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 (L2MS), 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 c...