EFFECTS OF SOURCE, THERMAL MATURITY, AND BIODEGRADATION ON THE DISTRIBUTION AND ISOMERIZATION OF HOMOHOPANES IN PETROLEUM

The distributions of C31-C35 17-alpha,21-beta-(H)-homohopanes in marine petroleums can be used to describe redox conditions in the source rock depositional environment. For example, high C35-homohopane indices [C35/(C31-C35)] are typical of petroleums generated from source rocks deposited under anoxic marine conditions, where the C35 backbone of the precursor bacteriohopanetetrol is preferentially preserved. Homohopane distributions have been used to distinguish petroleums from different organic facies of the same source rock. Homohopane distributions are altered by secondary processes. For example, the C35-homohopane index decreases with increasing API gravity and thermal maturity of related oils from the Monterey Formation, offshore California. Similar decreases in the homohopane index are observed for expelled oils from Monterey source rock during hydrous pyrolysis. Biodegradation can result in selective loss of high or low molecular weight homologs, apparently depending on the bacterial population in the reservoir. Bacterial demethylation of homohopanes appears necessary to explain 25-norhopanes in at least one biodegraded oil. Hydrous pyrolysis studies of the Phosphatic and Siliceous members in the Monterey Formation show that identical heating conditions result in different homohopane C-22 epimer ratios. Furthermore, a "reversal" in the trend of the epimer ratio at high experimental temperatures is observed for the Phosphatic, but not for the Siliceous lithology. This could be caused by the combined effects of isomerization and differential thermal destruction of epimers at high temperatures, possibly mediated by rock mineralogy.