Evidence for an oil-bearing sedimentary basin of probable Miocene age beneath {open_quotes}Silicon Valley,{close_quotes} California
- Geological Survey, Menlo Park, CA (United States); and others
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-dipping 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 ({delta}{sup 13}C{sub PDB}) composition of -23.32{per_thousand}, suggesting derivation from a Miocene source. Oil in mercury ore from the nearby New Almaden mine is slightly biodegraded, thermally altered, and shows a {delta}{sup 13}C PDB value of -24.70{per_thousand}, indicating a source different from that of the Los Gatos oil and perhaps of Miocene or Cretaceous age.
- OSTI ID:
- 425880
- Report Number(s):
- CONF-960527-; TRN: 96:004994-0521
- Resource Relation:
- Conference: Annual convention of the American Association of Petroleum Geologists, Inc. and the Society for Sedimentary Geology: global exploration and geotechnology, San Diego, CA (United States), 19-22 May 1996; Other Information: PBD: 1996; Related Information: Is Part Of 1996 AAPG annual convention. Volume 5; PB: 231 p.
- Country of Publication:
- United States
- Language:
- English
Similar Records
Historical oil wells in Los Gatos, Santa Clara County, California: Implications for tectonics, resources, and the environment
Chemistry and origin of Miocene and Eocene oils and tars in the onshore and offshore Santa Cruz Basins, California