--> Abstract: Coatings from Sediment Grains in the Navajo Sandstone, Black Mesa, Arizona, and Potential Implications from Water-Rock Interaction Kinetics, by Chen Zhu; #90078 (2008)

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Coatings from Sediment Grains in the Navajo Sandstone, Black Mesa, Arizona, and Potential Implications from Water-Rock Interaction Kinetics

Chen Zhu
Geological Sciences, Indiana University, Bloomington, IN

K-feldspar surfaces in the Navajo Sandstone at Black Mesa, Arizona are partially coated with tightly adhered kaolin platelets. Outside of the kaolin coating, feldspar grains are covered with a continuous 3 - 5 um thick layer of authigenic smectite, which also coats quartz and other sediment grains. Authigenic K-feldspar overgrowth was found on feldspar grains, and incipient quartz overgrowth on the quartz grains. These coatings were characterized with High-Resolution Transmission and Analytical Electron Microscope (HRTEM-AEM) and Field Emission Gun Scanning Electron Microscope (FEG SEM). Additionally, a 10-nanometer thick amorphous layer was identified on K-feldspar with HRTEM. The coatings may indicate two important aspects of reaction kinetics in water-rock systems, which in turn affect accurate predictions of reservoir quality, diagenesis, reaction rates and porosity/permeability changes during EOR. First, the secondary minerals rinding on feldspars are not at local equilibrium with fluid, as traditionally assumed, but their slower precipitation rates can raise the aqueous solution saturation state with respect to feldspars to very close to equilibrium, and hence retard feldspar dissolution, partially addressing the well-known apparent field-lab rate discrepancy problem. Dissolution and precipitation reactions in sedimentary systems are coupled and operate within a complex network of reactions, in which the rates of secondary clay precipitation plays a key role. Second, the presence of the amorphous layer on weathered feldspars requires re-consideration of the details of surface reaction controlled mechanism. We have conducted laboratory controlled experiments to quantify the roles of coatings. Examples of experimental results and geochemical modeling to interpret the field data and laboratory experiments will be given

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas