Geochemical Study of Petroleum Weathering from Sea Surface: Identification of Slicks' Sources and Implications for Petroleum Systems Analysis
Alexei V. Milkov1, Antonio Hospedales2, and Eric Thibodeaux1
1BP America, Houston, TX
2BP Trinidad & Tobago, Port of Spain, Trinidad and Tobago
Several oil sheens/slicks were observed and sampled at the sea surface in the Gulf of Mexico shelf after Hurricane Katrina. Some of these sheens/slicks were geochemically fingerprinted to petroleum samples taken from wells that were producing prior to the hurricane. This information was useful in identifying potentials sources of leaks from hurricane-damaged facilities. Basic gas chromatographic analysis revealed various levels of weathering of petroleum in sheens/slicks. Evaporation on sea surface is the main mechanism of oil weathering, and it affects composition of slicks/sheens quickly (within hours/days). Evaporation results in a gradual removal of lighter petroleum components, which was also reproduced in laboratory evaporation experiments. Light normal alkanes and isoprenoids up to nC20 were removed in most sampled slicks/sheens, which limited feasibility of their simple (e.g., using Pristane/Phytane ratio) geochemical fingerprinting to oils. For the least evaporated slicks/sheens (they spent only a short time on sea surface), detailed fingerprinting of inter-paraffin peaks in the nC20-nC23 range assisted in determination of sources of sheens/slicks. In more significantly evaporated sheens/slicks, inter-paraffin peaks after nC20 were altered, and more stable biomarkers were used to confirm the source of sheens/slicks. In the most evaporated sheen/slick, many source-specific (e.g., tricyclic terpanes) and maturity-specific (e.g., triaromatic steranes and methyldibenzothiophenes) biomarkers were affected. This finding implies that biomarkers should be used cautiously when one characterizes deep petroleum systems using sea surface sheens/slicks.
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