David A. Wavrek2
(1) University of Tulsa, Tulsa, OK
(2) Petroleum Systems Intl, Salt Lake City, UT
Abstract: Degradation of oil exposed in the southern “oil lakes” of the Kuwait desert
Oil spilt into the southern Kuwait desert nine years ago is still present as large surface “oil lakes”. In the absence of water, the oils have not been bacterially degraded and still show a full suite of longer chain normal alkanes. Comparison with oil samples from the Burgan reservoir (which was the source of the spilled oil) shows no differences in sterane and hopane distributions but does show loss of lower molecular weight compounds – those with less than about eight carbon atoms. This is attributed to evaporative loss. Evaporation of Burgan oil has been simulated in laboratory experiments and leads to oil compositions similar to those observed for the oil lakes. For temperatures ranging from 20 to 50°C and variable air flows the normal alkanes are lost more rapidly than other hydrocarbon types. Laboratory experiments show decreasing API gravities and increasing viscosities with increasing evaporative loss. Also stratification develops in the oil column with the most degraded oils at the surface. this degraded “skin” probably protects the bulk of the oil from further evaporation which is why the oil has survived at the surface for many years. GC-FPD analysis of oils from the surface of the oil lakes shows diminished amounts of benzothiophene and dibenzothiophene compared with the Burgan oil. Loss of these sulfur compounds is attributed to photo-oxidation at the oil surface and is consistent with the observed increases in hydrogen polysulfides. It appears that evaporation modified by photooxidation accounts for compositional changes in the Kuwait “oil lakes” and processes involving water (such as biodegradation and water washing) are not involved.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana