1) Raman spectroscopy was used to study the thermal maturity of solid bitumen from a sample with low maturity (0.61% reflectance). However, initial measurements showed intense fluorescence that obscured the typical Raman bands. 2) Repeated measurements at the same location caused the fluorescence background to decrease over time, revealing the Raman bands more clearly without artificially altering the sample. 3) Multiple measurements allowed the fluorescent component to be isolated from the Raman spectrum, producing a clean spectrum free of background interference. This method provides a way to apply Raman spectroscopy to samples that normally exhibit strong fluorescence.

1. 5th International Geologica Belgica Meeting
Mons, Belgium, 26-29 January 2016
Introduction
It is a highly important task in the petroleum industry: determining the
thermal maturity of carbonaceous material (CM). Raman Spectroscopy has
beenrecognizedasausefultechnique,duetoitsmanyadvantages :
It is quick and easy to conduct and not constrained to specific types of CM .
For CM with moderate to high temperatures, there is a good correlation
between the Raman spectra and thermal maturity, eg . However, at low T
the spectrum is lost under an intense background radiation, caused by
fluorescence. In this study we tested this on a sample with low-mature solid
bitumen(0.61%Bro)andfoundapossiblesolutiontothefluorescence.
[1]
[2]
[3,4]
Raman spectroscopy uses a laser to irradiate a sample. Carbonaceous
material in the sample scatters energy back, resulting in a spectrum with
typical bands. Most important are the graphitic G-band around 1580-1600
and disordered D1-band around 1350 cm-1 . But these bands are not well
visible is our first measurements. See spectrum 1 (black line) in figure 1.This is
due to the high fluorescence covering the spectrum. However, we found that
by repeating the measurement at the same location the fluorescence
decreasesandatacertainpointthespectralbandsappearmoreclearly.
[5]
Beyond the current limits of Raman Spectroscopy
controlling fluorescence in solid bitumen with low thermal maturity
1,3
Marleen Bouman ,
2 1 3 3 3
Pieter Bertier , Rudy Swennen , Thomas Goovaerts , Yves Vanbrabant , Kris Piessens
1
Department of Earth and Environmental Sciences, KU Leuven, Celestijnenlaan 200E, 3001 Leuven, Belgium
2
Clay and Interface Mineralogy, Energy & Mineral Resources Group, RWTH Aachen, Bunsenstrasse 8, 52072 Aachen, Germany
3
Geological Survey of Belgium (Earth and History of life), Royal Belgian Institute of Natural Sciences, Jennerstraat 13, 1000 Brussels, Belgium
Figure 2: Close-up of measurement number 8 to 21. The FWHM of the bands stays constant
(indicated by the blue arrows). Also the area ratio Ad/Ag under the D1 and G-band for each
spectrumstaysthesame.
Notes added in proof
Results and Discussion
Inconclusion,byexecutingmultipleRaman-measurements :
1)Theunwantedfluorescencesignaldecreases,revealingmoreclearlythespectralbands.
2)Thebandparameters(position,Ad/Agd,FWHM)seemtostayintact,sonoartificialalterationtakesplace.
onthesamelocation
Figure 3: SEM map and corresponding image under the Raman
microscope.SEMwasusedtolocatecarboninordertodetermine
the right measurement locations (containing carbonaceous
material).Carbonispicturedinred.
Raman view, 50x
A spectrum can be described by
several parameters: band
position (cm-1), full width at half
maximum (FWHM), relative
intensity (height) and area. The
shapes of the G- and D-bands
relative to each other change
with maturity. Because we do
not want to alter the sample by
the continuous measurements
(irradiation), it is important that
the parameters stay intact.
Figure 2 shows this is likely true.
The FWHM (blue arrow),
positions and relative area
Ad/Ag(shaded)stayconstant.
Intensity
Raman shift (cm-1)
G-band
-11600 cm
D1-band
-11370 cm
For this study we used a polished section (provided by the RWTH Aachen),
containing solid bitumen with approximately 0.61% Bitumen Reflectance.
RamanspectraweretakenattheGeologicalSurveyofBelgiumwith:
-aSENTERRA(Bruker)spectrometer
-Olympusmicroscope,objective50x
-Laserwavelength532nm,aperture25*1000µm,resolutionof9-18cm-1
-Laserpower5mW, integrationtime10*6sforeachmeasurement
Opus 7.2 software was used to analyse Raman spectra and determine
manually the maturity parameters. After measurement no damage was
visibleonthesamplewithamicroscope.
To check the validity of the results automated tests with several hundreds
consecutivemeasurementswereperformedatseverallocations.
Confirmed:thefluorescencedecreasesstronglyunderongoingirradiation.
Adjustment: given sufficient measurements, a small shift in the band
parameterstakesplacethatshouldnotbeignored.
Instrumental settings
sf
This alteration is predictable
and can be corrected for. At
t h e s a m e t i m e , t h e
fluorescent part of the
spectrum can be isolated. A
clean spectrum can be
obtained free of background
fluorescence (figure 4).
However, this method
requires additional validation
before it can be presented in
detail.
References
Potgieter-Vermaak et al. 2011 Quirico et al. 2005 Zhou et al. 2014
Liu et al. 2013 Tuinstra and Koenig, 1970
[1] [2] [3]
[4] [5]
10
20
30
40
50
60
Intensity
0
1100 1150 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650 1700 1750 1800 1850
Figure 4: Close-up of a reprocessing of the measurements, correcting
for artificial sample maturation and using the consequent
measurements to determine and extract background due to
fluorescence.
Figure 1: Raman spectrum of consecutive measurements at the same location. During this, the
backgroundsignal(fluorescence)decreasesandbandsat1370and1600cm-1appear.Theupper
blacklineisthefirstmeasurement,thelowerlinesaremeasurements number8to21.
-1Raman shift (cm )
G-band
-11600 cm
D1-band
-11370 cm
consecutive
measurements
Intensity