With lithification and burial, diatomaceous rock transforms from opal-A to opal-CT and then quartz. This study documents two different patterns of porosity reduction and their associated mineralogical and density changes using samples from the Monterey Formation, California. Samples that have one set of properties are called group 1, and those showing other trends are assigned to group 2. In one pattern, the samples (group 1) exhibit gradual porosity change with the opal-A/opal-CT transition. Opal-A and opal-CT coexist in the same samples, and samples bearing only opal-A or only opal-CT may have similar porosities. In the other pattern (group 2), the opal-A/opal-CT transition is associated with a sharp decrease in porosity. In this data set, opal-A and opal-CT are not present within the same sample. Samples bearing only opal-A have porosity above 55%, whereas those with only opal-CT have porosity below 43%. It appears that the two patterns separate from each other when opal-A dissolves and different minerals precipitate preferentially from the silica in solution. In group 1, the content of clay minerals, particularly illite and smectite, increases as porosity decreases. This suggests that porosity reduction is due to pore filling by authigenic clay minerals. In group 2, the amount of opal-CT or quartz increases with decreasing porosity, and the weight percent of clay minerals also decreases. This implies that porosity reduction occurs predominantly through addition of silica. Porosity in the opal-CT-bearing samples of group 1 is larger than that of the opal-CT-bearing samples of group 2. This fact has significant implications for exploration in these important oil-bearing rocks.