The aim of this study is to compare oils from known biodegraded fields offshore Norway to waxes and oils from an artificial cavern storage facility, to determine if the oil degradation processes observed in natural reservoirs offshore are in any way comparable to the processes occurring in oil storage facilities. The sample set contains wax and oil samples from an underground cavern crude oil storage facility and oils from the following fields: Svale, Heidrun, Falk, Draugen, Ula, Oseberg and one sample from the Gulf of Mexico. The analytical procedures used in this study are Iatroscan TLC-FID, GC-FID and GC-MS, and the chromatograms, chemical facies and maturity parameters are used to evaluate the origin and also the processes occurring in the storage tanks.
Wax deposition is a widespread and recurring problem that can occur during any stage of production, transportation and storage of crude oils. High molecular weight hydrocarbons (>C40) are generally believed to have the most significant impact on wax deposition problems; and certain case studies have demonstrated that a minimum wax content of 2% in an oil can result in wax deposition.
The samples from the caverns show only a very moderate degree of biodegradation compared to the biodegraded samples from offshore Norway, and they have a rather normal oil composition, very typical for slightly biodegraded oils. The moderate degree of biodegradation may be due to limited access to oxygenated meteoric water, which results in anaerobe conditions. It is uncertain if there is purely inorganic processes, i.e. some sort of gravimetric segregation that leads to the formation of these waxes, or if anoxic bacteria may be involved in the formation of long chained waxy compounds.
This study illustrates that the processes that affects oils in natural reservoirs may not be active in artificial underground oil storage facilities.