Molecular Composition of Middle Eastern Asphaltenes by Mass Spectrometry: Field vs Dead-Oil-Derived Deposits

Asphaltenes are complex mixtures of natural compounds that have proven to be notoriously difficult to analyze using routine methods. Here, a mechanically isolated asphaltene field deposit from an oil well obtained using a gauge cutter and dead oil-derived asphaltene from the same well in an offshore Abu Dhabi oilfield were analyzed by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). Atmospheric Pressure Photon Ionization (APPI), Laser Desorption Ionization (LDI), and Electrospray Ionization (ESI) were applied using a combination of positive (APPI, LDI, and ESI) and negative (ESI) ion modes, and the results were compared to bulk elemental ratios based on independent elemental analysis. Results indicate that the deposit was significantly enriched in sulfur relative to its dead-oil-derived counterpart. The latter was slightly enriched in nitrogenous species and also contained maltenes covering a wider compositional space than that of the field deposit. APPI resulted in marginally better agreement with bulk elemental analysis data for the field deposit and significantly better agreement for its dead-oil-derived counterpart. Whereas LDI generally preferentially ionizes organic nitrogen, the molar S/C ratio of the field deposit is better matched to the LDI-generated molar S/C ratio. We conclude that the optimal ionization method for mass spectrometric analysis is sample-dependent. Most importantly, we demonstrate that significant compositional differences exist between the dead-oil-derived sample and the deposit, raising concerns as to whether dead-oil-derived asphaltenes should be used in asphaltene-inhibitor evaluation studies.