Abstract: The Opal-CT/Ouartz Diagenetic Boundary Within the Monterey Formation of the California Offshore Santa Maria Basin: An Untapped Exploration Target

D. Mayerson, J. Crouch

Within the offshore Santa Maria basin much of the siliceous strata of the Monterey Formation has been diagenetically altered to opal-CT, and in areas where the strata are, or have been more deeply buried, they have been transformed to quartz. The density contrast created by the opal-CT/quartz diagenetic phase change is

identifiable on seismic-reflection profiles as a horizontal/subhorizontal reflector that typically cross-cuts stratigraphic reflectors. The opal-CT/quartz diagenetic transformation is believed to be a prerequisite for the attainment of adequately fractured chert reservoirs within the Monterey Formation. Also, within the siliceous-rich Monterey rocks, the opal-CT/quartz transformation occurs at about 80° C, which is close to the predicted onset of in-situ oil generation. Recognition and mapping of the opal-CT/quartz reflector can be used to predict the success of conventional exploration targets and may also be used to delineate areas of off-structure reservoir potential. Where the opal-CT/quartz reflector is above or near the top of the siliceous-rich Monterey Formation, the chan es of adequate oil generation and reservoir quality are high. In areas where the opal CT/quartz reflector is below siliceous Monterey strata, oil generation and fractured reservoir potential are low.

We have tied the opal-CT/quartz reflector to exploration wells and mapped this reflector throughout much of the offshore Santa Maria basin. This reflector is generally found at a depth of about 1.3 s to 1.7 s (two-way traveltime) beneath the sea floor. Across a number of the larger structures within the offshore Santa Maria basin, Monterey sediments were draped over preexisting basement highs and have remained at relatively shallow burial depths, above the opal-CT/quartz reflector. In contrast, where more deeply buried along structural flanks and within synclines, Monterey strata are below the opal-CT/quartz reflector. Hence, much of the remaining, producible Monterey oil within the offshore Santa Maria basin may reside below diagenetic permeability barriers in synclines, homoclines, nd along the flanks of anticlines. Many dry holes that were drilled in the largest, initially most attractive, exploration targets in the offshore Santa Maria basin occur within areas in which our proposed method predicts low hydrocarbon potential. However, wells have yet to be drilled within the synclines and along the flanks of these large structures where the hydrocarbon potential should be high.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994