Over many millions of years, gases, such as methane, tend to develop along with crude oil in natural deposits of petroleum. (sciencenewsforstudents.org)
Oil companies that today pay for CO2 to be delivered from natural deposits are in danger of losing money, because the current price of oil is so low. (scientificamerican.com)
Another complication is that the concentration of methane and other gases in natural deposits could be affected not just by chemistry but also by the speed at which they seep through the surrounding rocks. (newscientist.com)