Seismic Expressions of Monterey Formation Diagenesis: Examples from Offshore California

Malcolm B. Roy

Diagenesis of the diatomaceous rocks in the Monterey Formation in California coastal and offshore basins involves changes from amorphous biogenic silica to a stable crystalline quartz facies. In the intermediate stage, the transformation undergoes passage from the Opal-A to the Opal-CT phase. Associated with this diagenetic process is a marked increase in bulk densities between the different silica phases, owing to loss of porosity from compaction and solution recrystallization caused by increase in burial load and other physical factors. The sharp density contrast between the silica phases is manifested by an acoustic impedance boundary that may be expressed on seismic records. This seismic event can be distinct and independent of structural configuration, and in many pl ces cuts through stratigraphic boundaries. Several examples of seismic records from offshore California demonstrate the diagenetically caused reflection cutting through Monterey and post-Monterey formations.

Current and future exploration efforts in offshore California will continue to center on the widespread Monterey Formation. In addition to being the main source rock, the Monterey is also the reservoir rock. Recent discoveries indicate that oil production is mainly from the highly permeable, fractured, silica-rich sections. It is therefore important to recognize the diagenetic boundaries on seismic records and to delineate the more brittle quartz-rich facies where the reservoir quality is expected to be better than the intermediate Opal-A or Opal-CT facies. Furthermore, these boundaries could also provide good diagenetic traps off the flanks of structures where updip unaltered impermeable rocks hinder fluid migration.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.