BP Deepwater Horizon Oil Budget: 

What Happened To the Oil? 

The National Incident Command (NIC) assembled a number of interagency expert scientific teams to 

estimate the quantity of BP Deepwater Horizon oil that has been released from the well and the fate of 

that oil. The expertise of government scientists serving on these teams is complemented by 

nongovernmental and governmental specialists reviewing the calculations and conclusions. One team 

calculated the flow rate and total oil released. Led by Energy Secretary Steven Chu and United States 

Geological Survey (USGS) Director Marcia McNutt, this team announced on August 2, 2010, that it 

estimates that a total of 4.9 million barrels of oil has been released from the BP Deepwater Horizon well. 

A second interagency team, led by the Department of the Interior (DOI) and the National Oceanic and 

Atmospheric Administration (NOAA) developed a tool called the Oil Budget Calculator to determine 

what happened to the oil. The calculator uses the 4.9 million barrel estimate as its input and uses both 

direct measurements and the best scientific estimates available to date, to determine what has happened 

to the oil. The interagency scientific report below builds upon the calculator and summarizes the 

disposition of the oil to date. 

In summary, it is estimated that burning, skimming and direct recovery from the wellhead removed one 

quarter (25%) of the oil released from the wellhead. One quarter (25%) of the total oil naturally 

evaporated or dissolved, and just less than one quarter (24%) was dispersed (either naturally or as a 

result of operations) as microscopic droplets into Gulf waters. The residual amount — just over one 

quarter (26%) — is either on or just below the surface as light sheen and weathered tar balls, has washed 

ashore or been collected from the shore, or is buried in sand and sediments. Oil in the residual and 

dispersed categories is in the process of being degraded. The report below describes each of these 

categories and calculations. These estimates will continue to be refined as additional information 

becomes available. 

Figure 1: Oil Budget - Shows current best estimates of what happened to the oil. 
Explanation of Findings 

Unified Command Response Efforts: Response efforts to deal with the oil have been aggressive. As 

shown in the pie chart (Figure 1), response efforts were successful in addressing 33% of the spilled oil. 

This includes oil that was captured directly from the wellhead by the riser pipe insertion tube and top hat 

systems (17%), burning (5%), skimming (3%) and chemical dispersion (8%). Direct capture, burning 

and skimming remove the oil from the water entirely, while chemically dispersed oil remains in the 

water until it is biodegraded, as discussed below. 

Dispersion: Based on estimates, 16% of the oil dispersed naturally into the water column and 8% was 

dispersed by the application of chemical dispersants on and below the surface. Natural dispersion occurs 

as a result of the oil coming out of the riser pipe at high speed into the water column, which caused some 

of the oil to spray off in small droplets. For the purpose of this analysis, ‘dispersed oil’ is defined as 

droplets that are less than 100 microns — about the diameter of a human hair. Oil droplets that are this 

small are neutrally buoyant and thus remain in the water column where they then begin to biodegrade. 

Chemical dispersion also breaks the oil up into small droplets to keep it from coming ashore in large 

surface slicks and makes it more readily available for biodegradation. Chemical dispersants were applied 

at the surface and below the surface; therefore, the chemically dispersed oil ended up both deep in the 

water column and just below the surface. Dispersion increases the likelihood that the oil will be 

biodegraded, both in the water column and at the surface. Until it is biodegraded, naturally or chemically 

dispersed oil, even in dilute amounts, can be toxic to vulnerable species. 

All of the naturally dispersed oil and some of the oil that was chemically dispersed remained well-below 

the surface in diffuse clouds where it began to dissipate further and biodegrade. Previous analyses have 

shown evidence of diffuse clouds of dispersed oil between 3,300 and 4,300 feet in very low 

concentrations (parts per million or less), moving in the direction of known ocean currents and 

decreasing with distance from the wellhead. (citation: Federal Joint Analysis Group Report 1 and 2, 

http://ecowatch.ncddc.noaa.gov/JAG/reports.html). Oil that was chemically dispersed at the surface 

moved into the top 20 feet of the water column where it mixed with surrounding waters and began to 

biodegrade. 

Evaporation and Dissolution: It is estimated that 25% of the oil volume quickly and naturally 

evaporated or dissolved into the water column. The evaporation and dissolution rate estimate is based on 

scientific research and observations conducted during the Deepwater Horizon incident. 

Dissolution is different from dispersion. Dissolution is the process by which individual hydrocarbon 

molecules from the oil separate and dissolve into the water just as sugar can be dissolved in water. 

Dispersion is the process by which larger volumes of oil are broken down into smaller droplets of oil. 

Residual: After accounting for the categories that can be measured directly or estimated (i.e., recovery 

operations, dispersion, and evaporation and dissolution), an estimated 26% remains. This figure is a 

combination of categories all of which are difficult to measure or estimate. It includes oil still on or just 

below the surface in the form of light sheen or tar balls, oil that has washed ashore or been collected 

from the shore, and some that is buried in sand and sediments and may resurface through time. This oil 

has also begun to degrade through natural processes. 
Biodegradation: Dispersed oil in the water column and oil on the surface of the water biodegrade 

naturally. While there is more analysis to be done to quantify the rate of biodegradation in the Gulf, 

early observations and preliminary research results from a number of scientists show that the oil from 

the BP Deepwater Horizon spill is biodegrading quickly. Scientists from NOAA, EPA, DOE and 

academia are working to calculate more precise estimates of this rate. It is well known that bacteria that 

break down the dispersed and weathered surface oil are abundant in the Gulf of Mexico in large part 

because of the warm water, the favorable nutrient and oxygen levels, and the fact that oil regularly 

enters the Gulf of Mexico through natural seeps. 

Explanation of Methods and Assumptions 

Flow Rate: The Oil Budget Calculator starts with an estimate of the cumulative amount of oil released 

over the course of the spill. The newest estimates reflect the collaborative work and discussions of the 

National Incident Command’s Flow Rate Technical Group (FRTG) led by United States Geological 

Survey (USGS) Director Marcia McNutt, and a team of Department of Energy (DOE) scientists and 

engineers, led by Energy Secretary Steven Chu. This group estimates that approximately 4.9 million 

barrels of oil flowed from the BP Deepwater Horizon wellhead between April 22 and July 15, 2010, at 

which time the flow of oil was suspended. The uncertainty of this estimate is + 10%. The pie chart 

above is based on this group’s estimate of 4.9 million barrels of oil. 

Direct Measures and Best Estimates: The oil budget calculations are based on direct measurements 

wherever possible and the best available scientific estimates where measurements were not possible. 

The numbers for direct recovery and burns were measured directly and reported in daily operational 

reports. The skimming numbers were also based on daily reported estimates. The rest of the numbers 

were based on previous scientific analyses, best available information and a broad range of scientific 

expertise. These numbers will continue to be refined based on additional information and further 

analysis. Further information on these calculation methods is available in the Deepwater Horizon Gulf 

Incident Budget Tool Report from Aug 1, 2010 (available online). The tool was created by the US 

Geological Survey in collaboration with US Coast Guard, NOAA and NIST. 

Continued monitoring and research: 

Our knowledge of the oil, dispersants, ecosystem impacts and human impacts will continue to evolve. 

Federal agencies and many academic and independent scientists are actively pursuing better 

understanding of the fate, transport and impact of the oil. The federal government will continue to report 

activities, results and data to the public on a regular basis. Updates and information can be found at 

www.restorethegulf.gov, and data from the response and monitoring can be found at 

www.geoplatform.gov. 

DOI, NASA and NOAA continue to refine understanding of amounts of remaining surface oil. NOAA 

responders are working with the Unified Command on monitoring strategies for tar balls and near shore 

submerged oil, and researchers continue subsurface scanning and sampling to monitor the concentration, 

distribution and impact of oil there. EPA and NOAA have carefully monitored BP's use of dispersant in 

the Gulf and continues to monitor the air, water and sediments near the shoreline for the presence of 

dispersant and crude oil components with special attention to human health impacts. Numerous NOAAand NSF-funded academic researchers and NOAA scientists are investigating rates of biodegradation, 

ecosystem and wildlife impacts. DOI and DOE responders are working to ensure control of the well and 
accurate measurement of oil released and oil remaining in the environment. DOI is leading efforts to 

mitigate impacts of oil to terrestrial wildlife, natural resources, and public lands. 

Even though the threat to shorelines, fish and wildlife, and ecosystems has decreased since the capping 

of the BP wellhead, federal scientists remain extremely concerned about the impact of the spill to the 

Gulf ecosystem. Fully understanding the impacts of this spill on wildlife, habitats, and natural resources 

in the Gulf region will take time and continued monitoring and research. 
Deepwater Horizon/BP Oil Budget: 

What happened to the oil? 

Acknowledgements 

Authors 

Jane Lubchenco, NOAA, DOC 

Marcia McNutt, USGS, DOI 

Bill Lehr, NOAA, DOC 

Mark Sogge, USGS, DOI 

Mark Miller, NOAA, DOC 

Stephen Hammond, USGS, DOI 

William Conner, NOAA, DOC 

Credits 

The following scientists were involved in developing the Oil Budget Calculator tool: 

LT(jg) Charity Drew (USCG) – Original Excel spreadsheet and application inspiration 

David Mack and Jeff Allen (USGS) – Application development and engineering 

Rebecca Uribe (USGS) – Graphic design 

Bill Lehr (NOAA) – Lead mass balance and oil budget scientist 

Antonio Possolo and Pedro Espina (NIST) – Statistical oil budget model encoded as an R 

program 

LCDR Lance Lindgren, CDR Peter Hoffman, CDR Sean O'Brien, and LT Amy McElroy 

(USCG) – Application requirements and user stories 

Sky Bristol and Tim Kern (USGS) – Project vision and management 

Kevin Gallagher, Martha Garcia, and Stephen Hammond (USGS) – Executive sponsors 

The following experts were consulted on the oil budget calculations, contributed field data, suggested 

formulas, analysis methods, or reviewed the algorithms used in the calculator. The team continues to 

refine the analysis and this document will be updated as appropriate. 

Federal Scientists

Bill Lehr, NOAA 

Robert Jones, NOAA 

Antonio Possolo, NIST 

Independent Scientists

Ron Goodman, U. of Calgary 

Al Allan, SpilTec 

James Payne, Payne Env. 

Tom Coolbaugh, Exxon Mobil 

Ed Overton, LSU 

Juan Lasheras, UCSD 

Merv Fingas, Env. Canada (ret) 

Ali Khelifa, Env. Canada 

Pat Lambert, Env. Canada 

Per Daling, SINTEF 

Michel Boufadel, Temple Univ.