The use of hopanes to track in situ variations in petroleum composition in surface sediments

Alterations in the composition of a Light Arabian crude oil has been studied in situ during 12 months in an artificially contaminated site. After 12 months of experimentation, C17/Pr and C18/Ph ratios could not be used, because the isoprenoïds were no longer stable. In contrast, a high degree of stability of biomarkers, especially hopanes, was observed. These are very useful tools to assess alterations in other less stable classes of saturated compounds (n-alkanes and isoprenoïds) contained in petroleum. Marked degradation of both n-alkanes and isoprenoïds (using 17α21β C30 hopane as “internal standard”) has been evidenced. The use of biomarkers also made it possible to observe a burying process which occurred down to 6 cm deep after 12 months of experimentation.