Pacific Section AAPG Convention

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Pore Structure Of Opal-CT And Quartz Phase Porcelanites, Monterey Formation, California


We identify and quantify 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 using secondary and backscattered electron scanning electron microscopy of argon-ion polished or focused-ion beams' cut surfaces. Porcelanites are important reservoir rocks that demonstrate great differences in producability despite similar bulk physical characteristics. Previous studies have measured an order-of-magnitude lower permeability in opal-CT compared with quartz phase porcelanites, presumably due to difference in pore-throat size. In quartz-phase, we identify three porosity microfabrics. Silica-rich porcelanite (> 80% silica) has patchy/speckled and laminated microfabrics. The first displays low porosity matrix and high porosity lenticular patches. We measured 17-20% bulk porosity of (pore-diameter 0.05-3.00 microns). In contrast, a laminated silica-rich porcelanite (26% bulk porosity) has ∼100 μm-thick layers that alternate between highly porous (35% & pore size 0.01-3.7 microns) with good interconnectivity and low porosity layers (19%) with isolated pores (0.01-0.7 microns). The massive silica-poor porcelanites have porosity of 10% with (0.02- 0.03 microns) and poor connectivity. Our range of bulk porosities for quartz-phase porcelanite (10-26%) measured by microanalysis is similar to that determined by standard methods. Opal-CT porcelanite also reveals three different microfabrics: two in high-silica (>75%) and one low silica (<60%). One high-silica 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 cores are formed of highly porous granular opal-CT, but this porosity is mostly isolated by a virtually pore-free, surrounding impermeable mantle. The larger and better-connected interlepisphere pores are formed by larger, crosscutting and radiating bladed crystals. The second silica-rich, 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 last fabric is in silica-poor opal-CT porcelanite that has 18% bulk porosity with poorly connected pores (0.001-0.09 microns).