Composition and fate of gas and oil released to the water column during the Deepwater Horizon oil spill

Abstract

Quantitative information regarding the endmember composition of the gas and oil that flowed from the Macondo well during the Deepwater Horizon oil spill is essential for determining the oil flow rate, total oil volume released, and trajectories and fates of hydrocarbon components in the marine environment. Using isobaric gas-tight samplers, we collected discrete samples directly above the Macondo well on June 21, 2010, and analyzed the gas and oil. We found that the fluids flowing from the Macondo well had a gas-to-oil ratio of 1,600 standard cubic feet per petroleum barrel. Based on the measured endmember gas-to-oil ratio and the Federally estimated net liquid oil release of 4.1 million barrels, the total amount of C(1)-C(5) hydrocarbons released to the water column was 1.7 10(11) g. The endmember gas and oil compositions then enabled us to study the fractionation of petroleum hydrocarbons in discrete water samples collected in June 2010 within a southwest trending hydrocarbon-enriched plume of neutrally buoyant water at a water depth of 1,100 m. The most abundant petroleum hydrocarbons larger than C(1)-C(5) were benzene, toluene, ethylbenzene, and total xylenes at concentrations up to 78 μg L(-1). Comparison of the endmember gas and oil composition with the composition of water column samples showed that the plume was preferentially enriched with water-soluble components, indicating that aqueous dissolution played a major role in plume formation, whereas the fates of relatively insoluble petroleum components were initially controlled by other processes.

Fig. 1.

Photograph of fluids exiting the Macondo well being collected on June 21, 2010, with an IGT sampler deployed by the Millennium 42 ROV from the vessel Ocean Intervention III. The IGT sampler, located in the center of the picture, is held by a robotic arm seen on the right side of the image. At the time of collection, the kinked riser section directly above the blowout preventer had been sheared off and the LMRP Top Hat #4 containment system had been placed over the riser stub. The maximum temperatures measured when collecting MW-1 and MW-2 were 105 and 37 °C, respectively.

Fig. 2.

Plots of GOR as a function of (A) date and (B) daily oil collected by the recovery vessels Discoverer Enterprise and Q4000 (19).

Fig. 3.

Water column profiles of BTEX collected at (A) 2.3, (B) 6.1, (C) 16.5, and (D) 27 km from the Macondo well. These stations were within the region of the deep water plume identified by Camilli et al. (1) in June 2010 (See Table S4 for actual BTEX values in these water samples). BTEX composed 2.2% of the total oil in MW-1 (Table S2).

Fig. 4.

Fractionation index (F_i,benzene) as a function of aqueous solubility for 33 different hydrocarbon compounds observed in the southwesterly hydrocarbon plume observed by Camilli et al. (1) at four different stations ranging from 2.3 to 27 km distance from the Macondo well (same stations as shown in Fig. 3). A fractionation index value of 1 corresponds to material that has completely departed the ascending oil, relative to benzene, and contributes to the plume. A low fractionation index corresponds to material that is predominantly retained within the ascending oil. For compounds that are solids at ambient conditions, aqueous solubility refers to the subcooled liquid aqueous solubility; see SI Text for a more complete discussion of this term.