Subsurface Geology and Petroleum Potential of Shandon-San Juan Creek Area, Salinas Basin, California

D. M. Sparks

The area of northeastern San Louis Obispo County described in this paper is underlain and laterally adjacent to granite basement rocks of the Salinian block, with a Miocene to Pleistocene sedimentary section overlying the granite. The sequence of middle to upper Miocene Monterey and Santa Margarita Formations is virtually identical to the outcrop section flanking the La Panza Range to the south and west. The lower Miocene, however, consists of a very thick Vaqueros section, similar to the northern Carrizo Plain.

Two distinct structural regimes are present: (1) the late Miocene to present-day right lateral shear, reflected in the San Andreas and Red Hills faults; and (2) an earlier (pre-transform margin) phase, which created the grabenlike basement geometry underlying the Shandon area. The combination of these two tectonic phases has created a variety of complex stratigraphic and structural traps.

The Monterey formation is typically organic-rich and comprised of familiar chemical facies. A high geothermal gradient and observed thermal alteration effects suggest an oil generation threshold perhaps as shallow as 5,500 ft (1,676 m). Seismic mapping of the Monterey indicates present-day burial depths of 6,000-10,000 ft (1,828-3,048 m), suggesting that a significant hydrocarbon charge exists. To date, no wells have penetrated the Monterey within the ± 100-mi² (258-km²) area of maximum burial, and none have tested closed traps adjacent to the area.

As much as 100 million bbl of oil are estimated to be recoverable from traps in the Shandon area. That volume of oil could be contained in reservoirs trapped below 6,000 ft (1,828 m), precluding bacterial degradation and yielding expected gravities in the 20° and above range, with associated gas.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.