25th International Meeting on Organic Geochemistry IMOG 2011

P-384
Fingerprinting of oil sheens, slicks, and tarballs collected in
response to the MC 252 oil spill
Alexei Milkov 1 , Changrui Gong 2 , David Grass 2 , Mike Sullivan 2 , Tomieka Searcy 2 , Leon
Dzou 2 , Pierre-Andre Depret 2
1 BP Russia, Moscow, Russian Federation, 2 BP America, Houston, United States of America (corresponding
author:alexei.milkov@bp.com)
Petroleum leaked into the Gulf of Mexico (GoM) water
column as a result of Deepwater Horizon (DWH)
incident which occurred in Mississippi Canyon Block
252 (MC 252) on April 20th, 2010. 1553 samples of oil
sheens, slicks and tarballs were collected from the
sea surface and along the coastlines in Texas,
Louisiana, Mississippi, Alabama and Florida in
response to the incident from May 11th to November
21st, 2010. The samples were subjected to a
comprehensive geochemical analysis. At the time of
writing (January 18th, 2011), we analyzed and
interpreted 839 samples of oil sheens, slicks and
tarballs. The vast majority of the interpreted samples
(93%) were collected prior to August 15th, 2010.
Origin and source of collected oil sheens, slicks and
tarballs samples were determined based on a
geochemical fingerprinting workflow. First, ranges of
geochemical parameters in the source oils were
established. For the source oils we used three MDT
samples collected in the MC 252 well, nine samples
collected during oil production at the Enterprise and
Q4000, and 21 skimmed oils that were judged to be
sourced by MC 252 oil spill based on their physical
appearance, chemical fingerprint and geographic
location. The studied geochemical parameters include
ratios of alkanes and isoprenoids, δ 13 C of SARA
fractions, and weathering-resistant high molecular
weight biomarkers reflecting the depositional
environment as well as the maturity of source rocks.
The established ranges of parameters account for
variations in the source oils and accuracy of analytical
measurements. Second, geochemical parameters in
the studied weathered oil samples were compared to
those in the source oils. Finally, the weathering trend
was established based on the skimmed source oils
and samples of weathered oils and tarballs were
plotted to check the conformance to that trend.
Samples of oil sheens, slicks and tarballs were
interpreted to be probably from MC 252 if they had
the same geochemical fingerprints (accounting for
weathering) as MC 252 source oils. Among the
analyzed and interpreted 818 samples of unclear
origin, 466 samples are probably from MC 252. Such
samples occurred mostly within the areal extent of
sea surface oil and along coastlines of Louisiana,
Mississippi and Alabama where more oil emulsion
reached the shore. Based on a basic preliminary
statistical analysis of biomarkers in all samples, we
determined samples which have fingerprints not
identical but relatively similar to the known MC 252
source oils. Such samples may have originated from
yet unknown MC 252 source oils and were interpreted
as may be from MC 252. There are 47 such samples.
Samples that had geochemical fingerprints clearly
different from those in the source oils were interpreted
to be not from MC 252. There are 305 such samples,
and they were collected every week after the DWH
incident.
Our interpretation suggests that 305 samples of oil
slicks, sheens and tarballs of unclear origin, or 37% in
the studied dataset, did not match the DWH oil spill
fingerprint. The percentage of samples matching the
DWH fingerprint was higher for beaches near the
wellhead and lower for beaches further away. Some
of the samples were in a heavily weathered state,
indicating that they had been released months or
years before the DWH incident occurred. Some of the
samples had biomarkers that are not found in the
DWH fingerprint. Because this study did not include a
program to take representative samples of all oil
found in the DWH incident response, it does not
identify the percentage or volume of oil resulting from
the DWH incident, in comparison to other oil existing
in the GoM. There are many possible sources of
these non-MC 252 samples. Around 1,000,000
barrels of petroleum per year [1] or more may be
released into the GoM from natural seeps and some
of that oil ends up as tarballs along the Gulf shores.
Other sources include oil releases from pipelines and
wells in the Gulf Coast rich petroleum infrastructure
[2], spills from bypassing vessels through operational
discharges [3], cargo washing and direct spills [4].
[1] Kvenvolden, K.A., Cooper, C.K., GeoMar. Let., 23,
140–146.
[2] www.incidentnews.gov
[3] Oil in the Sea III, 2003.
[4] Dalton, T., Jin, D., Mar. Poll. Bull., 60, 1939-1945.
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