Evidence for an Oil-bearing Sedimentary Basin of Probable Miocene Age beneath "Silicon Valley," California
Richard G. Stanley, Robert C. Jachens, Keith A. Kvenvolden,
Frances D. Hostettler, Leslie B. Magoon, and Paul G. Lillis
Gravity anomalies, records from historical oil wells, and new organic geochemical results indicate the presence of a concealed sedimentary basin of probable Miocene age beneath a densely populated part of the Santa Clara Valley. A prominent isostatic gravity low extends about 35 km from Palo Alto to near Los Gatos and reflects an asymmetric, northwest-trending sedimentary basin comprising low-density strata that rest on higher-density rocks of the Franciscan Complex. Both gravity and well data show that the low-density rocks thin gradually to the northeast over a distance of about 10 km. The thickest basin fill (about 3 km) occurs along the basin's steep southwestern margin, which evidently is controlled by northwest-trending, southwest-dip- ping reverse and thrust faults of the seismically active Berrocal, Shannon, and Monte Vista fault systems. Near surface traces of these faults in Los Gatos, at least 12 wells were drilled between 1891 and 1929 to total depths as great as 840 m. No lithologic or paleontologic samples are available from the wells, but drillers' logs indicate thick intervals of brown shale and sandstone resembling nearby outcrops of the Miocene Monterey Formation. Small amounts of oil and gas were encountered in several wells, but no commercial production was established. Oil from a well in Los Gatos is highly biodegraded, contains biomarkers commonly found in oils derived from the
Monterey Formation, and yields a stable carbon isotopic (^dgr13CPDB) composition of -23.32^pmil, suggesting derivation from a Miocene source. Oil in mercury ore from the nearby New Almaden mine is slightly biodegraded, thermally altered, and shows a ^dgr13CPDB value of -24.70^pmil, indicating a source different from that of the Los Gatos oil and perhaps of Miocene or Cretaceous age.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California