Porosity reduction during diagenesis of diatomaceous rocks

With lithification and burial, diatomaceous rock transforms from opal-A to opal-CT and then quartz. This study documents two different patterns of por osity reduction and their associated mineralogical and density changes usin g samples from the Monterey Formation, California. Samples that have one se t of properties are called group 1, and those showing other trends are assi gned 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 sampl es, and samples bearing only opal-A or only opal-CT may have similar porosi ties. In the other pattern (group 2), the opal-A/opal-CT transition is asso ciated 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 hav e porosity above 55%, whereas those with only opal-CT have porosity below 4 3%. It appears that the two patterns separate from each other when opal-A disso lves and different minerals precipitate preferentially from the silica in s olution. In group 1, the content of clay minerals, particularly illite and smectite, increases as porosity decreases. This suggests that porosity redu ction is due to pore filling by authigenic clay minerals. In group 2, the a mount of opal-CT or quartz increases with decreasing porosity, and the weig ht percent of clay minerals also decreases. This implies that porosity redu ction occurs predominantly through addition of silica. Porosity in the opal-CT-bearing samples of group 1 is larger than that of t he opal-CT-bearing samples of group 2. This fact has significant implicatio ns for exploration in these important oil-bearing rocks.