Evidence for Shallow Hydrocarbons Offshore Northern Santa Cruz County, California: GEOLOGIC NOTES

Abstract

Analyses of high-resolution seismic reflection profiles and hydrocarbon samples indicate that natural hydrocarbon seepage is occurring along the San Gregorio and Monterey Bay fault zones offshore northern Santa Cruz County, California. A variety of anomalous seismic reflection features such as water-column anomalies, subsurface amplitude anomalies (bright spots), and seismic smears/wipeouts has been observed and mapped. More than 100 water-column anomalies (probable gas seeps) occur in the study area of approximately 270 mi2 (700 km2). Many of these seismic anomalies are associated with subsurface geologic structures, which suggests hydrocarbon migration from depth. Samples of natural gas collected from a shallow coastal water well contain 74 to 91% methane, 7 to 23% nitrogen, ~2% carbon dioxide, and < 1% ethane. The methane appears to be thermogenic in origin, having ^dgr13C values of -29.51 to -32.55^pmil PDB. Rock dredges from 2,300 ft (700 m) of water in Ascension Submarine Canyon have also recovered oil-saturated sandstones, further suggesting the seepage of hydrocarbons. We interpret the shallow occurrence of most of these hydrocarbons to be the result of migration from depth along active faults within the San Gregorio and Monterey Bay fault zones.