The effect of maturity and biodegradation on the enantiomeric composition of sedimentary dihydro-ar-curcumene and related compounds

The effects of maturity and biodegradation on dihydro-ar-curcumene and isodihydro-ar-curcumene enantiomers have been assessed by applying chiral GC–MS techniques to aromatic hydrocarbon fractions obtained from crude oils and sedimentary rocks. Dihydro-ar-curcumene of natural origins undergoes rapid racemisation in the subsurface, yielding a racemic mixture before the onset of significant oil formation. 1,2-Alkyl shifts on the aromatic ring also begin at an early stage to yield isodihydro-ar-curcumene and continues with increasing maturity. Laboratory experiments using proton catalysts and clay (Lewis acid catalyst) show that the alkyl shift reaction is catalysed by proton and Lewis acids and racemisation is only catalysed by Lewis acids. A moderately biodegraded crude oil has been shown to be depleted in the R enantiomer of dihydro-ar-curcumene and an extensively degraded oil has dihydro-ar-curcumene depleted relative to isodihydro-ar-curcumene. These results indicate preferential metabolism of both enantiomers and positional isomers by degrading micro organisms.