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Porosity Preservation by Microquartz

French, Marsha W.1; Worden, Richard 2
1 Exxon Mobil Upstream Research Company, Houston, TX.
2 Earth and Ocean Sciences, University of Liverpool, Liverpool, United Kingdom.

Authigenic quartz cement is the most common pore-occluding mineral in deeply buried (>2500 m) quartzose sandstones. Conventional cementation models often fail to predict the occurrence of deeply buried porosity confirming that our understanding of quartz diagenesis is still limited. We have utilized advanced analytical capabilities to improve our understanding of controls on quartz development.

In this study, several advanced analytical techniques were used to evaluate the crystallographic and compositional controls on the formation of microcrystalline quartz. SEM/Cathodoluminescence (CL) confirms that quartz overgrowths have a complex growth history. Previous workers (Kraishan et al. 2000) suggested that CL patterns in quartz cement are largely due to trace elements rather than defects and that aluminum varies consistently between each cement phase. Electron Backscatter Diffraction (EBSD) combined with Wavelength Dispersive Spectrometry (WDS) confirms that the complex banding visible in CL is not due to changes in crystallographic orientation but more likely variations in quartz composition associated with changes in pore fluid composition and/or reservoir conditions. Secondary Ion Mass Spectrometry (SIMS) analysis provides maps of ultra-trace element distribution that confirm that trace amounts of aluminum, iron, and titanium can be used as proxies for defect density and temperature, and thus provide insight into the initial reservoir conditions.

Integrating the results from these advanced analytical techniques has helped us develop our understanding of the processes controlling the formation of quartz cement and improved our ability to reconstruct the reservoir diagenetic history and /or trace element impact on quartz growth.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009