Geomicrobiology of Fossil Fuels
Although much of the organic carbon in the biosphere is continually recycled, a very signicant amount has become trapped in special sedimentary formations, where it is inaccessible to mineralization by microbes until it becomes reexposed to water through natural causes or human intervention. Microbial mineralization of such reexposed organic carbon also depends on the access to suitable terminal electron acceptors, that is, oxygen in air in the case of aerobes and inorganic electron acceptors in the case of facultative or anaerobic microbes. The trapped organic carbon exists in various forms. The degree of its chemically reduced state is related to the length of time it has been trapped and any secondary changes that it has undergone during this time. Some of this trapped carbon has value as a fuel, a source of energy for industrial and other human activity, and is exploited for this purpose. Because of the great age of this material, it is known as fossil fuel. The remainder of the trapped carbon is chiey kerogen and bitumen, some of which can be converted to fuel by human intervention. Fossil fuels include methane gas, natural gas (which is largely methane), petroleum, oil shale, coal, and peat. They are generally considered to have had a microbial origin (Ourisson et al., 1984).