consideration should be given to grain size in all routine examinations of rocks. 

 In thin section, diameters of grains, for the most part, are apparent diameters 

 rather than true diameters. The apparent average is less than the true average 

 size. This difference depends on such factors as grain size and relation of section 

 to bedding. The coarser the grain, the greater is the difference. Most sediments 

 of interest are moderately fine-grained and satisfactorily lend themselves to thin- 

 section study. A micrometer or grid eyepiece affords the most convenient method 

 of making grain-size measurements. Screen analyses normally yield the best re- 

 sults for grain-size studies and should be used where practicable. 



In lithofacies studies, consistent nomenclature is important, particularly 

 in textural investigations. For example, in the preparation of a sand-shale ratio 

 map, the criteria for distinguishing sand from shale must be defined clearly and 

 consistently followed, especially in regional studies that may involve the work 

 of several lithologic groups. The amount of silt that determines whether a rock 

 is called shale or siltstone must be decided arbitrarily. Sand-shale ratio maps are 

 very commonly prepared from electric logs. Thin sections provide a means of 

 evaluating the electric-log profile against the lithology. 



Diagenetic Changes 



In the migration and accumulation of petroleum, some of the most important 

 controlling factors are those changes that occur after or during deposition. These 

 changes (excluding metamorphism) are collectively termed diagenesis. Certain 

 processes such as compaction and cementation are lithifying diagenetic processes, 

 whereas others, such as leaching or replacement, are nonlithifying diagenetic 

 processes. These processes not only may limit or trap the flow of petroleum but 

 may conceivably force petroleum from one place to another. Here again, proc- 

 esses that may cause changes in composition may bring concurrent textural 

 changes. Some of the parameters that can be mapped are compaction, recrystal- 

 lization, cementation, leaching, and replacement. 



Metamorphic Changes 



The distinction between diagenesis and metamorphism is somewhat arbi- 

 trary. Generally, the concept of metamorphism includes the influence of elevated 

 temperature and pressure. Diagenetic changes, on the other hand, are believed 

 to occur at temperatures not much different from those at the surface. The be- 

 ginning of metamorphism is indicated by the development of certain diagnostic 

 minerals or by changes in fabric. iNear regional metamorphic terranes or con- 

 tact metamorphic terranes adjacent to igneous intrusions, the distribution of 

 petroleum may be critically affected by degree of metamorphism. In a given 

 area, it might be possible to determine the lithofacies criteria by which favorable 

 and unfavorable areas could be delineated. The trends established by plotting 



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