Geology Reference
In-Depth Information
bombardment of stable nuclei with gamma rays or neu-
trons. Gamma rays are detected by a scintillation counter
(Section 10.4.2) or occasionally by a Geiger-Müller
counter (Section 10.4.1) or an ionization chamber.
Radioactivity in borehole measurements is usually
expressed in API (American Petroleum Institute) units,
which are defined according to reference levels in a test
pit at the University of Houston.
Δ
V
11.7.1 Natural gamma radiation log
Shales usually contain small quantities of radioactive
elements, in particular
40
K which occurs in micas, alkali
feldspars and clay minerals, and trace amounts of
238
U
and
232
Th. These produce detectable gamma radiation
from which the source can be distinguished by spec-
trometry; that is, measurements in selected energy bands
(Section 10.4.3). The
natural gamma radiation log
conse-
quently detects shale horizons and can provide an esti-
mate of the clay content of other sedimentary rocks.
Potassium-rich evaporites are also distinguished. An
example of this type of log is shown in Fig. 11.12.
The natural gamma radiation log (or
gamma log
) mea-
sures radioactivity originating within a few decimetres
of the borehole. Because of the statistical nature of
gamma-ray emissions, a recording time of several sec-
onds is necessary to obtain a reasonable count, so the sen-
sitivity of the log depends on the count time and the
speed with which the hole is logged. Reasonable results
are obtained with a count time of 2 s and a speed of 150
mm s
-1
. Measurements can be made in cased wells, but
the intensity of the radiation is reduced by about 30%.
Fig. 11.11
The self-potential log.
boundary and generates a potential difference of a few
tens to a few hundreds of millivolts.
In sequences of sandstone and shale, the sandstone
anomaly is negative with respect to the shale. This SP
effect provides a sharper indication of the boundary than
resistivity logs. In such sequences it is possible to draw a
'shale line'through the anomaly maxima and a 'sand line'
through the minima (see Fig. 11.6). The proportion of
sand to shale at intermediate anomalies can then be esti-
mated by interpolation.
The main applications of SP logging are the identifi-
cation of boundaries between shale horizons and more
porous beds, their correlation between boreholes, and
the determination of the volume of shale in porous beds.
They have also been used to locate coal seams. In hydro-
carbon-bearing zones the SP log has less deflection than
normal and this 'hydrocarbon suppression' can be an
indicator of their presence.
11.7.2 Gamma-ray density log
In the
gamma-ray density
(or
gamma-gamma
)
log
, artificial
gamma rays from a
60
Co or
137
Cs source are utilized.
Gamma-ray photons collide 'elastically' with electrons
and are reduced in energy, a phenomenon known as
Compton scattering
. The number of collisions over any
particular interval of time depends upon the abundance
of electrons present (the
electron density index
), which in
turn is a function of the density of the formation. Den-
sity is thus estimated by measuring the proportion of
gamma radiation returned to the detector by Compton
scattering.
The relationship between the formation density
r
f
and electron density index
r
e
11.7 Radiometric logging
Radiometric logs make use of either the natural radio-
activity produced by the unstable elements
238
U,
232
Th
and
40
K (Section 10.2), or radioactivity induced by the
depends upon the ele-
ments present
