Studies of the amount of interstitial material may be made with various de- 

 grees of accuracy. For some purposes, a visual estimate of half a dozen different 

 slice areas may be sufficient. For a detailed micrometric treatment of quantita- 

 tive model analysis of thin sections, one may refer to Chayes (1956). In most 

 detrital sediments, there are no sharp breaks in sorting; therefore, if the inter- 

 stitial material is detrital, an additional problem arises. 



How can the interstitial material be sharply distinguished from the major 

 fraction? One approach is to select a size arbitrarily and to check the close grains 

 with a micrometer eyepiece. If the sorting is very poor, it is not practical to pur- 

 sue this kind of study ; it is better to use screen analysis, provided disaggregation 

 is possible. 



Kind of Cement 



The kind of cement or relative proportions of different kinds may be plotted 

 advantageously on maps. The same agents that influence petroleum movement 

 may also influence cementation. Under such conditions, changes in cement di- 

 rectly reflect conditions that improve or weaken chances of finding petroleum. 

 The most common cementing materials are calcite, dolomite, quartz, and chal- 

 cedony. In certain areas, evaporite minerals, phosphate minerals, or iron oxides 

 may be common. Knowledge of slight changes in composition, such as the pro- 

 portion of iron in ankeritic dolomite, may prove useful. 



Rock Composition 



Composition of the rocks is one of the most commonly used parameters in 

 lithofacies studies. An infinite number of variables in chemical as well as detrital 

 sediments can be mapped in this way. Gradual or abrupt changes from limestone 

 to dolomite, gypsum to anhydrite, quartz sand to feldspathic sand, or red beds 

 to gray beds are all equally useful for lithofacies studies. Few geologists question 

 the value of maps based on variations in composition. It is beyond the scope of 

 this paper to discuss the possible correlations between composition and petro- 

 leum occurrence. Clay minerals, by virtue of their composition and small grain 

 size, are very active chemically. They not only influence petroleum distribution 

 but are exceedingly important in drilling and production practices. Clay min- 

 erals, however, can be handled in thin section only in a general way ; other study 

 techniques are more applicable (Grim, 1953). Many changes in composition 

 lead to textural changes that may be of critical importance. 



Grain -Size 



Because petroleum migrates through and collects in rock spaces, any condi- 

 tion affecting these spaces (size, shape, distribution) is important. Texture in 

 sediments generally has not received the attention it warrants. More careful 



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