Eve S. Sprunt
Mobil Research and Development Corp., Dallas, TX
Amos Nur
Department of Geophysics, Stanford University, Stanford, CA
ABSTRACT
The pore structure of rocks has a large effect on many petrophysical properties that are measured by logs. Many techniques have been used to examine rock microstructure to improve our understanding and models of rock behavior. Cathodoluminescent (CL) petrography has many advantages as a method of examining pore structure and grain-relationships. The equipment required is much cheaper than that required for electron microscopy. Thin sections are used and the results can be readily compared to ordinary petrographic techniques. in CL petrography the pore spaces are clearly defined because the non-luminescing epoxy-filled pore spaces contrast sharply with the luminescing grains. Contacts
between grains, even grains of the same mineral, are distinct.
This paper focuses on quartz CL which is of interest because of the importance of sandstone reservoirs. Quartz CL is much Iess intense than carbonate CL so more careful experimental procedures are required.
A detailed description of a successful method of observing quartz CL is given along with examples of the advantages of using quartz CL petrography.
New work on what causes the colors of quartz CL offers hope that CL petrography may be a useful tool in unraveling the history and sources of sandstone reservoirs. Analysis of the impurity contents of quartz
shows that crystals with high titanium concentrations and low iron concentrations luminesce blue, whereas crystals with low titanium concentrations and high iron concentrations luminesce red. The electron
hole color centers which cause the quartz color in hand specimen are not responsible for the color of the cathodoluminescence.