315 



Holly had been thoroughly documented as an area of intense seepage by oil industry 

 surveys in the 1940's, 1950's, and 1970's (Fischer and Stevenson, 1973). Based on this 

 finding, a research grant was obtained firom the University of California's Energy Study 

 Program to acquire new high-resolution seismic data in 1995 to compare with data 

 obtained in 1973. The next overhead shows a comparison of data acquired between 

 Platform Holly and the seep tents in 1973 and 1995. 



The top of this figure (#3) shows the intense seepage around Platform Holly that 

 was recorded by an acoustic survey in 1973. The figure shows dark areas in the water 

 column that correspond to rising plumes of bubbles. The bubbles themselves are 

 composed mostly of methane, but also contain ethane, propane, and other constituents. 

 The bottom of the figure shows the seep activity in 1995 along the same line. Note the 

 dramatic reduction in seepage that has occurred over this section which covers a 

 horizontal distance of about one mile. Quantitative analysis of this data indicates about a 

 90% reduction in seepage within one mile of Platform Holly over the past 22 years. The 

 area around Platform Holly is the only area that has experienced a reduction in seepage, 

 based on our geophysical surveys. The reduction in seepage around Holly may result fi-om 

 a reduction in reservoir pressure due to withdrawal of oil. 



Another line of evidence indicating a decreasing seepage rate around Platform 

 Holly comes fi'om oceanographic surveys which measure dissolved hydrocarbon 

 concentrations in the ocean downcurrent fi-om the seep field. The dissolved concentration 

 can be used as a measure of seep activity, because seep gases partially dissolve in the 

 ocean as they rise fi^om the seafloor (Fischer, 1 978). Dissolved hydrocarbon components 

 are carried downcurrent, where they have been measured in past surveys. In 1995 we 

 measured dissolved hydrocarbon concentrations in an area that was surveyed in 1981 by 

 InterOcean Systems, Inc. of San Diego California. We have been woricing with InterOcean 

 in the synthesis of these data sets. This figure (#4) compares propane concentrations 

 between 1981 and 1995. The peak concentrations measured in 1981 were nearly ten times 

 as high as in 1995. Although ocean currents also influence hydrocarbon concentrations, 

 this is a larger reduction than can probably be accounted for by currents alone. However, 

 we are collecting oceanographic measurements and are using computer models to 

 understand the effects of currents on the concentrations. Some of the data that we are 

 using in our analysis has been generously supplied by the Coastal Oceanography Group at 

 the Scripps Institution of Oceanography. Our observation of a change in dissolved 

 hydrocarbon concentrations is consistent with a decrease in seep activity since 1981. 



If the hypothesis that oil production has reduced natural hydrocarbon seepage is 

 true, then some local and national implications need to be explored. At the local level, 

 reduced seepage will lower the total amount of reactive organic gases released into the 

 atmosphere offshore from Santa Barbara. It will also lower the amount of tar and oil in 

 local waters. Reactive organic gases are precursors to smog-forming ozone which is a 

 health hazard. Tar and oil wash up on our beaches and diminish their recreational and 

 aesthetic values. 



