Secondary and backscattered electron scanning electron microscopy of argon-ion polished or focused-ion beams’ cut surfaces reveal significant differences in pore size, shape and complexity between opal-CT and quartz-phase porcelanites, but also between rocks of the same silica phase with distinct silica content or sedimentary fabric. Previous studies of these important petroleum reservoir rocks measured an order-of-magnitude lower permeability in opal-CT than quartz-phase porcelanites. Detrital-poor quartz-phase porcelanite (> 80% silica) has massive or laminated microfabrics. The massive type consists of low-porosity matrix and high-porosity lenticular patches with 17-20% bulk porosity. In contrast, a laminated detrital-poor porcelanite (26% bulk porosity) has ∼100 µm-thick layers that alternate between well connected, highly porous (35% and low-porosity layers (19%) with isolated pores. Massive detrital-rich porcelanites have porosity of 10% with and poor connectivity.
Opal-CT porcelanites also have two detrital-poor (>75% silica) and one detrital-rich (<60% silica) microfabrics. One detrital-poor porcelanite with 30% bulk porosity has a pervasive lepispheric fabric in which lepispheric cores and interlepisphere porosity each comprise ∼1/2 of the total porosity (3-138 nm size). Lepisphere core are mostly isolated by a surrounding, virtually pore-free, impermeable mantle. The larger and better-connected interlepisphere pores are formed by larger, crosscutting and radiating bladed crystals. The detrital-poor, opal-CT porcelanite is characterized by extraordinarily large and connected vuggy pores with bulk porosity of 60%. This unique pore structure is associated with silica mobility during stalled burial or tectonic uplift of the opal-CT to quartz transition zone. The third fabric is in detrital-rich opal-CT porcelanite that has 18% bulk porosity with poorly connected pores.
|Advisor:||Behl, Richard J.|
|Commitee:||Francis, Robert D., Holk, Gregory J.|
|School:||California State University, Long Beach|
|School Location:||United States -- California|
|Source:||MAI 56/01M(E), Masters Abstracts International|
|Keywords:||Monterey Formation, United States|
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