350 Subsurface Geologic Methods 



lag in the samples and should make the sample log correlate very closely 

 with the drilling time and with the electrical log. 



Catching Samples 



There are many ways to catch representative samples from the re- 

 turning mud stream. The rotary shale shaker of the Thompson type has 

 become very common in recent years. This is a large cylindrical screen 

 through which the returning mud stream passes. The screen is turned 

 by a "water wheel moved by the mud stream. Attached to the large screen 

 is a much smaller screen with fine mesh through which a portion of 

 the main mud stream is diverted, off which comes a small portion of the 

 cuttings which is collected in a box at the end of the screen for visual 

 examination. This type of screen has the advantage that it requires 

 no outside power to operate it, and it catches a representative sample of 

 the cuttings without any further attention from the operator or crew. 

 However, unless a very fine screen is used on the sample catcher, it will 

 not catch all the fine sands. In some formations the mud will not wash 

 out of the cuttings without use of an excessive amount of water which 

 renders it unsuitable for use with low water loss muds. 



Another important type of screen commonly used is the vibratory 

 shaker. In this device the mud stream passes across a vibrating screen. 

 The mud passes through the screen into the pits while the cuttings are 

 vibrating off into another receptacle. In consolidated formations good 

 samples can be taken by placing a narrow box under a part of the end 

 of the vibrating screen so that a representative portion will fall into this 

 box. However, this screen is open to the same objection as the Thompson 

 machine in formations consisting of fine sands, since the fine sand tends 

 to pass through the screen and not be caught in the sample. 



The simplest and possibly most reliable sample-catching device 

 (fig. 147) consists of a small-diameter nipple welded to the bottom of 

 the mud-return line with a l|-inch or 2-inch line running to a box 

 1 foot by 1 foot by 3 feet, with a removable gate about 6 inches high 

 in the end. In this box a representative portion of the cuttings is collected 

 and may be shoveled out at the appropriate time into a bucket and the 

 box cleaned by removing the end gate and letting the mud stream 

 carry out the remaining cuttings. When these are washed away, the 

 gate is replaced and the collection of the next sample is begun. By 

 this means a representative portion of both the fine and coarse parts of 

 the cuttings is obtained. Sometimes such a box is set in the course of 

 the main mud stream, but so many cuttings accumulate that the space 

 behind the gate is filled with cuttings before drilling of the sample interval 

 is completed so that the last part of the interval is not represented by the 

 cuttings in the box. 



After the samples are caught, they must be washed properly. This 

 can be done in a bucket by filling it partly full of water and stirring the 

 samples vigorously, letting the clean part settle and decanting the fluid sev- 



