HR: 1340h
AN: OS13B-0538    [Abstracts]
TI: Factors Affecting the Temporal and Spatial Variability and Characteristics of Marine Hydrocarbon Seepage, Coal Oil Point, CA
AU: * Del Sontro, T S
EM: tonya_delsontro@umail.ucsb.edu
AF: Univ. California, Santa Barbara, Dept. Geological Sciences Webb Hall, Santa Barbara, CA 93106
AU: Leifer, I
EM: ira.leifer@bubbleology.com
AF: UCSB - Eng. Research Center, 6740 Cortona Dr, Goleta, CA 93117
AU: Luyendyk, B
EM: luyendyk@geol.ucsb.edu
AF: Univ. California, Santa Barbara, Dept. Geological Sciences Webb Hall, Santa Barbara, CA 93106
AB: The Coal Oil Point (COP) natural marine hydrocarbon (HC) seep field of the Santa Barbara Channel is one of the largest and most intensively studied marine HC seepage regions. Daily oil emissions were estimated at $\sim$100 barrels, while total gas emissions reach $\sim$100,000 m$^{3}$day$^{-1}$. The COP seep field consists of several intense areas of seepage that are each made up of dozens to hundreds or thousands of individual vents. Observations show that COP seepage varies spatially (cm-km), temporally (second - decade), and in magnitude (from trace to 10$^{4}$ m$^{3}$day$^{-1}$). The primary seepage trends (scale of 100s-1000s meters) lie above major WNW-ESE faults cutting the Coal Oil Point and South Ellwood anticlines. There is significant variation in seepage along these trends with the most intense emissions at intersections with NE-SW cross-cutting faults. Spatial variations on shorter scales (10s of meters) are related to fractures, exposed shale beds, and seabed characteristics (pebbles, sand, tar, etc.). Individual vent distribution varies on sub-meter scales. Seabed characteristics, seepage spatial distribution, and gas and oil emissions relate to seabed and subsurface geologic features. For example, bubble sizes are correlated with the substrate from which they are emitted. Small-sized bubbles ($<$0.1 cm diameter) emerge directly from fractures in the exposed shale. Larger-sized bubbles (0.1 $<$ 1.0 cm) tend to escape after passing through a sediment overburden, with the size of the bubbles increasing with the sediment coarseness. Giant bubbles ($>$1.0 cm) are associated with a tar-sand overburden. The bubble size is an important factor with regards to bubble-mediated transport of seep gas and oil to the sea surface. A network of small conical tents was developed and deployed in active seep areas to gather high time resolution (0.2 s) gas emission rates that also had high spatial resolution (1 m). The tents ($\sim$1-m tall, $\sim$2-m diameter) sit directly on the seabed over areas of seepage, typically spanning several vents. Rising bubbles generate an upwelling flow of water that spins a turbine with an optical encoder on its axis. The encoders are connected to a multi-channel datalogger. Measurements are recorded in revolutions per second and converted to gas volume flux based on laboratory calibration. Spectra of the seepage time series showed the effect of external forcings, including swell and tides. Responses to external forcing factors, such as a 1% swell variation, differ between multiple tents. Seeps with a higher flux exhibit a smaller response than seeps with a lower flux. Flux variations between tents demonstrate the complexity of the underlying processes of gas, oil, and tar migration through an inter-connected subsurface fracture network.
DE: 4219 Continental shelf processes
DE: 4251 Marine pollution
DE: 4294 Instruments and techniques
DE: 4504 Air/sea interactions (0312)
DE: 4599 General or miscellaneous
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting