Page 151 - Petroleum Geology

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because of the presence of 40K in clay minerals. These gamma rays are ab-
sorbed by the surrounding rocks, the rate of absorption being roughly pro-
portional to the rock’s density. Gamma rays emitted from the zone within
about 0.3 m of the wall of the bmehole are detectable in the borehole. The
Gamma Ray log is a log of the natural gamma radiation in the borehole as
measured by a scintillation counter in the sonde. It is thus essentially a log
of clay content, in the petroleum context, in the immediate vicinity of the
borehole. This is independent of the salinity of the mud of the borehole,
so the Gamma Ray log can be run instead of the SP when the salinitiesare
unfavourable for the SP.
The Gamma Ray log is recorded on the left-hand side of the log in ab-
solute units of microgram radium equivalent per ton (pg Ra eq./ton) or
API units (16.5 API units = 1 pg Ra eq./ton), increasing to the right. Clean
sands (with but few exceptions due to heavy minerals) have no radioactivity,
but mudstones may have 200 API units (12 pg Ra eq./ton) or more. A
Gamma Ray log therefore records a sandstone/shale sequence in the same
sense as would be recorded by a SP log if R,, > R,.
Because gamma rays will also penetrate a small thickness of cement and
steel, logs can be obtained from cased holes and used for geological purposes
(if the casing was run, for one reason or another, without electrical logging)
and for production purposes, to discover the exact depth at which casing
should be perforated to produce a sandstone or carbonate reservoir. When
run in casing, the Gamma Ray device is run with a Casing Collar Locator,
which is similar to the Induction device and detects the extra thickness of
steel in the casing couplings.
All radioactivity measurement has a statistical component, and all radio-
activity logs should record the statistical variations opposite a bed of low
radioactivity and opposite one of high radioactivity, so that real changes of
radioactivity level can be distinguished from statistical fluctuations. This is
achieved by holding the sonde opposite a sand, for example, while winding
film through for a minute or so.
The Gamma Ray log sometimes reveals beds of unusually low radioactivity
(e.g., coal seams) or unusually high radioactivity (e.g., the Kimmeridge Clay
in the North Sea) which may be useful for correlation - even when the cause
of the abnormality cannot be identified with confidence.
The Neutron log is obtained with a device that measures the response of
the rock around the borehole to bombardment with neutrons. The sonde
includes a source that emits fast neutrons, which are slowed down mainly
by collision with light nuclei, hydrogen and to some extent chlorine, until
they become “thermal” neutrons. These do not lose energy as they move
from one collision to another, but eventually they are absorbed by hydrogen
atoms, mostly, and gamma rays of capture are emitted. Those gamma rays
of capture that are not absorbed in the rock are detected in the sonde. The
Neutron log therefore reflects the proportion of hydrogen in the rock