Reaction of Organic Material to Progressive Geological Heating

Abstract

The generation of oil is a process which begins at some point during the burial history of a source rock. This “onset of maturation” is dictated largely by temperature and residence time. However, the nature of the source rock itself also influences the hydrocarbon product being expelled from the source rock. The vast majority of the world’s oil can be ascribed to source rocks of the following types: A. Marine mudrocks that were deposited under anoxic conditions and dominated by planktonic organisms and anaerobic bacteria. This type of source rock can have a carbonate or clay inorganic matrix and total organic carbon values from 1–30 weight % (commonly 4–10% when immature). Examples of this classical oil source rock are the source rocks of Western Canada, the Middle East, and the Malm of the North Sea. B. Specific coal facies such as torbanites and cannel coals containing a mixture of hydrogen rich plant detritus (for example, spores, pollen, cuticle, resin, and algae). Deposition was probably in open water areas of an overall coal swamp environment. Examples of these source rocks can be found in the Gippsland Basin, Canadian Beaufort Sea, and Southeast Asia. C. Lacustrine organic-rich deposits that are rich in freshwater algae, which ultimately result in high wax crude oils. Examples are relatively rare but include major source rocks in the Uinta Basin and China. This type of source rock will not be discussed further in this chapter.