Abstract The San Joaquin Valley lies between the Sierra Nevada Mountains and the Coast Ranges between North Latitudes 34° 54′ and 37° 50′ and West Longitudes 118° 36′ and 121°31′. It is a large synclinorium approximately 250 miles long and averaging between 50 and 60 miles wide. The main synclinal axis trends northwest and is on the west side of the valley. The sedimentary area is approximately 14,100 square miles; maximum depth to basement is in excess of 30,000 feet; and the volume of sediments is estimated at 31,000 cubic miles. Marine sands and shales comprise about 85 per cent of the total volume of sediments. The northeast or east side of the San Joaquin Valley is underlain by the Sierra Nevada granitic complex (Jurassic) and on the southwest or west side the unmetamorphosed rocks rest on the Franciscan series (also Jurassic) which is considered basement for the purpose of this symposium. The Franciscan rocks consist of dense, somewhat altered and metamorphosed sediments with basic igneous intrusives. Oil production is confined almost entirely to the south half of the valley on both the east and west sides. The bulk of the production has been from the west side with its thicker marine sedimentary section and more numerous, larger, and better developed folds. On the west side no tests have been drilled to basement and very few even to the Cretaceous. There remains a considerable untested area of Eocene and Cretaceous formations above the 20,000-foot limit. The Eocene is considered much the