THE ORIGIN OF LIGHT CYCLOALKANES IN PETROLEUM

It has been suggested that the light cycloalkanes in petroleum are generated through the thermal decomposition of heavier polycyclic natural products, such as the steranes and triterpanes. However, no support could be found for the assumption that the polycycloalkanes should decompose to light cycloalkanes at typical subsurface temperatures. For example, at 150°C, decahydronaphthalene-the bicyclodecyl unit fundamental to the steranes and triterpanes-has a half-life of approximately 30 billion years. At this same temperature, cyclohexane has a half-life of approximately 60 billion years. The surprising thermal stability of the cycloalkane ring can be traced to a prohibitively high activation energy for ring opening due to the steric strain associated with the β-elimination step. Cholestane undergoes thermal decomposition almost exclusively by loss of the alkyl side chain. Under thermal conditions sufficiently severe to break the carbon-carbon bonds of normal alkanes (weeks, 330°C), cholestane gives only insignificant amounts of light cycloalkanes. It is most doubtful, therefore, that the C5 to C9 cycloalkanes could be thermally produced from natural products like the steranes and triterpanes. An alternative hypothesis is offered in which the light cycloalkanes in petroleum are formed in a steadystate catalytic process. © 1990.