Deepwater Horizon Oil Sheen;  Photo credit: Chris Reddy, WHOI
Researchers from the University of California Santa Barbara (UCSB) and the Woods Hole Oceanographic Institution (WHOI) have ruled out leaks at the Mocondo well as the source of oil sheens found on the ocean surface near the Deepwater Horizon oil spill site.

A chemical analysis indicates that the source of oil sheens recently found floating at the ocean's surface near the site of the Gulf of Mexico Deepwater Horizon oil spill is pockets of oil trapped within the wreckage of the sunken rig, according to a report published in the journal Environmental Science & Technology.

First reported to the U.S. Coast Guard by multinational oil and gas company BP in September 2012, the oil sheens raised public concern that the Macondo well, which was capped in July 2010, might be leaking. However, both the Macondo well and the natural oil seeps common to the Gulf of Mexico were confidently ruled out, according to researchers from the University of California Santa Barbara (UCSB) and the Woods Hole Oceanographic Institution (WHOI).

The researchers used a recently patented method to fingerprint the chemical makeup of the oil sheens, and to estimate the location of the source based on the extent to which gasoline-like compounds evaporated from the sheens. Because every oil sample contains chemical clues pointing to the reservoir it came from, scientists can compare it to other samples to determine if they share a common source.

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The scientists analyzed 14 sheen samples skimmed from the sea surface during two trips to the Gulf of Mexico. Using comprehensive two-dimensional gas chromatography, the researchers first confirmed that the sheens contained oil from the Macondo well. But the sheen samples also contained trace amounts of olefins, industrial chemicals used in drilling operations. The presence of olefins provided a fingerprint for the sheens the scientists could compare to the samples they had analyzed during the last three years.

Olefins are not found in crude oil and their uniform distribution in the sheens indicated that the Macondo well was unlikely to be the source. The team surmised that the sheens must be coming from equipment exposed to olefins during drilling operations.

The researchers compared the sheen samples to other field samples, some of which they expected would contain olefins and some they thought would not. The reference samples included two pieces of debris from the Deepwater Horizon found floating in May 2010, as well as oil collected by BP in October 2012, during an inspection of the 80-ton cofferdam that had been abandoned at the seafloor after its use in a failed attempt to cover the Macondo well in 2010.

The team's gas chromatography analysis of BP's cofferdam samples definitively showed that it was not the sole source of the leak as there were no olefins present. Prior to the analysis the cofferdam had become the prime suspect as the source when BP found small amounts of oil leaking from its top.

BP scientists acquired oil samples from this leak point before sealing the leak, thinking they had resolved the problem. However, the sheens on the sea surface persisted, and the lack of olefins pointed to another source entirely. When researchers compared the chemical makeup of the sheens with debris found floating in 2010, they found a match. That debris, which came from the rig itself, was coated with oil and was contaminated by drilling mud olefins.

The chemical analysis also told researchers which sheens had surfaced more recently than others, allowing them to reconstruct a trajectory for local ocean currents that pointed back to the oil's source. By looking for sheens that showed the least amount of evaporation, they determined that oil surfaced closer to Deepwater Horizon wreckage than to the cofferdam site.

To explain how the oil might be trapped and released from the wreckage, the scientists point out that when the Deepwater Horizon rig sank, it was holding tanks containing hundreds of barrels of a mixture of drilling mud and oil.

Over time, corrosive seawater can create small holes through which oil can slowly escape to the surface. The researchers suspect that the containers on the rig holding trapped oil may be the source of the recent oil sheen.

The research team consisted of David Valentine and Matthias Kellermann of UCSB, and Chris Reddy and Robert Nelson of WHOI. The Gulf of Mexico Research Initiative, Woods Hole Oceanographic Institution and a Swiss National Science Foundation Postdoctoral Fellowship also funded the research.