General Principles
Introduction
Since its introduction in the 1930s, the dipmeter tool has found steadily increasing application in the petroleum industry. Used initially in exploration, the tool helped to locate and identify the major features of geologic structure serving as oil traps. As techniques became more refined and interpretation became more secure, the dipmeter's range of applications expanded, making it the principal logging tool for describing internal lithologic features and the sedimentological processes responsible for them.
The current emphasis on investigating sedimentary bedding conditions has further enhanced the utility of the dipmeter log. The high sampling density of 120 readings per foot of borehole depth makes the dipmeter tool virtually the only logging device that can supply the petroleum geologist with detailed information on finestructured sedimentary beds in the subsurface.
The dipmeter tool measures conductivity or resistivity changes, hole size, and sonde orientation-nothing more, nothing less. It does not directly measure the dip of bed boundaries or the dip of lithology changes. The conductivity changes are input into a computer program that correlates the recorded wiggle traces and computes apparent dip from the correlations. Computed dips are then corrected for sonde tilt and converted into true dips. The true dips are plotted and used to make inferences of structural dips, bed geometries, and depositional environments.
Dips displayed on the tadpole or arrow plot result from a combination of the original depositional dips, differential compaction and structural rotation during subsidence, and postdepositional deformation.
AS is true with other logs, information other than that contained on the dipmeter log is required to make the best interpretation. The minimum required input from the geologist is to describe missing sections and depositional environments. The more information available, the better the dipmeter interpretation.
The dipmeter tool operates on the following principle. A bedding surface cutting across a borehole at some angle causes microresistivity changes to be recorded at different depths on the individual dipmeter curves, which are recorded from electrodes on pads located at various circumferential positions around the borehole. Figure 1 shows a borehole intersected by a steeply dipping, thin resistive bed. 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, or azimuth, 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 pad electrodes.
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 and will 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.
Four Curve Dipmeter Tool
Tools Available
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.
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 under most 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 case result 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 for purposes of dip computation that the instantaneous velocity of the tool be known throughout the logging run. A fifth electrode (known as the speed 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.
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-bearing measurement 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 of low-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.
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.
Dipmeter Applications
UNDER CONSTRUCTION …!
Structural Dip
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 of bedding 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 of valuable 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. Dip azimuth 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 features crossed 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 further field development.
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°). At low 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
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 structural trend. 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 of change 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.
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 the zones 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.
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
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 determining structural 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 drilled with mud weights that were too heavy exhibit the same damage pattern.
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 the electrodes 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 where the deviation is less than or equal to 70° (with the pad pressure control at its maximum). The centralization assures tangential contact between pads and the borehole wall, ensuring that the electrodes on the pad maintain good formation contact. The formation-imaging tool also uses this sonde design.
Figure 1 shows a comparison of the measuring electrodes on the 4-curve tool, the 8-curve tool, and the 2-pad and 4-pad formation-imaging tools. For the 8-curve tool there are two measure electrodes on each of the four pads. The short spacing (3 cm) between the side-by-side electrodes results in a better curve likeness than that from the pad-to-pad configuration. This enables a larger number of high-credibility correlations to be made, with the result that shorter correlation intervals can be used to measure displacements between the side-by-side curves while maintaining a sharp and unambiguous curve match. By using processing methods that exploit the improved data-collection capabilities of the 8-curve tool, a fine vertical resolution of dips is achieved.
The 2-pad formation-imaging pad has the two side-by-side electrodes, plus an array of 27 resistivity buttons for detailed formation scanning. The 4-pad version has 16 electrodes per pad.
With previous pad-to-pad configurations of the 4-pad device, the lower limit for meaningful interval correlations was on the order of one dip computation per foot. Using the side-by-side correlation technique, this can be reduced to about 3 in. under favorable conditions, thus enabling more information on sedimentological dips to be derived.
The mechanical inclinometer in the 4-curve tool has been replaced in the newer tools by a triaxial accelerometer and three magnetometers. The three-axis accelerometer is housed in a single unit. The Al, A2, and A3 axes correspond to Pad 1, Pad 2, and the tool axis direction, respectively. Accelerometer information is used to derive tool axis deviation and make speed corrections to the recorded curves. The magnetometer has a separate unit for each of the above axes. By measuring the direction of the earth's 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 account during 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.
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 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) of Figure 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. 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.
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-based correlation 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 in 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 the analyst's 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 at the 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 other and agree 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.
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 or systems 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.
Data Presentation
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 plot is 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.
Listing In 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 column for trends in dip direction with depth. Conventional histograms of both dip and dip azimuth can also be presented ( Figure 2 ).
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 lowest dips becomes more apparent. This type of plot, popular for picking structural dip, is illustrated by Figure 3 .
Stick plots ( Figure 4 and Figure 5 ) show a series of short lines inclined to the horizontal. 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 the geologist's mapping requirements. 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 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.
Dipmeter plots may be interpreted by observing the variation of dip and dip azimuth with depth in conjunction with the openhole 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.
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 or tadpole plot. Each tadpole consists of a dot with an attached tail. In Figure 8 the position of the top dot shows a dip magnitude of 20°. 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. No tadpoles, or blank 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 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.
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, the sequence 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:
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
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.
Interpretation
1. Folded Structure. Figure 1 shows a folded structure. Note that in the shallow part of the well, dips are moderate and to the north.
In the deeper section, the well has crossed the axial plane of the fold and dips are more pronounced and to the south. At the point the well crosses the axial plane, dips are flat. It is here that a hydrocarbon trap exists. From the dips on the flanks it is possible to compute both the tilt of the axial plane and the plunge of the fold.
2. Unconformity. Figure 2 illustrates an unconformity. A series of sediments in the deeper part of the well was originally deposited flat. Thereafter, these sediments were tilted and then eroded and a new set of beds deposited. At the interface between the old and new sediments, there is an abrupt change of dip.
3. Faults. Faults may be picked from dip patterns by observing the drag patterns, if present, on either side of a fault. Figure 3 shows a normal fault with drag. Above the intersection of the wellbore with the fault, a red pattern will develop (dip increasing with depth). Below the intersection of the wellbore with the fault, a blue pattern will develop (decreasing dip with depth). At the intersection of the wellbore with the fault plane, the dip of the fault plane itself may be seen occasionally. Note that the fault dips down in the direction of the azimuth of the drag pattern. It thus strikes perpendicular to that direction.
4. Current Bedding. Among the sedimentary details that may be inferred from a dipmeter plot is the direction of transportation of sediments by streams. Figure 4 shows the sort of pattern to be expected in such a case. Here, blue pat- terns develop with the dip azimuth in the patterns pointing downstream. Depending on where the well is drilled, it may be of interest to move upstream toward the source or down- stream to finer sediments or broader deposits.
5. Channel Cut and Fill. A common type of deposit results when a channel is cut and refilled with reservoir sand. A red pattern will develop together with a characteristic Sp shape, broadening to the base. In drilling such plays, it is useful to know in which direction the channel extends and in which direction it thickens.
Note that the well in Figure 5 was drilled off the axis of the channel. Had it been drilled to the north, a thicker section of sand would have been found. To move to the center of a channel, therefore, offset the well in the same direction that the red pattern tadpoles point. To follow the channel, move at right angles to the red pattern dip azimuth, in this case either east or west.
6. Buried Bar with Shale Drape. Another common feature is a buried bar over which subsequent shale deposits have been draped. Here, dips within the sand body decrease with depth (blue), but, above the sand body, dips in the shale increase with depth (red) ( Figure 6 ). The SP usually shows a characteristic pattern, broad at the top. To follow the bar, wells should be offset at right angles to the dip azimuth seen within the bar. To drill a thicker section, a well should be offset in the opposite direction to the dip seen in the bar.
Another application of the dipmeter survey is the detection of fractures. There are many methods available for fracture detection, but no single method by itself is completely reliable. The use of the dipmeter for fracture finding, then, is just one of many methods, and should be used to complement the others.
The theory is very simple. A fracture may be invaded with mud filtrate and therefore offer a less resistive path to electric current. If one of the dipmeter pads happens to lie in front of a fracture, it will record a low resistivity value. Another pad at the same depth may not be in front of a fracture and will record a higher resistivity. Thus, comparison of adjacent pad traces should reveal the presence of a fracture if the two resistivity values are different.
Curves can be displayed in various ways to highlight such differences. Figure 7 shows one such presentation. Note that since the orientation of the dipmeter tool is known, the orientation of the fracture can be deduced.
Good dip information requires good raw data. To ensure such data the following guidelines are suggested:
- Recondition the hole prior to running the dipmeter.
Use a swivel-head adapter to reduce tool rotation while logging.
Log at 1800 to 2400 ft/hour to reduce tool jerking. Slow down even more if the tension on the line is erratic.
Reject sections of log where the tool rotates once in less than 60 ft of hole.
Make repeat sections and/or overlaps of 100 ft to 200 ft every time the logging is stopped for film or tape changes.
Inspect the raw log for dead correlation curves, insensitive curves, stuck calipers, etc. As a last resort, three good correlation curves are sufficient, but four are much better.
Carefully inspect the orientation curves for nonsense readings, such as a hole deviation less than zero.
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