Miscellaneous Subsurface Methods 619 



and the use of oil-base muds have contributed appreciably to increasing the 

 quality of data obtained from core analysis and have probably resulted 

 sometimes in obtaining increased core recovery. Tracers placed in the 

 circulating fluid may also ultimately help determine the extent of flush- 

 ing of cores by mud filtrate. 



In cable-tool drilling, when it becomes necessary to obtain a core, a 

 barrel such as is shown in figures 322 and 323 is used. It consists primarily 

 of an outer drilling barrel and an inner core-retaining tube, which does 

 not move upward after coring is started until the tool is pulled from 

 the hole. The outer drilling barrel slides on the core-retaining tube and 

 cuts away the formation around it. The core tube follows down over the 

 core of the formation, and the core catcher or trap ring traps the core as 

 the barrel is pulled upward. The core is brought to the surface merely by 

 pulling the barrel, which is usually attached to a wire line. Usually no 

 additional surface equipment is needed for this operation. 



Application of Coring to Geologic and Development Problems 



In the past it has been found that coring is an aid in the solution of 

 some of the problems in the geology and development of oil fields that 

 may arise when subsurface formations are prospected. Those problems 

 are usually general, or they may be related directly to either exploratory 

 drilling or field or proved-area drilling. The geologist must evaluate the 

 formations penetrated by the drilling, and to do this adequately he usually 

 utilizes the best tools and methods available. 



General geologic problems usually result from a well condition in 

 which a desired tool or method cannot be used. For example, frequently 

 it may be desired to obtain an electric log of a well, but the circulating 

 fluid in the well may be of such a nature that the logging instrument can- 

 not function properly, or, because of sloughing formations or other hin- 

 drances, the condition of the hole may be such that the instrument cannot 

 be lowered to the desired depth. Also, in many areas, the geologist pre- 

 pares a formation-sample log from data obtained by examination of for- 

 mation cuttings. At times, however, these cuttings may not be satisfactory 

 because (1) mud-circulation rates may be too low to get them to the sur- 

 face without their being reground; (2) the presence of caving or slough- 

 ing formations may mask the drilled cuttings; or (3) the formation cut- 

 tings may be so soft that disintegration occurs before they reach the sur- 

 face. In such circumstances coring, even though possibly less desirable, 

 would provide a means of obtaining substitute data for geologic use. 



In exploratory geologic work the geologist may use coring and core- 

 analysis data ta (1) obtain a detailed formation description of the beds 

 that are penetrated, (2) determine the dip and direction of dip of forma- 

 tion beds, and (3) determine the probable fluid content of prospective pay 

 sections. After a core has been obtained and removed from the well, it 

 is usually laid out in the same linear position it held in the core barrel so 



