Markers for secondary reactions of migrated crude oil on carbonaceous surfaces

A high abundance of ethyl substituted aromatic hydrocarbons (HCs) relative to their methyl counterparts is an unusual feature of some crude oils. Enhanced stability of ethyl aromatic HCs in the presence of tetralin was observed when individual ethylated compounds were heated with activated carbon in sealed tubes over a range of 170–340 °C. In addition, conversion of the common distribution of alkyl aromatic HCs to an unusual distribution, containing a higher relative abundance of ethyl compounds, was demonstrated by way of closed system heating of the aromatic fraction of a crude oil in the presence of activated carbon. The conditions for this unusual process require the presence of hydrogen donor components, which selectively limit the reaction and depletion of ethylated compounds relative to methylated compounds. The phenomenon has been shown to occur for substituted benzenes, naphthalenes, phenanthrenes and biphenyls. Enhanced abundance of ethyl aromatic HCs relative to their methylated counterparts is therefore proposed as an indicator for secondary reactions of migrated crude oil that has undergone thermal alteration after contact with carbonaceous surfaces in sediments. Application of these principles to selected crude oils and sediment extracts from the Carnarvon and Cooper/Eromanga Basins (Australia) indicates that significant secondary reaction of migrated crude oil has occurred.