occurs at each formation boundary. Generally the sandstones and limestones, 

 which constitute possible reservoir rocks for the accumulation of petroleum, are 

 low in radioactivity, whereas the shales, clays, and siltstones have appreciably 

 higher concentration of radioactive elements. The gamma-ray curve is thus 

 directly applicable to correlation and identification of the strata. By virtue of 

 the penetrating power of gamma rays, which are similar to X-rays and can 

 pass through steel, cement, and borehole fluids, the gamma-ray curve may be 

 obtained in wells which are lined with cement and casing as well as in open 

 holes. Likewise, the gamma-ray curve is obtainable regardless of the type 

 or condition of the borehole fluid. These important properties permit gamma- 

 ray logging to be conducted with equal success in old or new wells and under 

 conditions which may render other logging processes inoperative. 



The importance of obtaining, through casing, information relating to the 

 fluid content of the rocks led to the development of neutron logging. A neutron 

 curve is obtained by moving along the well a source which emits energetic 

 neutrons, together with a radiation detector spaced a fixed distance from the 

 source. The detector may be either one which responds to gamma radiation, 

 in which case the method can be properly described as a neutron-gamma 

 process, or one which is sensitive to neutrons. In the latter instance, the method 

 comprises neutron bombardment and neutron detection; hence, it is called a 

 neutron-neutron (n-n) process. In either instance, the results are nearly identical; 

 both types of neutron curve are influenced principally by the amount of hydro- 

 gen present in the vicinity of the apparatus, which, in turn, is related to the 

 fluid content of the rocks. The penetrating ability of the radiations employed 

 in neutron logging is such that satisfactory logs can be obtained through several 

 thicknesses of casing. The neutron curve is thus a natural companion to the 

 gamma-ray curve, and the majority of radioactivity logs consist of these two 

 curves. The gamma-ray and neutron curves are generally obtained simultane- 

 ously during a single traverse of the well by means of dual logging equipment. 

 Information from the radiation detectors in the subsurface instrument is tele- 

 metered to the surface via the logging cable. The log is recorded during the 

 upward traverse of the well to insure that the entire length of the hoisting cable 

 is under tension in order to obtain reliable depth measurements. Logging 

 speeds of 20 to 50 feet per minute are commonly employed. 



In cased wells it is customary to record the position of each casing joint 

 on the radioactivity log by means of a magnetic casing-collar locator incor- 

 porated in the logging instrument. These indications provide a series of depth 

 markers from which any chosen formation may later be located without de- 

 pending upon the precise measurement of a great length of cable. Later opera- 

 tions such as gun perforating may thus be performed accurately by locating 



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