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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Wireline Logs