BORE-HOLE INVESTIGATIONS 1083 



The latitude is the distance the hole travels north or south between successive 

 stations. Similarly, the departure is the distance the hole travels east or west between 

 successive stations. The coordinates are the values of the latitude and departure cumu- 

 lated from the casing shoe down to the depth for which the coordinates are given. The 

 coordinates actually give the position of the hole at that depth using the position of the 

 casing shoe as the reference point. 



The coordinate data are used to construct a curve or plot of the computed course 

 of the hole, starting from the casing shoe and going down to the bottom. 



SIDE-WALL SAMPLING 



The various logging records previously described indicate certain physi- 

 cal properties of the formation, and this information must be translated 

 into terms dealing with the constitution of the rocks and their fluid content. 

 It is often difficult, however, to interpret completely certain portions of the 

 logs, since there is not always an unequivocal correspondence between the 

 electrical measurements and the geological properties. Certain data, such 

 as porosity, permeability, paleontological characteristics, etc. can be deter- 

 mined uniquely only by the analysis of actual samples taken from the 

 formations. 



Inspection of the cuttings, brought continuously to the surface by the 

 circulation of the mud during the drilling, often is a valuable source of 

 information, but, in general, not sufficiently complete and accurate for 

 practical purposes. The conventional means of coring, by the rotary method, 

 is the best way of getting the most reliable information, but coring is 

 time-consuming and expensive, and only the horizons of most interest, 

 chiefly reservoir rocks and certain geological markers, are cored. One big 

 difficulty when coring is in estimating the probable depths of these horizons, 

 and, with a reasonable margin of safety, coring through those sections only. 

 In fact, key horizons may be passed inadvertently while the drilling is 

 being performed. It is oftentimes considered a safer and more economical 

 practice to proceed with the drilling, followed by electrical logging. After 

 the electric log has been run and has indicated the exact location of the 

 different beds, the side-wall samples are taken. 



Mechanical Samplers. — Various mechanical samplers are on the market. One 

 type is operated hydraulically by mud pressure. The instrument is mounted on the 

 bottom of the drill stem and is run into the hole to the depth at which it is desired to 

 secure a sample. While traversing the hole, the sampling blades remain inside the 

 body of the tool, but when the desired point is reached, pump pressure applied at the 

 surface pushes down on a piston which expands the blades outward and upward against 

 the formation, and with the pumps still running, the weight of the string of drill pipe 

 is slowly and steadily placed upon the tool. This causes the blades to penetrate the 

 formation, forcing cores of the formation into each of the sampler tubes. 



At no time during the coring operation is the drill string rotated in either direc- 

 tion. When the pump pressure is removed and the string raised, the blades close into 



