provide the lowest gamma-ray intensities. Halite (salt) is similar, but other 

 geologic knowledge usually permits salt zones to be differentiated from sand- 

 stone or limestone. Silty or shaly limestones or sandstones are generally higher 

 in natural radioactivity. Shale almost always records as variable but high in 

 gamma-ray activity. On the neutron curve, dense dry zones and gas-bearing 

 zones provide the highest intensities, whereas shales record at the lowest inten- 

 sity. Liquid-bearing porous zones are intermediate; the greater the hydrogen 

 content, the lower the response on the neutron curve. However, since shale 

 contains much hydrogen and is thus similar to oil or water in diminishing the 

 neutron curve intensity, the gamma-ray curve or other data must be applied 

 to determine whether producible liquid or shaliness is responsible for a 

 particular neutron-curve response. Increases of radiation intensity are uni- 

 versally recorded as deflections toward the right; thus low intensities correspond 

 to leftward positions of the curve, and high intensities correspond to rightward 

 positions. 



Figure 15-3 is representative of the behavior of the gamma-ray and 

 neutron curves throughout the world in responding to variations in rock 

 properties. Typical responses of the gamma-ray and neutron curves to variations 

 of hole diameter, casing thickness, cement, borehole fluid, and other physical 

 conditions pertaining to the well are shown in Figure 15-4. 



In contrast to the gamma-ray curve, which is affected only slightly, the 

 neutron curve is greatly influenced by borehole conditions. Not only can 

 variations of the borehole properties be detected and identified on the neutron 

 curve, but they must be identified and taken into account in order to interpret 

 properly the influence of the formations. Thus the heading of the log, which 

 contains a record of the well diameter, the casing, and the borehole fluid, should 

 be studied before attempting to interpret the radioactivity curves. Upon de- 

 termining that no borehole change occurs within the zone of interest, or if 

 this is not the case, upon locating and recognizing the effect of this change on 

 the log, the user may proceed toward interpreting the curve responses in terms 

 of formation properties. 



IDENTIFICATION Depending upon the particular problem at 



OF FORMATIONS hand, one or both of the radioactivity curves 



may provide the basic information required 

 to identify a particular rock. However, even though one curve may allow the 

 identification to be made with reasonable certainty, the other curve supplements 

 and confirms the conclusions. Some common identification problems are listed 

 below with an indication showing whether the gamma-ray curve, the neutron 

 curve, or a combination of the two supplies the basic information. 



336 



