wireline logs
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Wireline Logs
Wireline logs provide two basic functions for the geologist. They provide both the data for evaluating the
hydrocarbon-bearing Properties of a zone (formation evaluation) and the control for subsurface mapping. In
formation evaluation, logs are used to define physical rock characteristics, such as lithology, porosity and
permeability; to distinguish between oil, gas, and water in the reservoir; and to estimate reserves. In
subsurface mapping, logs are used to correlate zones, to construct cross sections, and to provide control forstructure and isopach maps.
There are, in addition to the above functions, two very important uses of well logs in facies analysis: as
direct indicators of vertical grain-size profiles by spontaneous potential (SP) and gamma ray curves, and in
interpretation of sedimentary structures by the dipmeter log. Used together, they can be a powerful tool in
environmental diagnosis.
Interpretation of Grain-Size Profiles from Well Logs
Certain types of sedimentary facies display characteristic grain-size distribution profiles. These profiles
may be revealed on spontaneous potential (SP) and gamma ray logs. The SP log records the voltage
differences between an electrode move along the wellbore and the potential of a fixed electrode at thesurface. This potential response to electrochemical factors within the borehole is brought about bydifferences in salinity between the mud filtrate and formation water within permeable beds. These factors
are essentially related to the permeability of the bed.
A major factor in the reduction of permeability in a formation is the presence of shale. The SP log response
is thus a measure of shale content. Because the amount of shale matrix in most sandstones tends to increase
with decreasing grain size, the SP log can be used as an indicator of vertical grain-size variations. The SP
curve, measured in millivolts and recorded on the left-hand side of the log display, varies between two
extremes a shale baseline and a line corresponding essentially to clean sand ( Figure 1 ,Example of SP
log in a sand-shale series).
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Figure 2
Use of the SP log as a vertical grain-size profile is valid only for sediments with primary intergranular
porosity. Thus, it is generally not a reliable indicator of vertical grain-size distribution in cemented
sandstones or most carbonates.
The second wireline log used to obtain vertical grain-size profiles is the gamma ray. Gamma ray logs
measure the natural radioactivity of formations. Shale-free sandstones and carbonates usually have low
concentrations of radioactive materials, whereas shale has relatively high concentrations of the radioactiveelements uranium, potassium, and thorium. The gamma ray log is thus used to estimate the amount of shale
in a formation. The gamma ray curve, like the SP curve, is recorded on the left-hand track of the log display
and records high concentrations of radioactivity by deflection of the curve to the right ( Figure 3 ,Example
of a gamma ray log, left track).
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Figure 3
As mentioned earlier, the amount of shale in a formation tends to increase with decreasing grain size.
Therefore, as in the case of the SP curve, deflections of the gamma curve to the right normally indicate
decreasing grain size.
The gamma ray log, like the SP log, has its limitations. Clean, shale-free sandstone may produce a high
gamma-ray reading if it contains potassium feldspars, micas, glauconite, or uranium salts. The high
readings produced in such cases can make a clean sand appear fine and shaly. Conversely, kaolin-andchlorite-rich shales, because of their low potassium content, may produce lower than normal gamma
readings.
As pointed out, no single environment displays a completely unique grain-size profile. Thus environmental
interpretation of SP/gamma ray curves should take into account as much supplemental data as possible.
Selley (1985) presented environmental interpretations for four basic SP/gamma log profiles that depend onthe presence or absence of glauconite, shell debris, carbonaceous detritus and mica. ( Figure 4 ,Four
characteristic gamma log motifs.
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Figure 4
From left to right: thinly interbedded sand and shale; and upward-coarsening profile with an abrupt upper
sand-shale contact; a uniform sand with abrupt upper and lower contacts: and, furthest right, an upward-
fining sand -shale sequence with an abrupt base. None of these motifs is environmentally diagnostic on its
own. Coupled with data on their glauconite and carbonaceous detritus content, however, they define the
origin of many sand bodies.)
Use of the Dipmeter in Facies Analysis
The standard dipmeter tool is a wireline logging device consisting of micro-resistivity electrodes mountedon four pads equally spaced at 90 from one another. The tool is gradually raised through the borehole and
the readings from each of the four pad electrodes are recorded as resistivity curves. A recording is also
made of the tool's position relative to magnetic north.
A resistivity anomaly is usually produced by a bedding plane intersecting the borehole, the character of the
anomaly being roughly similar on each of the four resistivity curves. A computer correlates the four curves
and calculates the vertical displacement of one curve to another (Figure 5 ,Mode of operation of the
dipmeter log showing how dip directions are calculated from the four mutually opposed resistivity curves).
The dip angle and azimuth of the bed are then computed and presented on one of several displays.
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Figure 5
The most common of these displays is the arrow "tadpole" plot (Figure 6 ). On a typical plot, dip is read by
the position of the tadpole base on the dip scale and the azimuth is read by the direction in which the
tadpole tail points.
Figure 6
In addition to its obvious importance in diagnosing structural characteristics, such as folds, faults and
unconformities, the dipmeter can be extremely valuable in facies analysis, particularly as an indicator ofsedimentary structures. It has been found on tadpole plots that dips arrange themselves into characteristic
patterns. When reflecting sedimentary structure these patterns, termed depositional patterns, consist of
three basic types: slope patterns, current patterns, and low-energy structural patterns. Combined with
SP/gamma ray profiles these patterns become extremely valuable indicators of depositional environments.
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Slope patterns are characterized by upward-decreasing dips (red dip pattern) generally having a common
direction. When generated within a sandstone they usually represent lateral accretion surfaces of a channel
sandstone.
( Figure 7 ,Idealized dip log pattern showing progressively lower slope amount (red motif) characteristic
of filled-in channels.
Figure 7
Tadpoles shown correspond to dips of major accretion surfaces - in this case, those of the point bar. Note
vertical exaggeration of cross section.) Such dips point in the direction of the stream channel and
perpendicular to stream flow.
Slope patterns may also be developed in fine-grained sediments where they represent drape or differential
compaction over more rigid underlying features, such as sand bars or reefs (Figure 8 ,Red pattern on
dipmeter resulting from differential compaction of shale over underlying rigid feature).
Figure 8
These dips point in a direction away from the crestal high of the underlying feature and are really more
structural than depositional in origin.
Current patterns areupward-increasing dips of common direction (blue patterns) generated by the concave-upward foresets of current-induced cross-stratification. They naturally point in a downcurrent direction.
Because of the limited thickness of many individual cross-strata sets, recognition by the dipmeter often
requires use of computer programs that calculate dip in very small vertical intervals. ( Figure 9 ,Dip
patterns related to current bedding produced by westward current flow. Examples C, D and E illustrate the
results of using a 2-ft correlation interval in beds of varying thickness.)
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Figure 9
Upward-increasing blue patterns are also produced by prograding deltas, barrier-island sequences, andsubmarine fans. In these cases, dip generally increases upward along with increasing grain size, and a
single pattern may extend over a large vertical interval.
Low-energy structural patterns are generally low-angle, parallel dip (green patterns), typically occurring in
shale. In addition to their presence in vertically extensive shale sequences, they occur in shale units
interbedded within sand bodies ( Figure 10 , Common dip patterns and coloring code).
Figure 10
Most shale is assumed to have been deposited on essentially flat, horizontal depositional surfaces.Therefore, any green pattern dips over two degrees or so are likely to represent postdepositional structural
tilting.
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Use of Porosity Logs as Indicators of Lithology
Much useful information on lithology can be gathered by using combinations of conventional porosity tool
measurements. The most useful combinations are:
crossplots such as bulk density versus neutron porosity, bulk density versus sonic travel
time, and sonic travel time versus neutron porosity.
M-N and MID plots, whereby three log readings (neutron density and sonic) are
reduced to two-dimensional crossplots.
It is possible to scale porosity logs so that two curves, when overlain and compared with a gamma raycurve, immediately give a visual indication of rock type.Figure 11 (Example of generalized lithology
logging with combination gamma ray neutron (CNL)-density (FDC) log) shows how a combination
gamma-ray, neutron-density log can be used as a tool for determining lithology.
Figure 11
Figure 12 (Example of a combination gamma ray (GR) neutron (N)-density (d) log showing corresponding
lithologies from the Ordovician Red River formation,
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Figure 12
Richland County, Montana ) is a combination gamma-ray, neutron-density log showing corresponding
lithologies within a carbonate sequence in the Williston Basin of Montana.
Gamma Ray Spectral Log
Figure 13 illustrates a gamma ray spectral log.
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Figure 13
Unlike the gamma ray log, which measures total radioactivity (left tracks), the spectral log reads the
relative concentrations of radioactive potassium, thorium, and uranium (right tracks). The thorium-uranium
ratio measured by this log has been found to be a valuable indicator of depositional environment (Fertl
1979).
A thorium-uranium ratio greater than 7 is thought to indicate a continental, oxidizing environment and a
ratio of less than 7 to imply marine deposits, most likely gray and green shales. For thorium-uranium ratiosless than 2, the presence of black, probably organic, shales deposited in anoxic marine environments is
suggested. For example, at point "A" on the log in Figure 13 , the thorium curve reads about 14 ppm and
the uranium curve about 8 ppm, yielding a thorium-uranium ratio of 1.75. Thus, a black marine shale is
indicated.
The gamma ray spectral log may also be used for lithological identification, particularly for clay-typing.The crossplot chart inFigure 14 (Thorium/ potassium crossplot for minerals identification ) maps a number
of radioactive minerals according to their thorium and potassium concentrations.
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Figure 14
Again, looking at point "A" on the log in Figure 13 , we see that the thorium curve reads about 14 ppm and
the potassium curve reads 2.5%. Applying these readings to the crossplot inFigure 14 , a clay of mixed-
layer composition is indicated.
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