Determination of stable carbon (δ¹³C) isotope systematics for alkylphenols and light aromatic hydrocarbons (BTEX) in petroleum formation waters and co-produced oils

Solid phase microextraction (SPME) techniques coupled with gas chromatography-isotope ratio mass spectrometry (GC-IRMS) were used to determine the stable carbon (δ¹³C) isotopic compositions of aromatic hydrocarbons (BTEX: benzene, toluene, ethylbenzene and xylenes) and phenol in a small suite of co-produced oils and waters from a North Sea oilfield. The δ¹³C patterns and the compound profiles observed are consistent with the theory that the concentrations and distributions of BTEX and alkylphenols in oilfield waters can simply be explained by partition equilibrium between oil and water. The large difference in δ¹³C signatures for phenol compared with benzene and toluene (7-8 %c vs. PDB) in both the oil and water phases strongly suggests that, at least for BTEX and phenols, reversible chemical reactions controlled by master geochemical variables (such as fO₂) do not appear to be important at the temperatures of most oilfields.