Document Type

Book Chapter

Publication Date


Journal / Book Title

Monterey Formation and Associated Coarse Clastic Rocks, Central San Joaquin Basin, California


The Miocene Monterey Formation is probably one of the main petroleum source rocks in the San Joaquin basin of California. As such, an understanding of the transformations occurring in the organic material in the Monterey Formation is of great importance. This may be enhanced by investigation of the diagenesis of associated minerals and the degree to which all the transformations are either chemically interrelated or independent. Constraints on geothermometric applications may also evolve. The present paper forms part of a study of this type and focuses on silica diagenesis.

ln a few exceptional California oil fields, notably Lost Hills, petroleum is produced directly from fractured Monterey formation "shales". Therefore, the Monterey formation in these fields has been cored extensively for reservoir eva1uation. Lost Hills field was chosen for the present diagenetic study because of sample availability.

There is an overall pattern of opal-CT d-spacing reduction with increasing burial depth, but the decrease is not monotonic. Perturbations may be caused in part by variations in the bulk composition of samples. A higher clay to biogenic silica ratio may increase the rate of d-spacing reduction in rocks at the same temperature. In addition, there may be further effects, perhaps due to carbonate and organic content, which have not been isolated or documented as yet in the literature. The progress1on of opal-A to opal-CT to microquartz is a good index for young (Cenozoic to late Mesozoic) and cool (<100 °C) siliceous sedimentary rocks. This could perhaps be extended to older and hotter rocks by employing the quartz crystallinity index. With increasing diagenesis, the QCI reduces to a minimum (upon complete conversion of opal-CT to quartz), but it then gradually increases as the biogenic quartz advances to a more ordered state.

Book Publisher

SEPM Pacific Section

Book Editor(s)

L. A. Williams and S. A. Graham

Published Citation

Kruge, M.A. and Williams, L.A., 1982, Silica diagenesis in the Monterey Formation (Miocene) in the Lost Hills oil field, San Joaquin Valley, California. In, Williams, L.A. and Graham, S.A., eds., Monterey Formation and Associated Coarse Clastic Rocks, Central San Joaquin Basin, California, Soc. Econ. Paleon. Mineral. (SEPM) Pacific Section, Los Angeles, p. 37-42.