A Platform Holly Seep Acoustic Observatory

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Cal State Lands Commission
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Hydrocarbon seepage to the ocean from underlying petroleum deposits is a natural process known to occur worldwide, and is estimated to release more than 600,000 tonnes of oil to the ocean annually (National Research Council 2003). The Santa Barbara Channel hosts natural hydrocarbon seepage that dates back at-least 40,000 years (Valentine et al. 2010; Hill et al. 2006) and likely much further (Boles et al. 2004), with seeping petroleum used by the Native American tribes prior to Spanish settlement of the area. The seepage in this region is especially prevalent in the vicinity between Campus Point and Coal Oil Point, the so-called Coal Oil Point seeps. The seeps at Coal Oil Point have been described as the world’s most spectacular (Hornafius, Quigley, and Luyendyk 1999), and are among the most voluminous – though most seeps remain unstudied. These seeps release oil and gas, with quantities once estimated at 100 barrels of oil and 1.7±0.3×105 m3 of gas, daily (Hornafius, Quigley, and Luyendyk 1999).

The offshore oil platform, Holly, is located within the Coal Oil Point seep field, where it has been producing oil and gas since 1968. The hydrocarbon extraction from Platform Holly has been linked to a reduction in flux rate of gas (Boles 2015; Quigley et al. 1999), for at-least one location, evidence that has been used to formulate the hypothesis that industrial hydrocarbon production reduces seepage rates of oil and gas proximal to the production area. While available evidence suggests a linkage between gas seepage and hydrocarbon production, several uncertainties have hindered the general acceptance of this hypothesis. These uncertainties include: i) reservoir connectivity – that is, produced wells must intersect the reservoir or conduits that feed seepage; and ii) the exclusive use of gas flux to relate production to seepage – that is, changes in oil flux from the seeps has not been rigorously quantified despite being the measure of greatest relevance to local stakeholders.

In May, 2015 the Refugio Pipeline Oil Spill forced a shut-in at Platform Holly, with abandonment planned imminently. Within the weeks following the shut-in exceptional quantities of oil were observed at the sea-surface and reported at the proximal beaches near Coal Oil Point. While largely anecdotal and subject to reporting bias, experienced observers noted increases in seep surface oil coverage, distance of slick travel, quantities of oil washing ashore, and instances of bird oiling. The highly publicized effects of these oiling anomalies again brought to the fore the linkage between historical production and seepage.

The shut-in at Platform Holly, combined with advances in broadband acoustic technology provide for a unique opportunity to assess the relationship between hydrocarbon production and natural seepage. Specifically, Platform Holly has been in production since the 1960’s and with production remaining shut in since 2015, the reservoirs are expected to increase in pressure with time with potential impacts to rates of natural seepage. This situation creates a unique opportunity to survey the current baseline of hydrocarbon seepage, using broadband acoustic technology, and through the establishment of an observatory, to monitor the time series changes in seepage for extended duration and at high resolution of data collection.

A collaborative team was assembled to conduct this research, including a seep expert from UCSB and an acoustician specializing in marine hydrocarbon seeps, from the University of New Hampshire. The team has worked together previously, including during the Deepwater Horizon event in the Gulf of Mexico, the 2013 Nautilus Live cruises, during the SEEPS 2015 expedition to the Gulf of Mexico, and for a US DOI funded precursor project. Collectively the PI’s have published over 100 papers in the peer reviewed literature, many of which are relevant to the research proposed here. A select listing of relevant papers are included in the biographies of the two PI’s (Exhibit A5). The research team provides complimentary expertise with Valentine providing local seep expertise in combination with gas and oil chemistry expertise, and Weber providing geophysical expertise in seep acoustics.