354 Subsurface Geologic Methods 



practicable to give the analytical detail that may be desirable in purely 

 scientific research. The geologist, nevertheless, should endeavor to make 

 the description as complete as possible in the time available. Necessarily, 

 the description should be more detailed on wildcat wells than on field 

 wells and much more detailed in the pay sections than in the upper sec- 

 tions. In field wells it is permissible to omit descriptions of long sections 

 of comparatively insignificant beds between key beds. The geologist 

 should remember that it is much better to have too full a description than 

 too meager a description. Experience has shown that full descriptions 

 have saved much time and money, whereas meager descriptions have re- 

 sulted in redescription of the cuttings several times. Nevertheless, some 

 redescription can not be avoided; therefore, samples should be carefully 

 preserved and indexed so that they will be available for future study. 



There are two principal ways of describing samples. The first of 

 these is the interpretative system, in which the geologist picks out the 

 ■ cuttings which he believes to be representative of the formation penetrated 

 and describes the entire sample as composed of this rock. The rest of 

 the sample is assumed to be cavings. This kind of description brings out 

 formational changes and is of greatest value in areas where the various 

 formations are of wide extent and relatively constant character, as in 

 the Paleozoic of the Mid-Continent region. In areas of rapid lateral 

 gradation in the lithologic character of formations, as in the Permian Basin 

 of west Texas, this method results in masking of lateral variations and 

 misinterpretation of the nature of the stratigraphic column. This, of 

 course, results in miscorrelation of the well logs. 



In regions of pronounced lateral gradation it has been found that a 

 second method of sample description is most satisfactory. This is the 

 percentage description, where the geologist describes all material in the 

 sample, disregarding obvious foreign substances and cavings. This sys- 

 tem, though making it difficult to determine formational boundaries from 

 the sample log, shows the gradations of the beds and often enables one 

 to trace a horizon through different sedimentary facies. 



Percentage of various constituents is estimated by the eye. It has 

 been found that experienced sample examiners, using good samples, will 

 agree very closely on the percentages of constituents. However, with 

 poor samples where some judgment must be exercised in disregarding 

 obvious cavings, the value of the description is dependent on the judg- 

 ment of the geologist who examines the samples. Mechanical counters or 

 calculators, such as the integrating stage, have not been found to give 

 enough additional accuracy to be worthwhile in ordinary work. Experi- 

 enced sample examiners can describe 100 to 300 samples in an 8-hour day, 

 according to the nature of the cuttings.^ 



The descriptions may be written out or plotted directly on a log 

 strip. For many purposes, it is desirable to make written descriptions, 



^ A full description of sample examination as applied to stratigraphic work is given by L. H. Lukert, 

 Oil and Gas Jour., pp. 49-51, June 1937. 



