USGS more cautious

In a 2002 assessment of petroleum resources in the Marcellus shale of the Appalachian basin, U.S. Geological Survey geologists came up with a mean estimate of 1.9 tcf of technically recoverable natural gas, a substantially lower figure than the 50 tcf estimate of Engelder and Lash.

The USGS assessment used the Greater Big Sand Devonian gas shale as a partial analogue for the Marcellus Shale, Robert Milici, USGS geologist and task leader for the 2002 assessment, told Petroleum News Jan. 23.

“At the time we conducted our assessment of the Marcellus shale in 2002, we had very little production data on which to base the assessment,” Milici said.

To derive an estimate for potential undiscovered resources in the shale, the USGS team added estimates of potential gas recovery from untested quarter-mile-square cells, he said.

“With sufficient new drilling and production data, especially from horizontal wells, we would be able to revise our previous estimate (about 2 tcf) of the technically recoverable gas resources in the area of thick Marcellus in Pennsylvania using the same methodology,” Milici said.

And Milici expressed caution about the Engelder and Lash findings.

“At this time, without a substantial amount of new drilling and production data, we regard the conclusions of the Penn State-Fredonia scientists regarding the potential of the Marcellus to produce 50 trillion cubic feet of gas as speculative, although not impossible,” he said.