USGS/BOEM study identifies method to differentiate between natural seepage and produced oils in waters off Southern California
Resource managers now have a method to determine the source of oil found in the waters off Southern California and differentiate between naturally seeped oils and those produced by offshore oil and gas production. This method is detailed in a new joint report from the US Geological Survey (USGS), the Bureau of Ocean Energy Management (BOEM), and academic and industry collaboration.
The report—Biomarker Chemistry and Flux Quantification Methods for Natural Petroleum Seeps and Produced Oils, Offshore Southern California—covers a joint USGS and BOEM 10-year series of studies of natural oil seeps mainly from the Santa Barbara Channel west of Los Angeles, California.
The study built up a set of initial studies in which biomarker and stable carbon isotope ratios were used to infer the age, lithology, organic-matter input, and depositional environment of the source rocks for 388 samples of produced crude oil, seep oil, and tarballs mainly from coastal California. The analysis resulted in a predictive model of oil source families that could be applied to samples of unknown origin.
Results of the original model identified three distinct types—“tribes”—of 13C-rich oil samples that were inferred to originate from thermally mature equivalents of the upper siliceous, middle shale, and lower calcareous units of the Monterey Formation.
Tribe 1 contains four oil families that have geochemical traits of clay-rich, marine-shale source rock deposited under suboxic conditions with substantial higher-plant input.
Tribe 2 contains four oil families that have intermediate traits, except for abundant 28,30-bisnorhopane, indicating suboxic to anoxic marine-marl source rock with hemipelagic input.
Tribe 3 contains five oil families that have traits of distal marine-carbonate source rock, deposited under anoxic conditions with pelagic but little or no higher-plant input.
Specific objectives of this new study were:
Identify new areas of hydrocarbon seepage that are known to occur near OCS platforms not sampled during the previous study;
Geochemically fingerprint new representative oils from the OCS platforms;
Geochemically fingerprint select coastal tar residues associated with unusual coastal oiling events;
Sample additional submarine seeps to strengthen correlations between offshore active seeps and coastal residues; and
Quantify the discharge rates of select natural seeps and attempt to scale such results into a regional perspective of natural oil and gas seepage rates.
The analysis found that oil from platforms north of Point Conception (Irene, Hidalgo, Harvest, and Hermosa) is chemically different than seep oil and can be unequivocally distinguished. Other platform oils need to go through a more extensive biomarker analysis to distinguish them from natural seepage.
During the course of study, oil samples were taken from offshore natural seeps and federal offshore oil and gas production platforms to develop a rigorous chemical fingerprinting method that will allow scientists to determine the origin of individual tarballs within the first month or two of seepage or spill deposition. This chemical protocol can be used to verify the source and environmental extent of oil deposited by natural seep events, which are known to occur regularly on Santa Barbara Channel beaches, or by a man-made source.
Our studies support the hypothesis that natural oil seepage from seafloor vents are responsible for the majority of tarball accumulation on southern California beaches. Oil fingerprinting provides the crucial tool to verify the origin of this deposited oil. While our study results are persuasive, they are not conclusive, because they depend on the assertion that beached or floating tarballs, by their inherent characteristics, are very recently deposited.
We found three primary areas of seepage currently active in the Santa Barbara Channel: Point Conception, Sacate and Gaviota beaches, and Coal Oil Point. We also found that only a small fraction of tarballs did not correlate with California derived oils and are most likely from unknown ship or land-based discharges into the ocean. Produced oil from offshore platforms can often be ruled out as the origin of tarballs through the fingerprinting process, because platform oil is not significantly biodegraded.
The ability to distinguish between biodegraded oils diminishes with time, and, under typical conditions, most spilled platform oil could resemble seep oil residues and seep-derived tarballs in about one month. The ability, however, to distinguish between seep-derived oil residues and platform oils within this time span is extremely valuable to regulators responding to an oil spill incident. The four platforms north of Point Conception produce oil that can be fingerprinted on the basis of chemistry alone without the need to consider biodegradation, and can thus be distinguished from known natural oil seeps in and offshore California. Finely calibrated multibeam sonar techniques can produce detailed images of discharging plumes that could possibly be modeled to obtain volumes and discharge rates.—Lorenson et al.
The hundreds of completed fingerprints derived from the studies populate a shared library now used by the US Coast Guard, California Office of Spill Prevention and Response, USGS and BOEM. The state is also using this chemical protocol to determine the origin of oil on oiled wildlife.
The research was conducted in three stages from 2001-2011 and was conducted by the Bureau of Ocean Energy Management, the US Geological Survey, the University of California at Santa Barbara and the Monterey Bay Aquarium Research Institute. A diverse review group with scientific experts in various disciplines from the State of California, County of Santa Barbara, academia and industry independently reviewed the data throughout the study.
Lorenson, T.D., Leifer, I., Wong, F.L., Rosenbauer, R.J., Campbell, P.L., Lam, A., Hostettler, F.D., Greinert, J., Finlayson, D.P., Bradley, E.S., and Luyendyk, B.P., (2011) Biomarker chemistry and flux quantification methods for natural petroleum seeps and produced oils, offshore southern California: U.S. Geological Survey Scientific Investigations Report 2011-5210, 45 p. and OCS Study BOEM 2011–016.