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 0 to 1.3–1.35%R 0 ) 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 0 to 3.6%R 0 indicates that initial liquid hydrocarbon generation commences at a vitrinite reflectance of 0.5%R 0 to 0.6%R 0 , and that a significant liquid hydrocarbon build‐up occurs from approximately 0.6%R 0 to 0.85%R 0 . The start of the “oil window” corresponds to a vitrinite reflectance of 0.85%R 0 ; 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 0 . Liquid hydrocarbon generation is negligible at a vitrinite reflectance of approximately 1.8%R 0 , and at 2.0%R 0 the hydrocarbon generative potential is exhausted. A vitrinite reflectance of 1.8%R 0 corresponds to the end of the “oil window”, and for general use a vitrinite reflectance range of 0.85–1.8%R 0 is suggested to define the “oil window” for humic coals and kerogen type III. The end of the “oil window” at 1.8%R 0 is supported by published data, indicating a much higher thermal stability of nC 15+ hydrocarbons than is generally accepted, and therefore limited oil‐to‐gas cracking at 1.3%R 0 . The re‐considered “oil window” between 0.85–1.8%R 0 has significant implications for evaluating the prospectivity of basins dominated by terrestrial organic matter, such as coals.