STUDYING INDIVIDUAL MACERALS USING IR MICROSPECTROSCOPY, AND IMPLICATIONS ON OIL VERSUS GAS CONDENSATE PRONENESS AND LOW-RANK GENERATION

A reflectance FT-infrared microscope spectrometer has been employed to study the chemical structures of individual macerals including vitrinite, resinite, cutinite and alginite (Botryococcus braunii and Tasmanites), which are then used to infer the petroleum generation characteristics of the macerals. The optical microscope was modified so that it is equipped with fluorescent illumination, and it serves as a sampling accessory attached to the infrared spectrometer. The system is capable of measuring infrared spectra of individual macerals with cross sectional areas as small as 20 x 20 microns. At similar maturities, alginite and cutinite are shown to contain the highest concentrations of aliphatic C-H(x), whereas vitrinite contains the most aromatic C=C. A closer look at the stretching vibrations of the aliphatic C-H(x) region reveals further structural differences. Maximum likelihood spectral restoration of the stretching vibrations of the aliphatic C-H(x) bands is employed to calculate the intensities of individual C-H(x) bands. It is found that vitrinite and resinite have significantly higher terminal CH3 relative to methylene CH2 in comparison to alginite and cutinite. A lower CH2/CH3 ratio signifies shorter and more branched aliphatic chains, which can reduce the bond dissociation energies of a kerogen. This may be able to explain the ''low-rank'' gas/condensate generation characteristics which are exhibited by resinite and vitrinite macerals contained in source rocks of the Mackenzie and Mahakam Deltas. The oil- versus gas/condensate-proneness of a kerogen can also be assessed by using the CH2/CH3 ratio.