Chemistry and Origin of Miocene and Eocene Oils and Tars in the Onshore and Offshore Santa Cruz Basins, California

Alan S. Kornacki and Robert I. McNeil

The Santa Cruz (La Honda) Basin is a small 'slice' of the San Joaquin Basin that has been displaced c. 300 km to the northwest by the San Andreas Fault. The poorly-explored offshore area that now lies within the Monterey Bay NMS includes portions of the Outer Santa Cruz and Bodega basins. A modest amount (c. 1.3 MM bbl) of variable-quality oil has been produced from Eocene and Pliocene pay zones in the La Honda Field. Much smaller amounts of light oil (>=40° API) have been produced from three other fields (Oil Creek; Moody Gulch; Half Moon Bay). Large tar deposits also outcrop near the city of Santa Cruz. Proven source rocks in this basin include the Eocene Twobar Shale and three Miocene units: the Lambert Shale, Monterey Formation, and the Santa Cruz Mudstone.

A high-gravity oil sample from the Oil Creek Field contains isotopically-light carbon (^dgr¹³C = -28.2 per mil) and has a relatively high pristane/phytane ratio. This oil was generated at high temperature (c. 140°C) by pre-Miocene source rocks (probably the Twobar Shale). The presence of isotopically-heavy carbon in all other oil and tar samples demonstrates they were generated by Miocene source rocks. But the C₇ oil-generation temperatures, sulfur content, vanadium/nickel ratios, and biomarker chemistry of these Miocene oils are significantly different than in Monterey oils from the prolific Santa Maria Basin (SMB). The sulfur content (8.0 wt%) and V-Ni chemistry of tarry petroleum recovered in the P-036-1 well (Outer Santa Cruz Basin) resembles the chemist y of very heavy (<15° API) oils generated by phosphatic Monterey shales in the SMB.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California