Occurrence and Distribution of the Monterey Opal-CT/Quartz Diagenetic Boundary in the Offshore Santa Maria Basin and Possible Implications for Future Prospect Delineation and Success

The offshore Santa Maria Basin is a prolific oil-producing region in Central California, with exploration dating back to the OCS-P 0060 #1 well in 1964 and continuing through the 1980s. While conventional structural plays within the Monterey Formation have been successfully exploited at the Point Arguello and Point Pedernales fields, little attention has been given to the relationship between the diagenetic alteration of the Monterey and hydrocarbon distribution within the offshore basin.

Research has shown that sediments of the Monterey Formation undergo diagenetic modification during burial, resulting in a progression of the siliceous lithofacies from opal-A to opal-CT to quartz based on burial time, temperature, lithology, and geochemical environment. Previous investigations of analogous Neogene siliceous sediments as well as the onshore Monterey Formation found both an association of hydrocarbon occurrence as well as the formation of hydrocarbon traps with the conclusion of the opal-CT/quartz diagenetic alteration.

Based on a variety of 2D and 3D seismic reflection data correlated with well logs, the opal-CT/quartz diagenetic boundary is interpreted to be present throughout much of the offshore Santa Maria Basin at burial depths of roughly 5000 feet beneath the seafloor. The seismic response at the opal-CT/quartz boundary is a high-impedance event that commonly cross-cuts stratigraphic reflectors. In unconventional exploration plays such as synclines and anticlinal limbs oil migrating up the flanks may become trapped at the opal-CT/quartz boundary as the highly fracturable and permeable quartz rich rock below the boundary changes to rock with less fracture permeability in the opal-CT zone above the boundary. The presence of the opal-CT/quartz boundary may provide a seismic indicator that can be used to high-grade prospects or to direct drilling to unconventional parts of the structure such as anticlinal flanks where hydrocarbons may be trapped due to a change in fracture permeability.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California