Modeling Trace Gases in Hydrocarbon Seep Bubbles. Application to Marine Hydrocarbon Seeps in the Santa Barbara Channel

Evaluating the importance of natural marine hydrocarbon seeps to global methane budgets requires correctly predicting the gas fraction lost by the seep bubbles during transit through the water column. In the Santa Barbara Channel, California, three widely differing seeps (depth, flux, oiliness) were visited and observations were made of seep gas partial pressures, Pi, near the surface for alkanes to n = 5 (pentane) as well as major atmospheric gases, and upwelling flows. It was found that alkane Pi decreased exponentially with alkane diffusivity. Seabed seep gas was available for one seep, and exhibited the same trend. For alkanes heavier than methane, the ratio of the surface to sea floor mole fraction showed a linear enhancement with increasing alkane number. Methane behaved differently because the water column became saturated. A numerical model was developed to study the sensitivity to environmental parameters of the bubble transport of seep gas to the surface. It was found that seep gas transport is highly sensitive to both upwelling flows, dissolved gas concentrations, bubble surface cleanliness, and bubble size. The model predicted that upwelling flows were increased bubble survivability and transport to the surface. It was also predicted that dissolved methane concentrations higher than 0.01 atm increased bubble survivability. When applied to simulating alkanes, the studies showed that only a narrow range in bubble size could explain the observed alkane enhancements. Thus model predictions were in agreement with the observation that a wide size range of bubbles was not observed at the sea surface.