Critical Relationships between Vitrinite Reflectance and Petroleum Formation
Vitrinite reflectance (%Ro) is a critical parameter in determining the amount of thermal stress (temperature and time) a source rock experienced during its burial history. Increasing reflectance of vitrinite is attributed to the growth of aromatic-ring clusters that results in their gradual alignment with increasing thermal stress. This process represents bond formation, as opposed to bond breaking in the process of petroleum formation. As a result, reaction kinetics for vitrinite reflectance are not the same as those for petroleum formation. For example, oil generation from high organic sulfur-bearing kerogen starts oil generation at a low thermal stress (<0.5 %Ro), and low organic sulfur-bearing kerogen starts oil generation at a high thermal stress (>0.8 %Ro). Therefore, it is critical to have good oil-generation kinetic parameters that can be applied in a thermal history that is based on vitrinite reflectance. This variable relationship between vitrinite reflectance and petroleum formation has been shown to be critical in the onshore Gulf Coast, Western Canada Sedimentary Basin, Mesopotamian Basin, Rocky Mountain basins, California Coastal basins, and Polish Carpathians. The variability between vitrinite reflectance and petroleum formation may also be accentuated by the occurrence of so-called suppressed vitrinite. The reflectance of this vitrinite increases significantly slower than that of vitrinite from humic coals subjected to the same thermal stress. For example, a thermal stress capable of increasing the reflectance of typical vitrinite from 0.5 to 1.0 %Ro may only increase from 0.5 to 0.6 %Ro for suppressed vitrinite. Therefore, it becomes critical to know whether a reflectance value is suppressed in determining the thermal history of a source rock and the boundaries for its primary (source-rock sourced) and secondary (oil-cracking sourced) gas generation. Source rocks with suppressed vitrinite have been observed in the Phosphoria Formation, Woodford Shale, Bakken Shale, and Alum Shale.
AAPG Datapages/Search and Discovery Article #90193 © 2014 Rocky Mountain Section AAPG Annual Meeting, Denver, Colorado, July 20-22, 2014