2002 年 67 巻 1 号 p. 30-37
Recently source rock potentials on coals in Japan have attracted interests by the discoveries of Yufutsu oil/gas field and a new gas-field at offshore in Sanriku District, in which coals were assumed as their source rocks. The gas generation mechanism and gas potentials of coal as a source rocks in the petroleum system is discussed in this paper. On the van Krevelen diagram, several Japanese coals are plotted in the area between type II and type III organic matter. Petrographical and ultimate analyses on coals revealed that the high hydrogen contents of Japanese coals are caused by abundant degradinite, a maceral of vitrinite group. The degradinite is observed such mixture as very small fragments of the virinite (type III organic matter) and exnite (type II organic matter) under the microscope. The oil generation is assumed to begin at Ro=0.5, but only dehydration and decarboxylation continue until Ro =1.0, and the demethylation occurs later along the curve on van Krevelen diagram, resulting in the decrease of H/C atomic ratio. It is difficult for the degradinite in the oil window (Ro= 0.5 to 1.0) to generate hydrocarbons because of its difficulty of primary migration from coals. The compositions of gases from Japanese coal mines showed that CH4 content in hydrocarbon was more than 99% with a few exceptions. It suggests that little hydrocarbons are generated and migrated from coals in the oil window. Coals and terrestrial organic matters have been treated as gas source rocks only, and acknowledged to generated gas after Ro = 0.5 in the previous studies . However in the petroleum systems for the basins of non-marine or deltaic environment, hydrogen rich coals, such as Japanese coals, should be awarded that non-migrating oil-drops in micron size could have generated light hydrocarbons in the gas generation window when a thermal clacking of oil and bitumen begun.