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The late Miocene to Pliocene argillaceous and siliceous shales of the upper Monterey Formation (the Brown Shale and overlying Belridge Diatomite) represent laterally continuous deposits of nearshore, diatom-rich muds and open marine, argillaceous, diatom-bearing muds on a developing shelf and growing anticline which flanked the San Joaquin basin. These units are laterally equivalent to submarine fan and turbidite sedimentary rocks deposited into the basin from the west, south, and east.

The lithology of the Brown Shale and Belridge Diatomite is controlled by original composition and subsequent burial diagenesis. The muds underwent diagenetic alteration with the development of silica phases in the sequence: (1) unaltered diatom frustules (biogenic amorphous silica or opal-A); (2) diagenetic amorphous silica (opal-A); and (3) diagenetic crystalline silica (opal-CT). In the oil field, the Belridge Diatomite grades downward from a massive or faintly laminated siliceous mudstone with diatom frustules and opal-A^prime, to a laminated and massive siliceous mudstone with opal-A^prime and some well-preserved diatoms. The Brown Shale grades from a massive or laminated siliceous mudstone with opal-A^prime and some diatoms to a well-laminated argillaceous, diatom-bearing mudston with opal-A^prime and some opal-CT. There is no distinct break in lithology between the Belridge Diatomite and Brown Shale reservoirs, but recrystallization and detrital clay increase downward across the boundary.

Porosity in the best diatomite is about 62%, and permeability is about 1 md.

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