116 Subsurface Geologic Methods 



persons who are thoroughly familiar not only with their objectives but also 

 with the difficulties that confront them. ... It is true that oil company micro- 

 paleontologists have invented some useful equipment but most of it is of minor 

 importance. Principally they have adopted inventions made by others; they 

 have modified equipment already manufactured; they have organized material 

 and personnel for systematic, rapid study. In other ways, however, their 

 contributions have been of great importance; by their refined correlations of 

 strata, they have aided in the discovery of oil and in the unraveling of the 

 complicated history of the earth.^^ 



DETRITAL MINERALOGY 

 GORDON RITTENHOUSE 



This section outlines briefly the methods used to study the mineralogy 

 of subsurface samples, some of the limitations of these methods, and the 

 uses to which the results may be put. Much has been written on the meth- 

 ods of studying and interpreting sedimentary rocks. Agreement is far 

 from unanimous on either the best methods of study or the meaning of 

 the information after it has been obtained. In this section the writer has 

 attempted to emphasize those methods that he thinks have been or will be 

 most useful in practical subsurface work and to present those interpreta- 

 tions of data that seem to him most logical and reasonable. 



The study of the mineralogy of subsurface samples is helpful in one 

 or more of the following: (1) correlating or identifying key beds or 

 producing horizons, (2) determining the extent, thickness, and lithologic 

 variations of beds or groups of beds, (3) selecting the best methods of 

 completing wells, (4) selecting the best methods for secondary recovery, 

 (5) determining the source or sources of various beds or various parts 

 of the same bed, and (6) determining the geologic history as it can be 

 deduced from the composition and physical properties of the rocks and 

 from the relations of different beds to one another. 



Much of the work on the mineralogy of subsurface samples has been 

 primarily descriptive: the composition, texture, color, and other obvious 

 characteristics are described in varying degrees of detail, and the rock is 

 given a name. Much exceedingly valuable information has been obtained 

 and will continue to be obtained in this way. One has only to consider 

 the great progress that has been made in subsurface geology during the 

 past three .decades to appreciate the value of such descriptive work. Al- 

 though geophysical logging methods have recently supplanted sample ex- 

 amination in part, subsurface samples will continue to be used extensively 

 in the future. 



As stratigraphic traps or combined stratigraphic and structural traps 

 become increasingly important sources of oil and gas, interpretation of 

 subsurface samples in terms of sources of sedimentary materials, the 



^^ Schenck, H. G., and Adams, B. C, Operations of Commercial Micropaleontological Laboratories: 

 Jour. Paleontology, vol. 17, no. 6, pp. 554-583, 1943. 



