Seven oil fields in the Santa Maria District produce oil from four different formations. The most prolific of these is the Monterey shale (middle and upper Miocene). Oil in the Knoxville sandstone, discovered in 1943, created a mild drilling boom, but was limited in areal extent. Lospe production is the latest economic interest in the district. Variation in the age of the Monterey cherty shale in the Santa Maria Valley and other fields makes correlations difficult. Ditch sampling of development as well as wildcat wells has been a valuable practice in stratigraphic control and aid in electrolog correlations. Microscopic study of washed samples shows definite lithologic changes that carry over wide areas.

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