A RE-CONSIDERATION OF THE "OIL WINDOW" FOR HUMIC COAL AND KEROGEN TYPE III SOURCE ROCKS

H. I. Petersen^*

^*Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark; email: hip@geus.dk

Based on natural data-sets and several hydrous-pyrolysis experiments on immature humic coals, the conventional "oil window" (0.5–0.6%R_o to 1.3–1.35%R_o) for humic coals is re-considered. A worldwide coal data-set comprising coals of Carboniferous – Tertiary age and having vitrinite reflectances from 0.32%R_o to 3.6%R_o indicates that initial liquid hydrocarbon generation commences at a vitrinite reflectance of 0.5%R_o to 0.6%R_o, and that a significant liquid hydrocarbon build-up occurs from approximately 0.6%R_o to 0.85%R_o. The start of the "oil window" corresponds to a vitrinite reflectance of 0.85%R_o; however, the start of the "oil window" can vary significantly for individual coal series, and it may range up to a vitrinite reflectance of 1.15%R_o. Liquid hydrocarbon generation is negligible at a vitrinite reflectance of approximately 1.8%R_o, and at 2.0%R_o the hydrocarbon generative potential is exhausted. A vitrinite reflectance of 1.8%R_o corresponds to the end of the "oil window", and for general use a vitrinite reflectance range of 0.85–1.8%R_o is suggested to define the "oil window" for humic coals and kerogen type III. The end of the "oil window" at 1.8%R_o is supported by published data, indicating a much higher thermal stability of nC₁₅₊ hydrocarbons than is generally accepted, and therefore limited oil-to-gas cracking at 1.3%R_o. The re-considered "oil window" between 0.85–1.8%R_o has significant implications for evaluating the prospectivity of basins dominated by terrestrial organic matter, such as coals.

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