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Transmission Electron Microscopy, Gas Adsorption, and High-Pressure Mercury Porosimetry Used to Describe Monterey Silica Diagenesis

David C. Hurd

Many diatomites in the San Joaquin Valley of California are predominantly biogenically precipitated silica, admixed with a fine-grained, well-mixed terrigenous component having feldspar, pyrite, volcanic glass, detrital quartz and illite, and mixed-layer clay. These diatomites are skeletally supported and have interparticle porosity between the frustules and intraparticle porosity within them. Pore-throat and total porosity variability occurs primarily because interparticle porosity is infilled by terrigenous material.

In the first stage of silica diagenesis, diatom frustules dissolve, and spherical, 50-A diameter, opal-CT particles precipitate. Both particle size and particle spacing increase with increasing depth until the silica particles are about 300 A in diameter. The opal-CT porosity during this stage is about 45%. The former intraparticle porosity of the diatom frustules becomes moldic porosity in the opal-CT zone. The relative amounts of small and large pore porosity can be quantified with gas adsorption and transmission electron microscopy, but not mercury porosimetry. Terrigenous material affects estimated pore throats in shallow opal-CT samples, but diagenesis overwhelms this effect at greater depth.

During the second stage of silica diagenesis, small (< 0.1 µm quartz bundles begin to appear. These bundles increase in size until they are about 5-10 µm in diameter; particle spacing also increases with increasing particle size. Small amounts of terrigenous materials have a pronounced effect on estimated particle spacing.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.