596 Subsurface Geologic Methods 



ment. A special type "M" drift indicator is used for orientation. The 

 retractable core-receiving tube contains a scriber and is connected directly 

 to the drift-indicator barrel mounted above it. A special disk cup has a 

 base which is positioned by a cylindrical key, so that an index pointer 

 in the disk cup is in accurate alignment with the scriber in the receiving 

 tube. The whole assembly illustrated, including the drift indicator, is 

 dropped or run into the drill pipe on wire line and seated. Previously, 

 the amount of time it will take to cut a core has been estimated and set 

 on the watch in the drift indicator. When this time has been consumed 

 in cutting the core, the drill pipe is held motionless while the picture is 

 taken by the drift indicator. The retractable assembly is removed from 

 the well. Ordinary drilling can be resumed after the core breaker is 

 dropped into the bit. The core thus cut is oriented, since the drift-indi- 

 cator disk shows the relation of the scribe mark on the core to the low 

 side of the hole, as in the second method described above. 



It should be noted that the last two methods are dependent upon a 

 drift angle in the well of 2° or greater for the orientation of cores. 



The core obtained by any of the three methods described above is 

 placed in the reader (fig. 307) . It is adjusted so that it is positioned in 

 space exactly as it was in the subsurface formation from which it was cut. 

 The center section of the reader is turned, and the arms are aligned parallel 

 to the dip of the bedding planes of the core. The true dip of the formation 

 is read directly from the vertical protractor, and the direction of the dip 

 is obtained from the horizontal protractor. 



Whenever possible, the data obtained are checked by cutting a second 

 oriented core from the formation immediately below the first one. 



Typical diamond-core bits used in oil-well coring are illustrated in 

 figure 308. Figure 309 is a photograph of two very hard sandstone cores 

 cut with different sizes of diamond-core bits from subsurface formations 

 in Wyoming. 



Design and material changes are being constantly made to simplify 

 and improve the orienting of cores by the "Corex" methods. Determining 

 the dips and strikes of formations by actually measuring them from cores 

 has proved to be the most accurate and reliable method yet devised. 



MAGNETIC CORE ORIENTATION 



M. G. FREY 



Many attempts have been made to develop a method by which cores 

 from oil-well borings can be accurately oriented. Normally it is impos- 

 sible to determine the original orientation of a core when it reaches the 

 surface and is released from the core barrel. Mechanical, electrical, and 

 magnetic methods have been used with various degrees of success in efforts 

 to orient cores. The purpose of this section is briefly to review the mag- 

 netic method and discuss its limitations. For a full description of the 



