Miscellaneous Subsurface Methods 585 



mable value. When depths measured in the well are considered as vertical 

 depths, erroneous conceptions are formed of geologic horizons, contours 

 of subsurface structures, the thickness of formations, and the depth and 

 location of faults, salt domes, and similar features. Not only the vertical 

 depth, but the location of the well bore throughout, is essential to the 

 correlation of wells to depict the actual geologic conditions underground. 

 Many of the assumptions made by geologists in older fields where drilling 

 was done before the general use of surveying are incorrect. Someone has 

 referred to the crooked hole as an "alibi for geologists." The intelligent 

 employment of electric-coring data is dependent upon knowledge of the 

 course of the hole throughout its length. 



In many wildcat-well operations oriented cores are taken at a con- 

 siderable expenditure of time and money. These cores are of little value 

 unless the dip and strike indicated are corrected for the angle and direction 

 of the hole itself. 



A number of mining companies doing exploratory work by diamond- 

 drill coring methods have realized the value of surveying small-diameter 

 core holes made in rock formations. A special l|^-inch diameter barrel has 

 been used in surveys made in shallow AX size holes drilled at Leadville, 

 Colorado, and in Missouri. Larger diameter holes were surveyed with a 

 regular single shot at Rifle and Aspen, Colorado. Special length barrels 

 were mauufactured for this purpose so that two instruments could be used 

 at once. Time necessary to make the surveys, run on drill rods or on 

 heavy wire line, was reduced by half by use of these long barrels. No 

 magnetic attraction was detected in the strata penetrated by the diamond 

 drills. 



The exact location of the producing portion of a well on a known 

 structure is of considerable economic importance. The petroleum geolo- 

 gist attempts to obtain the largest production with the greatest ultimate 

 yield consistent with sound economic development of the lease as a whole. 

 Surface spacing of wells may be made consistent with good practice in 

 the field concerned; however, if the wells are not drilled vertically, the 

 entire plan is defeated. As the surveying of new wells becomes standard 

 practice, it becomes evident that bottom-hole spacing is of far greater 

 importance than the orderly surface arrangement of wells. In fact direc- 

 tional drilling has made possible the surface spacing of wells at any 

 convenient location, with wells bottomed in almost as exact a pattern as 

 if they were at the surface. It is common knowledge that edge wells 

 are not very desirable because they are short-lived producers. Many wells 

 have been spudded high on the structure but bottomed in edge locations or 

 even too low on the structure to be worth producing. Consistent checking 

 of the well for drift and direction would have insured a better well or 

 obviated the extra expense of redrilling a portion of the well to get pro- 

 duction. 



When wells are drilled to bottom near faults or are caused to cross 



