GENERATION OF GAS AND OIL FROM COAL AND OTHER TERRESTRIAL ORGANIC-MATTER

Late Carboniferous-Permian sedimentary rocks contain the world's largest coal reserves which have generated and released gas but little if any oil. In comparison, organic matter deposited in lacustrine, fluvial, deltaic or brackish water sediments has sourced large quantities of waxy oils in countries such as China, Australia, the U.S.A., Argentina, Venezuela and Indonesia plus gas and condensate in deltas worldwide. Why is more oil coming from land derived organic matter disseminated in sediments than from the Carboniferous-Permian coal beds which also are land deposits? The answer appears to be in the hydrogen content and the ease of migration. When organic matter and coals have high hydrogen contents relative to carbon they can form oil. When the hydrogen is low, mainly gas is formed. Most of the world's coal is low in hydrogen whereas much of the terrestrial organic matter disseminated in sediments is higher in hydrogen so it forms more oil. Also, both gas and oil are absorbed on coal more strongly than on the disseminated kerogen of sedimentary rocks. This makes migration of oil out of the coal more difficult compared to migration from conventional shale source rocks.