Subsurface Laboratory Methods 157 



PETROFABRIC ANALYSIS 



WARREN R. WAGNER 



Although petrofabrics has not been widely used, it is a valuable 

 geologic tool that may be applied with marked success in deciphering 

 complicated structural conditions (surface and subsurface). The detail 

 to which it is carried depends upon the nature of the problem; its applica- 

 tion, however, may produce results on broad reconnaissance studies or on 

 large-scale detailed problems. The greatest value is probably obtained in 

 the latter. 



The idea that the petrofabric data for an entire area may be obtained 

 from one thin section and presented by making one or two orientation 

 (petrofabric) diagrams seems to be prevalent with some geologists. One 

 thin section an inch square may be the key to a particular part of the 

 problem, but the making of orientation diagrams alone does not get the 

 answer; it is their interpretation and their correlation with all other 

 petrofabric data of the area under study that complete the picture. 



The limited definition accorded the term "petrofabrics" by some 

 workers may have been responsible for this. The term has been defined in 

 various ways, and numerous students of the subject give it somewhat differ- 

 ent connotations. One of the most complete definitions is to be found in 

 Ingerson's short paper "Why Petrofabrics?" ^^ 



But the word as it should be understood in petrofabrics is much more 

 comprehensive. In this sense, it is analogous to the fabric of a building, that 

 is, its entire make-up from the structure of the individual bricks and the 

 mortar that holds them together, to the steel framework that binds the whole 

 into a unit. In other words, "fabric" includes all of the spatial relation of a 

 rock from the space-lattice of the individual mineral grains through cleavage, 

 fractures, joints, schistosity, lineation, and fold-axes. 



Therefore, petrofabrics to be complete should be approached as a 

 field problem supplemented by laboratory work. 



As defined above, all the details of a structural unit are brought 

 together. True enough, to acquire all data may require a tremendous 

 amount of tedius labor. But then, is not the price paid for any item 

 controlled by the returns? At the present moment, this tool is being em- 

 ployed chiefly by those more academically minded, although its applica- 

 tion to economic problems is advancing steadily. 



The metamorphic and igneous rocks have received the greatest atten- 

 tion, because the features and principles of petrofabrics are best developed 

 in them. Many of these principles probably hold true for the less-deformed 

 sedimentary rocks but are not so pronounced and thus have received less 

 attention. 



The purpose of this discussion is not to present the techniques and 

 methods of petrofabrics in detail but to summarize the rudiments with 

 references to literature on various phases. 



" Ingerson, Earl, Why Petrofabrics? : Carnegie Inst. Washington, Geophysics Lab., Paper 1081, 1944. 



