--> Abstract: Understanding Quartz Overgrowth Generation; #90063 (2007)

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Understanding Quartz Overgrowth Generation

 

French, Marsha1, Joann Welton1, Mike Braun1 (1) ExxonMobil Upstream Research Company, Houston, TX

 

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