|NASA Terra Satellite image of sun on the oil slick off of the|
Mississippi Delta (highlighted by the tree-like structure in green)
on May 24, 2010
Monday, February 14, 2011
Hydrocarbon gases estimated to have been 40% of the discharges from the BP oil well blowout
The BP oil well blowout last summer caused release of at least 50-100 million barrels of oil into the Gulf of Mexico. In other units, this was somewhere between 7 and 14 million tons of oil. This release was unusual geologically because the discharge rate was high compared to natural rates, it was extended (84 days), and it was deep (nearly 1500 meters). It was also unique because it contained another 40% by weight dissolved hydrocarbon gases, mostly methane and pentane. I featured an article on natural vs. Deepwater Horizon seep rates by Cutler Cleveland on this earlier post.
In a paper just released on-line by Nature Geoscience, Joye et al. have quantified this estimate of the amount of gas released, where it was released, and some of the implications. The reference is Nature (published on-line) 13 February, 2011, DOI: 10.1038/NGE01067. (I'll try to remember to come back and put in the formal publication reference!). I really like this paper--the authors dealt with some of the most atrocious scientific units possible to unravel this story: tons (or is it tonnes?), barrels, bopd's (barrels of oil per day), gallons ('a standard barrel of oil is 42 US gallons or 159 litres), BOE's (barrel of oil equivalent, in energy units of kilojoules or, in volume units, cubic meters or cubic feet),.... Definitely enough to drive an undergraduate chemistry class nuts for a few homework sets! Well done!
The bottom line is that hydrocarbon inputs into the gulf were 5,000-10,000 tons of carbon per day from this single well, and that "contrasts starkly" with inputs from natural seepage (220-550 t carbon per day over the entire 70,000 square kilometers of the Gulf of Mexico. The hydrocarbon gases seemed to appear in discrete layers between 1,000 and 1,300 m depth, where concentrations were up to 75,000 times background levels. The ultimate fate of these hydrocarbons is uncertain at present. However, if the geologic record of the past is an indication, it is possible that bacterial activity and consumption of the methane could lead to formation of oxygen-depleted bottom waters.