Abstract: Historical Oil Wells in Los Gatos, Santa Clara County, California: Implications for Tectonics, Resources, and the Environment

Richard G. Stanley

Between 1891 and 1929, at least 12 wells were drilled for oil and gas in Los Gatos. Many of the wells found small amounts of petroleum in shale and sandstone of the Miocene Monterey Formation(?), but no commercial production was established. At least one pump unit is still standing.

Several wells were drilled on the flatlands near surface traces of the seismically active Berrocal fault system; these wells provide subsurface control for structural cross-sections. Most wells encountered the base of surficial deposits (or the base of the Pliocene and Pleistocene Santa Clara Formation) at depths shallower than 400 ft (120 m). At least two wells reached total depths of 2,675 ft (815 m) or more in the Monterey Formation(?), suggesting that an adjacent negative gravity anomaly (the Cupertino low of Robbins, 1971) reflects a sedimentary basin containing low-density shale and sandstone deposited during Miocene subsidence. Eocene and Upper Cretaceous strata also may be present in the Cupertino low but have not been recognized in the Los Gatos wells.

The origin of the oil is unknown. Perhaps hydrocarbons were generated in organic-rich shale of the Monterey Formation(?) in the Cupertino low, and then migrated updip into the Los Gatos area where they accumulated in traps related to folds and thrust faults of the Berrocal fault system. Hydrocarbons associated with mercury ore at the New Almaden mines (about 11 km southeast of Los Gatos) may have migrated from the Cupertino low, or may have been generated locally from Miocene or older strata by hydrothermal heating and (or) burial beneath overthrust rocks of the Franciscan Complex.

The petroleum resource potential of the Los Gatos area is uncertain but probably small, based on comparison with the geologically similar but larger Sargent-Hollister basin. Further exploration near Los Gatos is unlikely owing to extensive urbanization. None of the old oil wells are known to have contaminated the surface environment, but drillers and public agencies should be aware that deep boreholes for water or other purposes in the Los Gatos area may encounter accumulations of hydrocarbons.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California