BORE-HOLE INVESTIGATIONS 



1105 



values of radioactivity. This is true also of volcanic ash and bentonite. Horizons of 

 unusual significance are formations classified as "radioactive sand or lime," because 

 these high radioactivity values are frequently misinterpreted. Such formations have 

 been found to be rather rare, but are worthy of note, especially if the horizon is a 

 possible producer of oil or gas in the area under study. The phenomenon has been 

 detected in both sandstone and limestone. Core analyses through zones of this kind 

 have shown the cause to be an extreme concentration of highly radioactive contamin- 

 ating material probably deposited under some unusual circumstance. 



The granite or basement rock is extremely radioactive and results in very high 

 gamma-ray curve values. 



Effect of Casing 



The effect of various bore-hole conditions on the gamma-ray curve is illustrated 

 in Figure 690. While each additional string of casing effects a slight reduction in 

 gamma-ray intensity, there may be cases where this is not always apparent. When 

 the casing is set in shale or other more highly radioactive formations, the greatest 

 curve shifts usually result. It is important to compare the casing record with the curve 

 to identify such shifts and eliminate possible misinterpretations. 



RADIOACTIVITY INCREASES 



GAMMA RAY CURVE (G/R) MAY 

 SHIFT WITH CHANGE IN CASING STRINGS 



NEUTRON CURVE IN) ALWAYS SHIFTS 

 WITH CHANGE IN CASING STRINGS 



A — ONE STRING OF CASING. CONTINUOUS UNIFORMITY. 



B — OPEN HOLE S CASING. NEUTRON CURVE SHIFTS TO LEFT OF CASING SEAT. 

 C — MULTIPLE CASING. NEUTRON CURVE SHIFTS TO LEFT AT EACH CASING SEAT. 

 — CASING a LINER. NEUTRON CURVE SHIFTS TO LEFT AT CASING SEAT a TO 

 RIGHT AT LINER. 



GAMMA RAY CURVE MAY ACT IN SAME MANNER AS NEUTRON CURVE BUT TO 

 A LESSER DEGREE. 



Fig. 690. — Effect of physical bore-hole conditions on gamma-ray and neutron curves. 

 (Courtesy of Lane-Wells Company.) 



The gamma-ray curve is normally not affected by the static fluid level or by 

 changes in the type of fluid. Satisfactory gamma-ray curves can be obtained in any 

 type of well fluid. 



Neutron Curve Interpretation 



The neutron curve is primarily a measurement of the amount of fluid surrounding 

 the measuring instrument. The word fluid denotes the total amount of liquid present, 



