Coast Guard investigating oil sheen at BP Deepwater Horizon disaster site - the sheen correlates to oil that originated from BP?s Macondo Well

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Coast Guard investigating oil sheen at BP Deepwater Horizon disaster site

Published: Wednesday, October 03, 2012, 5:30 PM Updated: Wednesday, October 03, 2012, 6:07 PM

By Mark Schleifstein, The Times-PicayuneThe Times-Picayune

The U.S. Coast Guard is investigating an oily sheen spotted in the Gulf of Mexico last month near the site of the 2010 Deepwater Horizon oil spill 40 miles south of the Mississippi River, a spokesman said Wednesday. Samples of the sheen taken near the site of the failed BP Macondo oil well have been sent to the service's Marine Safety Lab in Groton, Conn., to determine whether the oil is from the BP spill, said Lt. Commander Michael Wolfe.

The Coast Guard's National Response Center was first notified of the sheen on Sept. 16 in a report that said it was spotted on satellite imagery, and had not been visually confirmed.

"Attempting to confirm presence of sheen via visual observation and further satellite data collection," said that first report. "BP intends to mobilize a helicopter overflight of the area as soon as possible."

The sheen was later reported on a new National Oceanic and Atmospheric Administration web site tracking environmental incidents nationwide.
"Although the source of these sheens may be the wrecked BP Macondo well, this relationship has not been established at this time," officials wrote on the site, which is operated by the National Ocean Service's Office of Response and Restoration. "Activities include daily over-flights sponsored by BP, with USCG or NOAA observers on board intermittently."

The report also said BP was sending a vessel equipped with a remotely-operated underwater vehicle to the area to investigate the potential source of oil.

More...

http://www.nola.com/news/gulf-oil-spill/index.ssf/2012/10/coast_guard_investigating_sour.html

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NOAA SSC Update 5Oct12

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USCG Morgan City reports that there are still mystery sheens in lease block MC-252, but they are barely discernible. They are silvery streamers that start about 1/2 miles from the relief wells, ranging in size from 1/2 mile to 4 miles long and of varying widths. There is still no known source and the sheens are not recoverable. USCG took samples and submitted them to the USCG Marine Safety Lab in Groton, CT. Results are still pending. BP and the USCG have been doing overflights, satellite and sonar imagery. Depending on the results of the sampling, USCG will decide their next course of action.


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http://www.incidentnews.gov/entry/16595

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Re: Coast Guard investigating oil sheen at BP Deepwater Horizon disaster site

FOSC issues Notice of Federal Interest to BP and Transocean

October 10, 2012 | 9:55:12 PM EDT
NEW ORLEANS ? The Federal On-Scene Coordinator for the Deepwater Horizon oil spill in New Orleans issued a Notice of Federal Interest to BP and Transocean Tuesday.

Coast Guard Capt. Duke Walker issued the NOFI following sample results from an oil sheen located in the vicinity of where the Deepwater Horizon drill rig exploded and sank more than two years ago.

The sheen was first reported to the National Response Center Sept. 16 by BP based on satellite images from the 9th and 14th overpasses in the Mississippi Canyon, block 252, approximately 50 miles off the coast of Louisiana. The sheen is not feasible to recover and does not pose a risk to the shoreline.

The Coast Guard, in concert with BP and NOAA, has conducted regular assessments of the sheen by aircraft and boat since its discovery. The observed sheen size has varied over time depending upon the conditions present. Samples of the sheen were taken by Coast Guard Marine Safety Unit Morgan City Sept. 26 and sent to the Coast Guard Marine Safety Lab in New London, Conn. The Marine Safety Laboratory results indicate the sheen correlates to oil that originated from BP?s Macondo Well. The exact source of the sheen is uncertain at this time but could be residual oil associated with wreckage and/or debris left on the seabed from the Deepwater Horizon incident in 2010.

The NOFI effectively informs BP and Transocean that the Coast Guard matched the sheen samples to the Deepwater Horizon spill or sunken drilling debris and that either party or both may be held accountable for any cost associated with further assessments or operations related to this sheen.

The Gulf Coast Incident Management Team remains committed to the continued cleanup of the Gulf Coast and all shorelines affected as a result of the Deepwater Horizon oil rig explosion. The FOSC is determined to continue response activities to remove all oil where it is technologically feasible, environmentally beneficial and safe for workers to perform recovery operations.

The public is reminded to contact the National Response Center at 1-800-424-8802 to report all pollution incidents or the Coast Guard 8th District command center at 504-589-6225 in the event of any marine emergencies.

http://www.restorethegulf.gov/release/2012/10/10/fosc-issues-notice-federal-interest-bp-and-transocean

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Re: Coast Guard investigating oil sheen at BP Deepwater Horizon disaster site - the sheen correlates to oil that originated from BP?s Macondo Well

Sheen at Deepwater Horizon disaster site is BP oil, Coast Guard says

Published: Wednesday, October 10, 2012, 9:56 PM
Updated: Thursday, October 11, 2012, 9:16 AM
By Mark Schleifstein, NOLA.com | The Times-Picayune

Samples of oil taken from a sheen above the site of the sunken Deepwater Horizon oil rig and BP's Macondo 252 well matches oil released from the well during the 2010 spill...

There have been concerns since the sinking of the Deepwater Horizon that oil from the BP well could leak from storage areas on the ship or from the drilling riser piping that connected the well to the ship and now lies on the Gulf floor, a mile deep. Environmental groups also have raised concerns that the drilling of relief wells to cut off the flow of oil, or the failure of the original well, may have fractured the bottom sediments at the wellsite, which might eventually prove to be the site of new leakage from the well.

It's also unclear how much of the oil turned into droplets by the use of dispersants at the wellhead may have dropped to the bottom around the wellsite, another possible source of small amounts of oil floating to the surface.
...
"This new oiling at the well site contributes to Louisiana's concerns about the certainty of the ongoing oiling conditions and outstanding oil we have witnessed over the last several weeks," said Garret Graves, chairman of the state Coastal Protection and Restoration Authority and adviser to Gov. Bobby Jindal on the oil spill cleanup.
...

Full text:

http://www.nola.com/news/gulf-oil-spill/index.ssf/2012/10/sheen_at_deepwater_horizon_dis.html

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Re: Coast Guard investigating oil sheen at BP Deepwater Horizon disaster site - the sheen correlates to oil that originated from BP?s Macondo Well

New oil sheen in Gulf worries BP, lawmakers

By Harry R. Weber | Thursday, October 11, 2012
...
U.S. Rep. Edward Markey of Massachusetts, a top Democrat on the House Committee on Natural Resources, is calling for a full undersea survey of the Macondo site to be carried live on the Internet so independent scientists can see what is going on. A camera focused on the gushing well after the April 20, 2010, disaster, and when a massive cap halted the flow almost three months later.

"One can only hope that the nightmare well has not come back to haunt the people of the Gulf," said Markey, a frequent oil industry critic.
...
BP said Thursday it believes the oil came from equipment associated with the blown-out well and not the well itself. It said it has been working with the Coast Guard and other government officials to investigate the source of the oil. The company said flights over the area and vessel surveys suggest that the integrity of the cap on the Macondo well and the associated relief well are sound.

"The size of the sheen, its persistent point of origin and other factors indicate the most likely source is the bent riser pipe that once connected the rig to the well head, where a mix of oil, drilling mud and seawater were trapped after the top kill operation," BP said.

The top kill, which involved pumping material into the wellhead, was an early, unsuccessful attempt to stop the flow.

Matt Dundas of the environmental group Oceana said the news about is troubling.

"Here we are, two years and 5 million barrels of spilled oil later, and we're still dealing with BP's inability to keep oil out of the ocean," he said.
...

Full text:

http://www.chron.com/business/energy/article/New-oil-sheen-in-Gulf-worries-BP-lawmakers-3940826.php

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Re: Coast Guard investigating oil sheen at BP Deepwater Horizon disaster site - the sheen correlates to oil that originated from BP?s Macondo Well

Gulf Oil Sheen Blamed on 2010 Wreckage, Not Well

<!-- insert date & partner -->By CAIN BURDEAU Associated Press
NEW ORLEANS October 12, 2012 (AP)

...
A probe started after a sheen was discovered Sept. 16 in the waters near the site indicates the oil may have seeped from a mile-long metal tube, called the riser, which connected the Deepwater Horizon drilling rig to the Macondo well.
...
The tests of the sheen showed chemical signatures of Macondo oil mixed with drilling muds, which are lubricants used both in drilling the well and trying to plug it after the April 20, 2010 explosion.

"This led to the conclusion that the oil causing the sheen out at the surface is likely coming from the riser. The riser has drilling muds in it," said Frank Csulak, the scientific support coordinator for the National Oceanic and Atmospheric Administration.

He said at most the riser could contain about 75,600 gallons of oil, but experts expect less than that is trapped inside it. He said tests have ruled out the possibility that the new sheen came from oil sitting on the bottom of the Gulf because it was not weathered and degraded.

Csulak said BP and Transocean Ltd., the drilling company contracted by BP to drill the well, have been given until Friday to provide a plan of action. He said the companies could be asked to remove whatever oil is left in the riser.

"It would be possible to remove the oil; it's not going to be easy," he said. The amount of oil that leaked out is small and that once the oil reached the surface "it evaporates and dissipates within hours."

He added that surveys hopefully would determine what's going on deep down on the Gulf floor. "We have no idea if oil is coming out of one spot or multiple locations," Csulak said. It's unknown if it was a steady leak or intermittent.
...

Full text:

http://abcnews.go.com/US/wireStory/gulf-oil-sheen-blamed-2010-wreckage-17455692

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Re: Coast Guard investigating oil sheen at BP Deepwater Horizon disaster site - the sheen correlates to oil that originated from BP?s Macondo Well

FOSC for the Deepwater Horizon oil spill in New Orleans approved a joint plan from BP and Transocean

October 12, 2012 | 9:02:01 PM EDT

NEW ORLEANS ? The Federal On-Scene Coordinator for the Deepwater Horizon oil spill in New Orleans approved a joint plan from BP and Transocean Thursday.

The joint plan was required by Coast Guard Capt. Duke Walker to identify the source of a sheen located in the Gulf of Mexico that has been associated with Macondo 252 oil. The Notice of Federal Interest informed BP and Transocean that they may be responsible for costs related to identifying the source of the sheen and cleanup.

The proposed joint plan includes satellite observations and mobilizing remotely operated vehicles to examine the original Macondo well area including the wreckage, debris, and the riser on the sea floor.

The ROV is expected to be mobilized Sunday with direct Coast Guard oversight. Operations are anticipated to commence early next week with logistics and weather permitting.

The public is reminded to contact the National Response Center at 1-800-424-8802 to report all pollution incidents or the Coast Guard 8th District Command Center at 504-589-6225 in the event of any maritime emergencies.

http://www.restorethegulf.gov/release/2012/10/12/fosc-deepwater-horizon-oil-spill-new-orleans-approved-joint-plan-bp-and-transocea

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Re: Coast Guard investigating oil sheen at BP Deepwater Horizon disaster site - the sheen correlates to oil that originated from BP?s Macondo Well

Oct. 16, 2012 </FOOTER>

Ask for Underwater Rover Footage of Leak, Potential Size of Residual Spill

WASHINGTON (October 16, 2012) ? The two leading Congressional investigators into the BP oil spill today pressed the company to turn over any and all information related to the recent Gulf of Mexico oil slick that has been sourced from the Macondo well. Reps. Ed Markey (D-Mass.) and Henry A. Waxman (D-Calif.) wrote BP to ask them about the plan approved by the U.S. Coast Guard to inspect the well site, the wreckage of the Deepwater Horizon, and the surrounding area.

The two Democrats also ask BP what the size of the leak could be, if it is in fact coming from the sunken drill pipe, otherwise known as the riser; and to turn over any and all documents, maps, underwater rover footage, and any other materials related to this latest incident.

?This recent report of a new oil slick from the Deepwater Horizon?s riser raises questions about BP?s efforts to stem the long-term impacts of the 2010 oil spill,? write the two Democrats.

Rep. Markey is the Ranking Member of the Natural Resources Committee, and Rep. Waxman is the Ranking Member of the Energy and Commerce Committee.

The full letter to BP?s CEO Robert Dudley can be found HERE.

http://markey.house.gov/press-release/markey-waxman-press-bp-macondo-oil-slick

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Re: Coast Guard investigating oil sheen at BP Deepwater Horizon disaster site - the sheen correlates to oil that originated from BP?s Macondo Well

ROVs investigate possible sheen source

October 18, 2012 | 3:20:59 PM EDT

NEW ORLEANS ? Remote operated vehicles deployed from the offshore construction vessel Skandi Neptune collected oil samples on Wednesday from the underwater site of the Deepwater Horizon incident to determine the source of a surface sheen discovered last month.

The samples were taken after the ROV video showed apparent oil globules leaking from the containment dome at approximately 15 globules per minute, which is estimated to be less than 100 gallons per day. In 2010, the 40‐foot‐tall containment dome was used as part of an attempt to capture oil and allow it to flow through a pipe to a barge on the surface.

This technique was not successful and the equipment was moved away from the well head and riser pipe, and set in its current position approximately 500 meters from the original Macondo well head. It is entirely separate from the well head and any riser piping. Out of an abundance of caution, the ROV also inspected the original Macondo well area including the wreckage, debris, relief wells, and the riser on the sea floor and observed no oil leakage from that area.

The two collected oil samples will be used for lab analysis. One sample will be shared by BP and Transocean, and the second sample will be used by the Coast Guard. The lab analysis will help determine if the containment dome is the likely source of the recent sheening.

?The Coast Guard is further evaluating what is believed to be seepage from the containment dome to determine how best to respond,? said Capt. Duke Walker, Federal On‐Scene Coordinator for the Deepwater Horizon response.

The ROV operations were observed by the Coast Guard, Bureau of Ocean Energy Management, Bureau of Safety and Environmental Enforcement, the Department of Interior?s Trustee, BP, Transocean and state on‐scene coordinators from Louisiana, Mississippi and Florida.

The Skandi Neptune departed for the vicinity of the Deepwater Horizon incident Sunday to identify the origin of an oil sheen first reported to the National Response Center Sept. 16. The Coast Guard issued a Notice of Federal Interest to BP and Transocean after the reported sheen was correlated to the oil that originated from BP?s Macondo well.

Due to chain of custody procedures and processing approximately 72 hours of raw footage, the video of the ROV inspections is currently unavailable. It will be posted immediately upon processing.

The public is reminded to contact the National Response Center at 1‐800‐424‐8802 to report all pollution incidents or the Coast Guard 8th District command center at 504‐589‐ 6225 in the event of any marine emergencies.

http://www.restorethegulf.gov/release/2012/10/18/rovs-investigate-possible-sheen-source

[Emily]

Re: Coast Guard investigating oil sheen at BP Deepwater Horizon disaster site - the sheen correlates to oil that originated from BP?s Macondo Well

http://www.alaskadispatch.com/articl...n-rig-disaster

Gulf of Mexico Mystery: Why is oil leaking at site of Deepwater Horizon rig disaster?
Mark Guarino | The Christian Science Monitor | Oct 19, 2012

Oil is leaking into the Gulf of Mexico at the rate of about 100 gallons per day near the Macondo wellhead, the site of the oil spill over two years ago that became the worst environmental disaster in US history.

The US Coast Guard reports that the oil has been leaking since at least Sept. 16...

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Re: Coast Guard investigating oil sheen at BP Deepwater Horizon disaster site - the sheen correlates to oil that originated from BP?s Macondo Well

Coast Guard approves plan to secure containment dome

October 25, 2012 | 10:56:12 AM EDT

NEW ORLEANS - The Federal On-Scene Coordinator for the Deepwater Horizon oil spill in New Orleans authorized BP to proceed with a plan to cap and plug the containment dome.

In 2010, the 40-foot-tall containment dome was used as a part of an attempt to capture oil and allow it to flow through a pipe to a barge on the surface. The technique was unsuccessful and the equipment was moved away from the well head and riser and set in its current position approximately 500 meters from the original Macando well head.

The operation, which began Tuesday, included BP mobilizing a remotely operated vehicle from the offshore construction vessel Skandi Neptune to the containment dome to place a cap on top of the stove pipe and plug the ROV connection ports on the sides and top of the structure. The cap and plugs were successfully put in place and no further oil emissions from the containment dome were observed.

BP has collected data via satellite throughout the operation and will continue to do so for a five-day period following, in order to detect changes in the surface sheen and to evaluate the effectiveness of these actions in abating or eliminating the sheen. In addition, the Coast Guard plans an overflight of the area later in the week.

Representatives from the Coast Guard, Bureau of Ocean Energy Management and Bureau of Safety and Environmental Enforcement and Gulf Coast Incident Management Team's Department of Interior Liaison, as well as state on-scene coordinators from Louisiana, Mississippi, and Florida observed the controlled feed in New Orleans.

The FOSC has further directed BP to submit a feasibility plan that considers the next steps toward either removing or remediating the threats of oil posed by the riser pipe and containment dome.

The operation and proposed plan were in response to an oil sheen in the vicinity of the Deepwater Horizon incident first reported to the National Response Center Sept. 16. The Coast Guard issued a Notice of Federal Interest to BP and Transocean after the reported sheen was correlated to the oil that originated from BP's Macondo well.

The public is reminded to contact the National Response Center at 1-800-424-8802 to report all pollution incidents or the 8th Coast Guard District command center at 504-589-6225 in the event of a marine emergency.

http://www.restorethegulf.gov/release/2012/10/25/coast-guard-approves-plan-secure-containment-dome

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A reminder of this containment dome:

Image ? BP p.l.c.

Deep-sea ice crystals stymie Gulf oil leak
May 8, 2010 6:45 PM (42 mins ago) By HARRY R. WEBER and
SARAH LARIMER, AP

"A novel but risky attempt to use a 100-ton steel-and-concrete box to cover a deepwater oil well gushing toxic crude into the Gulf of Mexico was aborted Saturday after ice crystals encased it, an ominous development as thick blobs of tar began washing up on Alabama's white sand beaches.

The setback left the mission to cap the ruptured well in doubt. It had taken about two weeks to build the box and three days to cart it 50 miles out then slowly lower it to the well a mile below the surface, but the frozen depths were too much for it to handle."
More...

http://www.flutrackers.com/forum/showthread.php?t=145880

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Re: Coast Guard investigating oil sheen at BP Deepwater Horizon disaster site - the sheen correlates to oil that originated from BP?s Macondo Well

Is oil sheen in Gulf of Mexico coming from site of BP spill?

Posted on December 13, 2012 at 10:24 PM
Updated yesterday at 10:41 PM
Monica Hernandez / Eyewitness News

NEW ORLEANS - BP is looking into whether a massive oil sheen in the Gulf of Mexico is coming from the site of the 2010 spill.
...
?The only places that we have high suspicion that there might be contained oil is the containment dome where we already found some previously,? said Cpt. Duke Walker, Federal On Scene Coordinator for the Deepwater Horizon Response.

BP confirmed that an oil sheen spotted in mid-September was indeed leaking from the containment dome, an abandoned piece of equipment that capped the original spill. The company plugged that new leak, but days later, more slicks surfaced.

So far, investigators haven't found any new sources of oil, but haven't yet inspected the source of September's leak.

?We haven't been back to the containment dome yet, or the well heads, that's in our target package, and it will happen either [Thursday night] or perhaps [Friday],? said Walker.

This time, investigators also plan to look at the rig wreckage itself, which they didn't inspect after mid-September?s leak.

More...

http://www.wwltv.com/news/Is-oil-sheen-in-Gulf-coming-from-site-of-BP-spill--183455641.html

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Re: Coast Guard investigating oil sheen at BP Deepwater Horizon disaster site - the sheen correlates to oil that originated from BP?s Macondo Well

ROV wraps up investigation

December 18, 2012 | 3:40:33 PM EST

NEW ORLEANS ?Remote operated vehicles (ROV) investigating the recurring sheen near the MC 252 wellhead concluded operations Saturday, Dec. 15, 2012. ROVs inspected the sunken Deepwater Horizon Platform wreckage and the surrounding area.

No sources of leaking oil were identified. During operations, an unidentified substance inconsistent with oil was seen emitting from several areas of the rig wreckage, and samples were collected for further lab analysis.

?No apparent source of the surface sheen has been discovered by this effort,? said Capt. Duke Walker, Federal On-Scene Coordinator for Deepwater Horizon. ?Next steps are being considered as we await the lab results of the surface and subsurface samples and more detailed analysis of the video shot during the mission."

Out of an abundance of caution, the Macondo wellhead was also observed and, as expected, was found to be secure. On September 19, 2010 the well was closed off and cemented.

Satellite surveillance will continue to monitor the sheen while future steps are being considered.

The real time ROV operations were observed remotely by the Coast Guard, Bureau of Ocean Energy Management, Bureau of Safety and Environmental Enforcement, BP, Transocean and state on‐scene coordinators from Louisiana, Mississippi and Florida. Additionally, the Coast Guard had observers onboard the Olympic Triton which launched and controlled the ROVs during the operation.

Video of the ROV inspections can be found at the following links:

Well Heads
http://cgvi.uscg.mil/media/main.php?g2_itemId=1861633

Wreckage
http://cgvi.uscg.mil/media/main.php?g2_itemId=1861630

Riser Pipe
http://cgvi.uscg.mil/media/main.php?g2_itemId=1861396

Containment Dome
http://cgvi.uscg.mil/media/main.php?g2_itemId=1861393

The Gulf Coast Incident Management Team remains committed to the continued cleanup of the Gulf Coast and all shorelines affected as a result of the Deepwater Horizon oil rig explosion. The FOSC is determined to continue response activities to remove all oil where it is technologically feasible, environmentally beneficial and safe for workers to perform recovery operations.

The public is reminded to contact the National Response Center at 1-800-424-8802 to report all pollution incidents or the Coast Guard 8th District command center at 504-589-6225 in the event of any marine emergencies.

http://www.restorethegulf.gov/release/2012/12/18/rov-wraps-investigation

[Emily]

Re: Coast Guard investigating oil sheen at BP Deepwater Horizon disaster site - the sheen correlates to oil that originated from BP?s Macondo Well

Hopefully this is not new - just a summary of what's already been reported.

http://www.ksla.com/story/20440585/m...ks-from-bp-rig

Mystery substance leaks from BP rig
Posted: Friday, December 28, 2012 11:35 AM EST Updated: Friday, December 28, 2012 11:35 AM EST

KSLA News 12 HeadlinesMore>>
GULF OF MEXICO (WAFB) -

A mystery substance is leaking from the wreckage of BP's Deepwater Horizon.

The Coast Guard noticed it, but investigators said the substance is not oil...

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Re: Coast Guard investigating oil sheen at BP Deepwater Horizon disaster site - the sheen correlates to oil that originated from BP?s Macondo Well

Press Release 13-125
Study Identifies Source of Oil Sheens Near Deepwater Horizon Site
Chemical analysis shows that source is oil pockets trapped in wreckage of sunken rig

July 16, 2013

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.

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 results are published this week in the journal Environmental Science & Technology.
...
"This appears to be a slow leak from the wreckage of the rig, not another catastrophic discharge from a deep oil reservoir," says geochemist David Valentine of UCSB.

"Continued oil discharge to the Gulf of Mexico from the wreckage of the Deepwater Horizon rig is not a good thing, but there is some comfort that the amount of leakage is limited to the pockets of oil trapped within the wreckage of the rig."
...
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, a technique developed in Reddy's lab, 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 occurrence of these man-made olefins in all our sheen samples points to a single main source, which contains both Macondo oil and lesser amounts of the drilling fluids that harbor the olefins," said Valentine.

"This pointed us to the wreckage of the rig, which was known to have both, as the most likely source for the sheens."
...
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.
...

Full text:

http://www.nsf.gov/news/news_summ.jsp?cntn_id=128494&org=NSF&from=news