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From
Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
DOE Geoscience Research Program Symposium, September 2004 Theory for Quartz Cementation in Structurally Deformed Sandstones R. H. Lander, L. M. Bonnell, S. E. Laubach, and J. E. Olson Abstract: A comprehensive theory for quartz cementation must account for variations in the abundance and morphologies of cement in undeformed sandstones as well as in fractures of all scales. Our approach toward developing such a theory is to extend existing kinetic models by rigorously simulating the crystal growth anisotropy and nucleation surface area. For our analysis we assume that the rate limiting control on quartz cementation is the crystal precipitation rate (Walderhaug 1994, 1996, 2000). In a strict sense this assumption does not address whether the silica is derived from local diffusion or larger scale fluid advection but instead simply considers that dissolved silica is supplied at a rate that equals or exceeds the rate at which the crystals can precipitate. This assumption is reasonable for quartz cementation in near neutral pH fluids at temperatures in excess of ~80 °C but is not applicable to quartz precipitation in highly supersaturated fluids (e.g., in silcretes, saline-alkaline lakes, or in the presence of biogenic opal). We assessed the predictive accuracy of a kinetic model for quartz cement abundance in undeformed sandstones. The model (Touchstone™ version 5.0) considers the temperature effect on precipitation rates, the increase in surface area normalized rates of precipitation with nucleation surface area that have been documented in natural and synthetic quartz overgrowths (Makowitz and Sibley 2001, Bonnell and Lander in preparation, Lander et al in preparation), and the nucleation surface area and how it varies with sandstone texture, depositional composition, and diagenetic alteration. Quartz cement abundances are predicted to within four bulk volume percent of the measured values for more than 90% of structurally undeformed sandstone samples from diverse basin settings (rift, wrench, foreland/thrust belt, and cratonic), compositions (quartzarenite to litharenite to subarkose), thermal histories (maximum temperatures ranging from 108 to 244 °C for samples with Ordovician to Miocene depositional ages), and fluid overpressures (from hydrostatic pressure to near fracture gradient fluid overpressures) (Bonnell and Lander in preparation). We have extended this quartz modeling approach to explicitly account for the following effects of particular importance in structurally deformed sandstones: (1) substantially greater growth rates on surfaces that grow parallel to the c-crystallographic axis (pyramidal faces) compared to along the a-crystallographic axes (prismatic faces), (2) the order of magnitude increase in precipitation rate on non-euhedral surfaces compared to euhedral surfaces growing along the same crystallographic orientations (Lander et al in preparation), and (3) the complex interaction of repeated fracturing and cementation on nucleation surface area. Preliminary
results obtained using a 2D model incorporating these effects (Prism™)
reproduces quartz morphologies in fractures of varying scales as well
as in pores within undeformed sandstones. Results indicate that the abundance
and morphology of quartz cement within fractures depend on the relative
rates of quartz precipitation and fracture opening, the geometries and
sizes of grain surfaces and pores, and the orientations of crystallographic
axes with respect to fractures. Pervasive quartz sealing of fractures
occurs in the simulations when the rate of crystal growth on surfaces
of the slowest growing type (along the a-crystallographic axis) exceeds
the net rate of fracture opening. Euhedral quartz crystals rim otherwise
open fractures when the rate of fracture opening exceeds the fastest rate
of quartz crystal growth (on non-euhedral surfaces that grow parallel
to the c-crystallographic axis). Simulations suggest that quartz bridges
form where the following conditions are met: References R. H.
Lander and L. M. Bonnell, Geocosm LLC |