25th International Meeting on Organic Geochemistry IMOG 2011

O-61
The significant impact of weathering on spilled gulf of Mexico
mc252 oil chemistry and its fingerprinting of samples collected
from the sea surface and shore between May and September
2010
Changrui Gong 1 , Alexei Milkov 2 , David Grass 1 , Michael Sullivan 1 , Tomieka Searcy 1 ,
Leon Dzou 3 , Pierre-Andre Depret 1
1 BP America, Houston, United States of America, 2 BP Russia, Moscow, Russian Federation, 3 BP Asia
Pacific, Jakarta, Indonesia (corresponding author:changrui.gong@bp.com)
The spilled Gulf of Mexico MC252 reservoir fluid (oil with
dissolved gas) underwent tremendous changes to its
physical and chemical properties through multiple
processes, including, but not limited to, evaporation,
dissolution, dispersion, photooxidation, emulsification,
biodegradation and sedimentation, as it moved along a path
from the reservoir to the wellhead, then to the water column,
to the sea surface, and in some cases, to the shoreline.
As part of Gulf spill response effort, over 1500 samples of
source oil, sea surface slicks, emulsions, and tarballs were
collected (beginning from early May) for geochemical
fingerprinting (GC, MPLC, Isotope, GCMS, CSIR), and were
interpreted as ―PROBABLY‖, ―MAY BE‖, or ―NOT‖ related to
MC252 oil [1]. Five hundred and thirteen source oil, oil,
emulsion, and tarball samples ―PROBABLY‖ derived from
the MC252 spill available as of Jan 18 th were studied to
understand the relative importance of weathering processes,
and their impact on the oil budget at the surface and to
examine the effect of weathering on geochemical
parameters of MC252 oil.
In the first part of its path, from reservoir at 18,000 feet
deep to the sea surface, MC252 fluid lost up to 40% of its
weight (mostly gas range and soluble aromatic components).
Phase separation and dissolution in water were the main
processes responsible for this mass loss. These processes
substantially removed compounds smaller than C7. A
subsea hydrocarbon plume formed by suspended small oil
droplets was identified by multiple scientific survey ships [2]
from the MC252 spill. Respiration by methanotrophs and
other indigenous oil-degrading psychrophilic bacteria has
quickly removed dissolved hydrocarbon gas components [3,
4] and higher molecular weight hydrocarbons [5].
Oil droplets that survived the trip from well head to surface
appeared as surface slicks and emulsions. In the second
part of its path, from sea surface to landfall, evaporation,
and, less importantly, dispersion and dissolution, caused
further depletion totaling up to ~63% of the oil weight
affecting compounds up to C21. In the tar samples analyzed
from the spill significant biodegradation had affected a few
samples at the time of the sample analyses, but will play a
larger role in hydrocarbon‘s ultimate fate in the environment.
Weathering has a much bigger effect on aromatic
components than on saturate components due to the higher
solubility of aromatics in water and their susceptibility to
photooxidation. The impact of weathering on major aromatic
compound groups decreases from biphenyls to
naphthalenes, to dibenzothiophenes, and to phenanthrenes
(Fig. 1). Higher molecular weight compounds, especially
saturate biomarkers, were more resistant to these
weathering processes. Natural weathering seems to shift
isotopes of aromatic and asphaltene fractions to slightly
more enriched in 13C, and isotopes of resins to more
negative values (Fig. 2). The depletion of resin isotopes with
weathering might be due to continued conversion of
isotopically lighter saturate and aromatic fractions to resin
fraction during photooxidation [6] and, to a lesser extent,
biodegradation. In addition some commonly used
geochemical indicator ratios, such as MPI and C23tri/C30H,
are affected by the weathering processes.
Total naphthalenes/total dibenzothiophenes
100
10
1
0.1
Source oil Weathered oils
0.01
0.01 0.1 1 10 100
nC17/nC35
Figure 1. A decreasing ratio of total naphthalenes to total
dibenzothiophenes with increasing weathering as indicated by
decreasing nC17/nC35 ratios (nC35 conservative during weathering)
delta C13 of resin fraction
-25.8
-26
-26.2
-26.4
-26.6
-26.8
-27
-27.2
-27.4
Source oil Weathered oils
-27.6
0.01 0.1 1 10 100
nC17/nC35
Figure 2. A depletion in C13 for resin fraction isotope values with increasing
weathering
References
[1] Milkov et al. (2011) IMOG abstract volume
[2] Camilli et al. (2010) ScienceExpress. 19 Aug 2010
[3] Valentine et al. (2010) ScienceExpress. 16 Sept 2010
[4] Kessler et al. (2011) ScienceExpress. 6 Jan 2011
[5] Hazen et al. (2010) ScienceExpress. 26 Aug 2010
Dutta T.K. and S. Harayama, (2000) Environ. Sci. Technol. 34, 1500-1505
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