Sudipa Mitra-Kirtley

The Electronic Absorption Edge of Petroleum

The electronic absorption spectra of more than 20 crude oils and asphaltenes are examined. The spectral location of the electronic absorption edge varies over a wide range, from the near-infrared for heavy oils and asphaltenes to the near-UV for gas condensates. The functional form of the electronic absorption edge for all crude oils (measured) is characteristic of the “Urbach tail,” a phenomenology which describes electronic absorption edges in wide-ranging materials. The crude oils all show similar Urbach widths, which are significantly larger than those generally found for various materials but are similar to those previously reported for asphaltenes. Monotonically increasing absorption at higher photon energy continues for all crude oils until the spectral region is reached where single-ring aromatics dominate absorption. However, the rate of increasing absorption at higher energies moderates, thereby deviating from the Urbach behavior. Fluorescence emission spectra exhibit small red shifts from the excitation wavelength and small fluorescence peak widths in the Urbach regions of different crude oils, but show large red shifts and large peak widths in spectral regions which deviate from the Urbach behavior. This observation implies that the Urbach spectral region is dominated by lowest-energy electronic absorption of corresponding chromophores. Thus, the Urbach tail gives a direct measure of the population distribution of chromophores in crude oils. Implied population distributions are consistent with thermally activated growth of large chromophores from small ones.

Determination of Sulfur Species in Asphaltene, Resin, and Oil Fractions of Crude Oils

Sulfur chemical species have been determined for asphaltenes, resins, and oil fractions of crude oils by using X-ray absorption near-edge structure (XANES) spectroscopy. The prevalent sulfur species are thiophene, sulfide, and sulfoxide. The asphaltene of one crude oil was known to have a very high sulfoxide content; the issue addressed here is whether the sulfoxide polar group is also dominant for sulfur in the resin and the (typically nonpolar) oil fractions of the same crude oil. Results from this study show that large sulfoxide fractions are obtained for all components of this oil. Another crude oil, lower in oxygen, also shows similar sulfur composition in all three fractions; nevertheless, the oil fractions of both crudes tend to have somewhat larger sulfide fractions.

Sulfur Chemical Moieties in Carbonaceous Materials

X-ray Absorption Near Edge Structure (XANES) spectroscopy has been employed to characterize the different chemical structures of sulfur in kerogens, asphaltenes, and coals. Commonalities are found for the sulfur chemistry in these disparate carbonaceous materials. Most of the sulfur is organic, with thiophenic structures typically the most abundant and sulfidic forms also fairly abundant. Oxidized organic sulfur in lesser amounts is found in low rank coals, in the different fractions of one crude oil, and Type I kerogens. In addition, there is pyrite in the coals and pyrite/elemental sulfur in the kerogens. The sulfur chemistry is shown to reflect the carbon chemistry in kerogens and coals. Type II kerogens have a larger ratio than Type I kerogens of aromatic to saturated carbon. Likewise higher rank coals also have a larger ratio of aromatic to saturated carbon than lower rank coals. Here, it is shown that Type II kerogens and higher rank coals similarly have larger fractions of aromatic sulfur. This important result establishes a relationship between the carbon and sulfur chemistry of these materials. The nitrogen chemistry of the carbonaceous materials was also investigated with XANES. In all of the materials, the nitrogen is almost entirely aromatic with pyrrolic nitrogen being the most abundant. The presence of sulfur in carbonaceous samples is often an impediment in the processing and utilization of fossil fuel resources. Sulfur oxides released into the environment during combustion of these materials have been a matter of concern for decades. A knowledge of the sulfur chemistry in carbonaceous materials helps in the mitigation of sulfur-induced problems in resource utilization. In addition, elucidation of sulfur structures in carbonaceous samples, such as kerogens, helps one understand the complex maturation processes of these materials over geological time. Chemical properties of reservoired hydrocarbon fluids are now being utilized to a much greater degree to address major issues in the production of oil and gas. A new look at the chemistry of these fluids and of their source materials is mandated. Sulfur also affects specific properties such as the solubility