In vitro photooxidation of crude oil maltenic fractions : Evolution of fossil biomarkers and polycyclic aromatic hydrocarbons

Photooxidation is an important transformation pathway for petroleum spilled in the marine environment. The maltenic fraction of ten crude oils was exposed to sunlight. Acidic and non-acidic photoproducts were analysed by FTIR spectroscopy and by GC/MS. A formation of carboxylic acids, straight-chain ketones and alcohols was observed. The GC/MS analyses of residual alkanes showed a preferential alteration of branched alkanes. The analyses of biomarkers such as drimanes, hopanes and steranes indicated that the GC/MS indices widely used in geochemistry, presented a great stability after photooxidation except for the Tm/(Tm+Ts) ratio. Concerning the polycyclic aromatic hydrocarbons (PAHs), naphthalene and its derivatives were severely altered by photooxidation whereas, phenanthrene, dibenzothiophene and their derivatives were more recalcitrant. Methyl derivatives were more degraded than their parent molecules and in the case of methyl phenanthrenes, the 2-methyl phenanthrene had a great stability. The following sequence of increasing photodegradability of mono methylated phenanthrenes (MP) can be proposed: 2-MP<1-MP<3-MP<9-MP.