Conventional crude oil and heavy oil have also been defined very generally in terms of physical properties. For example, heavy oils were considered those petroleum-type materials that had gravity somewhat less than 20°API, with the heavy oils falling into the API gravity range 10°–15° (e.g., Cold Lake crude oil = 12°API) and bitumen falling into the 5°–10°API range (e.g., Athabasca bitumen = 8°API). Residua vary depending upon the temperature at which distillation is terminated. Atmospheric residua are usually in the 10°–15°API range, and vacuum residua are in the range 2°–8°API (Speight, 2000).
There have been several recent noteworthy attempts to classify crude oil (Bestougeff et al., 1984; Danyluk et al., 1984; Gibson, 1984; Khayan, 1984) using one or more of the general physical properties of crude oils. One method uses divisions by API gravity, which is already accepted by most workers, and also uses viscosity data (Khayan, 1984). This method is essentially a more formal attempt in which specific numbers are applied without recognition of the implications of these numbers.
In the author’s experience, the assignment of specific numbers to the classification of petroleum is fraught with difficulty. For example, heavy oil was considered those petroleum-type materials that had gravity somewhat less than 20°API, and generally fell into the API gravity range 10°–15° with tar sand bitumen falling into the 5°–10°API range (Speight, 2000). Using such lines of demarcation does not circumvent the question that must arise when one considers a material having an API gravity equal to 9.9 and one material having an API gravity equal to 10.1. Nor does the line of demarcation make allowance for the limitations of the accuracy of the analytical method. Cleary the use of one physical parameter be it API gravity, or any other physical property for that matter, is inadequate to the task of classifying conventional petroleum, heavy oil, and tar sand bitumen.