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%R₀ to 1.3-1.35%R₀) for humic coals is re-considered. A worldwide coal data-set comprising coals of Carboniferous - Tertiary age and having vitrinite reflectances from 0.32%R₀ to 3.6%R₀ indicates that initial liquid hydrocarbon generation commences at a vitrinite reflectance of 0.5%R₀ to 0.6%R₀, and that a significant liquid hydrocarbon build-up occurs from approximately 0.6%R₀ to 0.85%R₀. The start of the "oil window" corresponds to a vitrinite reflectance of 0.85%R₀; 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%R₀. Liquid hydrocarbon generation is negligible at a vitrinite reflectance of approximately 1.8%R₀, and at 2.0%R₀ the hydrocarbon generative potential is exhausted. A vitrinite reflectance of 1.8%R₀ corresponds to the end of the "oil window", and for general use a vitrinite reflectance range of 0.85-1.8%R₀ is suggested to define the "oil window" for humic coals and kerogen type III. The end of the "oil window" at 1.8%R₀ 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%R₀. The re-considered "oil window" between 0.85-1.8%R₀ has significant implications for evaluating the prospectivity of basins dominated by terrestrial organic matter, such as coals.