One thing that's sometimes overlooked is that natural, which produces less CO2 than oil when burned, often produces a lot of CO2 at the wellhead, which is vented to the atmosphere. In Wyoming, for example, the state govt. is finally beginning to force gas producers to find alternatives to dumping their CO2 in the air. These include pipelines to oilfields for EOR, and another technology that's ExxonMobil has been using for their feelgood ads on TV called Controlled Freeze Zone, where the CO2 is frozen out of the natural gas stream.