346 Subsurface Geologic Methods 



name of the farm, number of the well location, and depth of sample. 

 The hole is, of course, bailed clean each run of the bit, and only occasion- 

 ally are cavings a problem in consolidated formations. 



In drilling bentonitic shales, quicksands, or conglomerate, the hole 

 ordinarily caves readily so that the samples are not entirely reliable. When 

 high-pressure gas is encountered, the hole is commonly filled with water 

 or light mud to control the gas. This ordinarily results in samples being 

 finely ground and much material being washed off the upper parts of the 

 hole, an action which contaminates the samples. The same condition re- 

 sults from drilling ahead in a hole full of water coming from the forma- 

 tion. Generally, under these conditions, pipe is run before much hole is 

 made so that the number of poor samples is comparatively small. Where 

 wire line or tools have been lost in the hole and must be drilled up, some 

 samples contain large amounts of iron. This can be removed by means of 

 a magnet and the residue of the sample examined. Surface rock and other 

 materials sometimes are thrown into the hole in attempting to straighten 

 it. 



The problem of checking the depth at which samples are taken is not 

 critical with cable tools, since the depth is checked by sand line on the 

 bailer after each run. However, important datum beds in the section 

 should be checked for depth by stringing in the sand line or by running 

 a steel measuring line. Cable-tool samples differ from rotary samples 

 by the general flaky character of the harder formations and by the polish- 

 ing and rounding of many cuttings from attrition due to turbulence set up 

 by the bit, and the irregular intervals at which they are taken, since the 

 length of bit runs are determined by the character of the formation and 

 mechanical factors. 



Rotary Samples 



The collection of rotary samples presents many more difficulties than 

 the collection of cable-tool samples and for many years it was considered 

 impossible to obtain reliable samples by this method of drilling. It is 

 still difficult to obtain representative samples from rotary holes in uncon- 

 solidated formations, but a technique has been worked out whereby rep- 

 resentative samples can be obtained in consolidated formations. These 

 samples are taken from the returning fluid stream at regular intervals. The 

 sample interval is usually 5 or 10 feet, but may be as small as 1 foot or 

 as large as 30 feet. The chief problems in collecting samples of rotary 

 cuttings are to prevent contamination with upper beds, prevent powdering 

 of the sample, prevent loss of the sample, prevent elutriation, obtain 

 correct depth measurement, and wash and properly dry the samples. 



Contamination from Upper Beds 



Contamination from upper beds is the chief problem of collecting 

 rotary samples. In the early days of rotary drilling before mud building 

 was fully understood, the walls of the hole were poorly plastered and un- 



