The phase transitions from opal-A to opal-CT to quartz plays an important role in hydrocarbon trapping in regions with siliceous deposits, particularly where structural traps are absent. In 2010, a laboratory study was undertaken to determine the kinetics of the opal-CT to quartz phase transition under realistic subsurface geochemical conditions. Opal-CT-rich porcelanite from the Monterey Formation was subjected to hydrous pyrolysis at temperatures of 310°C, 333°C, and 360°C. A buffered aqueous solution was used to keep the pH at neutral or slightly alkaline conditions. Experiments by then graduate student Danica Dralus predict that at geothermal temperatures the phase transition will be slower than predicted by a previous study (Ernst and Calvert, 1969), pushing the predicted location of the transition region deeper.
Concurrent experiments performed on a clay- and TOC-rich Wakkanai Formation porcelanite from Hokkaidō, Japan, showed no transformation from opal-CT to quartz even after 20 days at 360°C. Monterey Formation porcelanite under the same conditions transformed completely in less than three days. The precise cause of the discrepancy in reaction rate remains unidentified, but it is clear that contaminating minerals and fluid chemistry profoundly affect the kinetics of the phase transition.
The kinetics of the opal-A to opal-CT transition and their response to TOC were explored in additional experiments conducted in 2014. Additional material from the Monterey Formation was collected from the same location as that used in the previous experiments but from an opal-A-rich bed with weathered (low-TOC) and unweathered (high-TOC) areas. A significant difference in transition rate was observed between the low- and high-TOC samples at laboratory temperatures even when other chemical parameters like clay content and pore fluid pH were equivalent.