measurable effects. However, many rocks do possess anisotropic features derived 

 from their environments. Some of these features are clearly visible to the un- 

 aided eye; — for example, foliation in schists. Others are detected only by sta- 

 tistical analysis with the universal stage. 



Petrofabric analysis consists of systematically traversing a thin section and 

 plotting positions of selected elements. The most commonly used elements are 

 the optic axis of quartz, poles to cleavage of mica, poles to twinning of lamellae, 

 and optic axes of carbonate minerals. Anisotropic fabrics may be either of depo- 

 sitional or of deformational origin. Inequant grains tend to be aligned under 

 the influence of current. If the current was strong and the grains pronouncedly 

 inequant, the anisotropy may be readily apparent. If the current was not so 

 strong, or the grains not so inequant, the anisotropy may be detected only sta- 

 tistically. Even quartz grains are slightly inequant, elongate along the c-axis. 

 Plotting the positions of these axes frequently demonstrates preferred orienta- 

 tion. This technique can be used in channel deposits to ascertain direction of 

 former stream flow. 



In a similar fashion, preferred orientations may be developed during de- 

 formation. When a simple fold is formed, there is a tendency for mineral grains 

 to orient themselves with certain directions parallel to the axis of folding. A 

 statistical study of the fabric of a specimen may reveal its relation to the structure 

 and indicate the trend of the fold axis at that point. 



Basement-Rock Studies 



Thin-section studies in subsurface petroleum geology should not be limited 

 to sedimentary rocks. Valuable and often critical information may be gained 

 by thin-section studies of basement rocks. The circumstances under which these 

 studies may be profitable fall into several categories. 



Recognition of a rock as a basement type may require thin-section examina- 

 tion. Occasionally wells have been bottomed far short of basement by misidenti- 

 fying or misinterpreting the penetrated rock. Determination that a rock is of 

 plutonic igneous or of metamorphic origin is generally sufficient to indicate base- 

 ment. One of the most common errors in the past has been the belief that all 

 quartzites are metamorphic. Actually many quartzites originate by the normal 

 sedimentary process of complete silification of a quartz sandstone. In their gross 

 features, these quartzites do not differ from metamorphic quartzites; however, 

 metamorphic quartzites can be recognized by the presence of metamorphic min- 

 erals or a crystalline texture as contrasted to clastic texture. 



Volcanic rocks, including pyroclastics, do not necessarily indicate basement, 

 and therefore, should be distinguished from plutonic igneous rocks ; however, in- 

 tensely hydrothermally altered volcanics, often called greenstone, generally con- 

 stitute basement. Although fresh granitic rock is usually recognized without 



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