Composition and fate of gas and oil released to the water column during the <i>Deepwater Horizon</i> oil spill

TLDR

Understanding the gas‑to‑oil composition of the Macondo well during the Deepwater Horizon spill is essential for estimating flow rates, total volume released, and the environmental fate of hydrocarbons. Samples were collected above the Macondo well on June 21 2010 using isobaric gas‑tight samplers and analyzed for gas and oil composition. The Macondo well released fluids with a gas‑to‑oil ratio of 1,600 scf / barrel, yielding ~1.7 × 10¹¹ g of C1‑C5 hydrocarbons; plume analysis showed enrichment of water‑soluble aromatics (benzene, toluene, ethylbenzene, xylenes) up to 78 μg L⁻¹, indicating aqueous dissolution dominated plume formation while insoluble components behaved differently.

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.