In California there are two regions in which fractured sedimentary rocks form important oil reservoirs. They are the Santa Maria Coastal district and the southwestern part of the San Joaquin Valley. This type of reservoir rock is limited essentially to fractured chert and siliceous shale of upper Miocene age. The productive areas, areal distribution, thickness, and lithologic variations of these strata are illustrated by three maps and seven stratigraphic sections. The sedimentary history and mode of origin of the fractures in the cherts and siliceous shales are discussed. A Pliocene sedimentary cover on these rocks appears to be important. Accumulations occur in all types or combinations of structural traps. Evaluation of potentially productive fractured zones requires t sts of some duration. Fracturing and consequent productivity are extremely variable.

Distribution of chert suggests it originated as a sedimentary facies. Fracturing in brittle siliceous shale sections may be caused by mechanical forces during folding, but fracturing in well developed chert sections appears to have a primary origin during chemical changes in the history of the rock.

Fractured reservoirs are in general characterized by low porosity and high permeability. Expected recovery per acre foot in such zones is relatively low.