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Mineralogical And Mechanical Characteristics Of The Opal-A To Opal-CT Transition Zone In A Diatomaceous Sequence Of The Miocene Monterey Formation, San Joaquin Basin, California.


In terms of porosity, permeability, density and other rock properties important to petroleum geologists, the opal-A to opal-CT transition zone is likely the most stratigraphically heterogeneous interval known to result from the burial diagenesis of fine-grained sediments. Investigation and quantification of rock properties within this zone is key to understanding of its potentially unique petroleum reservoir potential. In the western San Joaquin Basin, the upper Monterey Formation (Antelope, Belridge, Reef Ridge units) is originally composed of highly unstable diatomite or diatomaceous mudstone that is heterogeneously bedded or laminated. With burial, in situ transformation of biogenetic opal-A to diagenetic opal-CT occurs over an interval of 10s to 100s of meters with a relative timing that is largely controlled by the bulk composition of individual strata and maximum temperature reached. Particularly, smectitic clay content retards the transformation, resulting in diagenesis that occurs first in silica-rich lithologies and later in detrital-rich lithologies. Conversely, the presence of carbonate may accelerate the phase transformation. Both factors can lead to a complexly interbedded succession of opal-CT chert or porcelanite with opal-A diatomite or diatomaceous mudstone. The intercalation of highly fractured or fracturable opal-CT-phase beds with highly porous, moderately permeable diatomaceous beds could form an effective migration pathway or attractive reservoir for petroleum. We will complete a high-resolution study of this zone, relating the interstratified coexistence of opal-A and opal-CT-phase rocks with the resulting contrasting physical and mechanical properties and behaviors. Quantifying such contrast is important to understanding plastic-elastic deformation and fluid flow in natural or artificial hydraulic fracturing and the potential development of compartmentalization. Core samples from a transition interval interval in the western San Joaquin Basin will be evaluated for mineralogical content, porosity, and permeability. We intend to use this detailed lithologic and diagenetic stratigraphy to define the mechanical stratigraphy by performing laboratory rock strength/failure measurements of well-characterized samples. Correlation of these rock properties to well logs will be used to identify and evaluate the opal-A to opal-CT transition zone's character as a reservoir and provide a model for further log recognition and characterization.