the nearest casing joint and then raising or lowering the tools a few feet to 

 the desired position. 



The photon curve is a log made with an instrument similar to or identical 

 with a neutron logging instrument, except that the neutron source of the latter 

 is replaced by a gamma-ray source. Such a curve is sometimes run to aid in the 

 interpretation of a neutron log when borehole conditions are unknown and a 

 caliper log cannot be run — for example, in a cased hole. 



The conventional radioactivity log, consisting of a gamma-ray curve 

 together with a neutron curve, is widely used as a source of information con- 

 cerning numerous properties or characteristics of the strata or of the well. 

 Most of these properties are related only indirectly to the recorded radiation 

 intensities. The user must therefore combine the pertinent data from the radio- 

 activity log with other available information in order to obtain answers to 

 each of his questions concering the well or the rocks. In this way many 

 questions about the strata, such as the following, can be answered: 



What is the depth and thickness of a particular formation? 

 What type of rock is situated at a given depth? 

 How great is its porosity or fluid content? 

 How much shale does it contain? 

 Does it contain gas? 



Questions about the well, such as the following, can be answered: 



At what depth is the fluid level? 



Where is the casing seat? 



Is there a liner present, and, if so, where is its top? 



Direct and complete answers to most of such questions require the interpreter 

 to have a working knowledge of the typical responses of the gamma-ray and 

 neutron curves in the formations of his locality and to be familiar with the 

 effects of borehole conditions on the radioactivity log. Adding to this know- 

 ledge other specific facts concerning the well and the rocks enables him to 

 identify the stratigraphy and deduce the information from the indications 

 recorded on the log. 



Typically, the well conditions remain uniform for hundreds or thousands 

 of feet at a time; and the log responses throughout these sections, therefore, 

 represent variations in the character of the rock materials. With relatively 

 few exceptions, similar rocks all over the world produce similar responses on 

 the radioactivity log. Thus a particular formation can be identified from the 

 log with reasonable certainty. Clean limestones and sandstones nearly always 



332 



