The Pittsburgh City Council voted 6-3 on Monday to add a referendum to the fall ballot that, if approved by voters, would amend the city charter with a ban on all commercial natural gas extraction inside the city limits.

The referendum must get by Pittsburgh Mayor Luke Ravenstahl before making it onto the ballot. If the Democratic mayor doesn't act on the ordinance within 10 days, it would be automatically vetoed.

Ravenstahl opposed a previous ban enacted by the council last year, citing the economic benefits of development, but has not yet said whether he plans to sign or veto the recent referendum.

"I will review the legislation, taking into consideration its potential implications for Pittsburgh and the region," Ravenstahl said. "I also plan to speak with legal counsel, City Council members, experts and citizens, in order to make the most informed and responsible decision possible."

If Ravenstahl vetoes the bill within 10 days, the council would need six votes to override the veto.

Although the referendum covers all natural gas extraction, except existing wells, the measure is aimed at Marcellus Shale developments. The Pittsburgh City Council banned drilling in the city limits last November, but adding it to the city charter would make it harder to rescind in the future (see Shale Daily, Nov. 17, 2010).

The distinction could be irrelevant from a legal standpoint. In a pair of landmark cases in 2009, the Pennsylvania Supreme Court ruled that local oversight of oil and gas operations can't conflict with state regulations, but also confirmed that local governments are allowed to zone where drilling can occur.

The legality might also be irrelevant, though, because companies haven't sought to drill in the city.

Although some Pittsburgh landowners have leased property to natural gas companies, the Pennsylvania Department of Environment Protection (DEP) hasn't issued any permits for drilling inside the city limits.

Because Pittsburgh is more than twice as dense as Fort Worth, Texas -- perhaps the largest city in the country with drilling in its borders -- the city presents a major logistical challenge for any company looking to drill.

Some parks, cemeteries and industrial sites, though, could be large enough to support drilling operations.

Even Allegheny County -- home of Pittsburgh -- isn't seeing much activity, despite being in a wet gas corridor that is increasingly attractive to investors. Of the 37 drilling permits the DEP has issued in Allegheny County so far this year, none are in Pittsburgh and only two are for the Marcellus Shale. By comparison, the DEP issued 255 Marcellus permits in the five counties surrounding Allegheny County in the first half of 2011.