Cordilleran Section - 97th Annual Meeting, and Pacific Section, American Association of Petroleum Geologists (April 9-11, 2001)

Paper No. 0
Presentation Time: 3:50 PM


STANLEY, Richard G., U.S. Geological Survey, 345 Middlefield Rd., MS 969, Menlo Park, CA 94025 and LILLIS, Paul G., U.S. Geological Survey, P.O. Box 25046, MS 977, Denver, CO 80225,

Outcrops of oil-bearing Miocene strata occur east of the San Gregorio fault near Davenport (Santa Cruz County) and west of the fault near Point Reyes (Marin County). In both areas, much of the oil is found in sandstone dikes and sills that intrude fine-grained siliceous strata of the upper Miocene Santa Cruz Mudstone. The sandstone intrusions are thought to have formed when liquefied, petroleum- and water-charged sands from the upper Miocene Santa Margarita Sandstone were injected upward into the overlying Santa Cruz Mudstone.

Samples of oil from the Davenport and Point Reyes sandstone intrusions and from the nearby Half Moon Bay and La Honda oil fields were analyzed for stable carbon isotopic composition and biomarkers. The results show that (1) all oil samples analyzed were derived from source rocks of Miocene age, and (2) the Davenport and Point Reyes oil samples are similar in composition to each other but different from the Half Moon Bay and La Honda oil samples. The specific source rocks from which the oils originated have not been identified, but candidates in the Davenport and Point Reyes areas include the Santa Cruz Mudstone and Miocene Monterey Formation, whereas candidates in the Half Moon Bay and La Honda areas include the aforementioned two units as well as Miocene strata within the Lambert Shale.

These similarities in geologic occurrence and organic geochemistry suggest the possibility that oil-bearing sandstone intrusions in the Davenport and Point Reyes areas share a common origin and that the two areas were formerly contiguous. We hypothesize that oil in both areas was generated from thermally mature, organic–rich Miocene strata in the vicinity of the Davenport syncline northwest of Santa Cruz, that oil migration and sandstone intrusion happened about 9–7 Ma, and that the Davenport and Point Reyes areas were subsequently separated by about 115 ± 10 km of right–lateral displacement along the San Gregorio and northern San Andreas faults.