Physical and Mechanical Heterogeneity of Reservoir Rocks in the Opal-A to Opal-CT Transition Zone, Monterey Formation, Western San Joaquin Basin, California

Abstract

In terms of porosity, permeability, density, brittleness, 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. In the western San Joaquin Basin, the upper Monterey Formation (Antelope, Belridge, Reef Ridge units) is originally composed of diatomite or diatomaceous mudstone (a biogenic siliceous rock with 60-80% porosity) that is heterogeneously bedded or laminated in cyclic cleaning upward cycles. With burial, in situ bulk transformation of biogenetic opal-A (diatomite) to diagenetic opal-CT (porcelanite or chert) results in less porous and more brittle rock. This diagenesis is largely controlled by the composition of individual strata and maximum temperature reached. Smectitic clay content retards the phase transformation while the presence of carbonate may accelerate the transformation. Both factors can lead to a complexly interbedded succession of opal-CT chert or porcelanite with opal-A diatomite or diatomaceous mudstone. The adjunction of highly fractured or fracturable opal-CT-phase beds with highly porous diatomaceous beds would form an effective migration pathway or attractive petroleum reservoir. 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 deformation, fluid flow, and subtle changes in partial to complete transformation in this zone. Core samples from several transition zone intervals in a western San Joaquin Basin oil field 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 measurements of well-characterized samples. Correlation of these rock properties to well logs will be used to evaluate the opal-A to opal-CT transition zone's character as a reservoir and provide a model for further log recognition and potential for exploitation as a mixed reservoir of both high-porosity and high-permeability.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016