Abstract In the four oil fields of the Santa Maria district, namely, Casmalia, Cat Canyon, Lompoc, and Santa Maria, the reservoirs have been formed in the rocks where the oil and gas originated. The oil occurs in cross-faulted anticlines in which the faults have influenced the movement and location of the oil. In the Santa Maria district the oil comes from the diatomaceous Monterey shale of Miocene age. Although the diatoma-ceous shales have been commonly considered the source of the oil and gas, more recent studies have indicated that these highly organic sediments may have been deposited under conditions that would preclude their contribution of any hydrocarbons.