Many carbonate rocks are composed of carbonate grains and fragments 

 that have been deposited in essentially the same manner as sandstones or con- 

 glomerates. They are actually clastic rocks. If well samples were classified on 

 the basis of origin or mode of deposition, these limestones would have to be 

 considered with the clastic group. In certain investigations it is indeed necessary 

 to treat such limestones with the insoluble elastics, but to attempt to do so in 

 the examination and graphic presentation of well samples would only con- 

 tribute to the confusion that already exists. Therefore, all carbonates, regard- 

 less of origin, are here included in one class, and in no case are they considered 

 in the clastic group as previously defined. The rigidity of this presentation is 

 necessary if the basic colors of the strip log (yellow and blue) are to have a 

 constant meaning. 



On the basis of composition, which can be determined by the sample man, 

 the carbonate group is subdivided into two main classes: limestones and dolo- 

 mites. The term dolostone may be used in place of dolomite, but general ac- 

 ceptance of dolomite as a rock name, as well as a mineral name, makes the 

 term dolostone unnecessary. To the oil geologist, dolomite signifies a rock. There 

 is almost an infinite number of gradations between the nearly pure end members 

 of the group and the proportions of limestone or calcite, and dolomite in any 

 specimen may vary considerably in a few inches. Therefore, a quantitative an- 

 alysis would be of doubtful value. Within reasonable limits of accuracy it is 

 possible for the sample man and his limited means of determination to recognize 

 consistently two stages of the gradations, which are called dolomitic limestone 

 and calcareous, limy, or calcitic dolomite. These subdivisions are determined 

 by characteristic reactions in cold, dilute hydrochloric acid. 



Use 1 part C. P. hydrochloric acid to 7 parts water. 



Test the rock only in cold acid. 



Test in a depth of acid not less than 1/2 inch. 



Use chips about 1/4 inch diameter, i/£ inch thick. 



Observe reactions under the microscope, if the effervescence is slow. 



If there are no complications, the reactions will be about as follows (fig. 3-4) . 



Limestone: Violent effervescence; frothy audible reaction; specimen bobs 

 about and tends to float to the surface. 



Dolomitic Limestone: Brisk, quiet effervescence; specimen skids about on 

 the bottom of the container, rises slightly off bottom; continuous flow of C0 2 

 beads through the acid. 



Calcitic Dolomite: Mild emission of C0 2 beads; specimen may rock up 

 and down, but tends to remain in one place. 



Dolomite: No effervescence; no immediate reaction; slow formation of 

 CO2 beads on the surface of the rock; reaction slowly accelerates until a thin 

 stream of fine beads rises to the surface. 



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