Abstract: Identification and Correlation of the Opal-CT/Quartz Phase
Transition in Offshore Central California
Drew A. Mayerson, Catherine A. Dunkel, Kenneth A. Piper, Harold L. Cousminer
As part of the National Assessment of Undiscovered Oil and Gas Resources, compositional analysis of Neogene siliceous strata offshore central California has been performed to confirm the existence and stratigraphic location of the opal-CT/quartz diagenetic boundary. Infrared spectroscopic analysis was performed on ninety-nine samples from nine wells in four basins: Santa Maria, Ano Nuevo, Bodega-La Honda, and Point Arena. The results of these analyses confirm our hypothesis that the conversion of the siliceous strata from opal-CT to quartz correlates to a significant increase in bulk density observable on most well logs and an anomalous seismic reflector often observable on seismic Profiles.
This correlation is important because the phase
change from opal-CT to quartz increases the fracturability, and therefore reservoir quality, of the siliceous strata. Additionally, the maximum temperature at which the
phase
transition from opal-CT to quartz occurs (~80°C) is postulated to be the minimum temperature for oil generation within the Monterey Formation. Identification of the diagenetic reflector on seismic profiles, therefore, provides indirect evidence of reservoir quality and thermal maturity of the Monterey strata within the basins.
AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California