Geologic structures in the study area were identified using dart core data, seafloor bedding attitudes, 3.5 kHz and 1 kJ sparker seismic reflection data. Oil platform Holly lies above the anticline and produces oil from the Monterey Formation reservoir at 900-1350 meters subsea. Oil and gas in the Monterey Formation are trapped by the overlying Sisquoc formation cap rock, which is exposed on the seafloor. Oil and gas migrate through the cap rock along fractures on the anticlinal axis. To the north of the South Ellwood anticline, a similar yet shorter linear trend of seeps is controlled by what is interpreted to be the Coal Oil Point fold complex.
Seepage appears to occur where anticline axes and synclinal limbs intersect the seafloor.
More intense seepage occurs where axial surfaces are cut by north east-trending cross faults. Therefore, we conclude that seepage in the northern Channel is controlled by mostly folding but also faulting of the reservoir cap rock. SF6 was injected continuously via bubbles from a bottom-mounted source in the vicinity of the largest hydrocarbon seep. Surveys of hydrocarbons and SF6 were conducted for about 20 km downstream over a period of 2 days. The SF6 plume was resolved with more than 20 crossings approximately perpendicular to the current direction at three depth intervals.