Oil Generation Capacity of Oil Shales as a Function of Aliphatic Structure

Craig P. Marshall¹ and Alison N. Olcott^2
1Chemistry, The University of Sydney, Sydney, NSW, Australia
²Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institute, Woods Hole, MA

Petroleum generation potential from source rocks, coals, and oil shales are principally dependent on the availability of hydrogen, primarily hydrogen in the macromolecular structure, and the aliphatic chain length. The longer the aliphatic chain length the greater the potential for generating and expelling liquid hydrocarbons during thermal maturation. A series of Carboniferous, Permian, and Devonian oil shales have been analysed using Fourier transform infrared (FTIR) micro-spectroscopy to asses their ability to generate oil. The objective of the study was to determine if there was a correlation between petrographic constituents and the aliphatic chain length determined by FTIR micro-spectroscopy. It was found that all the petrographic constituents contain intense aliphatic C-H stretching bands in the 3000-2700 cm^-1 region. However, the relative intensities and line-shapes of these bands vary among the different oil shales. Curve fitting of this aliphatic stretching region yields five bands corresponding to the CH₃ anti-symmetrical stretching vibration at 2955 cm^-1, the CH₂ anti-symmetrical stretching vibration at 2920 cm^-1, the C-H stretching vibration at 2890 cm^-1, the CH₃ symmetrical stretching vibration at 2865 cm^-1, and the CH₂ symmetrical stretching vibration at 2850 cm^-1. Subsequent curve fitting of the stretching vibrations of the aliphatic C-H region provides a greater insight into the aliphatic structures of the oil shales. The CH₂/CH₃ intensity ratio can be calculated and used to assess the relative average aliphatic chain length and the degree of branching and hence the oil generation capacities of oil shales.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas