should be taken on the examination and the log plotted later. Each method 

 has certain advantages over the other. As mentioned earlier, the notes can be 

 typed when a number of copies are needed, or a strip may be plotted from them. 



From the microscopist's viewpoint, it is much better to plot the log directly 

 from the observation of samples. Colors do not require minute description, 

 because they are shown on the log by direct matching with the rocks. Small 

 differences in similar lithologies will be shown on the log, but they may be 

 neglected in a plot from a word description. The greatest advantage is that the 

 microscopist has always before him a graphic record of the rocks that have 

 been logged up the hole. This is of the utmost importance when suspected 

 cavings appear in the samples, for a glance up the log will indicate the depth 

 from which the cavings may have come. 



The written descriptions on strip logs follow the same general pattern as 

 the example given above. Inasmuch as there is very limited space on the 3-inch 

 strip, it is necessary to use abbreviations wherever possible. A list of abbrevia- 

 tions is presented at the end of this chapter. The strip may be plotted in standard 

 black-line symbols or in color. The black-and-white log has only one advantage: 

 that of duplication by photo-reproduction methods. Colored strips are more 

 legible, are more pictorial, and are easier to draw. They are used universally in 

 the oil industry, although the meanings of colors differ among oil companies. 



While acknowledging the advantages in various color symbolizations in 

 use, the system presented here is simple in principle, yet so adaptable that all 

 conceivable mixtures of lithologies can be shown. Briefly, it is as follows: 

 All carbonates are shown in sky blue; extremely dense (lithographic) limestones 

 or dolomites are shown in a darker shade of the same color. All coarse elastics 

 are represented by the basic color canary yellow. All evaporites (noncarbonate) 

 are shown by black-line patterns. Shales are shown in colors that match the 

 actual colors of the rocks. Siltstones and graywackes are diagonal bands of 

 natural color alternating with bands of canary yellow. In other words the 

 lithologies are midway between shales and sandstones, and the log representation, 

 likewise, is a combination of the respective colors. 



Minor constituents in the main rock mass are shown in the representative 

 colors of the lithologies in 45-degree bands drawn down from right to left. Thus 

 a sandy and calcareous shale would be shown by a yellow band and a blue band 

 across the shale color. 



The accessory constituents, such as mica and pyrite, in many cases are 

 represented by symbols suggestive of the minerals. These symbols are inked 

 in black. 



Colors of limestones are shown on the log only if unusual, such as red, 

 pink, or orange, by diagonal bands across the blue, extending downward from 

 left to right. The bands indicating color are not bounded by inked lines; 

 those showing minor constituents, crossing the color bands, are inked. There is 



43 



