Sure. Here's a recent one, for example (pdf hosted at motherjones but it's from the journal Climatic Change). http://www.motherjones.com/files/04-11shale_gas_footprint_fu...

And here's a short article in Time that mentions several studies: http://www.time.com/time/health/article/0,8599,2111562,00.ht...

The industry has disputed the data of some studies but also strongly resists efforts to shore up the data by better measuring methane leakage.

The direct greenhouse effects of methane vs. CO2 are not seriously in dispute, being a matter of basic physics. So I don't see how the level of methane emissions 200 years ago is relevant to the question of whether coal or gas is causing a greater greenhouse effect right now, per unit energy.

I'll mention though that while we may have less swampland, we've also got new methane releases from melting permafrost and clathrates.