Page 202 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 179

Fig. 5-27. Comparative fluorescence spectra of nine crude oils of different (from Purvis et al.,
1977)

Fluorescence

As an extension of light hydrocarbon gas analysis, UV fluorescence spectroscopy can
be used to measure the oil potential of near-surface sediments by analysing their
aromatic hydrocarbons. This is highly sensitive and selective method for the analysis of
oil components, particularly those containing one or more aromatic functional groups.
Using spectroscopic scanning, complex molecular aggregates, such as those found in
crude oils, can be rapidly characterized and quantified on the basis of their combined
intensity wavelength distribution or "fingerprint".
The fluorescence spectra of nine crude oils of different gravity are shown in Fig. 5-27
(Purvis et al., 1977). These two-dimensional fluorograms were produced by exciting at
265 nm and scanning from 250 nm toward the red end of the spectrum. The accepted
procedure for illustrating the change in the emission spectrum associated with different-
gravity crude oils is to measure the intensity of fluorescence at two wavelengths: 320
nm for light aromatic compounds; and 365 nm for the heavier, multiple-ring aromatic
compounds. The intensity of the fluorescence emission is proportional to the quantity of
aromatics in the extracted sample. The standard field method employs a rapid wet
extraction process which dissolves loosely-bound trace aromatics into hexane. This
extract generally favours the heavy oil fraction, which is in hydrophobic association with
the sediment.