The answer is natural gas: frequently present in oil or coal deposits. Natural gas is caused by the decay of organic matter sediments trapped in rocks down to the center of Earth and results in a mixture of light hydrocarbons in which methane is a prominent element. Natural gas is perhaps after oil the most important non-renewable source of energy today. It is created as a byproduct in oil extraction, the gas contains light gases that dissolved or associated with oil by the accumulation of organic matter over millions of years and are found in coal deposits.
Natural gas is present in fossil fuels like coal or oil. Deposits are found every year but they are limited and estimates tell us, that at the current rate of U.S. natural gas consumption in 2016 of about 27.5 trillion cubic feet per year, the US has enough natural gas to last around 90 years.