The southern coastal and mountain area occupies about 7,000 sq mi (18,130 sq km) in the southwestern corner of California. A narrow coastal strip of sedimentary rocks extends from the Mexican border northward to the vicinity of San Onofre, where it widens and continues north around the northern extremity of the Santa Ana Mountains and southeastward beyond Corona along the Elsinore trough. The rest of the area is a mountainous land underlain mainly by the great mid-Cretaceous batholith of southern California and by older metamorphic rocks. The coastal strip and the Corona vicinity, about 1,000 sq mi (2,590 sq km), are underlain by Late Cretaceous and younger sedimentary rocks, mainly marine. About 175 wildcat wells have been drilled in the area, but evidence of oil and gas is scant. Showings of oil and gas have been reported in a few wells, but petroleum has not been produced. Three possible future petroleum target areas are Corona, the eastern part of the Capistrano basin, and the San Diego coastal strip. The volume of possibly favorable sedimentary rocks has not been quantified, but scattered bits of evidence suggest that favorable Miocene and Pliocene strata, largely in the San Diego area, may aggregate about 50 cu mi (210 cu km).