360 Subsurface Geologic Methods 



Plotting 



After the description of the samples is made, it must be plotted in 

 graphic form to make the information easily available for correlation 

 and study. It is usual to plot this material on a narrow strip of heavy 

 paper or cardboard so that a large number of logs can be laid out to 

 compare and correlate the sections. A 3-inch width has been found to 

 be most convenient for this purpose. Upon this 3-inch strip the lithologic 

 characteristics are plotted on a column ^ to f inch wide with a vertical 

 scale of 1 inch equals 100 feet. On this column each 10 feet (1/lOth inch) 

 is marked so as to facilitate plotting. The rest of the log strip is used for 

 notes on lithologic characteristics that are not easily plotted as symbols. 

 It has been found in plotting the lithologic characteristics that strong 

 contrasting colors facilitate correlation and comparison of the sections. 

 While no standard system of symbols has been adopted, the following 

 colors are widely used. 



Light blue = Calcareous limestone 



Dark blue = Dolomite 



Red = Red shale 



Gray = Gray shale 



Yellow = Sandstone 



Green = Salt 



Purple = Anhydrite 



Other colors may be added for local necessities. It has been found very 

 helpful also to plot the drilling time next to the lithologic column on the 

 log strip. In the case of mechanically taken drilling time, a copy of the 

 graph is plotted; with manually taken drilling time, the plotting is done 

 as bar graphs — the common scale being 1 inch equals 100 minutes of 

 drilling time. 



Porosity and oil stain in the samples are indicated by symbols on 

 either side of the lithologic column. These symbols may be either in 

 ink or in suitable colors. It is very helpful to make some distinction as 

 to probable gas or water stain even though these fluids can not be differ- 

 entiated certainly from cutting examination. Sharp changes in lithologic 

 character should be indicated plainly on the plotted log. This indication 

 usually is done best by having the geologist who examined the samples 

 check the plotted log and indicate where he believes the formation boun- 

 daries should be, because their exact position is commonly difficult to 

 determine from the written description alone. 



All tests and showings should be indicated on the margin of the log 

 strip. These should include all drill-stem tests in rotary v/ells, and amount 

 and kind of fluid in cable-tool holes. It is also helpful to indicate any 

 swabbing tests taken, the acid used, amount of nitroglycerine used and 

 section shot, and perforations in the casing. Casing seats should be in- 

 dicated on the margin of the log with a notation of the diameter of the 

 casing and the amount of cement used in setting the casing. Total depth. 



