Alkenes commonly found in synthetic drilling-fluids were used to identify sources of oil sheens that were first observed in September 2012 close to the Deepwater Horizon (DWH) disaster site more than two years after the Macondo MC-252 well was sealed. Exploration of the sea floor by BP confirmed that the well was capped and sound. BP scientists and engineers identified the likely source as leakage from an 80-ton cofferdam abandoned during a failed attempt to control the Macondo well in May 2010. We acquired and analyzed sheen samples at the sea-surface above the Deepwater Horizon wreckage as well as oil collected directly from the cofferdam using comprehensive two-dimensional gas chromatography (GC×GC). This allowed the identification of drilling-fluid C16- to C18-alkenes in sheen samples that were absent in cofferdam oil. Furthermore, the spatial pattern of evaporative losses of sheen oil alkanes indicated that oil surfaced closer to the Deepwater Horizon wreckage than the abandoned cofferdam site. Lastly, ratios of alkenes and petroleum hydrocarbons pointed to a common source of oil found in both sheen samples and recovered from oil-coated Deepwater Horizon riser pipe buoyancy compensator module debris collected shortly after the explosion. These lines of evidence suggest that the observed sheens do not originate from the Macondo well, cofferdam, or from natural seeps. Rather, the likely source is oil trapped in tanks and pits on the Deepwater Horizon wreckage, representing a finite oil leakage volume.

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