Porcelanite and chert originate from marine diatoms as diatomite, which undergoes diagenetic conversion of amorphous opal (opal-A) to cristobalite and tridymite (opal-CT) and finally quartz. Porosity decreases during this process. However, permeability increases during transformation of opal-CT to quartz and can result in stratigraphic traps for petroleum like those discovered in the Rose and North Shafter fields in the San Joaquin Basin. The depth of the opal-CT to quartz phase transition is partly controlled by temperature, clay content, and water chemistry and occurs at ~7,000 ft (~2,100 m) subsea in the Monterey Formation in the Rose and North Shafter fields (Grau et al., 2003). Changes in silica phase can be identified visually in hand specimens, but are sometimes difficult to identify in the subsurface using seismic data. The ability to accurately predict locations of these diagenetic traps would be a valuable exploration tool.
As a graduate student, Danica Dralus determined kinetic parameters for the opal-CT to quartz transition based on hydrous pyrolysis of two natural samples: a weathered Monterey Formation porcelanite from Lompoc, California, which also contained significant amounts of dolomite; and a Wakkanai Formation porcelanite from Hokkaidō, Japan, which also contained quartz, clay, and some organic material. Temperatures were kept below the critical temperature of water, and the aqueous solution was buffered so that final fluid pH values measured between 7.0 and 8.2. Under these conditions, the samples showed a large variation in conversion rates. For instance, Ernst and Calvert (1969) predict that at a temperature of 360°C, full conversion should happen in approximately 35 days. In our tests, Monterey opal-CT was fully converted in less than three days, while the Wakkanai opal-CT showed no conversion after 20 days.
This figure shows an Arrhenius plot for the conversion of Monterey Formation opal-CT to quartz assuming a zero-order reaction. At laboratory temperatures, the transformation rates from our study are significantly faster than those found by Ernst and Calvert (1969). When the temperature dependence of the transformation rate is extrapolated to typical basin temperatures, our study predicts slower transformations rates, which yield deeper transition zone predictions when incorporated into a basin and petroleum system model.
Related: Kinetic Studies research.