<quote><i>/Geologist who works for a major oil company.</i>
So you're obviously a non-biased source.</quote>
So who am I to believe, The guy who spent 10 years getting his masters, or the guy who just spent 2 hours watching a movie?
It really is basic Geology, Ask someone from the USGS if you really want an unbias source. Seriously give them a call.
For example, The Marcellus Shale, ranges from a depth of 3000-7000 feet. The thing is Freshwater usually only goes down a few hundred feet. According to the DNCR the brine freshwater contact ranges from 200-1000 feet. http://www.dcnr.state.pa.us/topogeo/education/es3.pdf.
So You would at the very least need a 2000ft fault, But in more realistic cases you'd have to have a migration path of about 4000-5000ft.
What's scary is more along the lines of the drilling company fucking up and not casing the well right or something and the gas traveling up the well bore and into the water table. Of course this would be a major fuck up(Think Macondo).