INTRODUCTION

Shales represent about one-half of all sedimentary rocks in the stratigraphic column. They consist mainly of clay minerals, quartz, feldspars, and micas, with minor amounts of other minerals such as carbonates, iron oxides, zeolites, and sulfates. Thus, they are mineralogically and chemically more complex than are either sandstones or limestones. Owing to their fine grain size, only a few petrographic studies of shales have been published. In thin section, many of the constituents of shales cannot be resolved optically because of their small size and the intermixing of clay minerals. Clay mineral flakes are so thin that several may be stacked irregularly upon one another so that light passing through is diffracted and/or refracted irregularly, producing an image with poor resolution. In addition, opaque materials such as hematite and amorphous inorganic and organic particles make petrographic viewing difficult (Blatt, 1982). Some workers have succeeded in using the petrographic microscope to study shales (e.g., Folk, 1960, 1962), but the process is time consuming and requires considerable expertise. The literature dealing with petrographic study of shales prior to 1980 is summarized by Potter et al. (1980). Recently, petrographic microscopy studies have been published by Schieber (1986, 1989, 1994), Odin (1988), Weaver (1989), and Leckie et al. (1990), O'Brien and Slatt (1990), and Bennett et al. (1991 a).

In spite of some success with petrographic methods, the mineralogy and texture of shales is difficult to determine petrographically. Shale mineralogy can be effectively studied by use of semiquantitative X-ray diffraction; however, the X-ray diffraction technique cannot be used to study texture, which is one of the most important characteristics of shales.