Oil seeps and slicks studied near BP’s Macondo well
14th May 2012 · 0 Comments
By Susan Buchanan
Scientists say oil and gas seeps surrounding BP’s Deepwater Horizon well appear to be mostly natural and to pre-date the 2010 spill. But some experts wonder if at least some of the seeps might have been caused or expanded by activity at the well. They worry subsea rock near the well may have been fractured. And they’re waiting to learn more from data collected by the National National Oceanic and Atmospheric Administration and also from the Natural Resource Damage Assessment process—under way now in response to the spill.
Seeps begin in fractures in subsea rock formations containing oil and gas deposits. Hydrocarbons are gradually released by seeps into the marine environment, including the sea floor, the water column and sea’s surface.
In the northern Gulf, the geology of the continental slope promotes the release of hydrocarbons from the seafloor, said Harry Roberts, Boyd professor emeritus at Louisiana State University’s Coastal Studies Institute. “Using 3-dimensional, seismic data, over 22,000 natural seep sites have been mapped in the region, including quite a few in the vicinity of the Macondo well site,” he said. Natural seeps can be active for years, constantly releasing gas and crude oil.
Roberts said “gas ascending through the water column may go into solution before it reaches the sea surface, depending on the water’s depth.” Oil droplets and gas bubbles covered with oil reach the surface, and the oil spreads out to make sea-surface slicks. On calm days in the Gulf, when wave action is moderate, slicks can be observed in many places on the continental slope from satellite data, Roberts said.
Pilots and mariners see the slicks too, and report them to authorities.
How are seeps studied? “Hard bottom limestone, created by microbial oxidation of hydrocarbons at seep sites and by communities of seep-related organisms, make the seafloor highly reflective of acoustic energy,” Roberts said. “These seep sites can be identified and mapped by analyzing the seafloor return from 3-D seismic data.” NOAA’s ship Okeanos Explorer is in the Gulf now, mapping gas plumes from natural seeps, he noted.
Roberts, who has been studying the northern Gulf’s natural seeps since 1985, said he believes the blowout at the Macondo well had little effect on natural seep sites in the area.
In another opinion from LSU, emeritus professor of environmental sciences Ed Overton said “my understanding is that seeps caused by drilling and caps are infrequent relative to the large number of natural seeps in the Gulf.”
At Texas A&M, oceanography professor Norman Guinasso, deputy director of the university’s Geochemical and Environmental Research Group, said seeps near BP blowout predated the drilling of that well. “We’ve been studying oil and gas seeps in MC118, which is about ten miles from the Macondo site, for many years. These seeps were there long before the Macondo well was drilled.”
LSU oceanography professor Robert Carney, who studies the Gulf seafloor, said a key difference between natural seeps and industry leaks is that “in natural seeps, oil takes a long time to move up through rock and mud, whereas oil and gas from drilling will come up rather quickly.”
Associating seeps and sheens near the Deepwater Horizon with the well itself is a knee-jerk response, some experts said. Houston-based geologist and oil consultant Arthur Berman said “everybody wants to make a connection between seeps and the BP spill, and I’ve received a lot of calls about it.” But, he said, a great deal of oil enters the Gulf through natural seeps, with estimates running anywhere from 1,500 to 19,000 barrels per day. He added “it’s mostly around salt domes.”
Looking back two years, Berman said “as long as the Macondo well was still flowing, the probability of leakage would have been almost zero because fluids take the path of least resistance, and that was through the pipe. My experience tells me fluids went out the rupture at the well head. Given what we know about the Macondo well now, I don’t think it caused additional rock fracturing or new seeps.”
Meanwhile, BOEM has mapped over 22,000 active and inactive seep sites in the Gulf with 3-D seismic data, BOEM spokesman John Filostrat said last week. “Most are gas only,” he noted. He said sea-surface oil slicks must be reported to the U.S. Coast Guard, which then relays them to BOEM. “We map these reports” at BOEM, he said.
After a ten-mile oil sheen was spotted at a distance from Venice on April 12, BOEM’s sister agency, the Bureau of Safety and Environmental Enforcement, said a modest amount of oil was emitted from a natural seep in the sea floor—based on video from Royal Dutch Shell PLC, which used remotely operated vehicles to examine the sheen. The sheen appeared in the general area of two Shell platforms.
Among the sceptics that all seeps near the Macondo site are natural is Singapore-based geohazards expert BK Lim, who has been monitoring the Gulf from Asia since the BP spill. He suspects the Deepwater Horizon well has been leaking for two years. “I believe that some of the oil just took available pathways to the seabed,” he said. “Natural seeps do not as a rule produce sudden, oil slicks running for miles, as we saw in March 2011 and this April in the Gulf.” Oil from a 30-mile slick washed up from Grande Isle westward in late March of last year.
Lim said the March 2011 and April 2012 slicks in the Gulf may have been related to seismic events—earthquakes in Japan and Sumatra, respectively, in those months—that shook soft sediment near the Deepwater Horizon and discharged oil.
Yuba City, Ca.-based geologist Chris Landau said “it would have been easy to understand the Gulf oil blowout and its consequences if they would have just shown us the mud logs—a graphical, foot-by-foot map of what geologists make while drilling goes on. They are your diary and forensic tool—the black box that explains how and why the jet crashed,” he said in reference to airline disasters.
A mud log is a record of drilling activity prepared by a petroleum geologist’s assistant, or logger, during well drilling. The log contains geological information on the drilled hole, drill times, mud characteristics, and indications or “shows”of oil and gas.
“Mud log data for the Macondo well should be released on our website this June,” BSEE spokeswoman Chauntra Rideaux said last week.
Meanwhile, NOAA has gathered data on seeps that can be viewed by the public. Scientists on board NOAA’s ship Okeanos Explorer in the Gulf have tested multibeam sonar, or echo-sounding technology, to detect and map gaseous seeps in the water column, NOAA spokesman Fred Gorell said last week. Multibeam sonar can survey wide swathes of the seafloor and water column, he explained. “We obtain and share information for others to study and characterize,” he said. “The data is rich and voluminous. It’s not on our website, but can be accessed at NOAA data centers.” That spate of data collection was independent of the Natural Resource Damage Assessment process.
Federal and state government NRDA projects, funded by BP and often involving universities, are part of an effort to restore the Gulf to pre-spill conditions. Several professors contacted for this article were unable to talk because of their NRDA research work. NOAA is collecting information directly for the NRDA process, partly through research cruises, and some of the data will be used in the government’s legal case against BP. More on the agency’s role in NRDA can be found on www.gulfspillrestoration.noaa.gov.
Subsea rock formations and oil seeps and slicks near the Deepwater Horizon are among the many matters to be discussed in the federal trial against BP, scheduled to start on January 14 in U.S. District Court in New Orleans. “Anything and everything will come up in that trial,” Arthur Berman predicted.
This article was originally published in the May 14, 2012 print edition of The Louisiana Weekly newspaper