A few interesting articles for your reading:
"Democrats try again for state tax on Marcellus Shale," by Tom Barnes, Pittsburgh Post-Gazette 2/08
Some Democratic legislators, aided by environmental groups, are making another try at enacting a severance tax on natural gas pumped from the vast areas of Marcellus Shale in Pennsylvania.
Rep. Greg Vitali, D-Delaware, today proposed House Bill 33, which would impose a two-phase extraction tax. There would be a 5 percent levy on the gross sale value of each 1,000 cubic feet (mcf) of gas pumped from deep underground. There would also be a second tax, of 4.6 cents per mcf, which wouldn't vary according to the price of the gas.
Mr. Vitali estimated that if the tax is enacted this year, it would generate $245 million in 2011. That would rise to $320 million in 2012, $400 million in 2013 and $477 million in 2014, as the value of the natural gas and the amount of gas pumped rises, he estimated.
"Pennsylvania House approves 6-bill package aimed at reform," by Jan Murphy, Patriot-News February 08, 2011