Dipmeter Surveys

7/22/2019 Dipmeter Surveys

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Note that as the four pads ascend the hole, each

measure electrode contacts the thin bed at a

different elevation, giving rise to displacements, or

shifts, between curves.

The depth differences, or displacements between the curves, depend upon the dip magnitude and direction, orazimuth, of

the bedding surfaces. Mathematical correlation methods are applied to measure these displacements, either individual

features or short intervals being matched together. The dip and azimuth of the bedding can then be computed, and

corrected for the effect of the deviation of the borehole.

It should be noted that formation dip computations with the conventional 4-curve tool require that a bedding plane be

crossed by at least three of the four pads, since three points are needed to define a plane. This creates the constraint that

pad-to-pad correlation must be established between the resistivity curves recorded by at least three of the four padelectrodes.

Generally, in well-bedded or laminated formations, the recorded data allow the determination of formation dip and

azimuth. Pad-to-pad correlations are limited for many stratigraphic studies, however, because of the fine detail associated

with sedimentary features. Eight-curve and microelectric scanning tools incorporate a number of major improvements

over the 4-curve tool to overcome this limitation, and are specifically applicable to sedimentary studies.

Although the newer tools are replacing the 4-curve tool, many hundreds of the 4-curve logs have been run in the past andwill continue to be used for geologic and production studies. Therefore, for completeness, the 4-curve tool and field log

will be discussed first and the 8-curve dipmeter and formation imaging measurements will be covered later in more detail.

2. Tools Available

Figure 1


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A number of dipmeter tools are available. Three-arm dipmeter tools were used for many years, but these have now been

entirely superseded by four-arm and six-arm tools. Figure 1 illustrates a commonly used four-arm dipmeter tool.

All currently used dipmeter tools have the following common characteristics:

the orientation section measures tool deviation from vertical, tool azimuth with respect to north, and the orientation of

the reference electrode pad to either north or the low side of the hole

the caliper section measures two or more hole diameters

the microelectrode array records the resistivity of the formation in the very localized area where the pads contact the

formation;

the gross correlation device, such as a moderately deep resistivity curve or a gamma ray or SP curve

Until recently, orientation was measured using a pendulum to indicate deviation from vertical and a magnetic compass to

indicate tool rotation

relative to magnetic north. Recently introduced tools use flux gate magnetometers, gyroscopes, and/ or accelerometers to

deduce the tool position and orientation.

The microresistivity pads carry small "button" electrodes for water-base muds and "knife-edge blade" electrodes for oil-

base muds, although the latter are not always very effective.

In the field the norm is to supply a 5-in. print of the orientation curves, the correlation traces, and the caliper curves. All

data are recorded on magnetic tape.

Figure 1

Figure 2
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On the rare occasions when it may be desirable to compute dip results from the film rather than from the digital data tape,

a film on a very expanded scale (60 in. = 100 ft) is required. Figure 2 illustrates the far more detailed 60-in.

dipmeter presentation.

The 4-Curve Dipmeter Tool

The 4-curve device uses four identical microresistivity electrodes mounted on four pads. The four caliper arms are

actuated hydraulically from the surface with a force sufficient to maintain good pad contact with the borehole wall undermost conditions. The resistivity measurements are sampled 60 times per foot, or every 0.2 in. Such detail is essential,

because even 1 of structural dip may be significant in determining the location of hydrocarbon traps. A 1 dip across an

8-in. borehole causes a shift of 0.14 in. between curves.

The electrodes are small enough to resolve fine structure with linear dimensions down to about 0.4 in. (1 cm). Because

dipmeter correlations depend on variations in resistivity, the circuitry for the electrode output is arranged so that the curve

deflections are proportional to the electrode current. Current varies widely according to the contrast between the

resistivity of the formation in front of the electrode and the formation surrounding the sonde. The curves are recorded

with a "floating zero" on a nonlinear scale designed to accommodate large variations in local resistivity.

Figure 3 shows the four primary dip curves.

On this expanded depth scale, it is apparent that a consistent shift occurs between any two curves. The shifts in this caseresult from bedding planes intersecting the borehole at an angle of approximately 30. This angle is the dip with respect to

a plane normal to the instrument axis.

The cable speed at the surface is measured, but the velocity of the downhole tool may be different and may alternately

accelerate and decelerate with changes in friction because of the elastic properties of the cable. It is important forpurposes of dip computation that the instantaneous velocity of the tool be known throughout the logging run. A fifth

electrode (known as thespeed button)provides for this correction. The curve recorded by this electrode should very

closely correlate with the curve recorded by the electrode mounted below it on the same pad, and thus yield a

displacement equal to the separation between them. However, if the instantaneous tool velocity varies from the constant

surface cable speed, this apparent displacement also would vary, and velocity corrections must be made.

Figure 3
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Without knowing the orientation of the tool in space, the computed dip would be the slope of a geologic feature relative to

the plane defined by the four resistivity pads. To convert this angle to true dip, three continuously measured angles are

required:

deviation of the tool from the vertical (inclination)

hole-drift azimuth

azimuth of Electrode No. 1 from magnetic north

The deviation and the first of the two azimuths are measured directly.A relative-bearingmeasurement is also made (the

angular rotation about the axis of the tool of Electrode No. 1 from the upper generatrix of the hole), and it is from this

angle and the azimuth of Electrode No. 1 that the hole-drift azimuth is computed. The relationship is:

hole-drift azimuth = azimuth pad 1 - relative bearing

Deviation and relative bearing are measured with pendulum systems, and the azimuth of Pad 1 with a magnetic compass.

True north is the reference for the orientation of the tool. True north and magnetic north are frequently different; this

difference is referred to as magnetic declination. Maps showing current values of magnetic declination are available. At

point A on such maps, magnetic north is 20 east of true north; therefore, 20 must be added to the magnetic-north

bearing to obtain the orientation of the tool with respect to true north.

East declination refers to conditions in which magnetic north is east of true north. East declination requires that the

declination value be added to the magnetic-north azimuth measurement to obtain orientation with respect to true north.

West declination refers to conditions in which magnetic north is west of true north and requires that the declination value

be subtracted from the magnetic-north azimuth measurement.

True dip magnitude and the downdip direction with respect to true north is calculated from all of the previously

mentioned acquired data-i.e., dip curve shifts, caliper measurements, deviation, deviation azimuth, and azimuth of Pad 1.

The 4-Curve Dipmeter Field Log

At the wellsite, a field monitor log is recorded for each run of the tool. By carefully monitoring the four dip correlation

curves on this log, the field engineer can ensure the reliability of the final computation.

The log heading provides a review of definitions of the various angles measured and calculated for the tool. The choice oflow-angle or high-angle unit affects those definitions and calculations. The low-angle unit is for holes as much as 36

from vertical, the high-angle for holes up to 72 from vertical.

The angle called azimuth is:

the clockwise angle between magnetic north and the horizontal projection of the arm carrying the

reference electrode (No. 1) for a low-angle unit.

the clockwise angle from north to the horizontal projection of the axis of the tool-called DHD on the

log-for a high-angle unit.

The relative-bearing angle is measured clockwise from the high side of the tool to the reference electrode. Azimuth and

relative-bearing traces should move roughly parallel to each other in a low-angle unit.

The major part of the log, the right-hand side, contains the four correlation curves. The log heading shows the relative

position of each curve and indicates the direction in which resistivity increases.


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On the far right-hand side of the log are the two caliper curves, showing the hole diameter between Pads 1 and 3 as a

dashed line and that between Pads 2 and 4 as a solid line.

The depth scale appears in the center column of the field log.

3. Definitions of Formation Dip

The dipmeter survey records ways in which

subsurface layers of rock have been deposited and

subsequently moved. The raw data consists of

orientation information, showing where the

downhole tool is located with respect to vertical

and geographic coordinates; and correlation

information, used to determine the attitude ofbedding planes with respect to the tool. The field

log does not indicate formation dip. Computer

processing of the raw data is required before any

geological information can be extracted. The two

important computer-processed parameters, bed-dip

magnitude and dip azimuth, yield a great deal ofvaluable information when studied with regard to

how these parameters vary with depth.

Dip angle is the angle formed between vertical and a normal taken from a bedding plane. Thus, a horizontal bed has a dip

of 0 and a vertical bed has a dip of 90 (see Figure 1 ).

The dip azimuth is the angle formed between geographic north and the direction of greatest slope on a bedding plane. Dipazimuth is conventionally measured clockwise from north, so that a plane dipping to east has a dip azimuth of 90, and

one to west 270 (Figure 2 ).

Dipmeter surveys have a variety of applications. At the lowest level, the raw data may be used to compute (1) a

deviation survey, (2) true vertical depth, (3) the integrated hole volume (as an aid to fracture detection) and (4) thin-bed

definition.

At the intermediate level, computed dipmeter results may be used to determine the gross geologic structural featurescrossed by the wellbore, sedimentary details within sand bodies, the depositional environment, and true stratigraphic and

vertical thicknesses.

At the highest level, computed dipmeter results from many wells may be combined to produce structural cross sections

and trend surface maps.

The most important applications of the dipmeter survey are in exploration drilling, to help identify local structure and

stratigraphy, and in development drilling, to help map the productive horizons and indicate direction to follow for furtherfield development.

Figure 1
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Introduction

The primary, and sometimes the only, use of a dipmeter is for determining structural dip. Structural dip is the attitude of

the formations resulting from tectonic movements. Structural dip information might be used by the geologist for possible

whipstocking or deviating the present well or to locate a future well updip or downdip.

Structural dip determination from logs is not always obvious. It is possible to have two equally plausible trends; when this

occurs, additional information is necessary to determine the most probable trend.

In tensional areas, such as the U.S. Gulf Coast, offshore West Africa, and portions of the North Sea, structural dip

consists of a dip trend extending at least a thousand feet. The trend would remain constant or change gradually, unless a

fault or unconformity is crossed.

Thrust provinces tend to exhibit more stages of local structural deformation than tensional areas. This increased structural

deformation is due to tectonic or major erosional events, and it negates the thousand-foot structural dip rule.

As a general rule, structural dip extends horizontally no farther than it does vertically. When determining structural dip,

use the trends with the greatest vertical extent.

In addition to green groups, which indicate structural dip, red and blue groups are also useful for determining the

direction of structural dip. Red and blue groups are particularly helpful when dip magnitude is low (about 1 or 2). Atlow angles there is often a choice of trends; the most probable trend matches the majority of red and blue dip groups.

Low-energy environments allow deposition of horizontal sediment layers. The dip of layers that have undergone only

structural uplift indicates the structural dip.

Determining Structural Dip

Figure 2


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To determine structural dip from an arrow plot, examine the reduced scale tadpole plot for zones of low dip scatter. Use

either the 1-in. or the 2-in. scale. The zones of least scatter are derived from sediment layers deposited in low-energy

environments, and they produce dips indicating the structural dip.

From the zones of least scatter, pick a dip trend extending as far vertically as possible; this is the approximate structuraltrend. Next, use the 5-in. scale to determine the exact dip magnitude and azimuth of the trend ( Figure 3 ). Dips plotted on

the reduced scales are pooled; therefore, any trend determined from the 1-in. or 2-in. plots would be slightly in error.

Unless the logged section is short, there may be several structural dip trends on the log. Structural dip changes indicate

sections missing due to faulting or unconformities, or indicate the end of periods of postdepositional uplift. It is important

to determine the exact location of dip changes. Sometimes the point of change can be determined exactly; in other

conditions it may be difficult or impossible to determine.

One technique for locating points of change is to determine the obvious dip trends above and below the point of change,

then extend both trends toward each other as far as possible using isolated dips for support ( Figure 4 ). The point ofchange is located between the two extended trends. This technique does not locate the exact point of change, but it does

better define the zone in which the change occurs.

Hole Deviation as a Dip Indicator

Hole deviation may be used in some instances as a dip indicator. The hole tends to drift or walk when dip is present. The

following general rules can help in identifying structural dip.

Figure 3


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Compacted Formations Compacted formations cause the bit to walk or drift updip in a hole drilled with mud. Updip drift

occurs as the bit attempts to align perpendicular with the dip of the bedding planes. When air or gas is used for drilling,

the hole usually drifts downdip.

Uncompacted Formations Less compacted formations are more complex, but in uncompacted formations the hole

generally drifts downdip. In one offshore area the hole drifts downdip to about 6000 ft, then clockwise along strike as thezones become more consolidated. The clockwise drift results from bit rotation. Near 12,000 ft, the bit encounters more

compacted beds, and the bit drifts updip. Unless controlled, the hole follows a U-shaped path.

Prior knowledge of hole-drift tendencies can save rig time; the surface location can be offset relative to the proposed

bottomhole location, reducing the need to control the parameters that affect drilling rate.

Faults and Conformities Whenever a fault or unconformity is encountered, the bit will create a dogleg. The Plio-

Pleistocene example in Figure 5 illustrates the effect of a change in formation compaction on direction of hole drift.

There is a down-to-the-south-southeast growth fault at 8200 ft. Structural dip is to the north-northwest on both sides to the

fault. On the downthrown side of the fault the hole drifts east-northeast or 90 clockwise from the downdip direction. The

upthrown drift is south-southeast or updip. The hole-drift direction changes across the fault because of an abrupt change

in formation compaction.

Figure 4


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Flat Structural Dip When flat or almost-flat structural dip is encountered, the hole slowly spirals through 360. A

complete rotation may require up to 1000 ft of depth.

Low Structural Dip

Low structural dip is indicated by a tadpole cloud with its left edge at the zero dip line. This is illustrated in column B in

Figure 6 .

If the dip trend is flat, some dips would have magnitudes of a few tenths of a degree and very few actual zero dips would

be computed. Five or six tadpoles per hundred feet would be near zero (less than 1). Not every interval would contain

these few very low dips, since the beds were not deposited flat.

The directions of the red and blue dip groups also indicate the presence of very low dip trends. An area that was flat

during deposition would have red, blue, and green dip groups lacking a common azimuth.

Do not overlook a low dip trend when a few, almost-flat dips are present. Column C in Figure 6 contains a low (2)

southeast trend. When an obvious trend is present, honor it.

Difficult Environments

Figure 5


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The most difficult environments for determining structural dips are from sediments deposited in shallow water and on the

continental slope. Both environments produce a high degree of dip scatter.

In the shallow water environment, the scatter results from the initial high-angle depositions, reworking by waves, and

bioturbation. The scatter from beds deposited on the continental slope results from post-depositional deformation.

Fishing operations increase the difficulty of determiningstructural dip because of the damage they cause to

formations near the borehole. The dipmeter is a shallow

investigation tool, and its measurements are made from

the zone that is damaged during fishing jobs. Formation

damage increases the scatter on the tadpole plot; the

greater the formation damage, the greater the dip scatter.

Zones of least scatter with a 2 or 3 magnitude variation

may exhibit 10 or more after a fishing job. Wells drilledwith mud weights that were too heavy exhibit the same

damage pattern.

4. The 8-Curve Dipmeter Tool

The 8-curve tool emits a current from the entire lower section of the sonde into the formation. A small portion flows from

the electrodes to record the microresistivity dip curves. The rest of the current serves to focus this small electrode current,

providing a measurement with very good vertical resolution. Comparison of the detail of the microresistivity curves with

cores shows the resolution to be on the order of 0.4 in. (1 cm). All current is returned to the metal housing of the tool

string above the insulating sleeve.

The inclinometry cartridge fits inside the top of the sonde. Its axis is accurately aligned with that of the sonde and

includes a triaxial accelerometer and three single-axis magnetometers.

The four arms that carry the measure electrodes have a maximum diameter of 21 in. A simplified mechanical linkage is

used so that the electrodes describe arcs of circles as the caliper arms extend. The opposite arms are linked, making the

sonde self-centralizing in the hole. In an oval hole, however, each pair of arms opens to a different diameter, and so theelectrodes on them are noncoplanar. This noncoplanar geometry is accounted for in the computation process for dip

calculations. The 4-curve tool design uses a more complex arm geometry to keep all electrodes coplanar.

The bottom of the sonde, where the dipmeter pads are mounted, is decoupled from the weight of the electronics and

communications cartridges by means of a flex joint. Using a cross-linked arm arrangement, it remains centralized in holes

Figure 6


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speed corrections to the recorded curves. The magnetometer has a separate unit for each of the above axes. By measuring

the direction of the earths magnetic and gravity fields in relation to the tool axis, azimuth information is obtained.

The inclinometer gives accurate tool-deviation (0.2) and tool-azimuth (2) information. Also, since there are no moving

parts, there are no problems caused by friction or inertial delays as there were with earlier mechanical designs. The

response time of the system, therefore, is very fast, so that any sudden tool movement is recorded and taken into accountduring the processing of dip results.

At the wellsite, the computation program uses the microresistivity information from the two additional electrodes (or

speed buttons) to perform the speed correction. At the computing center, additional processing is performed and the speed

correction is further refined. The accelerometer data are first used to correct the eight dip curves and the two speed curves

for the effect of irregular tool movement. The displacements with the speed curves are then used to remove any remaining

minor speed fluctuations. The original dip curves can than be corrected to their true downhole depths.

The 8-curve tool has a sampling rate of 0.1 in., as compared with 0.2 in. for the 4-curve tool.

The total current (called Emex) that is sent into the formation is automatically controlled by the surface computer to allow

for major changes in formation resistivity. In this way the microresistivity curve activity is maintained in both high- and

low-resistivity zones so that good correlations can be made. In addition, the microresistivity curves may be played back

and re-scaled at the wellsite or computing center to remove the visual effect of variation in Emex current. This ensures

that information about grain-size or textural change in the formation is not obscured, as might be the case on the original

raw data curves.

The 8-Curve Dipmeter Field Log

A real-time field log is recorded during the logging runs. After listing details concerning the tool and recording system,

the log heading also identifies the various curves and scales. The following curves are presented:

Hole Deviation. This is computed from sonde deviation using values of sonde length and cartridge standoff. Either the

hole or sonde deviation can be presented (default is the tool deviation calculated with zero standoff).

Hole Azimuth. Displayed on a -40 to 360 scale.

Pad 1 Azimuth. Displayed on a -40 to 360 scale, this curve shows the azimuth of Pad 1.

Relative Bearing. Displayed on a -40 to 360 scale, this curve is presented as a cross-check between Pad 1 azimuth

(P1AZ) and hole azimuth (HAZI). The relationship is RB = P1AZ - HAZI

Dip Curves. These are the eight raw microresistivity curves before any Emex correction. The speed curves are not

presented.

Emex Curves. Both Emex current and voltage are displayed. As an aid to the field engineer, they allow the operation of

the Automatic Emex Control to be monitored during logging.

Calipers. Two caliper diameters set at 90 to each other are presented on a linear 20-in. scale.

The field log is readily used to evaluate the data quality. Dip curves should be visually similar in detail and activity. Any

departure from this norm may signal unusual conditions or faulty tool operation. The user of computed data is encouraged

to study the curves carefully when judging the quality of the computations.

5. Dipmeter Computation

Given that a plane cutting a wellbore produces resistivity anomalies at slightly differing depths on the wall of the

borehole facing up- or downdip, the computation of dip and dip azimuth is reduced to a problem of trigonometry. Any

plane can be uniquely defined by three points in space. A four-arm dipmeter provides four points. If the bedding planes


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are uniformly thick and plane at the intersection with the wellbore, only three of the available four points are necessary to

compute a dip. When one of the correlation traces is substandard due to hole conditions or recording techniques, the

fourth trace allows a margin of safety. Parts (a) and (b) ofFigure 1 show a cross section of a borehole with a four-arm

dipmeter tool, and a schematic of the correlation curves that might be recorded.

Figure 1

A comparison of displacements of an anomaly on two correlation curves is key to computing the formation dip. Figure 2

illustrates a dipping plane cutting across a borehole and the expected displacements.


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Figure 2

The starting point for dip computation is thus the correlation of one trace to another in order to discover the relevant

displacement. The correlation process can be made optically using the 60 in. per 100 ft record and a special apparatus

known as an optical comparator, or it can be done by computer. Optical correlation is rarely used anymore since it

requires a skilled specialist, takes time, and makes no allowance for tool acceleration and deceleration. Computer-basedcorrelation can be made using a variety of techniques, such as pattern recognition, Fourier analysis, and conventional

correlograms. The most commonly used technique builds correlograms. Three parameters are used to control the

correlation process, as illustrated inFigure 3 .
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Figure 3

They are the correlation interval, the search angle, and the step distance.

Correlation intervals may range from a few inches to several feet, depending on the information sought. For detailed

stratigraphy with high-quality raw data, a correlation interval of 3 in. to 2 ft may be used. For standard work, 2 ft to 6 ft is

good, while for structural information, 6 ft to 18 ft will do.

The search angle defines how far up and down the hole to seek a correlation and, depending on the hole size, reflects theanalysts guess of the highest expected dip.

The step distance defines the depth increment to be used between rounds of correlations. This is usually set to half the

correlation interval. Thus, a dipmeter computed on 4 ft x 2 ft x 35 means a correlation interval of 4 ft was used with a

step of 2 ft and a search angle of 35

Since only three points are required to define a plane, a four-arm dipmeter survey forms an overdetermined system. Any

three curves of the four can provide a dip.

Three items may be selected from a choice of four in twelve ways. Potentially, therefore, many dips may be computed atthe same depth. In practice, it is found that they do not all agree. For the same reason that four-legged stools tend to

wobble on an uneven floor, but three-legged stools do not, a number of dips are possible simply as a result of nature not

providing us with bedding planes that are perfect planes at the scale of one borehole diameter. Add to this the effects of

borehole rugosity, floating pads, and the like, and the result is a scatter of possible dips. The choice of the correct dip then

becomes an exercise in common sense. In general, this exercise has come to be known as "clustering." Simply stated: If at

any level in the well the majority of the possible dips agree with each otherandagree with the majority of the dips at

adjacent levels in the well, then those are the most probable dips to use. The criterion for judging the worth of any type of

dipmeter computation is, of course, its ability to reflect the known geologic facts.

Computing Dip

In the early days of the dipmeter, operators made dip measurements directly from readouts similar to the modern field

log. Conductivity curves were recorded in much greater detail at a scale of 1:20, or 60 in. = 100 ft.


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Each curve feature is the signature of a geologic event in the depositional sequence through which the tool passes. The

same event can often be recognized in each of the eight curves, though depth may vary because of dip. By measuring the

displacement of the event between each of the curves and knowing the precise depth scale, the actual displacement may

be read in inches or fractions of inches of borehole. The dip angle relative to the plane of the electrodes can be calculated

trigonometrically. Hole deviation and direction, the orientation of Pad 1, the true dip angle, and direction relative to a

horizontal plane can also be calculated.

Computer processing of dipmeter data has completely replaced the manual method for normal applications, but the basic

principles have remained. Visual correlation and inspection of detailed logs is still useful in quality control and in studies

of fractures and other specific geological events.

In the following discussion of dip computation systems, references are made to examples of dip results in order to show

the effects of computation type, tool type, and computation parameters. Here we provide an explanation of the

presentation method.

Other Presentations

Several approaches for processing raw dipmeter data and for displaying the results are available. The choice of system orsystems to use should be determined by the type of problem to be solved-structural, stratigraphic, or (as is often the case)

both.

In addition to the various arrow plots, azimuth-frequency diagrams, and formation-imaging displays that have been

described and illustrated, a number of other graphic and tabular presentations are available from dipmeter data. The more

popular ones are covered in the dipmeter interpretation sections of this manual.

6. Interpretation and Applications

Once a dipmeter has been computed, a number of ways of presenting the answers is available. These include:

tadpole or arrow plots

SODA (separation of dip and azimuth) plots

listings

azimuth frequency plots

histograms

polar plots

stick plots

stratigraphic plots

A typical tadpole plotis shown in Figure 1 .


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The dip magnitude is read from the position of the base of the tadpole on the plot. The dip azimuth is read by observing

the direction in which the tail of the tadpole points. The azimuth convention is to measure angles clockwise from north.

Thus a north dip points uphole, an east dip to the right, a south dip down-hole, and a west dip to the left.

SODA plots separate dip and azimuth as distinct points on separate tracks of the answer plot.

ListingIn addition to the dip and dip azimuth, these listings may include further details such as dip quality and hole

volume.

Azimuth frequency diagrams (or rose plots) present statistical information regarding some depth interval in the well,

usually 100 ft or 500 ft. Within that interval a polar plot is built with the number of dips having a dip azimuth of aparticular direction plotted in a circular histogram. These are most useful for making a quick scan of the geologic column

for trends in dip direction with depth. Conventional histograms of both dip and dip azimuth can also be presented (

Figure 2 ).

Figure 1


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Polar plots can be built in two ways. One way, the rose plot, has already been described. Another way is to scale the plot

with zero dip at the outside and 900 at the middle. Thus the azimuth of the lowestdips becomes more apparent. This type

of plot, popular for picking structural dip, is illustrated by Figure 3 .

Figure 3

Stick plots ( Figure 4 and Figure 5 ) show a series of short lines inclined to the horizontal.

Figure 2
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Figure 4

Each line represents the dip angle as projected in some line of cross section.


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Figure 5

A stick plot can be oriented whichever way the geologist wishes. If the orientation is changed, the new axes must be

relabeled. It is normal to distort the horizontal and vertical scales on these plots to fit the geologists mappingrequirements. Stick plots, normally used in multiwell projects to draw cross sections, are particularly helpful where the

interwell correlation is not immediately obvious from conventional logs.

Stratigraphic plots attempt to give a visual representation of the bed stratigraphy. Each dip may be represented by the

trace of the bedding plane on the borehole wall. If the trace could be "unwrapped" and laid on a flat surface, a sine wavewould be visible, its amplitude a reflection of the dip magnitude and its low point an indication of the dip azimuth.

Figure 6 illustrates such a plot.

Dipmeter plots may be interpreted by observing the variation of dip and dip azimuth with depth in conjunction with theopenhole logs. Here color helps highlight certain types of patterns. Conventionally, dips of more or less constant azimuth

that show an increase in dip magnitude with depth are colored red; those that show a decrease in dip magnitude are

colored green. Figure 7 illustrates these patterns.

Figure 6


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Broadly speaking, dip interpretation may be split into two parts, structural and sedimentary. Gross structural

characteristics, such as unconformities, folds, anticlines, and synclines, produce patterns that vary gradually over

hundreds of feet. Sedimentary characteristics, such as crossbedding, only appear within sedimentary beds and are

localized to a few feet to tens of feet. To become familiar with some of these patterns and their associated geologic

features, six cases may be considered.

Presentation of Dip Data

The basic method of presentation of computed dip answers is the arrow ortadpole plot. Each tadpole consists of a dotwith an attached tail. In Figure 8 the position of the top dot shows a dip magnitude of 20.

Figure 8

Figure 7


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Magnitude is the dip angle with respect to horizontal. The tail of the tadpole always points in the downdip direction in this

example-N60E, or 60 east of north. The computed dipmeter result is composed of many, often thousands, of tadpoles.

From the tadpoles it is possible to recognize changes in dip and direction up and down the well. Changes in magnitude

and direction are shown as depth increases.

During the computation process, the computer outputs quantities that are used to qualify the sharpness or reliability of the

correlation. This determination of answer quality is represented on the tadpole plot in three basic codes. Solid tadpoles

represent answers of high accuracy and confidence. Hollow tadpoles represent answers of a lesser degree of the same. Notadpoles, orblank zones, are intervals for which actual correlations were sufficiently in doubt that a decision could not be

reached. This method of plotting enables the user to make a judgment on data quality.

Figure 9 is a typical tadpole plot over 40 m of hole.

Note the solid tadpoles, hollow tadpoles,

and blank zone, as previously described.

The second set of tadpoles to the far right

indicates the hole-drift angle from verticaland the direction of drift. This

information can be very useful ininterpreting dip data and will be

addressed later.

An azimuth frequency plot (also known as a rose diagram) is shown on the same track as the dip tadpoles. Each of these

plots represents azimuth distribution of all dips between the arrowheads A and B.

Figure 9


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bioturbation

diagenesis--e.g., dolomitization of limestones or cementation of clastic rocks resulting in obliteration

of original bedding

deformation by creep, slumping, diapirism, or plastic flow

fracturing due to tectonic stress and movement

rubble in fault zones

in some cases, swelling of clay-rich formations adjacent to the borehole by absorption of drilling

fluids or modification of the rock stress by the drilling process

dips paralleling the hole axis

From the appearance of the plot we can infer formation characteristics related to sedimentary and tectonic processes that

further enhance the overall interpretation.

7.Interpretation

Interpretation and Applications

Once a dipmeter has been computed, a number of ways of presenting the answers is available. These include:

tadpole or arrow plots

SODA (separation of dip and azimuth) plots

listings

azimuth frequency plots

histograms

polar plots

stick plots

stratigraphic plots


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A typical tadpole plotis shown in Figure 1 .

The dip magnitude is read from the position of the base of the

tadpole on the plot. The dip azimuth is read by observing the

direction in which the tail of the tadpole points. The azimuth

convention is to measure angles clockwise from north. Thus a

north dip points uphole, an east dip to the right, a south dip down-hole, and a west dip to the left.

SODA plots separate dip and azimuth as distinct points on separate tracks of the answer plot.

ListingIn addition to the dip and dip azimuth, these listings may include further details such as dip quality and hole

volume.

Azimuth frequency diagrams (or rose plots) present statistical information regarding some depth interval in the well,

usually 100 ft or 500 ft. Within that interval a polar plot is built with the number of dips having a dip azimuth of a

particular direction plotted in a circular histogram. These are most useful for making a quick scan of the geologic columnfor trends in dip direction with depth. Conventional histograms of both dip and dip azimuth can also be presented (

Figure 2 ).

Figure 1


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Polar plots can be built in two ways. One way, the rose plot, has already been described. Another way is to scale the plot

with zero dip at the outside and 900 at the middle. Thus the azimuth of the lowestdips becomes more apparent. This type

of plot, popular for picking structural dip, is illustrated by Figure 3 .

Figure 3

Stick plots ( Figure 4 and Figure 5 ) show a series of short lines inclined to the horizontal.

Figure 2
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Figure 4


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Each line represents the dip angle as projected in some line of cross section.

A stick plot can be oriented whichever way the geologist wishes. If the orientation is changed, the new axes must be

relabeled. It is normal to distort the horizontal

and vertical scales on these plots to fit thegeologists mapping requirements. Stick plots,

normally used in multiwell projects to draw

cross sections, are particularly helpful wherethe interwell correlation is not immediately

obvious from conventional logs.

Stratigraphic plots attempt to give a visual representation of the bed stratigraphy. Each dip may be represented by the

trace of the bedding plane on the borehole wall. If the trace could be "unwrapped" and laid on a flat surface, a sine wave

would be visible, its amplitude a reflection of the dip magnitude and its low point an indication of the dip azimuth.

Figure 6 illustrates such a plot.

Figure 5


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Note the solid tadpoles, hollow tadpoles, and blank zone, as previously described. The second set of tadpoles to the far

right indicates the hole-drift angle from vertical and the direction of drift. This information can be very useful in

interpreting dip data and will be addressed later.

An azimuth frequency plot (also known as a rose diagram) is shown on the same track as the dip tadpoles. Each of these

plots represents azimuth distribution of all dips between the arrowheads A and B.

Figure 8


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From a series of these plots over a long interval, one may recognize major direction changes without studying the tadpole

plot in detail. The curves on the left of the figure are the two calipers and a computed resistivity. Gamma ray curves may

also be displayed. The calipers are a useful indicator of difficult logging conditions, particularly poor pad contact due to

hole irregularities. The calipers may also show an enlarged hole where the borehole intercepts a fault or fractured zone.

The resistivity curve can be used to positively tie the computed dip plot on depth with other openhole logs.

Tadpole Plot Characteristics

Figure 10 is a dipmeter plot of a section with excellent parallel bedding, less well-defined bedding, and a blank zone,

where no correlations could be found.

Note that a consistent trend of hollow tadpoles can give a high-quality interpretation although each individual dip may not

in itself imply high accuracy; this is the case within the top 15 m of the log.

The general appearance of the dipmeter plot when variables such as tadpole scatter, tadpole quality, and other trends are

considered reflects changes in bedding characteristics that are functions of depositional environment, tectonics,

diagenesis, rock stress, and other useful geologic factors not deduced from most other logging devices. Indeed, thesequence of those observable characteristics often can be repeated from well to well as consistently as can lithologic

sequences, and can provide additional geologic information about an area.

Note that during interpretation of any dipmeter plot, the major influence on the quality of the tadpole is the rock

characteristic. Poor bedding may be influenced by any of the following:

lack of textural or mineral stratification

small-scale heterogeneities--e.g., concretions, cross-laminations

Figure 9


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bioturbation

diagenesis--e.g., dolomitization of limestones or cementation of clastic rocks resulting in obliteration

of original bedding

deformation by creep, slumping, diapirism, or plastic flow

fracturing due to tectonic stress and movement

rubble in fault zones

in some cases, swelling of clay-rich formations adjacent to the borehole by absorption of drilling

fluids or modification of the rock stress by the drilling process

dips paralleling the hole axis

From the appearance of the plot we can infer formation characteristics related to sedimentary and tectonic processes that

further enhance the overall interpretation.

II Computation

1. Computation Methods

Figure 10


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One method used to obtain dip information from the raw data involves correlating intervals of the dip curves. To a

mathematician, a correlation coefficient is a measure of agreement between any two curves. Numerically, coefficients

may run from zero (representing two completely dissimilar curves) to one (representing two identical curves).

The computer calculates the similarity between a section of one curve and an equal section of a second curve. The length

of the interval on the first curve is the correlation length or interval. The computer then moves the first curve by somesmall, preset increment and recomputes the coefficient. This process is repeated many times.

When plotted with respect to depth, the resultant series of coefficients forms a function called the correlogram. This

correlogram shows a peak value where the curves have the best fit with each other ( Figure 1 ). The position of this peak

with respect to the center of the interval chosen on the first curve is the shift, or displacement, between curves.

Figure 1

The process is repeated for all curve pair combinations at that depth; the result is the relative position of correlated points

around the borehole, which (when combined with the other measurements such as tool orientation, drift, and caliper data)are used to calculate the dip answer for that depth. A new interval is then chosen on the first curve at a distance equal to

the step distance from the previous round of correlations just described, and the process is repeated to produce another dip

answer displaced in depth from the previous one by an amount equal to the step distance. This step distance is normally

chosen to be some fraction (usually 25 to 50%) of the correlation interval.

During the curve-to-curve comparison it is essential to prescribe for the computer the distance up and down the secondcurve to which the first curve is to be compared. This distance is fixed by the choice of the input parameter calledsearchangle.

Search angle is chosen according to the dip environment. For low structural dip areas, a 45 search is common, as most

stratigraphic dips fall within that range.

In tectonically disturbed areas, higher search angles may be required. The choice in such circumstances must be guided

by both local knowledge and close inspection of the dip curves. Large displacements may be visually evident and an

approximate dip range may be estimated.


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The user of the computed data should be aware of a particular characteristic of the interval correlation system. In order to

prevent some data from not being used in the computation, the step distance is normally (as mentioned above) less than

the correlation interval. This may allow a dominant anomaly (a large sharp peak or trough) to influence the dip answer for

each step in which it is included in the correlation interval. This can cause two or more adjacent dips to be essentially

identical, giving the user the impression that several parallel beds exist when in fact there may be only one. For example,

a 25% step may produce four similar dips from one anomaly, a 33% step may produce three similar dips, and a 50% step

may produce two similar dips.

If the user is aware of the parameters used for the computation he will recognize the duplications and interpret the dips

correctly. However, if the effect is not desirable, a method calledpoolingmay be used to present the results. In pooledplots, adjacent dips within a very small solid angle (2 to 3) are presented as one dip answer. Dips that do not pool are

still presented, so that no data is discarded.

Figure 2 shows another interval with both the unpooled and

pooled results side by side.

Note the groups of four dips on the unpooled data set that

appear as single dips on the pooled result. Also evident is the

marked decrease in dip density in the pooled data for the upper

half of the log. This can be a desirable presentation, particularly

when plotting data on reduced scales, such as 1:600 or 1:1200,

for structural dip analysis.

Computation Parameter Selection

There are three basic types of interpretation problems that users of dipmeter data may wish to solve:

structural interpretation

large-scale stratigraphic features

maximum detail, very fine stratigraphic features, as observed on detailed core inspection.

It is often desirable to interpret a combination of the above from a single dipmeter log. As a result, a variety of systems

have evolved to handle widely different requirements.

Figure 2


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Therefore, it is important to understand how the tadpole plot is affected by the choice of these parameters. For each step, a

single dip answer is produced, and all the data within that correlation interval are used to obtain that single dip. A 4-ft

interval may contain from 0 to more than 100 correlations, due to bedding contrasts, but only a single dip is calculated,

based on the best fit of the correlation curves. Large correlation intervals tend to smooth the dip results. Short correlation

intervals allow the system to find more detailed results.

Figure 3 contains a 4-m section of dipmeter computed in a sand section using several correlation intervals.

Note that although the dip direction trend is similar in each, the implied cross-sectional view of the formation issignificantly different.

Plot A clearly shows detailed internal sedimentary structures with a much better suggestion of environment than do B and

C. Plot B retains most of the characteristics

of Plot A, but with some apparent averaging and smoothing at dip magnitude boundaries. Plot C suggests large-scale,

almost parallel crossbedding. This plot fails to indicate the more complex internal sedimentary structures evident on the A

plot.

It is apparent from comparing these three computations that the choice of the computation parameters should be

influenced by the type of information required to support exploration and production programs. Although the basic

principles described in the foregoing apply to all correlation interval techniques, algorithms differ significantly for

different tool types, allowing the best adaptation to the data obtained by the tool.

2. Dip Computations with the 4-Curve Dipmeter Tool

For the 4-curve tool, two correlation techniques are available to determine the magnitude of the dip and the azimuth of its

direction: interval correlation (CLUSTER* Program), as described above; and feature correlation (GEODIP* Program),

Figure 1


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By a built-in order of precedence (e.g., first large troughs, then large peaks, then medium troughs, and

so on), the program first evaluates higher-order features, then when necessary also evaluates lower-

order ones. This is done during multiple passes through the four sets of elements.

Because geologic strata are deposited in succession, their boundaries do not cross. So, if event A

appears above event B on one curve, it cannot appear below event B on another. This is the rule of

noncrossing correlations.

If no correlation can be found within the specified search angle among all four curves, the program lowers its standardsand looks for 3-curve correlations instead. Planarity is monitored continuously, and if it fails to meet preset standards, the

program makes no attempt at 4-curve dips, but computes the four different 3-curve dips and displays them all.

Because the program works from identifiable features on the curve, each one corresponds to a geologic event and the

density of the output data depends on the density of geologic information at that level. This makes GEODIP processing

particularly successful in fine-structured sedimentary sections and for definition of lithological changes, such as scour

surfaces.

The calculation of dip angle at each depth is from displacements measured on boundaries rather than on feature centers.

These boundaries are shown on the correlation curves of a GEODIP log. They are themselves useful features for

interpreting lithology, as Figure 2 suggests.

Determining Data Quality

The geologic validity of each dip determination may be tested in several ways.

Closure If displacements are determined between each adjacent pair of curves, taken cylindrically (1-2, 2-3, 3-4, 4-1) they

should have an algebraic sum of zero. (Moving from one electrode to the next, you should return to where you began after

making a traverse of all four electrodes.) This condition is calledperfect closure. Small closure errors may be due to

inaccuracies in the computed displacements; large closure errors indicate that one or more of the correlations are in error.

Planarity Another test is forplanarity, the condition that the four points should define a plane. After four displacements

have been calculated, the lines joining diametrically opposed electrodes may fail to intersect, if there is an anomaly in thecalculation or in the bedding.

For the 4-curve tool, the geometry of the pad linkage ensures that distances between opposing adjacent pairs remain

equal. Displacements computed from opposite pairs of curves (h 1-2 and h 3-4 for example) must therefore be equal but

opposite if the bedding surface is planar. (The line segment connecting Pads 1 and 2 on the dipping plane parallels and

equals in length-but is oppositely directed to-the line segment connecting Pads 3 and 4, for example.) For perfect

planarity, h1-2+h3-4 = 0 and h2-3 + h4-1 = 0.

Likeness A third test is forlikeness, a quality derived from the correlogram, to compare the similarity of the curves. The

highest correlation coefficient computed over the search interval is the likeness of the two curves, and the trial

displacement of that maximum is the displacement retained for that interval of the curves. Since more than one cross

correlation is required to compute a dip, the credibility of the dip answer is roughly proportional to the lowest likeness of

all the correlations used.

Despite these tests, the results sometimes show excessive scatter that is not of geologic origin, particularly when shortercorrelation lengths are selected to improve resolution. The CLUSTER program reduces the scatter in the output by

statistically reducing the data. It is assumed that random noise does not repeat itself through small changes of the

correlation environment. Thus, at a given level the redundancy inherent in having four correlation curves allows the

curves to be grouped in various combinations in a search for consistency. In addition, coherence between consecutive

overlapping levels above and below each point in the hole is checked.

The program computes correlations between five of six possible pairings of the four curves, taken two at a time. To define

a plane, any two of these pairs must have one curve in common. The CLUSTER program, working with this output,

considers eight such solutions. Each of the eight yields a solution for the true dip plane, and generally each is slightly


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different. Calculations from an adjacent level yield another set of eight solutions. Since the correlation interval is greater

than the step distance, neighboring correlation intervals overlap. Comparison of dips from several overlapping levels

(eight solutions from each level) shows statistical scatter among the different solutions, but there should be a tendency for

many of them to "cluster" near some numerical value. When several solutions (not all from one level) fall within an

acceptable range of values, the program quotes the value for the group, rejecting those that scatter outside. As a result,

legitimate dip trends can be sorted from noise.

3. Computing Dip with 8-Curve Data

This section discusses the methods developed specifically for processing 8-curve data using the principles of interval and

feature correlation, the presentation of the results, and the presentations available at the wellsite and at the computing

centers.

The determination of formation dip measurements using the 4-curve dipmeter tool depends on the bedding plane being

detected by at least three of the four measure electrodes. This, in turn, implies that the formation is well-bedded or

laminated. Unfortunately this is not always the case, and for many formations pad-to-pad correlations are impossible to

establish, making sedimentary studies difficult or impossible. Also, pad-to-pad correlations may be difficult in highly

dipping formations or in highly deviated holes.

The 8-curve tool was designed specifically to overcome this limitation by providing two microresistivity curves, 3 cmapart, on each of the four pads. The density of the results is an order of magnitude higher than with previous 4-pad

hardware and processing. In addition, the improved sonde velocity correction, using accelerometer data to compute

instantaneous sonde speed and length of travel along the borehole, greatly increases the coherence of the results and helps

salvage data affected by severe hole conditions.

The processing methods discussed here have been developed to take advantage of the tool improvements. They provide

three independent computations of formation dip and allow adaptation of the interpretation of the results to the specific

problem of interest (e.g., structural, sedimentary, geometry of the sand body).

Programs for computing dip from 8-curve measurements include the basic interval correlation program, called mean

square dip (MSD), which uses all 28 possible cross correlations to compute 28 displacements (if all are successful). Since

only two adjacent displacements are needed to define a plane, considerable redundancy has been built into the

measurement system. The program thus tries to find a "best fit" plane that satisfies most of the displacements.

A second interval correlation method called continuous side-by-side (CSB) is also used. It only considers displacementscomputed from the side-by-side buttons on the pad. These four computed displacements represent the apparent angle of

the set of bedding planes that cut across the borehole.

Finally, feature correlation is provided by the LOCDIP* computation. These pad-to-pad correlations are made over short

intervals centered on bed boundaries, as defined by the major inflection points on the microresistivity curves. This

method is used to identify and then correlate major individual curve features. The correlation lines are displayed with the

actual microresistivity curves in a way similar to the GEODIP computation and presentation.

Mean Square Dip (MSD) Processing

At any one depth level, there are 28 possible cross correlations for the 8-electrode measurements, as compared to six for

the 4-curve recording. As in 4-curve processing, the correlation method for the eight curves requires defining an interval

length, a step, and a search angle; however, there is a significant difference in the way the cross correlation is made. In the

standard interval correlation program, a specific interval of a reference curve is defined and then slid along the interval of

the matched curve. For the 8-curve dipmeter tool, the MSD method considers the same depth interval on each curve anduses only the data within that interval to make correlations. In the case of low apparent dip, nearly all the data points

within the interval are considered when the correlation is made. As the apparent dip increases, fewer and fewer points

enter into the correlation. A limit is imposed when the search angle is increased until only half the points in the intervals

are being used. This corresponds to an apparent dip of about 72 in an 8-in. borehole with a 4-ft correlation interval.


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Side-by-side correlations are shown as thin lines, and, for

reference, the pad-to-pad correlations found for the same

interval are shown as thick lines. From this example, we see

that the number of side-by-side correlations is approximately an

order of magnitude greater than the pad-to-pad correlations, andthat the resolution is on the order of a few inches.

Another important feature, due to the proximity of the buttons on the pad, is that the displacements found by side-by-side

correlations are much smaller than pad-to-pad displacements. This allows the measurement of very high dips that are notdetected by pad-to-pad correlation. For such cases, once credible dips are found by CSB processing, they can be used as

input to the focusing option for the MSD program.

Figure 2shows a conventional pad-to-pad MSD correlation for a case of high apparent dip.

Figure 1


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Figure 2

The well is deviated about 35 to the southwest, in the same direction as the regional structural trend (30 to 40). Thus, agiven bedding surface would cut the borehole high on the northeast side and low on the southwest side. Obviously,

getting a good correlation is difficult, although the quality of the dip curves and the borehole condition is excellent.

Figure 3shows the results obtained with side-by-side CSB processing.

In this case, the 3-cm spacing of the buttons allows an unambiguous correlation to be made.

In the standard CSB computation, each pair of microresistivity curves (e.g., buttons 1-lA) is cross-correlated using short

correlation intervals of 12 in. or less, and under favorable conditions even 4 in. or 3 in. The step distance can be takenequal to half or three-quarters of the correlation interval. This gives a vector parallel to the dip plane. Under ideal

conditions (planar beds) another vector is found at the same depth by cross-correlating the microresistivity curves of an

adjacent pad (e.g., buttons 2-2A). These two vectors are then used to define a dip plane.


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With only four side-by-side correlations, a cross-check is needed to verify that the bed is indeed planar. If it is, then

displacements obtained using microresistivity curves from opposite pads (e.g., buttons 1-lA, 3-3A) should be equal in

value but opposite in sign, and the dip can be obtained from any two orthogonal pairs at that depth. If this is not the case,

however, a window is opened around the level under examination, and the vertical continuity of the displacements a

certain number of levels above and below it is checked. The pad showing the best vertical continuity is kept. A similar

procedure is then followed for Pads 2 and 4 and, again, the pad showing the best vertical continuity is kept. The

orthogonal pair showing the smoothest continuity within the window is used for dip computation.

In order to evaluate the credibility of the dip, a quality value ranging from 0 to 20 is assigned to each dip according to the

vertical continuity and the quality of the correlograms at the various levels or depths.

If the environment of deposition produces little contrast between beds or the formation is highly crossbedded with

sequences terminating over lateral distances of the same order as the borehole diameter, then pad-to-pad correlation may

be difficult or impossible due to curve dissimilarity. CSB provides an excellent solution to this problem.

Correlation intervals as small as 2 in. have been matched with detailed core information, although 6-in. to 1-ft correlation

intervals are most commonly used.

Figure 4shows the detail available from the CSB as compared to visible core features. To make this comparison the CSB

was processed with a 6-in.

correlation interval and a 2-in. step and then plotted on a scale one-quarter of full size in order to match with the core

photographs. Good dip agreement is apparent. Note the low contrast on the dip curves correlating to the fore-sets in the

lower one-third of the photo. The truncation visible on the core is also evidenced on the dip plot. Such detail would not bepossible with standard pad-to-pad correlation systems.

Figure 3


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The good likeness of the side-by-side curves is useful in cases of high apparent dip. Under these conditions it becomes

difficult to find an unambiguous curve match between the pads. Use of the side-by-side configuration allows reliable

measurement of displacements between the curves from the same pad and computed dip values.


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LOCDIP Computation

As discussed earlier, inflection points on the microresistivity curves describe geological events in the depositional

sequence of the formation. The purpose of the LOCDIP program is to detect the geological events, or boundaries, and

where applicable to associate a dip precisely at that boundary independent of dips at other depths. Instead of correlating

intervals of curves, it detects features (inflection points) on each curve and attempts to link these around the borehole, in a

manner somewhat similar to GEODIP processing. There are, however, some important differences:

To be retained as a LOCDIP result, an event must be recognized on at least seven of the eight

microresistivity curves; GEODIP logic requires only three out of the four curves. Thus, LOCDIP logic

is more demanding than GEODIP logic.

Figure 4


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A measurement of the planarity is derived for each of the possible dip planes at any level. The

retained value corresponds to the surface that best approximates the set of these planes. By convention,

a perfectly planar surface has a planarity of 100.

Some events are recognized on only a few of the dip curves. In this case, the available correlations are

traced across the applicable curves, with an "options" notation of "F" (fracture) or "P/L" (pebble or lens)

for single-pad events or two/three-pad events, respectively. These interpretations, however, are not to be

considered as certain, but rather as possible.

The processing of the 8-curve data is designed to extract the maximum amount of dip information from the raw curves. A

well may present several interpretation problems due to variations in lithology and bedding characteristics. A singlecomputation system may not offer the total solution. It is useful, therefore, to be able to combine the results of several

types of computation in one presentation.

DUALDIP* Presentation

The DUALDIP presentation for the 8-curve dipmeter tool allows results from more than one computation to be combined.

Figure 5is an example of multiple computations on a short section.

In the figure, the dips on the left side are side-by-side (CSB) results with a correlation length of 8 in. and a step of 4 in.

This produces three dips per foot, or about 10 dips per meter.

The tadpoles on the right are of two types. The round-headed tadpoles were computed from pad-to-pad correlations with a

correlation interval of 4 ft and a step of 2 ft. This is the MSD computation.

The triangular-headed tadpoles are LOCDIP computations, also known aspad-to-pad feature correlations. These dips

usually correspond to the more prominent bed boundaries, and are computed by the earlier mentioned pattern-recognition

system. For each LOCDIP computation which used seven or eight of the dip curves, a solid correlation line is drawn on

the plot showing exactly where the bed boundary was interpreted. For each of these correlations a local dip is shown. If

fewer than seven curves are correlated, then the correlation is shown as a dotted line, but dip is not computed.

Figure 5


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This presentation not only gives a visual impression of the frequency of stratification and its planarity and parallelism, but

it also allows the user to judge the validity of the correlations. This is of particular value in detailed studies of sedimentary

features.

All three systems may not, nor should they necessarily, give the same dip answer. This characteristic can be used to great

advantage in interpreting sedimentary features, particularly thin, highly bedded clastics.

In Figure 5, the two local dips at A and B correspond to the top and bottom of a distinct sedimentary unit. They suggest

the boundaries both dip at 1 northerly. All the finer bedding within these boundaries produced CSB or round-headed dips

consistently north-northeast between 4 and 10.

The internal bedding indicates sediment transport direction from south-southwest to north-northeast, with topset and

bottomset surfaces approximately 1 northerly. The CSB result is different from that obtained from LOCDIP and MSD

processing, whose computation system is restricted to major events, which can be correlated from pad to pad. The CSBlogic favors events with some continuity; individual single events are less likely to be computed, particularly where both

types are visible within the correlation interval. This tendency for different systems to favor different types of bedding

planes has been very useful, particularly in the interpretation of fluvial environments.

Note also that the 4-ft MSD correlation showed the dip at C to be southwest about 90 and consistent over 4 ft. This is

easily explained, considering the previous discussion of overlap effects, and it is supported by the LOCDIP computation

at that depth. This boundary presents a dominant anomaly to the 4-ft correlation system, and for fine stratigraphy would

be misleading by itself. When all bedding features, large and small, are parallel, all systems should give the same answeras at D.

4. Formation-Imaging Tool

Successful dipmeter interpretation depends greatly upon the accurate evaluation of geological features. The application of

the classic dip patterns is a relatively simple matter when geological events such as current bedding or lateral accretion

are known. In many complex environments this is a severe problem. A whole core over the zone of importance solves

these problems, but whole core availability is the exception rather than the rule. Formation imaging provides a continuous

oriented borehole representation that can be used in conjunction with a whole core or, in most cases, by itself to evaluate

geological events.

Interpretation The goal of formation-image interpretation is to characterize formation properties to assist

sedimentological interpretation, determine the presence of permeability paths and permeability barriers, help calculate netpay, plan perforation and fracturing, and to help decide whipstocks and where to drill next.

Formation images must always be interpreted after lithology has been fairly well defined, so supplemental data are

usually necessary to enhance the confidence of image interpretation. As with other dipmeter interpretations, the more

supplemental data available, the better the interpretation.

MeasurementThe clustered microresistivity buttons on two or four of the microscanner pads provide a continuous

electrical image of the borehole wall. The pads are oriented at right angles to achieve a three-dimensional perspective.These resistivity data are then mapped to a gray-scale or color "corelike" borehole wall image. This allows fine-scale

features to be described through essentially the same interpretation procedure as that used in the examination of slabbedcores. The images characterize many types of structural and stratigraphic features. These oriented features, combined with

a conventional dipmeter plot, are used to evaluate these events to extend the reservoir geometry beyond the wellbore.

Images of the rock formation exposed by the wellbore are processed from the microresistivity traces. Each image pad

covers 2.8 in. of the borehole wall. Thus, 22% of an 8-in. borehole can be imaged with two pads and 44% with four padson each logging pass. This coverage can be increased with multiple logging passes. The tool also contains a triaxial

accelerometer and three magnetometers for orientation and to enable speed corrections to be made on the acquired data.

Presentation of Images


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Several presentations are available for displaying the data. The vertical scale provides the most striking difference

between the formation-imaging presentations and other logs. The normal detail scale for logs is 1:240, while the

formation images are presented on a 1:5 scale. The standard presentations can be broadly classified into two types:

straight-line images and azimuthal images.

Straight-Line Images A straight-line presentation shows the images in a stationary horizontal scale ( Figure 1 ).

Figure 1

This presentation is divided into several sections. The left section contains the depth scale, the pad orientation, and theborehole deviation. The long arrow on the tadpole indicates the direction of borehole drift; the body of the tadpole

indicates the magnitude of deviation by its position on the horizontal scale. The small arrow shows the azimuth of Pad 1.

The next section contains the caliper and resistivity correlation curves. The calipers from Pads 1-3 and 2-4 are shown.

The resistivity curve is used only for correlation and not for quantitative purposes. Pads 3 and 4 of the 2-pad tool provide

the image. Both the raw microresistivity traces and the processed images are presented. The microresistivity traces are

from the 27 image buttons. The image traces are computer enhanced using 16 gray levels; they range from white

(resistive) to black (conductive).

Another popular presentation is shown in Figure 2 .

In this example, the formation images are displayed on the same depth scale as the dipmeter log. This scale is not as

effective for identifying individual sedimentary features but is better for displaying the overall features of a zone and

showing how they relate to dip patterns.


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Azimuthal Images A BORMAP presentation is shown

inFigure 3 .

The horizontal scale shifts according to the respective azimuths of each pad. Thus, multiple passes can be merged to

portray a more complete picture of the wellbore. In this example, images from two logging passes (from a tool with two

imaging pads) were merged to cover approximately 44% of the well-bore. There are vugs present at 4208.7 ft and at

4210.4 ft. This presentation is very effective for secondary porosity evaluation and for sedimentary structure

identification.

Image-Examiner Workstation

Image interpretation can be enhanced by means of a computer workstation equipped with image-examiner processing

programs. This allows such interactive processing features as

scale changes of both the vertical and horizontal, to enhance the interpretation

Figure 2


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a display of other logs for correlation on the same scales

graphic enhancement of specific features, such as bedding, texture, vugs, and fractures

dip computation of bedding surfaces, fault planes, and fractures

correlation of images to whole core sections, extending the interpretation to noncored sections

orientation of cores from features present in both the core and the formation images

quantification of images (such as sand count and calibration to core porosity) to increase

interpretation accuracy

Dip Computation/Thin Bed Definition Computation of the dip magnitude and azimuth of specific beds is essential to

many interpretations and can be performed on an image-examiner workstation. An example is shown in Figure 4 .

Figure 4

Figure 3


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The magnitude is measured from horizontal (0) to vertical (90). The azimuth of the downdip direction is measured from

true north. The thin sand shown at 6969 ft dips to the northwest. A sine wave is fit through both the upper and lower

surface of the sand, indicating a 39 dip magnitude and an azimuth of 317.These dips are "true dip", since hole deviation

is compensated. Apparent dips may be presented if a direct comparison with a whole core is required The actual thickness

of the sand stringer, measured be-the sine waves, is 1.61 ft.


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III Structural Dip Interpretation

1. Structural Dip Interpretation

Structural dip changes (and the lack of such changes) are good indicators of the type of structure present ( Figure 1 ). Thefollowing are guidelines for interpreting structure based on structural dip changes.

Figure 1

Structural dip decreases upward in structures uplifted contemporaneously with deposition. Constant dip over an interval

indicates postdepositional structural uplift. Structural trends that decrease to zero dip and reverse magnitude and azimuth

indicate structures with tilted axes. Deviated holes create the same effect by penetrating different parts of the structure

being explored.

Structural dip changes over short intervals indicate numerous faults. The beds between two faults only a few hundred feetapart commonly exhibit different dips from beds above and below the two bounding faults as a result of tilting.

If structural dip is changing rapidly in the horizontal direction, it is dangerous to extend the structural trends very far

horizon-tally. Only the geologist can decide how far the trend may be extended. When the dip of a structure is changing,

the feature interpreted as structural dip is the dip of a plane tangent to the mapping horizon.


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2. Salt Domes

Intrusive masses of salt form domelike features by penetrating overlying normally bedded sediments. Figure 1 is a sketch

of a typical salt dome.

Figure 1

A number of faults are present, most of which dip toward salt. Unconformities and pinchouts are common, as are steep

dips near the flanks of the salt dome. If the top of the dome is shallow enough, it may be overlain by caprock.

Not all domes resemble the one shown. Other features that lend themselves to dipmeter interpretation may be present;

these are presented on the following pages.

OverhangsFigure 2 illustrates a well that penetrated salt far below an overhang.
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Figure 3

Pre-Salt Uplift Growth Faults Another cause of dip into salt is the presence of a large pre-salt uplift growth fault. The dip

into the downthrown side of the growth fault can override any uplift-created dip away from salt. This feature occurs on

the south flank of the dome illustrated

Dipmeter Surveys

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