Ch. 31] 



SAMPLE LOGS 



561 



tools. Samples are brought to the surface by means of the mud 

 slurry moving up the hole outside of the drill pipe. Differential set- 

 tling, scattering of the fines, contamination from the walls, time lag, 

 and finally loss of disaggregated fines in screening at the slush pit may 

 combine to yield a sample that only faintly resembles the formation 

 penetrated. Thin key beds may be missed entirely, or their cuttings 

 may be so spread along the column 

 of returning drilling mud that it is 

 impossible to pick either the top 

 or the bottom. 



It is standard practice for the 

 geologist to examine the cuttings, 

 whether they be cable-tool or ro- 

 tary, with a binocular microscope. 

 He notes the depths of recognizable 

 formations and describes each unit. 

 This information may be kept in 

 notebooks, typed and kept in com- 

 pany files, or plotted on log forms 

 printed especially for this purpose. 

 On these forms, depths and litho- 

 logic characteristics of the strata in 

 neighboring wells may be directly 

 compared by laying them side by 

 side. 



Written records of the formations 

 penetrated, when plotted graphic- 

 ally to scale, are known as strip 

 logs. Such logs are customarily 

 plotted on strips of heavy paper or 

 lightweight cardboard, and the dif- 

 ferent types of rock are indicated by means of standardized symbols 

 or colors. Color symbols usually are employed by the geologist with 

 yellow representing conglomerate, gravel, sandstone, or sand. Gray 

 shale, slate, and clay may be left blank; red shale, slate, or clay, red; 

 limestone, dolomite, and chalk, blue; coal, black. Special features 

 of these different types of rock may be indicated by descriptions in 

 the blank spaces to the right of the color symbols. These features may 

 include principal lithologies with percentage of each color, texture, 

 angularity of grains, index fossils, nature and amount of cementing 

 material, estimates and nature of porosity, presence of secondary crys- 

 tallization, and frosting of sand grains. Such descriptive features of 



Fig. 3. A type of log form used in 

 plotting sample descriptions. The S 

 stands for section; T for township; 

 R for range, and "No." for well num- 

 ber. The vertical scale on the orig- 

 inal form is 1 inch = 100 feet. (Cour- 

 tesy of Mid-West Printing Co.) 



