The Santa Maria province in Santa Barbara and San Luis Obispo Counties, California, has been well explored during the past 70 years. Fifteen commercial oil fields, which had produced 556.5 million bbl of oil to January 1, 1969, were discovered between 1902 and 1952. No significant fields have been discovered during the past 20 years. The area and volume of Tertiary sedimentary rocks of the province are approximately 1,700 sq mi (4,400 sq km) and 1,100 cu mi (4,600 cu km), respectively. Production is from fractured upper and middle Miocene Monterey siliceous and cherty shale and chert, and from sandstone units of middle Miocene and Pliocene-Miocene ages. The Monterey is the reservoir of 75 percent and the source of at least 80 percent of the oil that has been produced. Mor than 95 percent of the cumulative production has come from fields in the central part of the province which produce from either anticlinal or overlap-truncation traps. Gravity ranges from 6° to 33° API and generally is lower than 25°. The quantity of known oil originally in place is approximately 4 billion bbl. It is estimated that from 100 million to 600 million bbl of oil in place will be found in the central area in reservoirs that are now productive, and near the northern and northeastern provincial borders in older (early Miocene to Late Cretaceous) clastic reservoirs.