Understanding
Quartz Overgrowth Generation
French, Marsha1, Joann Welton1,
Mike Braun1 (1) ExxonMobil Upstream Research Company,
To improve the ability to predict
porosity in deep sandstone reservoirs, a banded microcrystalline quartz (Louisiana
Banded Agate) was analyzed to understand quartz overgrowth development. The
chemical composition and crystal orientation of Louisiana Banded Agate was
investigated using a combination of advanced analytical tools such as Scanning
Electron Microscope (SEM), Electron Backscatter Diffraction (EBSD), and
cathodoluminescence (CL).
Band contrast images of the Louisiana
Banded Agate derived from EBSD show several zones of parallel alternating light
and dark bands, which are similar to natural quartz overgrowths observed in
deeply buried sandstones. The light bands are composed of fine-grained
crystalline quartz, whereas the dark bands represent amorphous or
micro-crystalline quartz with grain sizes smaller than the spatial resolution
of the SEM at the operating magnification. These observations indicate an
evolution from poorly crystalline silica to fully crystalline quartz during
growth events. The agate also exhibits iron staining between generations of
quartz growth, similar to natural quartz overgrowths in sandstone reservoirs.
EBSD demonstrates that how these surface defects and relative mis-orientations
control subsequent quartz growth. The compositional and crystallographic
variations are also compared with the spectral CL response.
Studying the chemical and structural
analysis of Agate provides important information on the mechanisms and kinetics
of how microquartz coatings and quartz overgrowths form in natural settings.
This fundamental information on quartz growth can be incorporated into forward models
to improve the ability to predict porosity in deep sandstone reservoirs.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California