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Generation of Oil from Coal and Carbonaceous Shale

Douglas W. Kirkland, T.-F. Tsui, M. L. Stockton

Coal and carbonaceous shale contain the remains of higher terrestrial (vascular) plants, remains commonly referred to as Type III kerogen. Given sufficient thermal exposure, such organic matter is commonly considered to generate only natural gas. Coaly sequences, however, are not always strictly gas producers. Many coaly sequences, particularly those of Tertiary age, have generated important volumes of oil.

Those oils are usually paraffinic and waxy, or naphthenic; have a pristane-to-phytane ratio of 2-12; contain definitive biomarkers such as tricyclic diterpenoids; and are low in sulfur (much less than 1%). These oils are clearly distinct from those derived predominantly from marine algal remains.

The principal source sequences of oils derived from coaly material occur in Tertiary deltas. Such source sequences contain abundant coaly material with favorable generative quality. Evaluation of generative quality is based either on geochemical analysis (e.g., atomic H/C > 0.9) or on petrographic analysis: source potential being proportional to the abundance and hydrogen richness of organic constituents (macerals). In approximate ranking, oil-generating potential of the hydrogen-rich macerals is exudatinite > alginite > resinite > liptodetrinite > cutinite > sporinite > vitrinite-B (fluorescing vitrinite).

Examples of basins containing both a major Tertiary deltaic sequence (hundreds of cubic miles) and major volumes of oil (billions of barrels) derived predominantly from higher terrestrial plant remains are: the Ardjuna and Kutei basins, Indonesia; and the Gippsland basin, Australia.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.