil.

In a statement on Friday, the EIA conceded its estimations of technically recoverable resources (TRR) were a “work in progress” that evolves with production technology but added that the biggest unknown involved the estimated ultimate recovery (EUR) per well.

“The TRR estimates provide context for the size of the resource, while projected production depends strongly on the number of wells, the EUR per well, other well characteristics and economics,” the EIA said, adding that TRR estimates include proved reserves and unproved resources. “As wells are drilled and field equipment is installed and productivity is assumed, unproved resources become proved reserves and, ultimately, production.”

In January the EIA stated in its Annual Energy Outlook 2012 (AEO2012) that unproved TRR for natural gas in the United States was 482 Tcf, down substantially from the 827 Tcf estimate of AEO2011 (see Shale Daily, Jan. 24). The Marcellus Shale made up 141 Tcf of the overall figure.

With its latest estimate, the EIA said the Marcellus Shale still holds 141 Tcf of unproved technically recoverable natural gas. The Haynesville/Bossier Shale held the second-highest amount of gas at 66 Tcf, followed by the Eagle Ford at 60 Tcf. The other shale plays mentioned were the Utica (16 Tcf), Fayetteville (13), Woodford’s Anadarko and Arkoma formations (both at 11), Pearsall (9), Chattanooga (2), and Caney (1). All other U.S. shale plays accounted for an estimated 163 Tcf.

For unproved technically recoverable tight oil, the EIA estimated that the Monterey/Santos Shale in California held 13.7 billion bbl, followed by the Niobrara formation with 6.5 billion bbl and the Bakken/Sanish/Three Forks with 5.4 billion bbl. Other shale formations that held significant tight oil resources included the Austin Chalk (2.7 billion bbl), Eagle Ford (2.5 bbl), Avalon’s Bone Springs formation (1.6 billion bbl), Spraberry (510 million bbl) and Woodford’s Anadarko formation (393 million bbl).

The EIA said uncertainty over calculating the EUR per well was compounded by the fact that most shale gas and tight oil wells were only a few years old and their long-term productivity is untested. Another factor complicating the issue is the fact that companies have already targeted the “sweet spots” of shale plays.

“Over time, estimates regarding a formation’s average EUR should become less uncertain as more wells are drilled across the entire formation and as more wells produce over a longer period of time,” the EIA said. “As a formation’s EUR estimate changes, so too will the formation’s TRR estimate. Consequently, TRR estimates are a work in progress, changing as more production experience becomes available and as new technologies are applied for extracting these resources.”

Earlier this month the EIA said it was forecasting a small drop in natural gas production over the next several months (see Shale Daily, July 12). But it also forecast an increase in natural gas production every year through 2035 because of the unconventional plays, whose resources have expanded through unconventional drilling technology (see Shale Daily, June 27).