Variation in alkylphenol distributions in a homogenous oil suite from the Dhahaban petroleum system of Oman

As part of a study to determine the effect of migration processes on the abundance and distribution of alkylphenols in crude oils, we have analysed a homogeneous suite of 15 oils from the Dhahaban petroleum system in Oman that lies along a secondary migration pathway in a foreland basin setting. Geochemical parameters based on aliphatic hydrocarbon biomarkers and aromatic hydrocarbon concentrations and ratios suggest a similar maturity and source facies for the oils, despite the wide variation in relative petroleum migration distance (up to c. 260 km). Given the similarity in source facies and maturity characteristics of the oils, we conclude that the observed variations in alkylphenol concentrations in this sample suite are controlled mainly by secondary migration processes. The majority of the oils are not degraded and in these samples we observe a significant decrease in total alkylphenol concentration within the first c. 50 km relative migration distance, supporting similar studies on North Sea petroleum systems and suggesting that alkylphenols may be useful as geotracers over shorter migration distances. There is little evidence of systematic migration distance-related variation in the relative abundance of individual alkylphenol isomers or homologue groups. The alkylphenol distributions in two biodegraded oils in the sample suite, both from relatively shallow reservoirs, were dominated almost completely by phenol, and this is discussed in the context of the microbial production and alteration of phenols, and the influence of water-washing on the content and distribution of phenols in crude oils.