University of California Energy Institute

Understanding the flows of fluids and carbons on continental margins is of great importance to understand their role in carbon budgets one important source of these fluid flows is marine hydrocarbon seeps (cold seeps). Seeps are found on all continental margins (Judd et al., 2002) and are important to global atmospheric budgets of the important greenhouse gas, methane, contributing an estimated 35-45 Tg yr-1 (Etiope and Klusman, 2002). Of this, marine seeps are estimated to contribute ~20 Tg yr-1 (Kvenvolden et al., 2001) or ~13% of natural emissions and primarily arises from methane hydrates and thermogenic source. Methane hydrates are a form of ice that is stable at high pressure and low temperature wherein methane gas is trapped in the ice crystal lattice. Methane hydrate deposits are estimated at 2000 Tg (Collett and Kuuskraa, 1998; Kvenvolden, 1999), and pose a significant climate threat should oceanic warming occur and lead to increased atmospheric greenhouse gases (Kennett et al., 2003; Leifer et al., 2006). Yet, despite methane’s importance of methane to global climate atmospheric budgets, significant uncertainty exists in the sources and sinks. Marine seeps are also an important source of petroleum to the ocean. During the 1990s, natural seeps annually emitted an estimated 600,000 tons (150 M gals) of oil into the ocean, approximately half the annual total oil entering the ocean, ~1,300,000 tons. For comparison, spills from marine vessels accounted for 100,000 tons, terrestrial run-off, 140,000 tons, and pipelines just 12,000 tons. In North America, seeps emit an estimate of 160,000 tons (NRC, 2005).

To date, few quantitative emission rates have been published for gas and even fewer for oil emissions. Gas emission rates have been quantified by sonar quantification (e.g., Hornafius et al., 1999), turbine-tent flow measurements (Leifer and Boles, 2004), and bubble emission 2 measurements (Leifer and MacDonald, 2003). Even fewer quantitative measurements of petroleum emission rates have been published. Methods include estimation from oil slicks (Clester et al., 1996) and direct capture from individual seep vents (Leifer and Wilson, 2004; 2006; Mikolaj and Ampaya, 1973). The latter study showed an increase in oil emission with decreasing tidal depth.Further, in areas where oil and gas are emitted together, petroleum increases the challenge of measuring gas emissions. In this study, we developed and field-tested an approach to allow simultaneous quantification of oil and gas emissions from shallow marine seeps in the Coal Oil Point seep field.