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Petroleum, one of the most complex mixtures on Earth, has been extensively studied for decades. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and the ultrahigh resolution it provides, however, has dramatically transformed our understanding of petroleum by providing detailed molecular characterization. Nevertheless, many properties of crude oil, and the molecular characteristics that drive them, remain poorly understood, especially in regards to the role and impact of oxygen in oils. In the work presented here in-depth characterization of oxygen in petroleum is explored, to augment understanding of petroleum behavior due to oxygen. Although current FT-ICR MS instrument easily provides ultrahigh resolution, there is a constant for more as the complexity of petroleum means there is also some molecule just out of the obtainable analytical reach. Spectral segmenting on petroleum, in which narrow mass windows are acquired instead of all masses at once as in conventional broadband mass spectrometry, was performed in order to increase resolution and dynamic range to explore the species typically “missed” in conventional FT-ICR MS analysis. From the performance improvements provided by spectral segmenting, it was observed that molecules with high heteroatom content, including extensive oxygen content, are observed at low abundances, and that they may play a critical role in petroleum behavior despite their low abundances. Because of this the behavior of oxygen in petroleum is further explored. Naphthenic acids (carboxylic acids) in petroleum are known to be corrosive, yet no known correlation between naphthenic acid content and specific petroleum corrosivity has ever been established, other than more acidic crudes are generally more corrosive. In one proposed corrosion mechanism, a ketone is formed with a structure related to the structure of the corrosive acid. A method to detect and characterize these ketones in petroleum is developed and validated with model acids in an oil matrix, and then applied to more complex mixtures of naphthenic acids and acids derived from a vacuum gas oil to provide insight into which acids are most corrosive. Oxygen in petroleum plays an important role when petroleum is exposed to the environment. FT-ICR MS is used with traditional gas chromatography mass spectrometry (GC/MS) techniques to highlight how it is possible to use oxygen, in addition to other properties and heteroatoms, to track the relation of petroleum as it flows from reservoir to an oil seep and ultimately at the sea surface. In addition to highlighting petroleum subterranean connectivity, an initial characterization is obtained from which information about how petroleum transforms as it migrates is provided. Asphaltenes are perhaps the most poorly understood fraction of petroleum, with their molecular structures under intensive debate. Analysis by high resolution GC/MS of pyrolysis products of “classical” asphaltenes from bitumen and of “environmental” asphaltenes from a tarball is performed. It is found that high resolution is necessary for accurate characterization of even “simple” pyrolysis products, and that environmental asphaltenes are enriched in oxygen relative to the classical bitumen asphaltenes. The exact oxygen structures observe also provides insight into the weathering processes that the tarball underwent in its formation.