Abstract The San Emidio Nose oil field has a reserve of 64 million bbl. The accumulation is in a stratigraphic trap in upper Miocene beds; the trap crosses the plunge of a subsurface anticline. The field was discovered in 1958 by the Richfield Oil Corporation (now Atlantic Richfield Company) after 24 years of exploration by six companies. Geographically, the field is located on the southernmost rim of the San Joaquin Valley. Structurally, it is located in the Maricopa subbasin and is one of several simple folds that were defined by geophysics and drilled during the 1930s. At San Emidio, however, the absence of suitable reservoir rocks on the crest of the structure was partly responsible for delaying the discovery until 1958, and six expensive dry holes were drilled before the sandstone geometry was fully defined. The trap is closely related to the westward thinning of the reservoirs (Reef Ridge and Stevens sandstones) and their enclosing shales up the east plunge of the fold. This thinning, which was shown geophysically prior to the discovery, is an elementary example of the stratigraphic application of geophysics.