PITTSBURGH — There’s been plenty of controversy over Marcellus Shale natural gas drilling in Pennsylvania, and now tax rates are part of the debate, too.

Gov. Tom Corbett’s energy executive is questioning research that suggests Pennsylvania’s Marcellus Shale impact fee will generate billions of dollars less in long-term revenue than the same natural gas production in West Virginia. But a Democratic candidate for governor said the math is correct.

The Pennsylvania Budget and Policy Center made the long-term comparison to what a 5 percent tax such as West Virginia’s would generate, but Energy Executive Patrick Henderson said in e-mails Monday and Tuesday that the group has a liberal bias.

Henderson said the Pennsylvania fee encourages investment and that “a job for a Pennsylvanian beats enacting a new tax.”

John Hanger, a Democratic candidate for governor and former head of the state Department of Environmental Protection, said “themath is the math” and that the Policy Center analysis is accurate.

“We have this ridiculous fee,” Hanger said. “First, it’s way too little. It amounts to a huge subsidy to the gas industry at the expense of schools and other vital services. We need a tax like Texas and West Virginia.”

The Pennsylvania fee is essentially based on the numbers of wells drilled and on the wholesale price of gas, so when prices plunged last year, the impact fee revenue declined, even though the amount of gas produced roughly doubled.

In an independent review, the AP found that at the current pace Pennsylvania gas production could generate the equivalent of $16 billion in company revenue in 2015, if wholesale prices are at the current $4. At West Virginia’s 5 percent tax rate, that would generate about $800 million.

But the policy center estimates the Pennsylvania impact fee will generate $237 million to $261 million in 2015, depending on the number of wells drilled and prices. By 2020, the center estimates that the impact fee will generate an equivalent effective tax rate of about 1.3 percent, so that over 20 or 30 years it may generate $10 billion or $15 billion less than a flat tax on production.