(1) Assessing the origin of oil spills and petroleum soil
contamination.
After the Exxon Valdez spill, geochemical analyses (by Exxon) of
shoreline oil residues in the Gulf of Alaska revealed that some of
the oil residues were not Alaskan crude from the Valdez, but rather
were California-derived oil that had been spilled in the Gulf of
Alaska at a much earlier date (Bence et al., 1995, 1996). This
example illustrates how the origin of an oil spill can be either
constrained or pinpointed by sophisticated chemical analyses that
distinguish between various oils. (Wang et al, 2006). We utilize
several approaches to determine the origin of oil:
- Whole oil gas chromatography, "Oil
Fingerprinting"*, of the spilled oil and of all of the produced
or pipeline oils in the immediate area of the spill can be used to
identify the origin of the spill (Sundararaman and Udo, 1998;
Staniloae et al., 2001). This approach requires collection and
analysis of all nearby potential source oils for the spill. When a
positive correlation of spilled oil and facility oil occurs, then
the origin of the spill has been determined. If the spilled oil has
undergone weathering, water washing, and/or biodegradation,
then biomarker
analysis and/or other techniques may be required to identify
the spill source, as described below.
- Biomarkers, molecular fossils
present in an oil, reflect the type and age of the source rock
that generated that oil. Specifically, biomarker distributions in
an oil reflect (1) the relative abundance of oil-prone vs.
gas-prone organic matter in the source, (2) the source rock age,
(3) whether the source was deposited in a marine, lacustrine,
fluvio-deltaic or hypersaline setting, (4) whether the source
lithology was a carbonate or shale, and (5) the thermal maturity at
which the source rock generated that oil (e.g., Peters and
Moldowan, 1993). Oils from different basins have different
biomarker distributions. Since different potential sources of a
spill may involve oil derived from different basins, biomarker
distributions can be used to either rule out or rule in
potential sources of a spill, and can be used to determine if oil
in a contaminated area actually represents more than one spill
(Stout et al., 2000, 2001). Biomarkers can also be used to assess
the origin of some refined hydrocarbon products (Peters et al.,
1992; Stout et al., 2005).
- Polycyclic aromatic hydrocarbons (PAH) are another group of
compounds present in oil that are especially useful in identifying
the source(s) of a spill. A subset of the PAH in oil are products
of the diagenesis of steroid, diterpenoid, triterpenoid and
hopanoid biological molecules originally deposited in sediments
(biomarkers). Several of these biomarkers are present as fully or
partially aromatized compounds with multiple aromatic rings;
therefore they are polycyclic aromatic hydrocarbons. These PAH are
resistant to biodegradation conditions typically encountered in
spill situations and have proved useful for defining a unique
fingerprint characteristic of a given oil; this fingerprint can be
used to correlate a biodegraded oil to a sample of its non-degraded
equivalent, and hence can be use to identify the source(s) of a
petroleum release (e.g., Burns, 1997; Stout et al., 2000,
2001).
- Other (non-biomarker) PAH, such as phenanthrene, alkyl
phenathrenes, pyrene, benz(a)anthracene, and similar compounds may
undergo biodegradation and weathering during post-spill conditions.
Analyses for these compounds are useful during the early stages of
an oil spill for source identification (Stout et al., 2000, 2001)
and at later stages for determining the extent of weathering and
biodegradation and the rates of several of the weathering and
biodegradation processes. (Lima et al., 2006; Reddy et al., 2002;
Farrington et al., 1982).
- Different oils commonly have different carbon isotopic
compositions. Therefore, carbon isotopic analyses of petroleum
samples from a contaminated area frequently can be used to
constrain the source of the contaminant and to determine if there
is more than one source of oil in a contaminated area (e.g., Bence
et al., 1996).
(2) Assessing the origin of gas seeps
Natural gas has two primary origins: (1) methane produced by
methanogenic bacteria (biogenic gas), and (2) hydrocarbon gas
produced by thermal alteration of sedimentary organic matter
(thermogenic gas). Thermogenic gas may or may not be co-genetic
with oil. Unlike thermogenic gas, biogenic gas is always very dry:
it does not contain significant ethane, propane or
higher-molecular-weight (i.e., "wet" gases). In addition, biogenic
methane contains isotopically lighter carbon (i.e., is more
depleted in 13C) than does thermogenic methane. As a result, geochemical analyses
can readily reveal if a gas seep represents thermogenic gas, or
whether it represents biogenic gas, such as forms from natural
degradation of soil organic matter or landfill material (e.g.,
Coleman et al., 1995; Schoell, 1983, 1984; Schoell et al.,
1993).
Microorganisms biodegrade different classes of compounds in
petroleum at different rates (e.g., Figure 3.62 in Peters and
Moldowan, 1993). As a result, the progressive biodegradation of an
oil spill can be monitored by periodic analyses of various
compounds in the oil-contaminated soil (e.g., Moldowan et al.,
1995; Bence et al, 1996). The early stages of oil biodegradation
(loss of paraffins and isoprenoids) can be readily detected by gas
chromatographic (GC) analysis of an oil. However, in heavily
degraded oils, GC analysis alone cannot distinguish subtle
differences in biodegradation due to interference of the unresolved
complex mixture (UCM or "hump") that dominates the GC traces.
Fortunately, in heavily degraded oils, one can use gas
chromatography-mass spectrometry (GC-MS) to quantify the
concentrations of biomarkers with differing resistances to
biodegradation (e.g., Moldowan and McCaffrey, 1995), allowing the
extent of biodegradation to be monitored over time. Recently, the
application of GC-GC (comprehensive two-dimensional gas
chromatography-gas chromatography) has been shown to be capable of
quantitatively resolving many of the compounds in the UCM and
providing useful information about weathering and biodegradation,
and will certainly prove useful in oil spill "fingerprinting"
(Freisinger and Gaines, 2001; Reddy et al, 2002). In addition, it
has been shown that several of the compounds now resolved from the
aromatic hydrocarbon UCM by GC-GC are toxic in laboratory toxicity
tests and may have deleterious effects in some oil spill situations
(Booth et al, 2007).
In an oil, the quantity of a biomarker that is resistant to
biodegradation increases as the oil is biodegraded, because such a
compound is "concentrated" in the oil by the loss from the oil of
the other less-resistant compounds. Therefore, by comparing the
concentration of such a resistant compound in a spill with the
concentration of the same compound in the original oil, one can
estimate how much of the oil has been degraded. For example, Prince
et al. (1994) used the concentration in oil of
17a(H),21b(H)-hopane, a biomarker which is relatively resistant to
biodegradation, to estimate the extent of biodegradation of
oils.
For more information on the geochemical techniques described
here, or to discuss a specific project, e-mail us at info@oiltracers.com, or call
us at (214) 584-9169.
* The term "Oil Fingerprinting" came into popular use during
the late 1960s and early 1970s with the application of gas
chromatography to analyses of spilled oil and potential sources. It
was a useful analogy to explain this type of forensic analyses for
spilled oil. However, it was recognized then, and remains true
today, that the analyses of spilled oils do not have the
statistical discriminating power of the human fingerprint in the
sense that each human has an individual fingerprint. Analyses of
spilled oils and potential sources are usually undertaken by
increasingly sophisticated chemical analyses until either all but
one potential source oil remains that cannot be distinguished from
the spilled oil, or all potential sources have been eliminated and
the spill is then a "mystery". The presumption for success using
fingerprinting is that a complete collection of possible sources
has been secured for the matching analyses. The term "passive
tagging" has been used in place of fingerprinting in the past to
describe the chemical analyses of oils. The term derives from the
process of using the chemicals naturally present in the oil as
"tags". The "passive" part of the term was used because there were
proposals and some experiments conducted in the late 1960s and
early 1970s to introduce "active tags" into various oil cargos to
allow for identifying the oils if they were spilled (e.g. see
Adlard, 1972; Zafiriou et al, 1973). Various chemicals were
proposed as active tags, but the obvious international
administrative and logistical effort needed to keep track of such
"active tags" prevented operational use of active tagging
systems.
References
ASTM (American Society for Testing and Materials), 1990a,
Standard Practice for Oil Spill Identification by Gas
Chromatography and Positive Ion Electron Impact Low Resolution Mass
Spectrometry; ASTM Designation D-5739-95: W. Conshohocken, PA, USA.
ASTM (American Society for Testing and Materials), 1990b, Standard
Test Methods for Comparison of Waterborne Petroleum Oils by Gas
Chromatography ; ASTM Designation D-3328-90: W. Conshohocken, PA,
USA.
Adlard, E.R. (1972). "Review of Methods for Identification of
Persistent Hydrocarbon Pollutants on Seas and Beaches." J. Inst.
Petrol. 58(560):63-74.
Bence, A. E. and W. A. Burns (1995). "Fingerprinting
Hydrocarbons in the Biological Resources of the Exxon Valdez Spill
Area." American Society for Testing and Materials: 84-140.
Bence, A. E., K. A. Kvenvolden, et al. (1996). "Organic
Geochemistry Applied to Environmental Assessments of Prince William
Sound, Alaska, after the Exxon Valdez Oil Spill- a review." Organic
Geochemistry 24(1): 7-42.
Booth, A. M., P. A. Sutton, C. A. Lewis, A. C. Lewis, A.
Scarlett, W. Chau, J. Widdows, and S. J. Rowland (2007). Unresolved
complex mixtures of aromatic hydrocarbons: thousands of overlooked
persistent, bioaccumulative, and toxic contaminants in mussels.
Environ. Sci. Technol. 47, 457-464.
Brodskii, E. S., and S. A. Savchuk, 1998, Determination of
petroleum products in the environment: J. Anal. Chem., v. 53, p.
1070-1082.
Burns, W. A., P. J. Mankiewicz, A. E. Bence, D. S. Page, and K.
R. Parker, 1997, A principle component and least squares method for
allocating polycyclic aromatic hydrocarbons in sediment to multiple
sources: Environ. Toxicol. Chem., v. 16, p. 1119-1131.
Coleman, D. D., C.-L. Liu, et al. (1995). "Isotopic
Identification of Landfill Methane." Environmental Geosciences
2(2): 95-103.
Farrington, J.W., B.W. Tripp, J.M. Teal, G. Mille, K. Tjessem,
A.C. Davis, J.B. Livramento, N.A. Hayward and N.M. Frew (1982).
Biogeochemistry of aromatic hydrocarbons in the benthos of
microcosms. Toxicology and Environmental Chemistry, 5:331-346.
Frysinger, G. S. and R. B. Gaines (2001). Separation and
identification of petroleum biomarkers by comprehensive two
dimensional gas chromatography. J. Separation. Science. 24:
87-96.
Henry, C. B., P. O. Roberts, and E. B. Overton, 1997, Advancing
forensic chemistry of spilled oil: self-normalizing fingerprint
indexes, Proceedings of the International Oil Spill Conference, p.
936-937.
Lima, Ana L. C., John W. Farrington, and Christopher M. Reddy
(2005). Combustion-Derived Polycyclic Aromatic Hydrocarbons in the
Environment - A Review. Environmental Forensics 6:109-131.
Moldowan J. M. and McCaffrey M. A. (1995). A novel hydrocarbon
degradation pathway revealed by hopane demethylation in a petroleum
reservoir. Geochimica et Cosmochimica Acta, 59(9), 1891-1894.
Moldowan J. M., Dahl J. E., McCaffrey M. A., Smith W. J., and
Fetzer J. C. (1995). Application of biological marker technology to
bioremediation of refinery by-products. Energy & Fuels, 9(1),
155-162.
Peters, K. E., G. L. Scheuerman, et al. (1992). "Effects of
refinery processes on biological markers." Energy and Fuels(6):
560-577.
Peters, K. E. and J. M. Moldowan (1993). The Biomarker Guide,
Interpreting molecular fossils in petroleum and ancient sediments,
Prentice Hall.
Prince, R. C., D. L. Elmendorf, J. R. Lute, C. S. Hsu, C. E.
Haith, J. D. Senius, G. J. Dechert, G. S. Douglas, and E. L.
Butler, 1994, 17a(H),21b(H)-hopane as a conserved internal marker
for estimating the biodegradation of crude oil: Env. Sci. and
Technol., v. 38, p. 142-145.
Reddy, C. M., T. I. Eglinton, A. Hounsel. H. K. White, L. Xu, R.
B. Gaines, G. S. Frysinger (2002). The West Falmouth Oil Spill
after thirty years: The persistence of petroleum hydrocarbons in
marsh sediments. Env. Sci.and Technol. v. 36: 4754-4760.
Sauer, T. C., and A. D. Uhler, 1994, Pollutant source
identification and allocation: advances in hydrocarbon
fingerprinting: Remediation, Winter 1994-1995, p. 25-50.
Stout, S. A., W. P. Naples, A. D. Uhler, K. J. McCarthy, L. G.
Roberts, and R. M. Uhler, 2000, Use of Quantitative Biomarker
Analysis in the Differentiation and Characterization of Spilled
Oil: SPE Paper No. 61460.
Stout, S. A., A. D. Uhler, and K. J. McCarthy, 2001, A Strategy
and Methodology for Defensibly Correlating Spilled Oil to Source
Candidates: Environmental Forensics, v. 2, p. 87-98.
Stout, S. A., A. D. Uhler, and K. J. McCarthy (2005). Middle
distillate fuel fingerprinting using drimane-based bicyclic
sesquiterpanes. Environmental Forensics 6: 241-251.
Schoell M. (1983). Genetic characterization of natural gases.
American Association of Petroleum Geologists Bulletin 67,
2225-2238.
Schoell M. (1984). Stable isotopes in petroleum research. In:
Advances in Petroleum Geochemistry. (J. Brooks and D. H. Welte,
Ed.), Academic Press, London. 1, 215-245.
Schoell M., Jenden P. D., Beeunas M. A. and Coleman D. D.
(1993). Isotope Analysis of Gases in Gas Field and Gas Storage
Operations. Society of Petroleum Engineers #26171 , 337-344.
Staniloae, D., B. Petrescu, and C. Patroescu, 2001, Pattern
Recognition Based Software for Oil Spills Identification by Gas
Chromatography and IR Spectrophotometry: Environmental Forensics,
v. 2, p. 363-366.
Sundararaman, P., and O. T. Udo, 1998, Abstract: Geochemical
Approach to Identifying the Origin of Oil Spills: a Case Study From
Nigeria: AAPG Bulletin, v. 82, p.1883-1984.
Wang, Z., M. Fingas, and D. S. Page, 1999, Oil spill
identification: Journal of Chromatography, v. A 842, p.
369-411.
Wang, Z. D. S. A. Stout, and M. Fingas (2006). Forensic
fingerprinting of biomarkers for oil spill characterization and
source identification. Environmental Forensics 6: 187-196.
Zafiriou, O.C., J. Meyers, and R. Bourbonnire (1973). Oil
Spill-Source Correlation by Gas Chromatography: An Experimental
Evaluation of System Performance. Proceedings of Joint Conference
on Prevention and Control of Oil Spills. USEPA, API, USCG. American
petroleum Institute, Washington, DC pp. 153-159.