BPSM Scientists Ken Peters and Oliver Schenk, along with graduate student Danica Dralus, built a prototype module in commercially available petroleum system modeling software to determine the depth of the opal-CT to quartz phase transition along a cross-section from the SJ-6 seismic line (Bloch, 1991) in the east-central portion of the San Joaquin Basin. Ernst and Calvert (1969) determined zero-order kinetics for this transition based on hydrothermal experiments using distilled water. However, the opal-CT to quartz phase transition is a dissolution and re-precipitation process (Stein and Kirkpatrick, 1976), where both the silica dissolution rate and solubility contribute to the rate of the reaction. It is now known that the chemistry of migrating fluids affects the depth and temperature of silica transformations (Ireland et al., 2009). By using kinetics for the opal-CT to quartz phase transition that are currently being determined by hydrous pyrolysis experiments on porcelanite collected from Lompoc, California and Hokkaido, Japan, we hope to predict the locations of stratigraphic petroleum accumulations in siliceous source rocks like those recently discovered in the Rose and North Shafter fields in the San Joaquin Basin (Grau et al., 2003). The kinetics and software module may be useful to identify silica-phase-transition stratigraphic traps throughout the Pacific Rim where siliceous source rocks are common (e.g., Miocene-Oligocene near Sakhalin Island).
The figure shows preliminary results from a test of the prototype opal-CT to quartz module along the SJ-6 seismic line in the San Joaquin Basin, California. This hybrid simulation (Darcy flow and flowpath migration) is based on stratigraphy and geohistory for multiple wells along the line, appropriate kinetics for oil generation from the siliceous source rocks, and kinetics for the opal-CT to quartz phase transition from an early paper (Ernst and Calvert, 1969). Note the present-day petroleum accumulation (green) in the siliceous McLure Shale Member of the Monterey Formation at the depth of the calculated transition from opal-CT to quartz.