PARAMETERS OF COAL AND OIL FIRES

Although the oldest bitumen fire known occurred 16 Ma in the Sinai Peninsula, prehistoric coal fires occurred predominantly during the Pliocene-Pleistocene interglacial periods (2.5-1.8 Ma), in the mean and high latitudes of Asia and North America. These fires occurred in areas characterized by rugged relief and steeply dipping coal-bearing strata. Combustion in the presence of fuel and oxygen occurred within 500 m of the surface. Prior to the Quaternary, numerous fossil-fuel horizons were isolated from atmospheric oxygen. Subsequently, tectonic activation and uplift resulted in caustobiolith oxidation and combustion. Pyrogenic landscapes, up to 1000 km² in area, are a distinctive feature of many coal and oil basins. Two examples are the Hatrurim and Kuznetsk basins in Israel and Russia, respectively. The evolution of such landscapes is dependent upon repeated cycles of combustion – gas outburst – rock crushing – cooling –shrinkage fracturing – oxygen intake – ignition – combustion. As caustobiolith-bearing strata burn, flame fronts develop along steeply dipping beds and the fire descends quickly to depth. On rare occasions, coke forms and is itself later ignited. Underground coal fires are estimated to spread at the rate of 5-17 meters per year. During oil fires, however, gas ignition in narrow “chimneys” is responsible for flames that may spread as fast as tens of meters per second. The basic mechanism of heat and mass transfer during pyrogenesis is gas convection. Country rock has been heated by coal fires to temperatures as high as 1300^oC, whereas bitumen and oil fire temperatures may reach 1600^oC and 2000^oC, respectively. Such fires usually burn at less than 25 bar lithostatic pressure. During combustion, pyrolysis and sublimation from the gas occurs. Pyrometamorphism is responsible for mineral dehydration, amorphization, carbonate decomposition reactions, and it may result in the partial or complete melting of country rock.