Naphthenic acids in groundwater overlying undeveloped shale gas and tight oil reservoirs

The acid extractable organics (AEOs) containing naphthenic acids (NAs) in groundwater overlying undeveloped shale gas (Saint-Édouard region) and tight oil (Haldimand sector, Gaspé) reservoirs in Québec, Canada, were analysed using high resolution Orbitrap mass spectrometry and thermal conversion/elemental analysis - isotope ratio mass spectrometry. As classically defined by C_nH_2n+ZO₂, the most abundant NAs detected in the majority of groundwater samples were straight-chain (Z = 0) or monounsaturated (Z = -2) C₁₆ and C₁₈ fatty acids. Several groundwater samples from both study areas, however, contained significant proportions of presumably alicyclic bicyclic NAs (i.e., Z = -4) in the C₁₀-C₁₈ range. These compounds may have originated from migrated waters containing a different distribution of NAs, or are the product of in situ microbial alteration of shale organic matter and petroleum. In most groundwater samples, intramolecular carbon isotope values generated by pyrolysis (δ¹³C_pyr) of AEOs were on average around 2-3‰ heavier than those generated by bulk combustion (δ¹³C) of AEOs, providing further support for microbial reworking of subsurface organic carbon. Although concentrations of AEOs were very low (<2.0 mg/L), the detection of potentially toxic bicyclic acids in groundwater overlying unconventional hydrocarbon reservoirs points to a natural background source of organic contaminants prior to any large-scale commercial hydrocarbon development.