OTC 4624
Applications of Total Scanning Fluorescence to
Exploration Geochemistry
by J.M. Brooks, M.C. Kennicutt II, L.A. Barnard, and G.J. Denoux, Texas A&M V., and
B.D. Carey Jr., Tenneco Oil Co.
Copyright 1983 Offshore Technology Conference
This paper was presented at the 15th Annual OTC in Houston, Texas, May 2-5,1983. The material is subject to correction by the author. Permission to
copy is restricted to an abstract of not more than 300 words.
ABSTRACT
A total scanning fluorescence technique is described for correlation (oil/oil and oil/source rock)
and surface geochemical prospecting studies.
The
fluorescence system acquires a total fluorescence
spectrum of emission, excitation and intensity for
wavelengths between 200 and 800 nm using a computer
controlled
UV-Spectrofluorometer
(Perkin-Elmer
650-40). The resulting matrix of intensity values,
obtained at specific emission/excitation wavelengths,
can be viewed in a three-dimensional or contour
presentation. Similarity indices are calculated to
compare spectra for
correlation studies.
The
method can also be used as a regional evaluation
tool for surface geochemical prospecting.
Shallow
surficial (>2 meters) sediments collected over oil,
condensate, and gas provinces exhibit distinct
fluorescence signatures.
INTRODUCTION
Fluorescence spectroscopy is a technique that
has had wide application in characterizing hydrocarbon mixtures. Ultraviolet (UV) fluorescence is inherently more selective for aromatic compounds than
conventional absorption measurements and at least an
order of magnitude more sensitive.
Fluorescence
methods are particularly useful for the detection and
measurement of organic compounds containing one or
more aromatic functional groups. A number of workers
have used fluorescence techniques to estimate petroleum hydrocarbons in marine waters 1 ,2,3,4,5 and sediments. 6 ,7
Since all oils contain a significant
amount of aromatic compounds, with one to four (or
more) aromatic rings and their alkylated analogues,
oils exhibit distinctive fluorescence "fingerprints".
These "fingerprints", used in conjunction with other
analyses, can provide significant information for
typing oils, shale extracts and sea bottom sediment
extracts.
Conventional fluorescence analyses have traditionally used fixed emission/excitation wavelengths
or fluorescence emission spectra (at a fixed excitation wavelength) to characterize aromatic mixtures,
References and illustrations at end of paper.
qualitatively and quantitatively.
Gordon et a1. 2
and Wakeham 7 used a synchronous scanning technique
developed by Lloyd 8 to "fingerprint" aromatic
compounds in marine sediments. The excitation and
emission monochromators are simultaneously varied
with the excitation wavelength offset 20-30 nm lower
than the emission wavelength in synchronous methods.
This technique was an improvement over simple
emission spectroscopy for analyzing complex mixtures because larger ring number aromatic compounds
generally fluoresce at successively higher excitation wavelengths. Synchronous scanning fluorescence
approximates ring size distributions for aromatic
mixtures.
Fixed wavelength and synchronous scanning
fluorescence suffer from non-selectivity and are
generally ineffective in structural elucidation of
mixtures.
Despite the ability to select both the
excitation and emission wavelengths, conventional
fluorescence methods have limited applicability and
are difficult to interpret because spectra of
complex mixtures cannot be satisfactorily resolved.
In an attempt to overcome these problems, a methodology for total scanning fluorescence was developed.
A three-dimensional, contour and tabular presentation of the data is possible.
A total scanning fluorescence system has several advantages over simpler scanning methods: (1)
the acquisition of multiple fluorescence spectra is
faster; (2) the amount of fluorescence data per
sample is greatly increased; (3) the stored data can
be extensively manipulated by computer; and (4) individual excitation spectrum can be retrieved from
the total fluorescence spectrum and analyzed for the
intensity and wavelength of maximum excitation
and/ or emission fluorescence.
Viewing the total
fluorescence spectrum, using a three-dimensional or
contour presentation, is a powerful and useful
"fingerprinting" tool.
HARD AND SOFTWARE
The Perkin-Elmer
(PE) 650-40 Fluorescence
Spectrophotometer {s automated via a "SCANR" software program.
The hardware system includes a
393
PE 650-40 Spectrofluorometer with RS232C Communications Interface, a PE Model 3500 or 3600 Data Station
with 64K memory and dual disk drives, and a PE Model
660 graphics printer. The "SCANR" software runs on
both PE 3500 and 3600 Data Stations and can provide a
hardcopy of the generated three-dimensional plots via
the PE 660 graphics printer. The matrix can also be
transferred to a mainframe AMDAHL computer via a
"TERM" program (PE copyright) for statistical analysis and contour presentation.
A typical threedimensional presentation is shown in Fig. 1. Also
shown is a contour presentation of the same matrix of
points produced by a graphics package of NCAR 9 , 10
subroutines for the AMDAHL system. 11
The data acquisition time for a 200 to 600 nm
grid for both emission and excitation, with 10 nm resolution, is approximately 30 minutes.
The total
fluorescence excitation/emission wavelength array is
filled for each sample by sequentially stepping the
excitation monochromator over the wavelength range of
interest and scanning with the emission monochromator.
Programming flexibility permits acquisition
speeds of from 60 to 450 nm/min on each monochromator.
Up to 2500 discrete intensity values may be
acquired per spectrum and 2 nm resolution may be
attained at slower scan speeds. After acquisition,
the data array is permanently stored on a 5~" microfloppy diskette. The data can be transformed via a
three-dimensional conversion routine adapted from
Gottlieb 12 and a spectral plot generated with a
graphics package (PE copyright).
EXTRACTION AND GRAVIMETRIC ANALYSIS
Approximately 20 grams of well cuttings or sediment are required for analysis. A subsample is lyophilized, ground to a uniform size with a mortar and
pestle, and Soxhlet extracted for 12 hours in a 100 %
hexane solvent system.
All glassware and alundum
thimbles are precleaned with Micro Cleaning Solution,
washed with nanograde solvents, and combusted at
500°C for at least 4 hours. The extracts are concentrated, using a Buchii Rotovapor R, to a volume of
about 5 mI. Care is exercised at all times to ensure
that the extract is not brought to complete dryness
to prevent volatilization of lighter sample components. The volume of the extract is brought up to 7
ml and stored at 4°C in the dark until further analysis.
This treatment minimizes photolytic losses
and chemical interactions between the extracted compounds. A total system blank is routinely run for
every set of samples processed and checked by both
fluorescence and gas chromatography to ensure acceptable blank levels. Oil samples are generally directly diluted with hexane, although the oil can be
Soxhlet extracted if a direct comparison with source
rock or sea bottom sediment extracts is desired.
compounds illustrates the molecular variability in
excitation/emission spectra with structure (Fig. 2).
The fluorescence signatures of four typical oils
are shown in Fig. 3.
A statistical comparison of fluorescence in two
samples can be made using a Pearson product-m.oment
correlation coefficient calculation that provides a
point to point comparison producing a similiarity
index (S. I.) . Two examples using fluorescence in
oil/oil correlations are presented.
The first
example is ten pelagic tars collected in the South
Atlantic Ocean and the second example is nine oils
from the USGS North Slope of Alaska Intercomparison
Study. These examples demonstrate the usefulness of
fluorescence as a correlation tool.
It should be
noted that fluorescence is only one of many techniques used to generically type oils, and at all
times more than one parameter must be evaluated in
order for proper correlations to be made.
Three distinct types of tars were delineated in
a group of sea surface oils collected in the western
South Atlantic Ocean. The tars were grouped based
on their fluorescence similarity indices, i.e. S.I.
;;:0.90 (Table 1, Fig. 4). Type I tars occurred at
low concentration levels «0.01 mg/m 2 ) and appear to
represent low-level chronic oil pollution. Type II
tars were less degraded based on gas/liquid chromato~raphy, occurred at higher concentrations (>0.01
mg/m ), and were found in more coastal-influenced
waters.
These groupings were confirmed by carbon
isotopic compositions, molecular compositions (gas
chromatography) and biological marker "fingerprints"
(steranes and triterpanes), except in one case where
the tar appeared to be a mixture of Types I and II.
The third tar type was a single sample that was
significantly different by all parameters measured
and was probably due to a localized event (i.e., a
spill, tanker washings, etc.).
As part of the USGS North Slope of Alaska Intercomparison Study, nine oils were analyzed for a
variety of parameters in order to type the oils.
The majority of the oils were two basic types as
defined by total scanning fluorescence, with two
other light condensate oils being distinctly different from these two types (i.e., samples 006 and 024;
Table 2, Fig. 5). The two basic oil groups (Types I
It is
and II) were very similar (S. 1. ~ 0.8).
probable that Type II oils are different from Type I
oils due to varying proportions of co-sources and
biodegradation.
All Type II oils were highly
degraded as determined by gas chromatography.
Isotopic and biomarker "fingerprints" supported
these oil groupings.
In conclusion, two closely
related types of oils were present plus two
distinctly different condensate fluids.
APPLICATION AS A CORRELATION TOOL
As part of the North Slope Study, fifteen shale
samples were also analyzed for source rock/oil correlations.
Based on fluorescence, other chemical
parameters and geological considerations, three
conclusions were arrived at: 1) the Torok formation
was infiltrated with deeper sourced oil, primarily
from the Pebble Shale Unit; 2) Type I oils were predominantly sourced in the Kingak shale with contributions from the Pebble Shale Unit and minor contributions from the Shublik formation; and 3) Type II
oils were predominantly sourced in the Pebble Shale
Unit with contributions from the Kingak Shale and
again possible minor contributions from the Shublik.
The total scanning fluorescence technique has
been successfully used as both an oil/oil and an oil/
source rock correlation tool. The complex mixture of
aromatic compounds in oils, source rock extracts and
sea bottom sediment extracts produce characteristic
fluorescence "fingerprints".
Fluorescence "fingerprints" differ because the aromatic composition of
oils are highly variable and different aromatic compounds produce fluorescence at significantly different excitation and emission wavelengths.
The
fluorescence signature of several authentic aromatic
394
-
----- ----- ------ ----------
-: --=-::"'-,..--..------
-~-----=
Oil 006 had no source in the samples analyzed and oil
024 had a source similar to Type II oils, but
appeared to have a higher contribution of Shublik
sourced oil.
APPLICATION IN
GEO~HEMICAL
istic of oil and gas/condensate regions.
CONCLUSIONS
Total scanning fluorescence is a useful correlation technique for grouping oils and evaluating
oil/source rock relationships.
It no doubt has to
be used in conjunction with other correlation tools.
However, it appears to be an inexpensive, fast, and
effective correlation tool.
The technique is
another parameter that can be used in geochemical
prospecting to differentiate oil and gas prone
offshore areas. Sea bottom sediments sampled over
oil and gas areas appear to have distinctive fluorescence signatures with gas/condensate zones characterized by two ring aromatic signatures and sediments
overlying oil reservoirs characterized by three to
five ring aromatic signatures.
Total scanning
fluorescence may also have application to soil
geochemical prospecting.
PRO~PECTING
Surface geochemical prospecting for oil and gas
has had a long history of use in the petroleum industry and has been received with varying degrees of
acceptance. Most work, to date, has been based on
variations of the soil gas techniques pioneered by
Horvitz 13 '14 using fixed wavelength fluorescence and
the molecular compositions of gases. Early studies
often used questionable sampling and analytical
techniques which lead to ambiguous results. Recent
publications by Stahl et a1. 15 and Richers et a1. 16
have confirmed that hydrocarbons do migrate to the
surface over petroleum deposits and that surface
manifestations of deeper accumulations can be used to
assess the potential of a given area. These soil gas
techniques have been extended to sea bottom sediments
and combined with other parameters such as carbon
isotopic analysis of the methane 15 and total scanning
fluorescence.
ACKNOWLEDGEMENTS
The authors wish to acknowledge the assistance
of Tenneco Oil Company for providing much of our
initial fluorescence instrumentation and for supporting our application of fluorescence to geochemical exploration.
Fixed wavelength and single wavelength scanning
fluorescence has been used extensively in geochemical
exploration. Kartsev 17 , in the Russian literature,
describes a prospecting technique using fluorescence,
and more recently Hebert 18 has reviewed the use of
fluorescence as a geochemical prospecting tool.
Fluorescence methods have had limited success as surface prospecting tools, due no doubt to the limited
fluorescence data obtained by conventional techniques
and to interfering substances.
REFERENCES
1.
2.
Data obtained with the total scanning fluorescence technique described here indicates that one can
distinquish oil and gas/condensate signatures in sea
bottom sediments that reflect deeper accumulations of
these hydrocarbons.
As a geochemical prospecting
tool this method is useful because of its specificity
for aromatic hydrocarbons.
The hexane extraction
used removes primarily non-polar compounds from the
sediments, of which aromatic hydrocarbons are generally the major components which strongly fluoresce.
The technique detects aromatics in sediments which
result from the upward migration of subsurface petroleum accumulations and from petroleum pollution
occurring at the surface.
Very few aromatic compounds are known to be produced in situ or deposited
by biogenic agents in shallow recent sediments.
3.
4.
5.
Our studies have established several prerequisites for attaining useful fluorescence data in
surface geochemical prospecting for area evaluation:
1) deep penetration of the sediment column to obtain
non-anthropogenically contaminated samples (>2 meters
in many areas); 2) multiple samplings within the
sediment column to minimize intra-core variability;
3) good areal coverage of the investigation area; 4)
complimentary
geophysical
evidence
to evaluate
migration pathways; and 5) total fluorescence spectra
to differentiate oil from gas/condensate areas.
Fig. 6 shows a typical oil signature from a sea
bottom sample obtained in the Buccaneer Oil and Gas
Field located ca. 50 kID south southeast of Galveston,
Texas. The sample was not affected by surface oil
pollution. Fig. 7 shows a condensate signature from
6.
7.
8.
9.
a sea bottom sample obtained offshore South Texas in
ca. 50 m of water.
Levy, E.M.:
"The Presence of Petroleum Residues off the Coast of Nova Scotia, in the Gulf
of St. Lawrence and the St. Lawrence River,"
Water Res. [1971] 5, 723-733.
Gordon, D.C., P.D.-Keizer, and J. Dale: "Estimates using Fluorescence Spectroscopy of the
Present State of Petroleum Hydrocarbon Contamination in the Water Column of the Northwest
Atlantic Ocean," Mar. Chern. [1974] 2, 243-256.
Keizer, P.D., D.C. Gordon, Jr., and J. Dale:
"Hydrocarbons in Eastern Canadian Marine Waters
Determined by Fluorescence Spectroscopy and
Gas-Liquid Chromatography," J. Fish. Res. Bd.
Can. [1977] 34, 347-353.
Gordon, D.C.~P.D. Keizer, and J. Dale: "Temporal Variations and Probable Origins of Hydrocarbons in the Water Column of Bedford Basin,
Nova
Scotia,"
Estuarine Coastal Mar. Sci.
[1978] 7, 243-256.
Law, R:-J.:
"Hydrocarbon Concentrations in
Water and Sediments from U.K. Marine Waters
Determined
by
Fluorescence
Spectroscopy,"
Mar. Pollut. Bull. [1981] 12, 153-157.
Hargrave, B.T., and G.A. Phillips: "Estimates
of Oil in Aquatic Sediments by Fluorescence
Spectroscopy,"
Environ. Pollut.
[1975]
~,
193-215.
Wakeham,
S.G.:
"Synchronous
Fluorescence
Spectroscopy and its Application to Indigenous
and Petroleum-Derived Hydrocarbons in Lacustrine Sediments," Environ. Sci. Technol. [1977]
11, 272-276.
Lloyd,· J.B.F.:
"Synchronized Excitation of
Fluorescence Emission Spectra," Nature [19711
231, 64-65.
------Adams, J.C., A.K. Cline, M.A. Drake, and R.A.
Sweet:
"NCAR Software
Support Library,"
TN/lA-lOS, NCAR Technical Note [1975] Vol 1,
Chap 2.
10. Adams,
These two patterns are character-
J.e.,
and R.A. Rotar:
"NCAR Library
Routines Manual," TN/IA-67, NCAR Technical Note
395
Richers, D.M., R.J. Reed, K.C. Horstman, G.D.
Michels, R.N. Baker, L. Lundell, and R.W.
and Soil-Gas Geochemical
“Landsat
Marra:
Study of Patrick Draw Oil Field, Sweetwater
k. Assoc. Petrol. Geol.
Wyoming,”
County,
Bull.
[1982] @
903-922.
“Geochemical Methods of Pro17. Kartsev, A.:
specting and Exploration for Petroleum and
Natural Gas”, Translated by P-A. Witherspoon
and W.D. Romney, Univ. of California Press,
Berkeley [1959] Chap. VIII.
18. Hebert, C.: “Geochemical Prospecting for Oil
and Gas. Usin~ Hydrocarbon Fluorescence Tech‘;In sfipo~ium on Unconventional Methods
in Exploration for Petroleum and Natural Gas,
Southern Methodist University,
16.
[1975] 5.1-5.47.
“Computer Graphics Software for the
Reid, T.:
AMDAHL,” Data Processing Center, Texas A&M
University [1980] 203 pp.
12. Gottlieb, M.: “Hidden Line Subroutines for Three
[1978] ~, 49-58.
Dimensional Plotting,” ~
“On Geochemical Prospecting,”
L.:
13. Horvitz,
Geophysics [1939] $ 210-228.
““Near-Surface Hydrocarbon and
L.:
14. Horvitz,
Petroleum Accumulation at Depth,” Mining Eng.
[Dec. 1954] 3-7.
15. Stahl, W., E. Farber, B.D. Carey, and D.L.
Kirksey: “Near-Surface Evidence of Migration of
Natural Gas from Deeo Reservoirs and Source
[1981]
Am. Assoc. Pe~rol. Geol. Bull.
Rocks,”
~, 1543-1550.
11.
~
Table 1.
Station
1
1/2
2
2/3
3
3/4
5
5/6
6
7
Similarity Index Based on Total Scanning Fluorescence for Tar Balls in the
South Atlantic Ocean.
1
1/2
2
2/3
3
3/4
1.00
1.00
0.93
1.00
0.79
0.68
1.00
0.93
0.93
0.87
1.00
0.85
0.78
0.97
_
0.93
1.00
5
0.73
0.62
0.94
_
0.77
0.90
1.00
5/6
0.31
0.22
0.32
0.26
0.28
0.31
1.00
6
7
0.92
0.97
_
0.79
0.97
_
0.87
0.27
0.27
1.00
0.76
0.65
0.96
m
0.92
_
0.98
_
0.31
0.76
1.00
Stations 2/3, 3/4, 5, 7
Type I:
Type II: Stations 1/2, 2, 3, 6
Type 111: Station 5/6
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INTENSIN MAXIMUM DF T520 AT AN EMISSION WAVELENGTH
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DF 4XINM AND AN EXCITATION WAVELENGTH
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from the
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T4796
14.99 GR SEDIMENT EXTRACTED
125
[
~1
t,,
/“,!
tloo :
175
f-$..,,
y,,
Intensity
Maximum Of 121 At An Emission
And An Excitation
Wavelength
Fig. 6—A typical
oil signature
of a sea bottom sediment
Wavelength
Of 330nm.
extract
~
of 370nm
over Buccaneer
oil and gas field.
t\,.
1,
‘\
i ~ II\,..,p,
~, “’,,, !,, \,
> \i,,
11 \, ‘~, i, ‘i\,
(.,
(
!
//
}Jj
it, ‘~,,‘\ ‘f\,
y,
,.,, ‘,, $!,~,‘;,
[ t. ‘, i ~.+-
600200
INTENSITY MAXIMUM
RI (365/320)= .47
OF 278 AT AN EMISSION WAVELENGTH
Fig. 7—A typical
gas/condensate
signature
OF 291)NM AND AN EXCITATION
of a sea bottom sediment
extract
from offshore
WAVELENGTH
south Texas.
OF 270 NM