Detection of Porphyrin Features in Visible and Near-Infrared Reflectance Spectra of Oil Shales

Peter Newhall Holden, Michael J. Gaffey

Features caused by porphyrin organo-metallic compounds, important biomarkers in oil to oil and oil to source rock correlations, have been detected in the whole-rock visible and near-infrared reflectance spectra of various oil shales.

Visible and near-infrared reflectance spectroscopy is a remote sensing and laboratory technique that has been primarily used to determine mineralogy and trace-element chemistry of rock bodies. Recent investigations using oil shales in the laboratory have shown that absorption features due to nickel and vanadium porphyrin organic ring structures are detectable in the whole rock reflectance spectra even at very low concentrations (1 ppm). The predominant feature seen is the d-pi transition near 0.4 µm. Two smaller features are also seen near 0.55 µm due to d-shell transitions in the complexing metal ion. All of these features shift, depending on the complexed metal ion making nickel porphyrins distinguishable from vanadyl porphyrins.

The relative ease and economy of this technique combined with the fact that nickel and vanadyl porphyrins are indicators of depositional environment create some interesting possibilities with regard to the evaluation of oil shales as oil source rocks. It may be possible to analyze core samples of oil shales in the field or to identify significant outcrops of oil shales and their type from aerial surveys or to simplify an existing laboratory procedure for oil shale evaluation.

The porphyrin features seen at 0.4 and 0.55 µm are the first features due to individual organic species identified in visible and near-infrared reflectance spectra. The porphyrin class is an important biomarker as an indicator of depositional environment and as an oil to oil and oil to source rock correlator.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.