The research team 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 early 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 sea floor after its use in a failed attempt to cover the Macondo well in 2010.
The team's GCxGC analysis of BP's cofferdam samples definitively showed the cofferdam was not the sole source of the leak 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 the top of the cofferdam. BP 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 Reddy and Valentine compared the chemical makeup of the sheens with Deepwater Horizon debris found floating in 2010, they found a match. That debris, which came from the rig itself, was coated with oil and contaminated with drilling mud olefins.
"The ability to fingerprint synthetic hydrocarbons allowed us to crack this case. We were able to exclude a number of suspects and match the olefin fingerprint in the new oil slicks to that of the wreckage from the sunken rig," said Valentine.
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 source of the oil. By looking for sheens that showed the least amount of evaporation, they determined that oil surfaced closer to the Deepwater Horizon wreckage than the cofferdam site.
To explain how the oil might be trapped and released from the wreckage, the scienti
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Woods Hole Oceanographic Institution