A. Electric Well Logging
B. Gamma Ray Logging
C. Caliper Logging
D. Overview of Well Logging
Applications
E. Electric Logging Equipment
F. Logging and Groundwater
Investigations
G. Logging of Limestone and
Dolomite Formations
H. Identifying Lignite Coal Beds
I. Locating Minerals
Potential
In electrical well
logging, two electrical properties are measured in the borehole: potential and resistivity. It has been observed that in a borehole, the
electrical potential varies according to the nature of the beds traversed. For
example, salt water sands and brackish water sands are usually more negative
than the associated shale or clay. On the other hand, fresh water sands may
either be more negative or more positive than the associated formation.
Borehole
potentials are caused by electrochemical reactions taking place between the
formations and the mud column. Potential measurements are made by recording, in
terms of depth, the potential changes between an electrode in the hole and
another electrode at the surface, usually in the mud pit. An idealized
potential (S.P.) curve is represented on the left side of Figure 1. From a
potential curve it is possible to pick up the boundaries of many formations and
to obtain information on the nature of some of these formations. Potential and resistivity are simultaneously recorded.
Resistivity
The electrical
conductivity of a bed is controlled by the nature, quantity and distribution of
the water contained in the bed. Because these factors vary appreciably from one
bed to another, conductivity measurements made in a borehole can be used to
pick up formation changes and to obtain information on the nature of the
formation traversed.
In practice it is
not the conductivity but its reciprocal, the resistivity,
which is measured. The resistivity curve is obtained by
recording either the resistance changes of an electrode placed in the hole (Single Electrode method), or the apparent resistivity given by a multiple-electrode arrangement. The
measurements are plotted in terms of depth and the resulting record is called a
resistivity curve. An idealized resistivity
curve is shown on the right side of Figure 1. It can be seen that fresh water
sands and dense formations have a much greater resistivity
than salt water sands, clays and shales.
The equipment
required to make single-electrode measurements is much simpler and less
expensive than that needed for multiple-electrode measurements, but
single-electrode measurements have less lateral penetration than
multiple-electrode measurements have, therefore they do not always permit
distinguishing oil sands from water sands invaded by mud. On the other hand, a
single-electrode curve is as good, if not better, than a multiple-electrode resistivity curve for all other problems, especially for
obtaining correlation between wells and for determining the depth and thickness
of each bed.
Alternating
current of low frequency is used for this measurement. As the logging electrode
travels in the hole, changes in formation resistivity
cause changes in the electrode resistance, which in turn cause voltage changes
in the logging circuit. These changes are rectified and recorded as the resistivity curve.
The lateral
penetration of a single-electrode measurement is about ten times the electrode
diameter, i.e., 18" with the electrodes having a diameter of 1-1/2"
and a height of 8". These dimensions were selected to give approximately
the same resistivity values that would be obtained
with a short normal resistivity curve.
Electric Log
The combination of
a potential curve and of one or several resistivity
curves placed side by side constitutes an electric log. Such logs are extremely
valuable for geologic studies (correlation between wells, subsurface mapping,
research on sedimentation), for seismic problems (determination of the best
shooting point in a shot hole), for the location of fresh water bearing beds,
for determining the exact thickness and position of sand, clay or shale beds,
etc.
Requirements to
Obtain Good Logs
It is not
possible, with conventional logging instruments, to obtain a good electric log
in the section of the hole that does not contain water or water base mud. It is
therefore necessary that the hole be filled with water base drilling mud or
water in the section where the electric log is needed. If this is not possible,
a gamma ray probe may be used to obtain a good log.
Unusual Logging
Conditions
If the hole is
losing mud to the formation and the mud level has dropped appreciably at the
time the electric log is to be made, the hole should be filled before the
measurements are started. This is usually done by dumping water in the hole. If
the composition of this water is different from that of the mud used for
drilling, the potential and resistivity curves will
probably exhibit a shift at the interface, and the amplitude of the kicks in
the section having the saltiest water may be less than in the other section.
These differences are usually small unless one water
is much saltier than the other.
If some of the
water or mud enters permeable beds or fractures during logging, the potential
curve will be unstable and it will probably not repeat well in the part of the
hole above the lowest point taking water. It may also exhibit a considerable
drift. Such potential drift and instability are observed no matter the type of logging
equipment used, and cannot be suppressed. Drift is generally encountered only
in shallow formations, i.e., where the potential curve is generally flat and
not very useful, even when the water level does not drop. Because the resistivity curve is generally not affected by this
movement of water the usefulness of the log is not impaired. The same
instabilities are observed also in artesian wells and they cannot be suppressed
either, unless the flow of water is stopped.
Logging Shallow
Formations
Even when the hole
is well conditioned for electric logging measurements and there is no loss of
mud into the formation, it frequently happens that the potential curve drifts
appreciably to the left in the upper part of the hole. This is a natural
phenomenon and it cannot be corrected.
When logging fresh
water sands, it sometimes happens that the potential curve "reverses"
i.e., the potential in sands is more positive (i.e., it kicks more to the
right) than that in clay. This reversal may happen, in particular, when the
drilling mud is saltier than usual. The usefulness of a reversed potential
curve is not impaired when the logging operator is aware of this possibility.
Nothing can be done about this condition, except replacement of the used mud by
fresh mud.
Radioactivity
Radioactivity is
the emission of rays caused by the spontaneous change of one element into
another. Although several types of rays are emitted, only gamma rays have enough
penetration to be of practical use in logging the natural radioactivity of
rocks.
Radioactivity
of Rocks
All natural rocks
contain some radioactive material. However, compared to that of uranium or
radium ore, even of low grade, the radioactivity of most rocks is very small.
The radioactivity of a rock is usually expressed in terms of equivalent amount
of radium per gram of rock required to produce the same gamma ray intensity.
Although there is no fixed rule regarding the amount of radioactivity a given
rock may have, shales, clays and marls are generally
several times more radioactive than clean sands, sandstones, limestones and dolomites.
Because shales, clays and marls have radioactivities
that are of the same order, the term "shale" will generally be used
here to denote any of these three formations. Similarly, for the sake of
simplification, the term "sand" will be used to denote either sands, sandstones, limestones,
and dolomites, since these four rocks have radioactivities
of the same order.
The radioactivity
of clean sands, i.e., sands free of shaly materials
(shale, clay, marl) is generally very low. Sands that
contain some shaly material have a somewhat higher
radioactivity, and the increase is proportional to the amount of shale
contained. Therefore, shaly sands and sandy shales generally have a radioactivity that is between that
of clean sands and that of shale. In a given area, the radioactivity of shales does not generally vary too much, so that a gamma
ray log is an approximate measurement of the quantity of shale contained in a
formation.
The radioactivity
of shale varies from area to area. In the tertiary and more recent formations,
i.e., those usually found in the
Some organic
marine shales have a much
higher radioactivity than the ordinary shales in the
same area. However, they are generally relatively thin and are not found too
frequently. When present, marine shales make
excellent geologic markers.
Interpretation
of Gamma Ray Logs
The interpretation
of gamma ray logs can be summarized as follows:
1. In a given
area, only the relative radioactivity of the various rocks is of significance.
2. Rocks of low
radioactivity include primarily clean sands, sandstones, limestones,
and dolomites. Anhydrite, salt, lignite and coal have also a low radioactivity.
Their radioactivity increases when they are shaly.
3. Ordinary shales have a much higher radioactivity than the rocks
listed above. The radioactivity of sandy shales is
less than that of shales. Shales
are sufficiently high in radioactivity and can generally be easily
distinguished from the other rocks on a gamma ray log.
Cased Holes
Most of the gamma
rays emitted by the formation can penetrate casing, so that a gamma ray curve
can be obtained in cased holes, although the amplitudes of the curve are
somewhat reduced. For example, a 5/16 inch thickness of steel reduces the gamma
ray intensity about one fourth.
Effect of Mud
The mud has two
effects on the gamma ray curve:
1. It absorbs a
small percent of the radiation and therefore reduces the log amplitude; unless
the hole diameter is very large (more than 24") this effect is very small
and can be ignored.
2. The shale or
clay contained in the mud increases the radioactivity background, so that even
clean sands show a slight radioactivity on the log. If the mud is uniform, this
small increase is constant from top to bottom. However, if the shale has
settled at the bottom of the hole there will be an increase in the
radioactivity measured in this interval that has to be considered when
interpreting the log. The effect of the mud on the gamma ray log is the same
whether the mud is fresh or salty. Because this effect is usually very small, a
gamma ray log is very useful in wells containing salty mud since, in this case,
the electric log is generally poor.
Effect of Hole
Size
The larger the
hole, the smaller the gamma ray intensity reaching the probe. However, this
effect is small and can generally be neglected.
Application of
Gamma Ray Logs
Gamma ray logs are
used in the following instances.
1. To log cased holes
(no electric log can be obtained in cased holes).
2. To log dry
holes (no electric log can be obtained in holes that do not contain water or
mud).
3. To log holes
containing salt water or salty mud (the electric logs obtained in such holes
are generally poor).
4. To supplement
the information given by the electric log (identification of formations,
estimating the amount of clay in sands, etc.)
5. To locate
radioactive ores, uranium in particular.
6. To help locate
lignite and coal beds.
7. To help locate
clay and fresh water sands.
The caliper
logging system provides a means for a continuous recording of borehole diameter
versus depth. These logs are generally run in uncased wells, but for many
applications have proven very valuable when used within casing. The caliper
tool has three arms that may be motored open or closed by control from the
surface. Caliper arms are ordinarily supplied in two lengths to provide a
maximum extension of either 15 inches or 30 inches. Caliper logs are used to
determine hole and casing diameter, locate caved zones, casing, and the absence
of casing; and permit the recognition of mud cake. (i.e., permeable zones). An
example of a caliper log is shown in Figure 2.
D. OVERVIEW OF WELL LOGGING
APPLICATIONS
Geophysical logs
and, more particularly, electric logs give a detailed and continuous picture of
the formation penetrated in the course of drilling and are one of the most
useful tools currently available for subsurface investigation. They are used
in:
1. Core Holes
2. Shot Holes
3. Water Wells
4. Oil & Gas Wells (conventional and progress logging)
5. Mineral Exploration
6. Soil Mechanics
Core Holes
An electric log
gives a detailed picture of all the beds penetrated by the drill. By
correlating the logs obtained in the wells of a given area, accurate geological
maps are obtained showing structures, faults and changes in lithology
and sedimentation.
Seismograph
Shot Holes
Electric or gamma
ray logs made in shot holes provide the seismologist with invaluable
information. The logs permit selecting the best shooting point and they supply
qualitative indications of changes in surface velocity. Further, by correlating
the logs made in the area of interest, an accurate geologic map can be
obtained.
Water Wells
An electric log
run in a water well permits determining the exact depth and thickness of each
aquifer and estimating the quality of the water.
Oil & Gas
Wells (Conventional Logging)
Once an oil or gas
field is discovered and the basic reservoir data are obtained in several key
wells, all the information that is generally needed in the subsequent wells is
an electric log that gives, besides correlations, the exact depth and thickness
of the producing zones. Electric loggers are perfectly adapted to this type of
logging. Either single-electrode or multiple-electrode equipment can be used.
The latter are also useful to decide the depth of the oil-water contact. The
logs are also invaluable in secondary recovery work (water flooding) and for
underground storage investigations.
Oil & Gas
Wells (Progress Logging)
In certain areas
several electric logs are run in each well as the drilling progresses. This is
done for the sole purpose of ascertaining, by means of correlation with the
logs of other wells in the vicinity, the stratigraphic
position of the well as the drill goes deeper. For this purpose, a
single-electrode electric log or a simplified multiple-electrode log is
sufficient and can be obtained to depths of approximately 8000 feet with
portable loggers. Skid units and truck mounted equipment are also available for
deeper wells. This procedure permits making appreciable savings, not only on
the logging costs but also on rig time since the equipment is always available
when needed.
Mineral
Exploration
Electric logs can be
used for locating coal, lignite and certain ores.
Soil
Engineering
Electric logs are
used frequently for investigating the foundations of dams, bridges, highways,
etc.
There are several
types of electric logging equipment and they are generally classified in two
groups: Single-electrode equipment and multiple-electrode equipment. Each group
can be further divided depending on whether the equipment is portable.
Single-Electrode
Equipment
This is the
simplest and least expensive. Further, it can generally be operated by any
member of the drilling crew. The single-electrode equipment gives a Potential
curve (S.P.) and a single-electrode resistivity
curve. The single-electrode resistivity curve is
equivalent to a very short normal curve and, therefore, it gives good detail
under normal conditions, even in very thin beds. Single-electrode equipment is
recommended for correlation work (core holes, shot holes, progress logging),
water wells, mining applications and soil mechanics studies. It is also used
with success in oil and gas wells when the main purpose of the logs, besides
obtaining correlations, is to determine the depth and thickness of the beds
penetrated by the drill. For the estimation of petroleum saturation and/or
formation porosity, and for logging wells that contain salt water or salty mud,
single-electrode equipment is not generally recommended; multiple-electrode
equipment is preferable. However, in holes of the same size, drilled with the
same type of mud through the same producing horizons, changes in fluid contents
and porosity are often reflected on the resistivity
curve obtained with single-electrode equipment. Because the latter is
relatively inexpensive, many operators have been prompt to take advantage of
this empirical method for making quantitative determinations.
Multiple-Electrode
Equipment
When it is
essential to obtain true resistivity data,
Multiple-Electrode equipment should be used. This equipment gives the S.P. and
two, or more, resistivity curves. Multiple-Electrode
equipment is also recommended for the logging of wells that contain salty mud
or salt water.
Gamma Ray Logs
Gamma ray logs are
used in the following instances:
1. In air drilled
holes and in cable tool holes.
2. In cased holes
(instead of electric logs that are meaningless).
3. In holes that
contain salty mud where even multiple-electrode logs may not give a usable
record.
4. When data on
the clay or shale content of certain formations are needed, for example for a
better interpretation of electric logs.
5. For locating
coal or lignite beds that, sometimes, are not clearly seen on electric logs.
6. For uranium,
potash and phosphate exploration (these minerals are generally more radioactive
than the other types of rocks).
Gamma ray logs can
be obtained whether the well is cased or not. Gamma ray logs can always be
obtained no matter the nature of the fluid in the hole (air, water, mud, oil).
Caliper Logs
Caliper logs,
either alone or with other types of logs, are useful for solving certain
problems. Their main applications are:
1. To compute the
amount of cement needed for a cementing operation.
2. To compute the
amount of gravel needed for gravel packing.
3. To help
identify certain formations subject to caving. For example, shales
are generally more subject to caving than sandstones and limestones,
therefore they can readily be seen on a caliper log. A somewhat similar
condition exists where potash is associated with common salt. The salt
saturated brine used for drilling dissolves more potash than salt,
consequently, the hole enlargements shown on the caliper log in the salt
section generally correspond to the potash layers.
Advantages of
Single-Electrode Resistivity Curve.
An important
advantage of single-electrode resistivity curves is
that they give considerable detail: under usual conditions beds having a
thickness of one foot or more can be located and their boundaries can be
accurately picked.
Another advantage
of this curve is that its response is dependable: for every increase (or
decrease) in formation resistivity there is an
increase (or decrease) on the resistivity curve.
Conversely, every increase shown by the curve corresponds to an increase in the
formation resistivity. On the other hand, the
response of multiple-electrode logging systems is not always consistent, for
example, increases in formation resistivity are
sometimes recorded as decreases. Also there are sometime spurious deflections
on multiple-electrode logs several feet from bed boundaries.
The two foregoing
advantages of the single-electrode resistivity log
result in curves that sharply delineate lithology
changes. With this log supplemented by the S.P. curve, it is generally possible
to identify the type of formation traversed by the well and, in the case of
water bearing beds, to estimate changes in the groundwater salinity. The
qualitative interpretation of the data is easy and does not require charts.
Single-electrode
equipment gives curves from the bottom of the hole up to the casing shoe or to
the mud level, whichever is deeper.
Single-electrode
equipment is small (and generally highly portable), rugged, simple, relatively
inexpensive, and can be operated by a member of the drilling crew after a few
hours' instruction.
Shortcomings of
Single-Electrode Resistivity Curve.
There are two
cases when single-electrode resistivity measurements
may not be as efficient as multiple-electrode ones. One is when the hole
diameter is larger and the mud is salty. In this case, the curves lose some of
their detail. Thin beds cannot be seen and the boundaries of thick beds cannot
accurately be picked.
The other case is
when true resistivities are needed, for example, when
it is necessary to estimate the oil saturation in petroleum reservoirs. The
single-electrode resistivity curve is not well
adapted to this problem for the following reasons:
1. As pointed out
above, a single-electrode resistivity measurement is
basically identical to that obtained with a very short normal device. A glance
at conventional departure curves for beds of finite thickness shows that the
resolving power of a very short normal device for determining true resistivities is small, more particularly for beds whose resistivities are much greater than the adjacent formation resistivity, and/or those which are invaded by mud. In the
case of invaded beds, not only is the resolving power poor, but true resistivity determinations cannot be made, even in theory,
unless the extent of invasion and the resistivity of
the invaded zone are known.
2. Apparent resistivities obtained from single-electrode measurements
are greatly affected by changes in hole diameter. Therefore, it is illusory to
try to determine the true resistivity of a formation
unless a caliper log is available or unless it is known that the formations of
interest do not appreciably cave.
These shortcomings
are found in any type of single-electrode equipment, but they can be remedied
if a special logging procedure is used and if special features are embodied in
the logger.
F. LOGGING AND GROUNDWATER
INVESTIGATIONS
Today's water well
drillers operate over larger areas and they sink deeper wells than their
predecessors of some forty years ago. They use rotary drilling equipment
instead of cable tools. For these reasons yesterday's approach to the drilling
of water wells is now quite often inadequate.
In order to take
full advantage of present day's opportunities, the progressive water well
driller is borrowing techniques that are now standard in oil well drilling.
Among those, electric logging is probably the simplest and most effective. This
technique is no longer too expensive for water well work because portable
loggers have brought its cost to a level permitting its regular use in ground
water development work.
Driller's Log
In continuously
cored holes the examination of cuttings permits obtaining logs that are reasonably
accurate. This is not so when rotary drilling is used, as the samples taken
contain cuttings from several feet of hole and debris resulting from the
erosion of the exposed formation by the mud stream. Electric logging, because
of its accuracy, reliability and simplicity, is the usual answer.
Electric Log
Conventional
electric logs consist of a potential curve and of one or more resistivity curves recorded side by side as a graph.
Electric logs can only be recorded in open holes.
The potential
curve is the continuous recording in terms of depth of the natural electric
potential in millivolts. It is sometimes called the
S.P. curve (Spontaneous Potential). The potential in clay (or shale) is
generally used as reference, for convenience. Potential readings in aquifers
vary in direction and amplitude according to the respective salinities of the
drilling mud and the formation water.
A resistivity curve is obtained by lowering one or several
electrodes and recording - also in terms of depth -appropriate electrical
measurements. As the logging tool travels in the hole the recorded variations
of formation resistivity constitute a resistivity curve. When only one electrode is in the hole
the graph obtained is called "single electrode resistivity
curve". The graph obtained when the downhole
tool carries several electrodes is called "
Electric Logs
Basically, an
electric logger consists of a downhole tool - sometimes
called logging electrode or "sonde" -
connected to a reel mounted logging cable, a depth measuring device, a control
panel and an automatic recorder. The other end of the logging cable is
connected to the surface control panel and the recorder by a means of a
collector. The measuring device is mechanically or digitally connected to the
chart drive mechanism so that the chart moves in synchronism with the sonde. Recording pens moving back and forth, according to
the amplitude of the signals (potential and resistivity)
received from the sonde, draw the corresponding
curves.
"Two
Curve" loggers are the simplest and least expensive. They permit recording
a potential and a single electrode resistivity curve
and are satisfactory for usual water well drilling applications. "Multiple
Electrode" loggers allow recording a potential and one or more resistivity curves. The latter loggers are somewhat more
complicated and expensive but they permit obtaining quantitative information
regarding the fluid content and other characteristics of the formation
penetrated by the drill. Loggers of both types are available.
The more electric
logs are made in an area and correlated with pumping tests and production
performances, the more efficiently the subsequent wells can be completed. For
example, because the composition of the water of a given formation is
relatively uniform over a large area, it is generally possible to calibrate,
for the area, the readings of the electric log in terms of water salinity. If
water samples and an electric log are available in a test hole, it is a simple
matter to determine limiting resistivity and
potential values beyond which the water can be used for domestic or industrial
purposes. This information can then be applied for all the wells drilled in the
general area.
Although electric
logs cannot determine the yield of an aquifer, they are extremely useful in
helping solve this problem since they make it possible to "count" the
net sand (or gravel) thickness of the aquifer. This information, combined with
the pumping and other tests made in the area, generally permits fairly accurate
prediction of the yield of most aquifers.
After several
electric logs are available in a given area, cross sections and maps can be
prepared, from which it is possible to predict the net sand (or gravel)
thickness at any location. From this information, supplemented by the pumping
tests and production performances of the wells, the depth at which other wells
must be drilled in order to produce at a given rate can be predicted. This is
important since the drilling costs can thus be estimated accurately in terms of
the quantity of water wanted.
As seen from the
foregoing discussion, there are many advantages to making electric logs in all
wells drilled for water. These advantages are enhanced if proper consideration
is given to the few limitations of this modern technique. If these limitations
are kept in mind, the electric log, in conjunction with the driller's log, can
be used to great advantage in eliminating guesswork in the development of
ground water supplies. It can thus be appreciated how the routine use of
electric loggers in water wells will supply a wealth of information rapidly,
simply and at very reasonable cost.
Gamma Ray Logs
Gamma ray logging
equipment is used to record the variation of natural radioactivity of the
formation in open and cased holes. Gamma ray logs can be obtained whether the
well is filled with water or any type of drilling fluid.
Clay and shale are
moderately radioactive while clean sand, gravel, sandstone are much less
radioactive. The gamma ray curve is therefore a "formation log" and
permits distinguishing clay and shale beds from sands, gravels, sandstones and limestones. Shaly and clayey
sands or gravels are generally mediocre aquifers. They can usually be
identified by their intermediate radioactivity and comparison between electric
and gamma ray logs of the same well.
In many areas,
desirable aquifers are separated from unwanted ones by very thin impervious
beds that are impossible to locate on the driller's log and difficult to pick
up on the electric log. It is often possible to locate such thin beds on the
gamma ray log.
Caliper Logs.
Particularly when
drilled with rotary equipment, the size of the hole does not remain constant.
Caliper logs give the means for determining the volume of the hole for
operations such as gravel packing or casing cementing.
G. LOGGING OF LIMESTONE AND
DOLOMITE FORMATIONS
Limestone and
dolomite have essentially the same physical properties. Therefore, the
following discussion of limestone logging applies also to dolomite.
Electric Log
As far as electric
log interpretation is concerned, limestones can be
classified in three types: 1, dense limestone, 2, limestone having intergranular porosity and 3, fractured limestone.
Dense Limestone
Dense limestone
has a very high resistivity and generally very little
S.P.
Limestone with Intergranular Porosity
This type of
limestone gives the same electric log as that obtained in sandstone, i.e.,
1. The resistivity is less than that of dense limestone, but
generally greater than that of clay or shale. The resistivity
decreases when the porosity increases and when the formation water salinity
increases.
2. The S.P., with
respect to that of associated shale or clay, is small if the limestone contains
fresh water (from minus 50 to plus 50 mv). If the
formation water is brackish or salty, the S.P. is generally negative and large
(more than minus 50 mv).
Fractured
Limestone
The resistivity of fractured limestone is less than that of
dense limestone, and it decreases when the porosity increases. Generally, the
S.P. is very small, regardless of the type of formation water.
Gamma Ray Log
An electric log
does not always permit distinguishing a fractured limestone from clay or shale,
but the distinction can be made if a gamma ray log is also available.
Limestone, whether porous or dense, generally has a very low radioactivity
provided it does not contain shaly material. Its
response on the gamma ray log is, therefore, the same as that of sand and
sandstone. On the other hand, clay and shale have a high radioactivity so that
it is possible to distinguish limestone from clay or shale with a gamma ray
log.
Note that the
gamma ray log is affected neither by the type of formation water nor by the
salinity of the drilling mud.
H. IDENTIFYING LIGNITE COAL BEDS
Because coal and
lignite generally have a very high resistivity, they
can be located with an electric log. The single-electrode resistivity
curve is usually preferable because it gives a very detailed log. Although
dense limestone and similar rocks have also a high resistivity,
the differentiation between them and coal or lignite can be made with the
driller's log since coal and lignite are soft while dense rocks are hard to
drill.
There are a few
coals (some cannel coals) and lignites (in certain
parts of the
With very few
exceptions, coal and lignite have an extremely low radioactivity while all the
associated formations have a higher one, especially clay and shale. Therefore,
a gamma ray log is a very useful supplement to the electric log for locating
coal and lignite beds.
Since coal is
often friable, the hole usually exhibits enlargements opposite coal beds. These
enlargements are readily detected on a caliper log.
Conductive
Minerals.
A number of
minerals have a much greater conductivity than the usual rocks and can,
therefore, be located from resistivity measurements
made in wells which have cut them. The most common conductive minerals are:
Graphite, Pyrite, Chalcoprite, Pyrrhotite
and
If the ore body is
not cut by the borehole, it can be located only if:
1. There is
continuity of conductivity, so that the ore body acts as a single conductive
mass of large size,
2. The volume of
the ore body is large enough,
3. The ore body is
not too far from the borehole in which the measurements are made.
Native copper,
although extremely conductive, cannot be directly located from electrical
measurements - unless the concentration is extremely high - because the copper
nuggets are generally separated by rock that is much less conductive than the
copper.
Resistive
Minerals
Many minerals
(hematite, limonite, blende, coal, etc.) have a high resistivity, generally much higher than that of the
formation in which they are found. and they can be located from resistivity measurements made in wells which have cut them.
If the mineral is not cut by the borehole, it can be located only if it is
close to it and if it is relatively thick, as explained above for conductive
minerals.
Radioactive
Minerals
Radioactive
minerals (most uranium ores, many phosphates) can be located by gamma ray
logging, provided they have been cut by the well or are only a few inches from
it.
Indirect
Location of Minerals.
It is possible
sometimes to locate indirectly certain minerals, in particular.
1. If they are
associated with a formation that can be identified on the log, or
2. If the mineral
has produced, by electrochemical action or by dissolution, a halo around the
deposit. This halo may be detected by resistivity
and/or S.P. measurement.
J
R Associates,
Ph: