A re-consideration of the oil window for humic coal and kerogen type III source rocks

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.