Impact of Laboratory-Induced Thermal Maturity on Asphaltene Molecular Structure

The structure of asphaltenes of various maturities prepared by semiopen pyrolysis of Green River Shale is measured by elemental analysis, laser desorption laser ionization mass spectrometry ((LMS)-M-2), surface assisted laser desorption ionization (SALDI) mass spectrometry, sulfur X-ray absorption near edge structure (XANES) spectroscopy, and infrared (IR) spectroscopy. These measurements demonstrate systematic changes in the composition of asphaltenes during thermal maturation. At low maturities, the evolution of asphaltene composition is dominated by changes in the heteroatoms: total sulfur as well as carbon oxygen, sulfur oxygen (sulfoxide), and aliphatic carbon sulfur (sulfide) bonds are lost, while the molecular weight increases. At high maturities, the sulfur content and speciation as well as molecular weight are relatively constant while the evolution in composition is dominated by changes in the carbon backbone: the abundance of aromatic relative to aliphatic carbon increases, the length of aliphatic chains shortens, and the abundance of aromatic C-H bonds increases greatly. The distribution of different carbon oxygen functional groups is relatively unchanged over the entire maturity range. These changes sometimes mirror and sometimes oppose compositional changes in the bitumen, suggesting that the composition of the asphaltene and maltene fractions of bitumen evolve differently. The observed changes in asphaltene structure are not fully independent of one another, as the composition of asphaltenes is constrained to maintain a balance of the strength of intermolecular forces to ensure solubility in aromatic solvent and insolubility in aliphatic solvent (the definition of asphaltenes). That constraint leads to a decrease in sulfoxide content (weakening intermolecular forces by reducing dipole interactions) concurrent with an increase in molecular weight (strengthening intermolecular forces). These trends in asphaltene composition with maturity are expected to occur in naturally occurring source rocks, such as some tight-oil formations, but not necessarily in conventional reservoir rocks where the asphaltenes escape from the source rock and enter the reservoir during maturation.