Document Type

Book Chapter

Publication Date


Journal / Book Title

Petroleum Generation and Occurrence in the Miocene Monterey Formation, California


Major portions of the Miocene Monterey formation of California were deposited under low oxygen conditions, with restricted clastic influx, beneath waters with high phytoplankton productivity. The resulting diatomaceous and organic-rich sediments underwent diagenetic modification as they were buried.

A suite of core samples was collected from eight wells in the Lost Hills oil field ranging in present depth of burial between 535 and 2285 meters. In one well, the entire Monterey section was sampled. Cores taken from the remaining wells sampled the Reef Ridge and Antelope Members of the Monterey Formation at various burial depths.

Silica mineralogy was studied using x-ray diffractometry and optical microscopy. Silica phases exhibit a clear diagenetic progression with depth from the opal-A of the diatom frustules to opal-CT and ultimately to microquartz. The d(101) spacing of opal-CT decreases from 4.088 to 4.044 Å with depth. No opal-CT was detected below 1470 meters.

Organic material was studied by bulk pyrolysis (Rock-Eval) and optical methods. Organic carbon is abundant, comprising 1.4-10.2% of the rocks, mainly in the form of Type II kerogen (which reflects a dominantly marine origin) and soluble bitumens. A different organic facies is apparent in rocks sampled higher in the stratigraphic section, evidenced by admixed Type III kerogen.

Maturation parameters (maximum pyrolysis temperature, visual T.A.I., and vitrinite reflectance) indicate that none of the samples has yet reached the onset of the main phase of oil generation. Diagenetic transformation of biogenic silica to quartz is therefore complete before the kerogen begins to generate appreciable petroleum. Amounts of detectable free hydrocarbons (S1) and hydrocarbons from kerogen pyrolysis (S2) are consistently high. Heavy bitumens are common and in many cases inflate S2 values. Where this occurs, pre-pyrolysis extraction is necessary to avoid misinterpretation of Rock-Eval data. Oil is produced only in the limited intervals where S1/(S1+S2) is high (~0.4). This suggests that most of the oil migrated and accumulated rather than formed in situ, although some of the heavy bitumens may be indigenous.

Book Publisher

SEPM Pacific Section

Book Editor(s)

Isaacs, C.M. and Garrison, R.E.

Published Citation

Kruge, M.A., 1983, Diagenesis of Miocene biogenic sediments in Lost Hills oil field, San Joaquin Basin, California. In, Isaacs, C.M. and Garrison, R.E., eds., Petroleum Generation and Occurrence in the Miocene Monterey Formation, California, Soc. Econ. Paleon. Mineral. (SEPM) Pacific Sect., Los Angeles, p. 39-51.