118 Subsurface Geologic Methods 



lo obtain information about grain size or heavy-mineral content is time 

 wasted insofar as correlation is concerned, although such examinations may 

 yield valuable information on other problems. In general, one should 

 start with the simplest and fastest procedure and try successively more 

 complex and slower procedures until one is found that will provide the 

 needed information. A little forethought and a preliminary examination of 

 a few representative samples may not only save time and money but 

 determine the ultimate success or failure of an investigation. 



Characteristics of Different Types of Subsurface Samples 



Subsurface samples are usually of three types, namely, rotary cuttings, 

 cable-tool cuttings, and cores. The way in which these three types of 

 samples are procured limits the amount of data than can be obtained from 

 them and the interpretation of that data. The brief and perhaps over- 

 simplified explanation that follows outlines some of the major points to 

 be considered in studying these three types of samples. 



In rotary drilling the cutting action is provided by the abrasion and 

 downward pressure of a steel bit attached to a hollow drill pipe. Drilling 

 mud is pumped down through the drill pipe and returns to the surface in 

 the space between the drill pipe and the sides of the hole. The drill 

 cuttings carried to the surface in the drilling mud are usually separated 

 from the mud as it passes over a vibrating wire screen or through a shaker. 

 Samples of the accumulated cuttings are taken at intervals. Rotary cut- 

 tings have the following characteristics that limit their usefulness: 



1. Some rocks, particularly poorly cemented sands or bentonitic 

 shales that break into small fragments or disintegrate into mud, pass 

 through the screen or shaker and are not present or are present in only 

 small amounts in the samples. Sometimes more representative cuttings 

 can be obtained by diverting a part of the mud and cuttings into a con- 

 tainer and washing out the drilling mud after the cuttings have settled. 



2. Because rotary wells usually are not cased until drilling is com- 

 pleted, cavings from above may form a very large porportion of the 

 sample, especially when mud consistency has not been carefully controlled. 



3. Large and small cuttings, or cuttings of different specific gravity, 

 tend to be carried upward at different rates, and as a result the cuttings 

 from different horizons are mixed. 



4. Owing to the time required for the cuttings to be carried from the 

 bottom of the hole to the surface, the samples come from a somewhat 

 higher horizon than is being drilled at the time they are collected. A 

 depth correction may or may not be made at the well. A rule-of-thumb 

 correction of ten feet per 1,000 feet of depth is commonly used. 



5. The drill cuttings are usually less than three-eighths inch in dia- 

 meter. Hills,^^ whose recent paper on examination of subsurface samples 



^' Hills, J. M., Sampling and Examination of Well Cuttings: Am. Assoc. Petroleum Geologists Bull., 

 vol. 33, no. 1, pp. 73-92, Jan. 1949. (See pp. 344-364 of this Symposium). 



