1106 EXPLORATION GEOPHYSICS 



without regard to whether it is oil, gas, distillate or water, since the neutron curve 

 responds only to the hydrogen content of fluids. This fluid may be considered to be of 

 two types : that contained within the bore hole, and the connate or invaded fluid content 

 of the formations. 



The effective thickness of fluid surrounding the instrument normally remains con- 

 stant except where there are changes in bore-hole diameter or at the fluid level. The 

 bore-hole fluid (while permitting accurate location of such points) has negligible influ- 

 ence on the formation fluid response of the neutron curve. The principal effect regis- 

 tered on the neutron curve, therefore, is the amount of fluid present in the adjacent 

 formation. The behavior of the curve is such that the greater the amount of fluid the 

 lower the intensity of the curve. The neutron curve, as a result, reflects formation 

 porosity rather than permeability. 



Shales in general contain the greatest amount of total fluid since total fluid includes 

 chemically combined as well as interstitial water. Shales therefore represent a reliable 

 base or reference value on the neutron curve. Shale has been found to have as much 

 as 44% total water. Sands do not contain chemically combined water, and the most 

 porous sands often show a low neutron curve response approaching that of shale. Non- 

 productive limes and dolomites may contain very little, if any, actual fluid and therefore 

 record as the highest intensity values. 



Proper interpretation of the neutron curve is simplified when it is used in combi- 

 nation with the gamma-ray curve. (The resistivity or natural (self) potential curves 

 of the electrical log may be used where available in place of the gamma-ray curve.) 

 Formations which show relatively similar response on the gamma-ray curve may 

 frequently be differentiated on the neutron curve. For example, limestone response is 

 usually higher than sand response on the neutron curve. The presence of large amounts 

 of silt or bentonite in certain productive sands may result in very high intensity 

 response of the gamma curve, leading to their interpretation as normal or highly radio- 

 active shale. The neutron curve, by showing response higher than shale, permits their 

 proper identification as sand or lime. 



Fluid content studies of sand and lime zones by the combined use of gamma-ray 

 and neutron curves are extreinely valuable. Frequently such zones may be either 

 extremely dense or fluid-bearing throughout, or may be dense on top or at the base. It 

 is not unusual to have a dense streak within a sand or lime body effectively creating a 

 condition of separate reservoirs which might require completion by other than conven- 

 tional means. In such cases the gamma-ray curve serves to identify the extent of the 

 sand or lime body and the neutron curve indicates the dense and fluid-bearing zones, 

 thus permitting proper completion measures to be applied. Many case histories have 

 proved that the fluid zones in a productive horizon can be recompleted according to 

 neutron curve intensity values to provide longer production life. 



Under certain conditions the neutron curve has been found useful for detecting 

 gas-fluid contacts. Dry gas sands (containing less than approximately one gallon of 

 fluid per 10(K) cubic feet of gas) normally record as highs, similar to dry, dense forma- 

 tions of the neutron curve. Since the fluid-bearing zones register as medium to low 

 values, the gas-fluid contact is recorded on the log as a sharp shift to the right. 



Interpretation of the neutron curve may be clarified by reference to Figure 689, 

 which indicates the neutron curve response corresponding to typical formations. The 

 solid, dotted, and dashed lines indicate the range of neutron curve response to be 

 expected for formations differing widely in amount of fluid content. Particular atten- 

 tion is called to the zones marked "fluid". These zones show the method of identifying 

 such porous zones. It is important to note that the neutron curve response to fluid- 

 bearing zones of less than 2 feet in thickness may not show the pronounced throw-back 

 experienced in thicker zones. This results from the loss of detail caused by the length 

 of the ionization chamber and should be considered when interpreting the curve. 



A second type of information supplied by the neutron curve, the accurate location 

 of casing shoes, parted casing, liners, and fluid levels is shown in Figure 690. 



