Mineral-Catalyzed Formation of Natural Gas during Coal Maturation

Kinetic data from the literature were used to predict formation rates and product yields of oil and gas at typical low-temperature conditions of coal maturation. The data indicate that gas formation rates from hydrocarbon thermolysis reactions are several orders of magnitude too low to have generated known reserves of coalbed gas. By contrast, acid-mineral-catalyzed cracking, transition-metalcatalyzed hydrogenolysis of liquid hydrocarbons, and transition-metal-catalyzed CO₂ hydrogenation form gas at sufficiently high rates in geologic time and at geologic conditions to account for formation of enormous reserves of coal-bed gas. Rates of gas production in these reactions are 5 to 10 orders of magnitude higher than those predicted from thermolysis. The gaseous product compositions for metal-catalyzed hydrogenolysis of hydrocarbon liquids and for CO₂ hydrogenation are nearly the same as those of natural gases, while those from thermal and catalytic cracking are vastly different. From chemical analysis of a pair of gas-producing and non-gas-producing coals it was found that significant, comparable amounts of iron are present. The available data are consistent with a model involving thermal and catalytic cracking of kerogen to oil followed by iron-metal-catalyzed hydrogenolysis of oil to natural gas; in CO₂-containing coal gases, natural gas may also be formed by iron-catalyzed CO₂ hydrogenation.