YUN, JANET W., Earth Sciences Department, University of California, Santa Cruz, CA; MICHAEL E. FIELD, U.S. Geological Survey, Menlo Park, CA; SARA S. FOLAND, Farallon Energy Group Ltd., Denver, CO; DANIEL ORANGE and J. CASEY MOORE, University of California, Santa Cruz, CA

Abstract: Gas Occurrance From the Subsurface to the Water Column: An Integrated Study in the Offshore Eel River Basin, CA

An integrated approach is used to investigate the occurrance of gas from the subsurface to the water column in the offshore Eel River Basin in northernmost California. Inferred locations of gas in the subsurface are correlated with seafloor and water column data in an effort to better understand fluid migration pathways. Integration of multiple datasets provides information about fluid migration that would not be observed using single datasets independently.

Gas occurrance in the subsurface is inferred primarily by use of seismic reflection data of varying frequencies, ranging from commercial multichannel seismic reflection data (~20 Hz) to high frequency single channel data (~1 kHz). Inferred locations of gas from seismic reflection surveys are correlated between data sets, and with seafloor bathymetry, side-scan sonar, and acoustic backscatter data. These data are additionally correlated with water column geochemical data and water column gas plumes inferred from very high frequency (3.5 kHz) seismic reflection data. Gas seeps identified by geophysical methods are then groundtruthed using remotely operated vehicles.

We find that indicators of gas in the subsurface, surface, and water column vary in location, and are not always coincident. Complex structural geology identified in seismic reflection records can be correlated with some fluid expulsion sites, but is not deterministic of the presence of gas in the subsurface. Additionally, seafloor indicators of gas expulsion, such as pockmarks, suggest that fluid expulsion occurs episodically and catastrophically on the upper continental slope. However, active gas venting observed on the continental shelf is not correlated with identified pockmarks, and some vent sites are correlated to underlying faults.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California