Proceedings Volume Cover
Fidelity Union Bldg.  
Dallas, Tex.  
Gamma Ray-Density Logging  
Albin J. Zak and Joe Ed Smith, Junior Members AIME  
Core Laboratories Inc., Dallas, Tex.  
In the practical logging tool, a beam of  
gamma rays is emitted into the formation.  
small fraction of these photons find their way  
back to the detector. Normally, the detector  
used is Geiger-Muller counter or scintillation  
data. The basic theories and physical principles counter. Studies of other detecting devices,  
underlying the techniques of density logging with such as cloud chambers and ionization chambers,  
gamma rays are discussed along with some of the have shown them to be unsuitable for this purpose.  
problems associated with the instrumentation of source for this tool is  
radioactive cobalt (CobO ). The source is Shielded  
This paper presents  
methods of interpreting and applying gamma ray•  
density logs in evaluating fundamental reservoir  
a review of current  
The most common gamma  
such tool. An effort has been made to evaluate  
the instrument calibration techniques, and discus• from the detector with lead such that the majority  
sion and figures are presented which describe the  
effect of borehole conditions, pressure, tempera• those back-scattered from the formation.  
ture, etc. The relationship of natural density,  
of the gamma rays counted by the detector are  
grain density, and interstitial fluid density to  
porosity is presented. Field data illustrate how  
density logs can be correlated with porosity and  
grain density data. With the aid of adequate core  
Initial calibration of the instrument is made  
in special testholes in which the density of the  
calibrating material is carefully controlled.  
These experiments are made in various hole sizes  
in each of several different mud densities. From  
this, an adequate prediction has been established  
as to the effect of borehole diameter and borehole  
fluid density on the log response. To date, there  
are no valid correction factors proposed for the  
effects due to poor borehole geometry. The rules  
and limitations of logging speed, time-constant,  
bed thickness, etc., that apply to conventional  
radioactivity logs also apply to density logging.  
The effects of temperature and pressure are those  
effects that are imposed on the response of the  
gamma ray counter, and the effects on the densi•  
ties of the interstitial fluids.  
analysis data,  
formation porosity can be obtained from density  
log interpretation. The paper also offers  
cussion of the limitations of the density log as  
porosity tool. The current applications of the  
density log to geologic work are cited, and the  
prospective uses and future developments are  
briefly explored.  
reasonably accurate estimate of  
The principle of gamma ray absorption as  
function of density was adapted to the logging of  
petroleum formations through the utilization of  
phenomenon known as "back-scattering". This tech•  
nique uses tool which contains both a gamma ray is fairly simple; the possible applications are  
The basic interpretation of the density log  
source and detector. The detector measures the  
intensity of the gamma rays emitted from the  
source that have been back-scattered to the de•  
tector by the formation. After proper instrument  
calibration, the log response can be used to  
compute the natural density of the formation.  
most prominent denSity log applica•  
tions are: T(h1e) lithological eorrelations, (2)  
determination of density grad:"ents for gravity  
meter surveys, (3) determination of borehole fluid  
denSity for gradient surveys, (4) location of  
casing shoe and cement top, (5) location of casing  
leaks, (6) aid in the interpretation and evalua•  
tion of other logs, and (7) method of estimating  
Since there is  
a definite relationship between  
natural density and porosity, it is possible to  
predict formation porosity from density logs. It  
is necessary to correlate carefully density log  
data with core analysis data before the direct  
correlation of density log response with porosity  
can be made.  
formation porosity. Application  
the most important and most widely used.  
7 is probably  
Maximum use of the density log nas not been  
fully realized. Although there are some severe  
limitations in its use, there are many cases where  
References and  
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