Why Do Microquartz Coatings Preserve Sandstone Reservoir Quality?
Robert H. Lander1, Richard E. Larese2, and Linda M. Bonnell1
1 Geocosm LLC, Austin, TX
2 Clastic Petrology Consultant, Durango, CO
Sandstone reservoir quality can be preserved at high temperatures due to the inhibition of “normal” quartz overgrowths by microquartz coatings. Previous explanations for this curious phenomenon invoke the high solubility of micron-scale microquartz crystallites (Aase et al 1996, Jahren & Ramm 2000). It is argued that this heightened solubility retards silica mass transfer because of the lack of a chemical gradient between microquartz bearing sandstones and silica sources such as stylolites. Thus rates of “normal” quartz growth should be lower in sandstones with microquartz than comparable ones without. This hypothesis, however, is inconsistent with Touchstone simulations that indicate the surface area normalized rate of “normal” quartz precipitation in Miller Field sandstones with microquartz is equal to or greater than that in interbedded sandstones lacking microquartz (Bonnell et al 2006).
The experiments of Bonnell et al (2006) suggest that microquartz inhibits “normal” quartz growth because it grows at much slower rates. To better understand this rate effect we conducted additional hydrothermal experiments involving quartz overgrowth formation on a single crystal that was covered by copper foil with holes ranging from 50 μm to 3.2 mm. The experiments show a near linear dependence between surface area normalized growth rate and nucleation substrate size. The evolution in crystallite morphologies with growth indicates that this dependence reflects an ~20x decrease in growth rate that occurs when non-euhedral surfaces become euhedral. The dependency of net growth rate on nucleation substrate size occurs because, by Steno's Law, overgrowths that form on smaller substrates achieve euhedral termination with less growth. Our 2D model of quartz cementation that accounts for this effect (Prism2D) reproduces both the experimental observations and the inhibitory effect of microquartz coatings in Miller Field sandstones.