1110 EXPLORATION GEOPHYSICS 



that the formation breaks and details shown on the neutron curve may be sharper than 

 those shown on the gamma-ray curve. In all cases, regardless of the magnitude of the 

 break, the midpoint, or center of the minimum-maximum intensity value on the curve 

 will usually indicate the formation contact. See Figure 694.t Other factors such as 

 the lack of uniform distribution of radioactive materials may influence the sharpness 

 with which formation contacts can be measured. Effects of this nature cannot be easily 

 determined, but experience indicates that they generally do not materially influence the 

 accuracy of determining formation breaks. 



Cement Detection. — Gamma-ray logging may be used to locate the presence of 

 cement behind the casing by mixing carnotite (containing about 10 per cent uranium 

 oxide) with the cement.J 



Obtaining Stratigraphic Information 



Early experience with the gamma-ray curve indicated that a reliable picture of the 

 stratigraphy of the section covered by the log was obtainable. The location of upper, 

 cased-oflf, potential producing zones presented a logical field for this type of logging, 

 particularly where no information was available or where the available information was 

 somewhat doubtful. The location of the top of a producing zone for bottom water shut- 

 off, the correction of some of the earlier drillers' logs, the location of upper sands for 

 salt water disposal, the supplying of additional information where cores were not com- 

 pletely recovered or were lost, the location of upper potential fresh water sands, and the 

 location of the top and bottom of an oil-producing zone for gas-oil ratio control are all 

 applications of this type. 



Structural Studies 



In most areas, one or more outstanding, easily-identified horizon markers have 

 been found. In some instances, they are exceptionally clean sands or limes. In other 

 cases, some highly-radioactive shale, bentonite or zone of volcanic ash is present as a 

 persistent marker which can be located on the gamma-ray curve. In such cases surveys 

 are run on key wells, or over selected cross-sections of the area being studied. With 

 a few accurate points scattered over a pool, a much better idea of subsurface conditions 

 is obtained. Figures 695 and 696 illustrate cross-sections in studies of this kind. 



In Figure 695 the two marine shales of each well have been accentuated by the 

 heavier lines and through proper correlation show the traces of the two apparent fault 

 planes of the block. It is reasonable to assume that, with the block lying between wells 

 C and D, another test drilled between them would find the 8040 foot sand higher in the 

 fault block. By locating a fault through well C which is not present in well B, corre- 

 lation indicates an interruption in the normal dip. The other fault is located between 

 C and D by correlation of radioactive shales and confirmed by obvious correlation on 

 the sands. 



On many of the large ranches in West Texas, New Mexico and other areas a large 

 number of cased water wells are scattered over the range land. Good structural 

 markers are present in some of these wells. Gamma-ray curves have provided the 

 information for structural studies based on these markers. 



Locating Zones of Porosity 



The porous limestones and sandstones usually contain fluid, and these porous zones 

 may be located by a combination of gamma-ray and neutron curves. The gamma-ray 

 curve defines the stratigraphic breaks in the section with limes and sands recorded as 

 minimum values. The response of the neutron curve is proportional to the amount of 

 hydrogen within the neutron field. The greater the amount of hydrogen within a forma- 

 tion, the lower the value recorded on the curve. It follows, therefore, that dense, barren 

 horizons will be recorded on the curve as comparatively high values. Shales, because 



t Note similarity with Figures 636, 643, 657 of electrical logs. 



t L. G. Howell and A. Frosch, "Detection of Radioactive Cement in Cased Wells," A.I.M.E. 

 Pet. Tech.. Vol. 136, 1940. 



