Petroleum Systems of the Santa Cruz County Coast, California

Hosford Scheirer, Allegra; Magoon, Leslie B.; and Graham, Stephan A.
[email protected]

Numerous asphalt-saturated sandstone deposits and naturally occurring seeps on and near the Santa Cruz County coast in central California confirm the presence of an active, but previously undefined, petroleum system. These tar occurrences lie about 25 to 50 km south-southeast of the La Honda Basin, where 5 small oil fields have produced nearly 2 million barrels of oil and 300 million cubic feet of gas. The close proximity of the Santa Cruz County coast to the proven petroleum system of the La Honda Basin indicate that similar elements are at work along the coast. The likely source rock for these asphalt deposits is California’s “superstar” oil producer, the Monterey Formation, in the Outer Santa Cruz Basin offshore. Shell Oil Company drilled two exploratory test wells within the basin in 1967. Both of these wells were drilled on anticlines, penetrated at least 3,000’ of Monterey Formation (although some of this is probably the result of structural thickening), and recorded hydrocarbon shows. It has been suggested that the Santa Cruz Mudstone overlying the Monterey Formation could also serve as a local petroleum source rock. However, overburden thicknesses in the two offshore wells range between 1,800’ and 2,500’, making burial depths sufficient for thermal maturation and petroleum generation unlikely.

To better understand the petroleum system in this area, we constructed two burial history models at the location of the offshore wells. The models extend from the Pigeon Point Formation, a Late Cretaceous sandstone, to the Pliocene Purisima Formation and unnamed overlying sediments. On the basis of previous modeling work in the offshore Sur and northern Santa Maria Basins to the south, the Monterey Formation was subdivided into 3 distinct facies, and each was assumed to be a petroleum source rock. Model results will be discussed.

At present, the large-offset San Gregorio Fault separates the Outer Santa Cruz Basin from the onshore region, making timing for petroleum generation and migration critical. Uplift and erosion associated with active strike-slip faulting in the area has likely breached any existing traps in the region. Additional complexities to be explored in the modeling include both structural thickening and diagenetic alteration of the Monterey Formation. We will also explore whether the asphalt was generated as heavy oil or suffered post-generation biodegradation.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013