(24e) The Compositional Continuum of Petroleum: Detailed Molecular Characterization of Heavy Crude Oils and Asphaltenes by Ultrahigh Resolution FT-ICR Mass Spectrometry

The world's current and future dependence on fossil fuels will shift the types of materials slated for production. Light, sweet crude will diminish as their reserves approach exhaustion and the production of heavier, more heteroatom-rich crudes will increase to meet world demand. As petroleum producers increase oil production to meet the world's growing energy needs, they will inevitably access an ever-widening quality range of petroleum materials (i.e., bitumen and heavy oils) and tap into deeper and deeper offshore reservoirs. Canada has continuously ramped up oil sand mining in Alberta over the past two decades to surpass the Middle East as the primary supplier to the United States. Richer in heteroatom content and present in severe environments (temperature and pressure), these materials pose major flow assurance challenges. Thus, detailed information on oil composition is paramount in understanding its reactivity and production behavior. Oil companies sell molecules and characterization at the molecular level is vital to determine how an oil will behave during production. However, the inherent complexity of crude oil makes it difficult to provide compositional information on even the most likely culprits in common production and refining problems. In fact, many of the most common characterization techniques are based on bulk properties or solubility. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) allows detailed characterization of complex petroleum samples at the level of elemental composition assignment. Ultrahigh resolution (450,000-700,000 at m/z 500) enables identification of isobaric species that differ in mass by 3 milliDalton or less, and high mass accuracy (better than 300 ppb mass error) allows for unambiguous molecular formula assignment to each of more than ten to forty thousand peaks in each mass spectrum. Thus, it is now possible to identify, sort and monitor simultaneously thousands of elemental compositions as a function of boiling point as well as provide a comprehensive analysis of non-boiling species.

Proposed over 20 years ago, the Boduszynski model describes the evolution of petroleum composition as a function of atmospheric equivalent boiling point [1]. Based on his model, Boduszynski concluded that ?most of petroleum components do not exceed a molecular weight of about 200.? He acknowledged that the results are controversial, ?These findings are significant because of the existing controversy over whether there is an appreciable concentration of molecules in petroleum having molecular weights greater than 2000 Da. Data show there is not.? However, a definitive proof of Boduszynski's model requires direct, complete compositional characterization of complex distillate cuts unavailable at that time. If substantiated, the Boduszynski model would impose strict limits on molecular weight distribution for both distillable and nondistillables petroleum fractions that contradict many previously published assertions about petroleum molecular weight and composition.

Here, we present our cumulative efforts in heavy oil and asphaltene characterization by (+/-) electrospray and atmospheric pressure photoionization FT-ICR mass spectrometry; summarize class, type, and molecular weight trends for distillable/nondistillable petroleum species; and compare results to proposed asphaltene structural models. In addition, structural characterization of nickel and vanadyl porphyrins on from an unfractionated asphaltene will be presented in a single mass spectrum for the first time. Detailed characterization of an Arabian heavy crude oil distillation series defines maltene and asphaltene compositional space (aromaticity and carbon number), and calculation of the hydrogen to carbon ratio (H:C) bounds the distillable space for all identified classes, revealing that projection of the compositional continuum to high carbon number cannot account for bulk H:C ratios. Chromatographic fractionation of crude oils from various geographic regions with varying total acid number and heteroatom content followed by subsequent mass spectral characterization reveals compositional differences and provides further insight into the separation and characterization of nonpolar and polar species in crude oils. Characterization of entrained species in asphaltene aggregates will be discussed. Compositional information for distillables and nondistillables and its implications for asphaltene aggregation will be discussed in detail.

References [1] Boduszynski, M. M.; Altgelt, K. H., Composition and Analysis of Heavy Petroleum Fractions. CRC Press: New York, NY 1994.

Keywords: APPI, petroleum, Fourier Transform, Ion Cyclotron Resonance, ICR, FT-ICR, FTMS