An integrated approach is used to investigate the occurrence 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 occurrence 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.
2000 AAPG Annual Meeting
Marching into Global Markets -- A World of Resources