The Coal Oil Point Seeps

The Coal Oil Point Seep Field

Introduction

The Coal Oil Point (COP) seep field is one of the largest and best studied areas of active marine seepage in the world, and is located a few kilometers from the University of California at Santa Barbara campus. Studies have quantified the COP seep field area (Allen et al., 1970; Fischer & Stevenson, 1973) and emission fluxes in the Santa Barbara Channel (e.g., Hornafius et al., 1999; Quigley et al., 1999; Clark et al., 2000) based on sonar techniques, flux buoys, and direct gas capture. Since the late 1990s, the UCSB seep group has mapped the seeps in the area using sonar and quantified seepage flux from sonar and direct gas capture using a flux buoy (Washburn et al., 2001). Results indicate that ~1.5 x 10⁵ m³ dy^-1 (5 x 10⁶ ft³ dy^-1) of seep gas escapes from ~3 km² of sea floor to the atmosphere (Hornafius et al., 1999) with roughly an equal amount injected into the coastal ocean (Clark et al., 2000).

Map of the Coal Oil Point seep field and informally named seeps in Santa Barbara Channel. A) shows the southwest US coast; B) shows the Santa Barbara Channel. Gray rectangle indicating the location of the seep field. C) shows details of the seep field with gray areas for high bubble density from sonar returns (Hornafius et al., 1999). from Leifer and Boles (2005).

Most seepage is located along linear trends above faults or fractured anticlines. The shallowest seepage is in 5 15 m water nearshore to COP and includes the Cliff House Seeps and Sands Seeps, and do not release oil. The inner trend is at ~20-m depth and includes the Farrar Seep, IV Super Seep, and Shane Seep. A second trend at ~40-m depth includes the Horseshoe and Coal Oil Point Seeps. The deepest trend is at ~70-m depth and includes the La Goleta Seep and Seep Tent Seeps as well as Platform Holly. This trend corresponds to the intersection of the South Ellwood Fault with the ocean floor (Fischer, 1978). Seepage continues to occur along the S. Ellwood Fault despite recharging of sub-hydrostatic reservoir pressure by seawater moving down the fault (Boles & Horner, 2003), indicating that buoyancy drives the flow along large fractures.

History

The hydrocarbon seeps in the Santa Barbara Channel have a long history. Oil and gas leaks from fractured Miocene-age Monterey formation through faults, fractures and outcroppings to the seabed and coastal lands. The largest and most intense seepage occurs at the northern edge of the Santa Barbara Channel, in the Coal Oil Point seep field. Geologic evidence suggests that seepage has been ongoing for at least 500,000 years.

The history of the seeps is intertwined with that of human exploitation of this useful, natural resource. Approximately 7,000 years ago, the Chumash Indian community was solidly established along the California coast including the Santa Barbara-Ventura basin. Early explorers noted that the natives used the tar to caulk their canoes and make other items water proof. In 1769 Father Crespi observed the Chumash building boats near active tar seeps and named the area "La Carpinteria".

These natural petroleum seeps proved to be a lucrative resource for Santa Barbara and California. During the first half of the 19th century, tar and oil of the natural seeps in the Santa Barbara County and Channel region of the basin was used to "lubricate wagon wheels and farm machinery" and slowly replaced whale oil. Asphalt mining flourished within Goleta and Carpinteria during the mid-1800s, paving the first streets of San Francisco and Santa Barbara. The first oil wells in Santa Barbara County were located in Summerland, dug by hand in 1877, and in 1898, the world's first offshore oil well was drilled from a pier.

Oil seepage was used by the early explorers to identify accessible petroleum, and helped California become the dominant oil production state in the early 1900s in the US. Offshore production peaked in the late 1920s with the Rincon and Elwood (original spelling) fields. By 1930, 29 wells along over a dozen piers made the Elwood field credible for 6% of all California's oil production. Production of the South Ellwood field began in 1967 with the erection of Platform Holly. Tar was mined from areas in Santa Barbara Channel, including a tar mine located on the current UCSB campus. This tar played an important roll in paving the streets of Stanta Barbara in the early days of the 1900s.

Below is an aerial image of the Coal Oil Point seep area in the late 1960s. Oil seepage is clerly identifiable at the sea surface, extending in a linear trend about 10 degrees west of north. The trend is related to a crossing fault. While today there is significant seepage in this area including oil, the line of surfacing oil slicks have not been observed for many years.

Aerial image of the linear trend of seepage offshore of Coal Oil Point (looking North). This trend extends for ~1/4 mile (Wilkinson, 1972).

The Coal Oil Point seep field has seeps spanning a phenomenal range of emission fluxes, oil to gas ratios, and tar emissions. Seeps in the field can be single streams of bubbles, to intense plumes that generate strong upwelling flows and other important plume processes. While some seeps are pure gas, others produce copious oil emissions. Chemical analysis has shown enormous variation in the level of weathering of the emitted oil in the freshly surfaced oil at the sea surface, as well as tar and asphalt. Most of this tar ends up on the beaches at the Coal Oil Point, which contains at least 100 times more tar than any other location (Allen et al., 1970). However, tar from Coal Oil Point can be found throughout the Santa Barbara Channel, and even as far away as Alaska.

References

   Allen, A., Schlueter, R., and Mikolaj, P. (1970) Natural oil seepage at Coal Oil Point, Santa Barbara, California. Science, v170, pp974-977.
   Bartlett, W.L. (1998) Elwood oil field: Santa Barbara County, California. Structure and Petroleum Geology of Santa Barbara Channel, CA. Pacific Section AAPG Miscellaneous Pub. MP-46, pp 217-237.
   Clark, J.F., L. Washburn, J.S. Hornafius, and B.P. Luyendyk, Natural marine hydrocarbon seep source of dissolved methane to California coastal waters, Journal Geophysical Research - Oceans 105, 11,509-11,522, 2000.
   Galloway, J. (1998) Chronology of petroleum exploration and development in the Santa Barbara Channel area, offshore southern California. Structure and Petroleum Geology of Santa Barbara Channel, CA. Pacific Section AAPG Miscellaneous Pub. MP-46, pp 1-12.
   Hornafius, J.S., D.C. Quigley, and B.P. Luyendyk, The world's most spectacular marine hydrocarbons seeps (Coal Oil Point, Santa Barbara Channel, California): Quantification of emissions, Journal Geophysical Research - Oceans 104 C9, 20703-20711, 1999.
   Leifer I., and J. Boles (2005). Measurement of marine hydrocarbon seep flow through fractured rock and unconsolidated sediment. Marine Petroleum Geol., 22(4), 551-568.
   Quigley, D.C., J.S. Hornafius, B.P. Luyendyk, R.D. Francis, J.F. Clark, and L. Washburn, Decrease in Natural Marine Hydrocarbon Seepage near Coal Oil Point, California Associated with Offshore Oil Production, Geology, 27 (11), 1047-1050, 1999.
   Wilkinson, E.R. (1972) California offshore oil and gas seeps. California Division of Oil and Gas, Misc. Publication C.
   Washburn, L., C. Johnson, C.G. Gotschalk, and E.T. Egland, A gas capture buoy for measuring bubbling gas flux in oceans and lakes, Journal of Atmospheric and Oceanic Technology 18, 1411-1420, 2001.