Physical properties and silica diagenesis

Silica diagenesis in the Monterey Formation is of interest to the petroleum industry because this unit is a major California hydrocarbon source and reservoir rock. Measurements of porosity, density, and ultrasonic velocity from dry core plugs in three Monterey Formation reservoirs in California's San Joaquin Valley are analyzed along with their corresponding well logs. Furthermore, these observations are compared to available data from other siliceous rocks of the Monterey Formation, the North Sea and the Sea of Japan. This work describes two distinct patterns of porosity reduction in siliceous rocks.;The two patterns can be distinguished using density, porosity, and mineral content data (Chapter 1) and therefore identified using outcrop or cores. Most dramatically, the two patterns can be identified by falling on one of two dry bulk density-porosity trends. The clearest difference between the two patterns is that one shows a gradual change in porosity at the opal-A/opal-CT transition while the other exhibits an abrupt decrease in porosity.;The two porosity reduction patterns can be distinguished in trends of density or porosity versus impedance or elastic moduli (Chapter 2) and therefore identified using well log or seismic data. Furthermore, effective medium modeling (Chapter 3) supports my hypothesis that the two porosity reduction patterns correspond to two textures.;There are several practical uses in identifying which porosity reduction trend best applies to a particular dataset. Because the two bulk density-porosity regressions have a high correlation coefficient, porosity can be predicted from bulk density if the porosity reduction trend is already known. A tool to calculate porosity from density in well logs is useful because one commonly used method, density porosity, calculates porosity by incorrectly assuming that grain density is constant. Porosity calculated using my density-porosity transform approximates direct measurements much better than the neutron porosity tool and at least as good as density porosity. This transform is not only effective in characterizing siliceous rocks from the Monterey Formation, but is also effective at calculating porosity in siliceous rocks from other locations, such as the Norwegian North Sea and the Sea of Japan.