Chips cut from the bottom of the hole usually range from i/g inch to Y% mcn 

 across and are normally flaky in shape. The sizes of the chips depend on the 

 character of the rock, the sharpness and type of the bit, the rate of bit rotation, 

 and the weight on the bit. Unless the rock is extremely soft, such as chalk or 

 some shales, the chips are quite angular. The cuttings from any given lithology 

 tend to be of rather uniform size. 



In contrast to fresh cuttings, cavings may occur in pieces as large as two 

 inches. Since caving is more likely to develop in the softer rocks, the caved 

 fragments are often well-rounded. Recirculated material, if soft, will be very 

 well-rounded; but if the material happens to be from resistant rocks, it may be 

 very difficult to distinguish from the fresh cuttings. 



Depth Lag of Samples 



When samples are caught at the surface, the depth of the hole is recorded 

 on the bag. This depth is measured from either the top of the derrick floor or 

 from the top of the rotary table. While the hole is shallow, there is little dis- 

 crepancy between the recorded depth and the actual position from which the 

 sample was cut. As the hole deepens the error not only becomes larger but 

 also is subject to considerable variation, according to the rate of drilling and the 

 rate of mud circulation. The two examples given below illustrate how the 

 rate of drilling affects the lag. 



1. Assume that at a depth of 6500 feet it requires 30 minutes for cuttings 

 to travel from the bottom of the hole to the surface, where they are caught. The 

 drilling rate is 20 feet per hour. Therefore, while a chip is traveling to the 

 surface, the bit drills to 6510 feet. Although the depth recorded on the sample 

 bag is 6510 feet, the lowest sample recovered is actually from a depth of 6500 

 feet. The lag in this case is 10 feet. 



2. At 6510 feet a hard, slow-drilling stratum is encountered, and the drill- 

 ing rate falls to 4 feet per hour. Cuttings from this rock reach the surface 

 when the bit is at 6512 feet. The lag here is only two feet. 



It is evident that a lag factor cannot be applied to sample depths solely 

 on the basis of the depth of the well, except where the rocks being penetrated 

 are very uniform in drilling characteristics. In order to make lag adjustments 

 correctly, it is necessary to know the rate of drilling and the rate of mud cir- 

 culation. These data usually can be obtained from the drilling record of the 

 well. Inasmuch as these records are not ordinarily available to the microscopist, 

 it is better that he plot on the log the lithologies at depths shown on the sample 

 bags even though it is known that some lag exists. If the necessary data become 

 available subsequently, the corrected tops of main stratigraphic units can be 

 noted on the log strip. 



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