Abstract

The Half Moon Bay oil field was first developed in the 1880’s and further drilling has been done in each “oil boom” of the past 100 years. The field has produced an estimated 58,000 barrels of oil from about 19 wells within a maximum area of 155 acres. Recent peak field production was 11 BOPD in 1985. Efforts to develop new production have been severely limited by Coastal Zone restrictions.

The main reservoir in the Half Moon Bay oil field consists of very thin sandstone layers within lower Pliocene (and upper Miocene?) mudstones of the Purisima Formation, at depths of 240 to 3,085 ft (73 to 940 m). Oil is high gravity: 43°-55° API. Petroleum is trapped on two separate structural features. Recent drilling has been concentrated in the Verde area on the northwest-trending Purisima anticline, where the Purisima Formation overlies a thick upper Miocene sequence. The potential for Monterey Shale production on that structure has not been adequately evaluated.

Most of the earlier wells and production were located to the northeast in the Purisima Creek area in a fault block on which the Purisima Formation lies unconformably on lower Miocene and Eocene beds. The Eocene Butano Sandstone in that block has produced a very minor amount of oil. The northwest-striking faults which bound that block had significant pre-Pliocene offset. They appear to extend north to join the San Gregorio fault, and south to merge with a previously mapped major fault in the La Honda area. Similarities between the stratigraphic sections in the Half Moon Bay and Point Reyes areas support earlier estimates of about 44 mi (70 km) of right slip along the San Gregorio fault since the end of Miocene time.

[Note: this paper is partially excerpted from A. J. Horn (1983), “The Resurrection of the Half Moon Bay Oil Field, San Mateo County, California”. Because that article emphasized the Purisima Creek area of the oil field, additional sources have been utilized to extend and update its coverage.]