FROM NON-ECONOMIC INTO PRODUCING FIELD, A CASE STUDY IN KETALING BARAT FIELD, INDONESIA

Convention Bandung 2004 (CB2004)
The 33rd Annual Convention & Exhibition 2004

Indonesian Association of Geologist
Horizon Hotel, 29-30 Nov, 1 Oct 2004, Bandung

FROM NON-ECONOMIC INTO PRODUCING FIELD,
A CASE STUDY IN KETALING BARAT FIELD,
INDONESIA

Bob W.H. Adibrata(1), Y. Hirosiadi(2), E. Septama(3), A. Rachmanto(4)

1bobwikan@pertamina.com, 2yosihiro@pertamina.com, Geology Section, Technology Support Division,
Pertamina Upstream, Kwarnas Bld 15th Fl, Jl. Medan Merdeka Timur No. 6, Jakarta 10110 INDONESIA
3erlangga@pertamina-sumbagteng.com, Exploitation Section, Pertamina DOH Sumbagteng, Bajubang, Jambi
36611, INDONESIA
4ambar@pertamina-dohsbs.com, Recent address: Exploitation Section, Pertamina DOH Sumbagsel, Prabumulih,
Sumatera Selatan, INDONESIA

Abstract

Focus of this study is re-activity of a non-economic field into production by combining
old vintage 2D seismic data with current 3D seismic data, supporting with archival,
conventional log data and limited sidewall core and thin section analysis. The
reservoir consists of bioclastic wackestone overlying by coral bindstone in the Upper
Miocene Equivalent of Baturaja Formation, at Ketaling Barat field, Jambi, Indonesia.
The objective of this study is to evaluate and test a multiple attribute analysis
whereby carbonate facies can be determined and to characterize the distribution of
potential carbonate reservoir.

Introduction

Ketaling Barat field is located 5 km East of Jambi, Jambi Province, Indonesia (Figure
1). Activities in this field started in 1959 by Dutch’s NIAM N.V., by drilling 3 wells,
Ketaling 1, 2 and 3, which mainly based on geological field work in the surrounding
area. From those three wells, only Ketaling-2 gave respond with gross of 220 bbl fluid
per day (95% water). With the unsatisfactory result, this field was then abandoned as
a non-economic field for about 40 years. The first 2D seismic data acquisition taken
in 1982, and the vintage set was then re-processed in 2000 and re-interpretation was
held during the same year, with the result of Ketaling Barat (KTB)-04 that has been
drilled in 2001. The appraisal well of KTB-04 gave a significant result of 3600 BOPD,
with no water. Lithology variation between KTB-04 and previous wells, confer the
idea that there are two different stages in carbonate development in this area,
defined as Phase-1 and Phase-2. Phase-1 is platform carbonate where oil produced
from KTB-04, and Phase-2 is reefal carbonate, where oil from Ketaling-2 came from.
Four other wells has been drilled during 2001-2002 period, KTB-05 (Phase-1
reservoir, 1.5 MMSCF), KTB-06 (Phase-2 reservoir, 420 BOPD, 70 % water, 0.5
MMSCF), KTB-07 (Dry hole), KTB-08 (Phase-1 reservoir, Oil show, Technical
problem, P&A). Highly variation in result which represents reservoir heterogeneity,
lead to a mini pilot project (2 x 4 sq. km) of 3D seismic acquisition and processing
that was conducted in 2003 to enhance reservoir characterization.

Improved interpretation has achieved using 3D data volume. The Equivalent of
Baturaja Formation can be determined more clearly into two different stages. The two
stages, Phase-1 and Phase-2 has been mapped respectively. Phase-1 developed as
an isolated platform directly on top of basement, controlled by normal fault, spread out throughout the area with average thickness of 35 meter. Phase-2 developed as
reefal build-up facies, distributed mostly in the center of Ketaling Barat field, with
average thickness of 45 meter. Paleomorphology also worked as the main control on
carbonate development and distribution in the area.

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Dipmeter Surveys (Reef Interpretation )

Reef Interpretation

Introduction

Buried topography may significantly influence the thickness, sedimentation, and dip attitude of beds overlying topographic features. One of the first stratigraphic applications of dipmeter data was to determine the positions of wells drilled on buried topographic features. The factor contributing to the interpretation of these situations is the drape of beds over the underlying buried topography. Although this chapter deals primarily with reef interpretation, many of its basic principles apply to buried ridges, knobs, and depressions covered in the next chapter. The differences between them lie in the application of the interpretation.

The dip of reef surfaces is interpreted from the drape of sediments over the reef, particularly where the reef underwent considerable vertical growth. Dipmeter data obtained very near the reef or on the reef slope exhibit dip anomalies that help to describe the reef slope. The magnitude of the dip at any point above a reef varies depending on the following:

the slope of the reef surface

the height of the reef above the surrounding platform

the distance of the point above the reef surface

the type of rock above the reef

the total historical overburden

the position over the reef at which the measurements were taken (crest, flank, or toe)

Figure 1 shows the cross section of a barrier reef complex.

Dipmeter interpretation will be described for wells drilled

in pinnacle reefs overlain by shale

in pinnacle reefs overlain by low-compactibility formations

on the forereef slope

on the crest of the barrier complex

 

The dip patterns may be somewhat different in each of these cases because of the relief of the feature as well as the lithology of the enclosing formations.

Well-correlation and dip data have shown that the slopes on reef flanks can vary from 2° or 3° to as high as 45°. As evidenced by current reefs, higher slopes are possible, but are not generally observed in the subsurface. It is possible that erosion of steep and irregular slopes prior to burial produced reef flanks of reduced dip angle. Large accumulations of reef talus material near some reefs may support this premise.

Of the factors influencing the dip above a reef, the most important are reef topography and the compactibility of overlying beds. If all dip patterns above reefs conformed to the same model, interpretation would be straightforward. Unfortunately, identical dip patterns may imply quite different reef slopes in different environments.

Figure 2 is a photograph of a pinnacle reef in a Cambrian zone in West Texas. Some significant features show in the enclosing beds. Note the drape of the overlying beds in the flank position with the direction of dip sharply away from the reef mass.

 

It is useful to note that subsidence of the mass into the underlying platform has caused dip below the reef to be toward the reef mass itself.

Finally, within the reef there is generally a lack of distinct stratification.

Comments on Reef Interpretation

The tectonic and geological history of an area generally determines a maximum height of buried topography above a reference datum. This is particularly true of reefs, because the controlling conditions for vertical accumulation may prevail over a large portion of a basin.

Further, unfavorable conditions cause cessation of growth over large areas, and the limited vertical height becomes common to many reefs simultaneously. Therefore, a maximum expected height of reef crest above a datum may be well established.

Reef falls, or pinnacles that for whatever environmental reason ceased growth early, may have any thickness less than maximum. The length (and shape) of the slope pattern in wells drilled on the flank position may be interpreted qualitatively as a guide to the vertical size of the reef. "Typical" dipmeter patterns for the area are very useful in this respect.

The accuracy of interpretations discussed in this chapter depend largely upon knowledge of sediment compaction around reefs and other buried topography. The relatively simple procedures employed to estimate compaction may not apply directly to all areas.

Extrapolation of dip assuming a linear slope may be used best where experience with seismic and well-to-well correlations support this approach. Extrapolating updip beyond the elevation geologically possible or likely for a particular feature could be misleading and expensive. The basic concept should prevail, however, and persons employing these basics in their analyses are advised to consider all available data and experience.

Techniques

Reef Interpretation

The pattern of drape over a reef may be determined by a number of factors, including

• compaction
  

compaction with deposition

solution of surrounding salts

solution with deposition of overlying sediments

gypsum to anhydrite conversion

combinations of the above

Compaction

In most circumstances, compaction plays the key role in causing drape over buried topography. It is useful therefore to refer to a model to understand and interpret the dip patterns.

The simplest model, and one with good independent support from well correlation, is illustrated in Figure 1 , which shows a simple reef mass of constant slope of 30° surrounded by shale.

 

If we know the compaction factor of the shale, and we assume that all shale compaction occurred after deposition and that the reef is rigid, then we can accurately calculate the present attitude of the shale bedding plane.

For this model it is assumed that present shale thickness is 50% of the original precompaction thickness; therefore, the resulting compaction factor is 0.5.

From these assumptions we can conclude that the dip of the shale bedding plane is the angle of the tangent, which is 50% of the tangent of the reef angle.

The equation is

tan^-1(0.5 tan 30°) = shale dip = 16.1°

The general equation is

shale dip = tan^-1 [(1 - C) tan reef dip]

where:

,

and

1 - C = compaction

Solving for the reef dip, which cannot be directly measured, we have

reef dip = tan^-1 

reef dip = tan^-1 

In this example, with a measured shale dip of 16.1°, we can calculate the reef dip to be 30°. Considering the range of local changes in compaction due to lithological changes, locally changing reef slope, or the fact that compaction may not be totally postdepositional, a simple solution is to divide the shale dip by an estimate of compaction.

reef dip =  = 32°

As shale-dip values increase, as in the case of very steep-sided reefs, the simplified solution becomes less accurate and the general equation should be used; however, slumping, fracturing, and sliding may render interpretation more difficult and the precision of reef slope less significant.

In the previous simplistic model where all compaction was assumed to occur after deposition, the theoretical dip pattern would be a constant 16.1°. Where beds are now essentially parallel, they may be considered to have been paralleled during deposition and therefore equally compacted. This model is applicable in these cases.

In general, however, the drape of beds over a reef produces a red pattern on the dipmeter plot if the well is drilled in the flank position. The existence of the red dip patterns implies that compaction cannot be assumed to be postdepositional except over limited intervals where the dip magnitude is relatively constant. In this case, the compaction may only be invisible over short intervals because of the low rate of change of dip with depth. Provided other factors remain constant, reefs with large relief tend to produce long red patterns above; those with low relief produce shorter patterns. In any case, it is the analysis of the red pattern that allows us to calculate the reef slope.

Estimating Height of Nearby Reefs

Two basic dip patterns have been observed in shale-enclosed reefs. The first is a long, slowly increasing red pattern. The dip of the shale above the reef is some fraction of the reef dip, and the reef dip is estimated using the earlier derived equation. The interval from 1315 to 1365 m shows little dip change, which indicates that it was deposited prior to most of the compaction process. This zone is therefore a candidate for the simple model approach.

The second dip pattern observed in shale-enclosed reefs is characteristic of steep-sided, probably curved surfaces of high-relief pinnacles ( Figure 2 ).

 

Statistically, this pattern has two sections. The lower section is characterized by a sharp slope pattern immediately above the reef. The upper section is a long and slowly decreasing slope pattern. Where the sharp slope pattern and the slowly decreasing dip pattern join is the approximate height of the nearby reef.

Estimating the Reef Slope

The reef slope should be estimated using two methods. First, applying your knowledge of compaction to the upper dip section, calculate the dip. Second, extrapolate the lower red dip pattern to the reef surface. This dip is a good estimate of the reef dip at the contact, but it may not persist over long horizontal distances. If the two dips agree within a few degrees, confidence in the answer is high. If the two dips do not agree within a few degrees, they at least establish a range of possible dip. Knowledge of the seismically defined size and shape should be integrated into the final solution.

Where overlying beds are of low compaction, the overlying dip also is less. For example, a formation with compaction of 20% and a dip over the reef of 4° would imply a reef dip of approximately 20°. In this case the red pattern would not be nearly as striking as in the previous examples.

Some interpreters may be tempted to find the exact depth of the contact, and they may seek a particular tadpole to define the dip of the surface. This approach can give quite erroneous results, because of the local irregularities existing on any weathered surface. These irregularities may be of a size on the order of the borehole diameter.

In this case the dip at the contact would be entirely misleading, and the general dip trend is better defined from dips sufficiently above the surface, because the small features would have been compensated by sedimentation and compaction.

As a general rule, dip patterns should be extrapolated horizontally in the same order as the vertical length of the pattern. This implies that any single-dip tadpole should not be extrapolated beyond the borehole.

Reef Detrital Material

Dip within a reef is not generally very well ordered. There is one exception, however: reef detrital material.

Reef detrital material is often found in accumulations near the base of the reef, and dip patterns within this material may have the appearance of foreset bedding.

This bedding may have dips greater than the reef dip, but the direction should be generally downslope. This information is particularly useful where it supports draping dip in overlying beds and in situations of low reef dip or low compaction.

The only indication of this detrital material may be from the dip-meter plot, as there is little mineralogical distinction between detritus and the main reef mass. This information may be significant when estimating the depth of the reef top, particularly where the detrital section is quite thin.

Reefs Surrounded by Salt

Reefs surrounded by salt are not likely to exhibit strong dips in the overlying sediments. If salt solution occurs simultaneously with or subsequent to deposition of these sediments, a dip pattern is produced. This pattern is determined by the rate and timing of the removal of salt. If salt removal commenced after deposition of some of the overlying beds, then these beds would have collapsed to more or less conform to the reef surface. Their dip would then be equal to the reef dip, and a pattern of essentially constant magnitude would be formed. This is illustrated in interval A in Figure 3 .

 

If the beds of interval B were being deposited during the removal of salt, a red pattern over that interval would be produced on the dipmeter plot. Interval B could be expected to thicken in the downdip direction.

The dip pattern in the figure would be further modified by compaction during and after deposition, but the basic pattern would be recognizable. Reef dip in this case would be approximately equal to or slightly greater than the constant dip value of interval A.

If salt removal occurred contemporaneous with the deposition of some of the overlying beds, most of the interval above the reef would exhibit a red pattern, as illustrated in Figure 4 . The dip of the reef would approximately equal the trend of the red pattern extrapolated to the reef contact. Extrapolation of red patterns of drape over reef or weathered surfaces is necessary, because dips near the surfaces may be difficult to ascertain due to bedding destruction by fractures, slump, or sliding and local irregularities of the surface.

 

Dipmeter Interpretation

Figure 5 illustrates the dipmeter pattern of a well drilled in the flank position of a reef where salt removal around the reef played a significant role in the final dips of the overlying beds. The pattern may be analyzed as follows:

 

 

Interval A contains a section of relatively constant dip with an average value of 15° to 18° east. This interval was probably deposited prior to salt removal, and it represents the minimum dip of the reef.

Interval B contains a red pattern that indicates the period of salt removal.

Interval C contains a long, gentle red pattern that finally disappears well above the top of the figure. This pattern is probably a reflection of compaction during deposition, and it would be superimposed on the patterns of intervals A and B.

Because regional dip in this area is less than 1° to the southwest, the consistent east dip above 3900 ft is interpreted as part of the overall drape on the reef. The long drape feature, over 1000 ft in length, suggests that the reef feature is not small.

Based on the seismic interpretation and the dip data, it was decided to whipstock the well to contact the reef 300 ft to the west. The result was to gain 90 ft of elevation on the reef. A straightline correlation between the two contacts implies a reef dip of 16-1/2°, approximately the mean value of the tadpoles in interval A.

Exercise 1.

Figure 1 shows a Devonian reef which has a long, linear slope in this area. Regional dip is 1° SW.

Compaction is 40 to 50%.

What is the reef dip?

What is the direction of dip?

How far and in what direction must an offset be drilled to gain maximum reef?

 

Solution 1:

The long red dip pattern terminates at 6° just above the reef talus interval and would extrapolate to the reef surface at about that angle.

If compaction = 50%,

reef dip 

If compaction = 40%,

reef dip 

Direction of dip = NE.

Additional reef available is 190 ft.

If dip = 12°, go 890 ft.

If dip = 15°, go 710 ft.

Calculated from offset = 

As shown in the accompanying figure, the actual offset well gained 160 ft in 700 ft offset from a reef dip of 13°. Compaction of the shale calculates to be 40% from the data in these wells.

 

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Dipmeter Surveys (Fault Interpretation)

Fault Interpretation

Introduction

Faulting occurs when beds are in tension or under compression. Such forces produce normal faults and reverse or thrust faults. In areas that have undergone mainly tension (such as the northern Gulf of Mexico), almost all of the faulting is normal. In areas that have undergone both earlier tension and later compression, both normal and reverse/thrust faults may be present in the same well. For dipmeter interpretation, input of the local geology is required to define the actual model.

In order for a fault to be detected by the dipmeter, either some sort of distortion must be present near the fault plane or one fault block must be tilted more than the other. When tilting is present, the location of the fault is indicated by a sudden change in dip magnitude and/or direction.

Typical forms of distortion near both tensional and compressional faults are shown in Figure 1 . Each of these is covered in this section. Normal faulting (beds in tension) is discussed first and reverse/thrust faulting (beds under compression) last.

 

Growth Faults

Faults that were active during the time of deposition are called growth faults. The downward-moving block provided a low area that acted as a sediment sink and accumulated thicker layers of sediments than the equivalent upthrown zone. Tension-created slumping into the downthrown side of the fault also aided the downthrown thickening processes. Downthrown thickening, which begins some distance from the fault, increases toward the fault plane, with the maximum amount of thickening found immediately downthrown. This thickening into the fault, plus sinking of the increasingly heavier downthrown side of the fault, produces a rotation effect that also increases the dip of the beds into the fault.

Rollover

The cumulative effect of downthrown thickening, slumping, and rotation, which is called rollover, produces a trend of downward-increasing dips that dip toward the upthrown fault block ( Figure 1 ). This trend terminates at, or shallower than, the fault plane. It is this dip trend that allows growth faults to be located, and their attributes identified, by the dip-meter tool. The downward-increasing dip trend produces a red dip pattern whose azimuth is toward the upthrown fault block and normal to the strike of the fault. Although not routinely found associated with growth faults, strike slip rotates the azimuth in a opposite to that of block movement. The vertical extent of the red pattern can be used as an indicator of the minimum displacement of the fault. Displacement is usually greater than the vertical extent of the fault; it is rarely less.

 

Subsidence Effect

When the rate of deposition is greater than the rate of subsidence, a system of progressively younger faults in a seaward direction is created. When the rate of deposition equals the rate of subsidence, a fault with a very large displacement is produced, assuming of course that the system is stable for a considerable period of time. When the rate of deposition is less than the rate of subsidence, progressively younger faults are created in a landward direction.

Bed Thickness

Figure 2 is a cross section illustrating the effect of a growth fault on bed thickness. Wells 1-3 penetrated the upthrown block of a down-to-the-east growth fault. Both sands A and B are the same thickness in both wells. Part of sand A is faulted out in Well 4, while sand B, which is still located upthrown, remains the same thickness. Well 5 penetrated sand A in a downthrown position in the rollover zone, so the sand is much thicker than its upthrown equivalent. Sand B was faulted out of Well 5. Well 6 penetrated both sands in a downthrown position within the rollover zone, so they are thicker than their upthrown equivalents. The downthrown thickening continues to decrease to the east until Well 9 is reached. This well is located beyond the eastern limit of the rollover zone, so both sands are the same thickness as their upthrown equivalents.

 

Growth Fault Examples

Figure 3 is an example of a large growth fault which cuts the Vicksburg formation of a South Texas well. The fault, which cuts the well at a depth of 14,890 ft, is downthrown, or dips, to the southwest. Therefore the rollover zone, which dips toward the upthrown block, dips to the northeast. The rollover zone (the zone that creates a downward-increasing dip trend) extends upward to 13,750 ft; the minimum displacement of the fault is approximately 1000 ft.

 

Figure 4 is an example of an offshore Louisiana fault whose displacement is smaller than that of the fault in Figure 3 . This offshore Louisiana Miocene example illustrates the rollover created by a small growth fault. The fault, which by correlation has a displacement of only 120 ft, is located at 12,638 ft and is downthrown, or dips, to the south-southeast and strikes normal to the pattern dip direction, or east-northeast west-southwest. Because of shattering of sediments near the fault plane, only a scattering of dips were recorded immediately downthrown.

 

Some fault examples show even more extensive shattering and washing out of the hole on the downthrown (most active) side of the fault plane. The dip trend which begins at the base of the blank zone is recorded from the upthrown block, so the fault cut is no deeper than the bottom of the blank zone. Usually it is picked at the base of the blank zone.

One of the "eyeball" indicators of a possible missing section, which is present in these examples, is a borehole dogleg. Any time the bit crosses a formation compaction change it reacts by creating a change in the amount and/or direction of well drift. Since compaction changes are almost always present across a fault or unconformity, a change in well drift azimuth and/or magnitude can (but does not always) indicate the presence of a fault or unconformity.

As long as a fault continues to grow, downthrown thickening is produced. This in turn produces an increasing-with-depth red dip pattern. During periods of relative nongrowth resulting from changes in depositional processes, the beds are rotated at a constant rate. This in turn produces constant dip zones within rollover-created red dip patterns. Figure 5 is an example of such a fault. The increasing-with-depth rollover zone begins at about 8700 ft. From 9100 to 9250 ft the dip trend remains constant. From 9250 down to 9350 ft the dip again increases downward, indicating a period of renewed growth.

Structural Dip Imprint

The dips from a zone of distortion are changed when structural dip is imprinted on them, so it may be necessary to remove structural dip before determining the attributes of a fault, just as it is necessary when making stratigraphic interpretations.

Figure 6 is a theoretical example of the appearance of a dip-meter plot when structural dip is imprinted over dips created by rollover. (A) shows a west-dipping red dip pattern created by rollover into a down-to-the-east growth fault. (B) shows that a moderate amount of east structural dip was added to the west dipping red pattern of (A). The resultant dip pattern, moving down the hole, is a decreasing dip trend or blue pattern. The dip decreases to zero, then increases in the opposite direction until a maximum is reached at the fault. A typical red pattern is formed below the zero crossing point. Below the fault cut, only east structural dip is seen.

(C) illustrates an even stronger east structural dip imprinted over the west-dipping red pattern. At the point where the west-dipping structural dip and the strong east structural dip start to oppose each other, a decreasing dip trend, or blue pattern, begins. In this case, the trend decreases down to the fault cut but never quite reaches zero dip. As soon as the fault is crossed, only strong east structural dip is recorded.

 

Figure 7 is a dipmeter example of the first type of imprint, where a strong red pattern opposes moderate or low structural dip in the opposite direction. Structural dip is about 10° south-southeast. This opposes the northwesterly dipping red pattern dipping into a down-to-the-southeast growth fault. The resulting dip patterns are ones of decreasing dips (blue pattern) from 13,700 ft down to the zero crossing point at 13,790 ft, then ones of increasing dips (red pattern) in the opposite direction down to the fault cut at the base of the blank zone at 13,880 ft.

 

Deviated Wells

Deviated wells are sometimes drilled parallel to fault planes. As a well periodically gets closer to the fault and, in some instances actually bumps the fault, the dips increase and then decrease. Some dip scatter is created by formation shattering and hole conditions near the fault.

Platform wells may be deviated in a direction and an angle such that they cross normal faults from the upthrown to the downthrown sides instead of in the usual manner.

Postdepositional Precompacted Faults (No Distortion)

Normal faults that occur after deposition but before formation compaction usually exhibit no distortion near the fault plane. Such faults can be recognized on the dipmeter plot only if a change in structural dip occurs across the fault. Because there is a change in the degree of formation compaction, the borehole doglegs even though there is no distortion of the beds near such a fault.

The sudden downward decrease of structural dip is one of the "eyeball" indicators used to differentiate between faults and unconformities. Most of the time, in areas that have not undergone strong tectonic deformation, the downward decrease indicates faulting. In order to have lower dip below an unconformity, two different centers of uplift are required.

Instead of downthrown rotation, some faulted areas have undergone rotation of the upthrown fault block. This creates a sudden structural dip increase in a downward direction. This is the same dip pattern created by the presence of an angular unconformity or a rapid, postdepositional structural uplift. Therefore, other information is needed to determine which of the three features is present when a sudden downward increase in structural dip is noted on the dipmeter plot.

A lack of distortion near a fault plane can occur with both tensional and compressional faults. Therefore, unless there is a structural dip change at the fault cut, faults of this class cannot be seen on dipmeter plots.

Postcompaction Faulting (Drag)

Normal faulting that takes place after some degree of deformation has occurred usually develops drag, or beds dipping in the same direction as the fault, near the fault plane. In some areas, drag is found only on the downthrown side of the fault; in others, drag may be found in beds on both the downthrown and upthrown sides.

Since the relative motion of the upthrown and downthrown fault blocks creates a drag zone whose dip is in the same direction as that of the fault plane, the maximum dip of the resulting red dip pattern may be used as a minimum dip of the fault plane. Local experience is used to determine whether or not the maximum dip of a drag-generated red pattern is in fact a reasonable value for the dip of a fault plane. Figure 1 is a theoretical example of a normal fault with drag only on the downthrown side. The red pattern, which was generated by the downthrown drag zone, dips in the same direction as the fault and normal to the strike of the fault. The maximum dip of the pattern may be used as the minimum dip of the fault plane. As happens with a growth fault, the hole doglegs within a hundred feet or so of the depth at which the fault cuts the well. Since drag is present only on the downthrown side, structural dip is recorded on the upthrown side of the fault.

 

Figure 2 illustrates a normal fault with drag in both the upthrown and downthrown beds adjacent to the fault plane. The dip in the downthrown block created a dip pattern similar to the one in the previous example. However, drag, which is also present in the upthrown beds, creates a pattern of downward decreasing dips or a blue pattern. The fault cuts the well at the junction of the two patterns. Once again, the maximum dip of the red pattern may be used as a minimum dip of the fault plane.

The amount of rollover present on the downthrown side of a nonburied growth fault decreases upward. It disappears at a point corresponding to the time at which the fault ceased to be active. The amount of drag created by any period of movement remains relatively constant over the entire interval. The termination point may be a point corresponding to the end of the active faulting period, or, if buried, to an unconformity.

Faults with Hybrid Dip Patterns near the Fault Plane

Some faults begin as growth faults with downthrown rollover zones. Either continued movement along the fault plane or movement that began after compaction occurred then created a downthrown drag zone. Since rollover and drag-generated dips oppose each other, dip patterns like those illustrated in Figure 1 are created by continuing or later fault movement. A red, or downward-increasing, dip pattern begins at the point at which the hole penetrated the rollover zone. The dips increase down to the point at which the drag-zone dips begin to oppose the rollover dips. The trend then decreases downward to the zero crossing point. Below that point, another red pattern dipping in the same direction is formed. The dips continue to increase in magnitude down to the fault cut. On the upthrown side of the fault the dips may return immediately to a structural trend, or indicate an upthrown drag zone.

 

Buried Faults

Growth faults die out gradually in an upward direction. Postdepositional faults may extend to the surface, where they create cliff-like scarps, or they may end abruptly at an erosional surface. Such faults are called buried faults.

In addition to ending abruptly at an unconformity, disconformity, or diastem, buried faults may change displacement across deeper unconformities. The buried-fault creation process is illustrated in Figure 1 . First, a fault extending to the surface is formed. Later, erosion removes the elevated portion of the upthrown block; the land surface is once again level across the fault zone.

 

The original amount of uplift is labeled a. Still later, deposition begins again, and horizontal sediment layers are deposited above the erosional surface. Subsequently, movement again occurs along the fault plane. This movement creates a displacement labeled b. The displacement of the beds below the unconformity is now a + b.

Erosion has removed the beds that were originally displaced by amount a. Therefore, only displacement b extends across the unconformity on the upthrown side. If erosion occurs again, the beds that were uplifted above the surrounding land surface will be eroded to a flat surface.

The displacement below the shallowest unconformity is equal to b. The displacement below the deepest or oldest unconformity equals a + b. This cycle may be repeated.

Figure 2 illustrates a buried fault example from eastern Venezuela. A down-to-the northwest growth fault is located at 3842 ft. The fault terminates at a depth of 3760 ft, which is the unconformity separating the Paleozoic from the Lower Cretaceous.

 

This growth fault was originally active in Paleozoic time. Any scarp that existed was eroded before Lower Cretaceous sediments were deposited above the unconformity. No subsequent movement occurred along the fault plane.

Since both types of distortion (rollover and drag) commonly found near normal faults create similar dip patterns, some local knowledge is useful in determining which type to use when making an interpretation. In any given area one type of distortion is found near most of the area faults. For example, in the northern Gulf of Mexico and in Nigeria, rollover is the dominant distortion type. In Mississippi and North Louisiana, drag is most often found on the downthrown side of normal faults.

Here are some rules of thumb for determining the type of distortion present near a normal fault:

If the vertical extent of the downthrown mega-red dip pattern is more than 200 ft, rollover is assumed. Normal fault drag rarely extends vertically more than 200 ft.

When the vertical extent of the mega-red dip pattern is less than 50 ft, drag is assumed. In areas where rollover dominates, this assumption can lead to incorrect interpretation about 30% of the time, since many small growth faults do exist. When the vertical extent of the mega-red pattern is between 50 and 200 ft, use the dominant type of distortion known to exist in the area.

Deviated wells may cause the extent of the dip pattern to be expanded by 50% or more. Hole deviation must be taken into account when using the vertical extent of a mega-red pattern as input into one of the rules of thumb.

Semicontemporaneous antithetic fault systems that help accommodate rotation are often found associated with growth fault systems. These faults usually exhibit downthrown drag that creates red dip patterns dipping toward their downthrown blocks. The dipmeter example in Figure 3 shows three such faults. These faults are down-to-the-northwest so the northwest dipping red dip patterns found on their downthrown sides are the result of drag rather than rollover. These antithetic faults are dipping into a large down-to-the-southeast growth fault which is below the total depth of this well.

 

Compressional Faults

Faults that result from compressional forces may, depending on the fault angle, be called reverse or thrust faults. The fault angle of reverse faulting is 45° or more, while thrust fault angles are less than 45°

The main form of distortion found near a reverse or thrust fault is drag on both sides of the fault. Drag, which is the result of movement of compacted beds, may be additionally modified by horizontal movement, or strike slip.

The compressional fault attributes that may be available from dip-meter plots are depth, strike, direction of overthrust, and fault angle. Figure 1 illustrates the expected dip patterns near compressional faults. Such faults commonly show up very well on dipmeter plots. A mega-red dip pattern is usually found in the overthrust block. Its azimuth is in the direction of overthrust, assuming, of course, that no strike-slip has occurred. The downthrown pattern is one of downward-decreasing dips, or a blue pattern. These dips also point in the direction of overthrust. Both dip patterns are the result of drag on both sides of the fault. The fault is located at the junction of the red and blue patterns.

Figure 2 is a dipmeter example from western Venezuela showing a reverse fault. Structural dip above 7800 ft is 12° southeast. From 7800 to 8450 ft westerly dipping beds in the overthrust drag zone oppose the southeast structural dip. This produces a decreasing-with-depth pattern down to the zero crossing point at 8030 ft; the pattern then increases downward to the fault cut. The dip azimuth reverses across the zero crossing point.

 

The maximum dip of this example is recorded at the fault. The blue pattern generated by the downthrown drag zone decreases rapidly. The dip patterns near major compressional faults are rarely symmetrical. The overthrust pattern usually has the greatest vertical extent. If the displacement is small (i.e., less than 100 ft), the red and blue dip patterns tend to be more nearly symmetrical.

The dip direction on both sides of the fault is the same as the direction of overthrust, which is to the west in this example. The strike of the fault, north-south, is normal to the direction of dip patterns.

In this example, the direction of structural dip in the overthrust block and the direction of overthrust are opposite, so the dip trend decreases to the zero crossing point and then increases in the opposite direction. Had the direction of structural dip and of overthrust been the same, the dip trend would have continued to increase in the overthrust drag zone. Horizontal movement of one block relative to the other (strike-slip) may also occur. The drag-created dip patterns, which dip in the direction of over-thrust, would be modified by any horizontal movement. When such movement occurs, the drag dip patterns are rotated in the trailing direction, which is opposite to the direction of movement, and so no longer indicate the direction of overthrust.

Both compressional faults and overturned folds create repeat sections on logs. If the repeat is right side up, it is the result of faulting. If one repeat is upside down or a mirror-image of the other, it is the result of folding.

Thrust Fault

UNDER CONSTRUCTION … !

Exercise 1.

See Figure 1 .

 

There is a missing section in this well between 14,000 and 14,100 ft.

Where is the dip change that indicates the location of the fault?

Where is the top of the rollover zone or top of the mega-red dip pattern?

What indicator suggests the presence of rollover rather than drag?

What is the minimum displacement of this growth fault?

In what direction is the fault downthrown?

What is the strike?

Is a dogleg present?

Solution 1:

The dip change corresponding to the base of the rollover zone is at 14,049 ft.

The top of the mega-red dip pattern, which corresponds to the top of the rollover zone, is at 13,680 ft.

The vertical extent of the red pattern is more than 200 ft; therefore, rollover rather than drag is present.

The minimum displacement of the fault, which is equal to the extent of the red pattern, is 370 ft.

The rollover zone dips into the fault; therefore, the fault is downthrown to the south and strike is EW.

There is a dogleg. The hole is vertical at 13,000 ft and drifts to the SE a maximum of 3-1/2° at 14,000 ft. The hole then begins to straighten.

 

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Dipmeter Surveys (Structural Dip Interpretation )

Structural Dip Interpretation

Structural Dip Interpretation

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

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 feet apart 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.

 

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. 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.

Overhangs Figure 2 illustrates a well that penetrated salt far below an overhang. Note the following:

Dips are generally highest closest to salt.

Dips increase as an overhang is approached from above.

Dips, then, decrease below the overhang.

 

 

There is another downward increase as the well approaches the main salt stock.

One of the uses of the dipmeter on wells drilled near a salt dome is to indicate the presence of overhangs, which warrant further investigation by an ULSEL survey.

The ULSEL device is an electrical logging system with long electrode spacings allowing formation investigation up to 2000 ft from the wellbore. ULSEL measurements combined with induction log and dipmeter data provide the information necessary to compute the distance, direction, and profile of the nearest salt dome.

Vertical and Overturned Beds Vertical, near-vertical, and overturned beds are found near salt domes and in areas of over-thrusting. Straight holes are rarely drilled through vertical beds. The apparent dip has a computed value of less than 90°. The dips become vertical only after correction for sonde tilt.

The steepest dips near a salt dome are generally found under an overhang, and some beds may be overturned indicating a horizontal and vertical component to salt movement ( Figure 3 ). The illustrated well was sidetracked under the overhang, and it penetrated increasing easterly dipping vertical beds, overturned beds, and, finally, high easterly dips again.

 

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 in Figure 4 .

Gouge Zones Some salt domes are covered by a thin gouge zone, usually less than 100 ft thick. These gouge zones contain a mixture of the various sediments the dome has penetrated. When the resistivities of the normally pressured, bedded shales around a dome are approximately 1 ohm-m, the gouge resistivity averages approximately 1.2 ohm-m. Gouge is a mixture of sands and shales, and it has a "hashy" appearance on the SP and short-spaced resistivity curves.

A blanket of diapiric clay is sometimes found draped around one flank of a salt dome. This is a high-pressure, low-resistivity clay. Resistivities within Gulf Coast diapiric clay domes are commonly less than 0.5 ohm-m. Dips within gouge zones and diapiric clays tend to be random or nonexistent.

Clay Domes

Clay domes are formed in the same manner as salt domes. Source beds are masses of low-density shales. The density of these shales can be less than the density of salt: 2 g/cc versus 2.16 g/cc. These low-density shales floated upward through zones of weakness to form clay domes. The penetration of younger overlying beds created dips away from the clay dome.

In the northern Gulf of Mexico the top of a clay dome is indicated by a downward decrease in resistivity. The half-ohm shale point was used as an indicator of the top of the clay dome in the Eugene Island Block 198 field.

Resistivities within domes may be as low as 0.2 ohm-m in the U.S. Gulf Coast region. In Nigeria, a 1 ohm-m value is more common.

It is currently more difficult to identify clay domes than it was in the 1960s. At that time, a constant dip trend matching the dip of the domal surface was recorded within the dome. As the dome was approached from above, the dip trend increased in magnitude, just as if the flank of a salt dome were being approached. After the clay dome was penetrated, a constant dip trend was usually recorded. This is illustrated in Figure 5 .

 

Since the late 1960s clay dome dips have become more elusive. Instead of constant dip trends within the dome, only blank zones are found on dip plots. One explanation for this change has been advanced by a major company geologist. He suggests that the current lack of dip data within the clay dome results from formation damage caused by increased mud weights. Dips detected within clay domes were probably derived from cleavages or compaction surfaces, not from bedding planes.

Dips are still found within high-pressure, low-resistivity shales in their normal stratigraphic position. After shales have been uplifted, they may be more susceptible to mud-weight induced damage.

Structural Dip Deletion

Formation dip results from the original depositional dip, compaction and postdepositional deformation, and structural uplift or subsidence. The magnitude and direction of structural dip are removed before making fault or stratigraphic interpretations.

If the dip in the zone of interest is less than the structural dip, structural dip should be deleted from each of the dips on the tadpole plot.

If the dip in the zone of interest is equal to or greater than structural dip, but with a different azimuth, structural dip should be deleted.

Results of Dip Deletion

Figure 1 is an actual dipmeter plot that illustrates the results of structural dip deletion. The dips opposite the pay zone are less than structural dip, so structural dip should be deleted before attempting a stratigraphic interpretation. After deleting the 22° of north-northwest structural dip, the dips in the zone of interest form a south-southeast dipping red pattern. If structural dip is not deleted prior to stratigraphic interpretation, the interpretation will be in error.

Instead of being a fan deposited by a north-northwesterly flowing current, the sand was deposited as fill within an east-northeast, west-southwest striking channel, with the axis lying to the south-southeast.

 

 

Benefits of Dip Deletion

Structural dip deletion serves as an indicator that the correct structural trend was identified and deleted. The structural dip on Figure 2 was selected as 35° at an azimuth of 90° down to 7150 ft. Below 7150 ft, the structural dip was selected as 35° with an azimuth of 117°.

 

After a structural dip of 35° at 117° was deleted over the entire interval, an apparent northeast structural dip trend remained above 7150 ft. Almost all apparent structural trends disappeared below 7150 ft. This indicates that 35° at 117° was the correct structural dip below 7150 ft but incorrect for the interval above. Another deletion pass was made over the entire interval to delete 35° at an azimuth of 90°. The apparent trend above 7150 ft disappeared, indicating that the correct structural dip was deleted. The incorrect structural dip deletion below 7150 ft produced an apparent southwest structural trend.

Another benefit of structural dip deletion is the identification of dips resulting from erroneous correlations. These dips tend to be higher than structural dip, and they typically remain unreasonably high after deletion.

The Process of Dip Deletion

If the magnetic recording tape is available, structural dip deletion is a relatively easy process, and a tadpole plot with structural dip removed can be quickly generated. If the answer tape is not available, the processing must be recomputed, or a "stereo net" or hand calculation must be performed. Programs are available for the HP-25, HP-41C, HP-75, and the TI-59 calculators. For logs with more than a few points requiring structural deletion, log recomputation is strongly recommended .

Deleting Uplift Effects Gulf

Coast salt domes may have undergone several periods of uplift, both contemporaneous and postdepositional. Dips have reversed as the salt being uplifted at one location masked the dip from a nearby salt spine that had been uplifted earlier. Prior dips in directions different from those of current dips indicate the existence of fossil structures in the area. These structures may still be productive.

To determine structural dip at any specific time, the effect of structure must be removed a single uplift at a time. The shallowest structural dip should be removed first. The remaining dips indicate the attitude of beds prior to the youngest uplift ( Figure 3 ). After selecting a new structural trend for the shallowest remaining interval, delete the trend. The remaining dips indicate the attitude of the beds at the time of the second-youngest uplift. This process is continued until the end of the dipmeter log is reached.

 

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Dipmeter Surveys (Dip Patterns)

Dip Patterns

Basic Types

Introduction

There are several graphical methods of plotting dip computations. This chapter covers interpretation rules based on the common "tadpole" plot. The head of the tadpole indicates dip magnitude and is plotted on a dip scale ranging from 0° to 90° versus depth. The tail of the tadpole, which points in the downdip direction, is plotted on a compass rose (north, up; east to the right; south, down; and west to the left).

The two or more tadpoles forming a group are derived from the internal structure of sediment layers deposited in a single depositional environment. All dips on a tadpole plot can be assigned to one of four basic groups. These groups are the building blocks used to create megapatterns. Mega-patterns, lithology, and knowledge of the depositional environment are used to make interpretations.

Dip Groups

The four dip groups are the red (slope), blue (current), green (constant), and random. These basic groups are the building blocks of megapatterns, which are used to identify missing or repeat sections and to interpret stratigraphy. The red, blue, and green patterns are illustrated in Figure 1 , which shows the borehole, formation-imaging, and dipmeter tadpole plots for each.

 

Red (Slope) Groups

Red dip groups are composed of two or more adjacent tadpoles with constant azimuths and downward increasing magnitudes. These groups are generated from sediments deposited on a sloping surface or from sediments with dips that have been altered by postdepositional movement.

Most red groups result from downdip thickening. Beds deposited on a sloping surface thicken and become wedge-shaped in the downdip direction; therefore, the dip direction of red groups indicates the direction of thickening.

Blue (Current) Groups

Blue dip groups are composed of two or more adjacent tadpoles with constant azimuths and downward-decreasing magnitudes. These groups are generated mainly from sediment layers deposited as foreset beds. The dip directions of the foreset-generated blue groups indicate the directions of current flow during deposition. Some blue groups are generated by weathering beneath erosional surfaces; this process creates downward-flattening features.

Green (Constant) Groups

Green dip groups are composed of two or more adjacent tadpoles with constant magnitudes and azimuths. These groups are derived from parallel crossbeds or from sediments that were deposited flat and have subsequently undergone structural uplifting. Green groups are the only dip groups indicating structural dip today and are the groups sought within zones of least scatter.

Random Groups

Random dip groups are composed of adjacent tadpoles with random magnitudes and azimuths. These groups are derived from sediment layers deposited in high-energy environments, such as shallow water less than 50 ft deep; from layers that have undergone reworking by bioturbation; and from layers that have undergone postdepositional movement.

Megapatterns

These dip groups are used as building blocks in identifying megapatterns. Megapatterns, lithology, and depositional environment information are used for determining the location of attributes of faults, unconformities, and stratigraphic features.

Mega-Red Dip Patterns

The mega-red dip pattern is a family of the basic dip groups characterized by an increasing downward magnitude trend and a constant or gradually rotating general azimuth ( Figure 2 ). Individual basic dip groups may exhibit dips that do not match the general trend. The features in the subsurface that create mega red patterns on the dip plot include distortions near a fault plane, sand bars, beach ridges, reefs, and channels of all types.

 

Normal Faults Two distortion types, rollover and drag, may be present near a normal fault. Rollover, with dip into the fault, results from sediments slumping into the downthrown side of a fault that was active at the seafloor during the time of deposition.

Drag zones contain beds dipping in the same direction as the fault plane. The mega-red dip pattern results from friction between the active downthrown block and the passive upthrown block. Most of the distortion occurs in the active or down-thrown fault block; however, upthrown drag is occasionally noted.

Mega-red dip patterns are not always found near the fault plane. Some faults have no downthrown or upthrown distortion; in these situations there is no dipmeter indication unless there has been tilting of one of the fault blocks.

Reverse Faults Reverse thrust fault usually exhibit drag on both sides of the fault. The drag zone in the overthrust block creates a mega-red pattern dipping in the direction of overthrust. The downthrown dip pattern, if one exists, is a mega-blue pattern. If drag is present on only one side of the fault, it occurs on the more active, overthrust side.

If a missing or repeat section is at or near the base of a mega-red pattern, the pattern probably results from some type of distortion near the fault plane. If there is no indication of a nearby missing or repeat section, then the mega-red pattern probably has a stratigraphic origin.

Additional information from other logs about the depositional environment and lithology is necessary to determine the stratigraphic feature generating a mega-red pattern.

Mega-Blue Dip Patterns

Mega-blue dip patterns are formed when the dip magnitudes of families of basic groups decrease downward but their azimuths remain the same or rotate slowly ( Figure 3 ). As is true of mega-red patterns, a few individual basic dip groups may exhibit random azimuths. Also, local data about depositional environment and lithology obtained from other logs are required to make a stratigraphic interpretation.

 

Mega-blue patterns result from foreset deposition, weathering under erosional surfaces, and compaction caused by the sinking of a relatively dense mass, such as a sand or coral reef, into softer underlying beds. Dip direction of foreset beds indicates the direction of sediment transport or current flow. The dips created by compaction indicate the direction to the thickest portion of the overlying mass. Fore-set deposition occurs in delta-dominated environments, tide/ wave-dominated environments, longshore current sand waves, submarine fans, tidal flats, and at or near the axes of any type of channel.

 

 

Identifying Megapatterns

Basic dip groups that do not form megapatterns terminate at or near a vertical or slightly inclined line ( Figure 4 ). Basic dip groups that form megapatterns terminate at successively higher magnitudes-e.g., higher downward for a red, higher upward for a blue--for the length of the pattern. If the deepest dip group of a megapattern has an azimuth different from the azimuth of the general pattern, the azimuth of the pattern, not the basal group, should be used.

 

Theoretical Dip Patterns

A series of theoretical dip-versus-lithology patterns can easily be created for any specific environment. Since the same dip patterns can be created by different stratigraphic features, the theoretical sketches are grouped by depositional environments (nonmarine, deltaic, interdeltaic, and deepwater) . A missing and repeat dip response is also included.

The interpretation process can be carried out step by step in the following sequence:

• Determine structural dip.
  

Delete structural dip if necessary.

Identify and describe the attributes of missing and repeat sections.

Make stratigraphic interpretations using lithology and knowledge of the depositional environment.

If independent knowledge of the depositional environment is unavailable, local "rules of thumb," using such parameters as bound water resistivity, shale resistivity, and dip scatter, can be used as environmental indicators.

Structural Dip In order to represent structural dip today, any bedding plane or sediment layer must have been deposited flat and have undergone only structural uplift since the time of deposition. Structural dip trends are selected from zones of least dip scatter, since such zones are most likely to have been deposited in a low-energy environment and thus are most likely to represent structural dip today. A good rule of thumb is to assume that structural dip trends picked from the dipmeter display extend horizontally no farther than they do vertically. Dip trends that extend 1000 ft or more can usually be extended as far horizontally as the closest offset well. However, if numerous faults and unconformities are present, it may be impossible to find a dip trend that extends 1000 ft vertically.

Structural dip should be deleted before fault and stratigraphic interpretations are made if the dip magnitude in the zone of interest is less than that of structural dip, or if the dip azimuth in the zone of interest is different from that of structural dip.

Missing or Repeat Sections

After determining and (if necessary) deleting structural dip, the next step is interpreting missing and repeat sections. Missing sections result when normal faults, angular unconformities, disconformities, or diastems are present. Repeat sections result from compressional faulting and folding.

Since stratigraphic features and faults can generate identical dip patterns, an independent input as to locations of probable missing sections is desirable before making missing-section interpretations. Normal faults may generate red dip patterns that dip either toward or away from the fault plane. Dip patterns on the downthrown side of growth faults, which result from rollover into the fault, dip toward the fault plane. The vertical extent of such patterns can be used as a minimum fault displacement indicator. Nongrowth normal faults that occurred after some formation compaction had taken place create red dip patterns that dip in the same direction as the fault plane. These result from a drag zone immediately downthrown to the fault.

Reverse and thrust faults, which generate "right-side-up" repeats on the logs, create red-over-blue dip patterns. The patterns dip in the direction of overthrust, and the fault plane is located at their junction. Overturned folds also create log repeat sections, but one repeat is a mirror image of the other.

From the bottom up on Figure 1 , the first missing and repeat section is a diastem or disconformity. Since the angular difference across such features is less than one-half of a degree, they are not easily recognized on dipmeter plots. The small blue pattern shown beneath the missing section is the result of some type of weathering.

 

The next repeat section results from an overturned fold. The log response of the repeat section produces a mirror image with the repeat section upside down with respect to the first log response. In this example, there is a dip reversal across the fold; this is not always the case.

A reverse or thrust fault also produces a repeat log response, with the repeat right side up with respect to the first log response. Both the red pattern in the upper, or overthrust, block and the blue pattern in the downthrown block are the result of drag. The dip direction of the overthrust red pattern is the same as the direction of over-thrust (to the east in this example).

The next upward dip decrease is the result of a period of postdepositional uplift that created a portion of the underlying 25° northeast structural trend. There was no erosion of the uplifted beds. Deposition, which continued without a break, then produced onlapping beds. The overlying 20° east structural trend was produced by a later period of uplift. Such features are common in sediments deposited in deepwater environments.

The next upward dip increase, from 20° east to 10° east, occurs across an angular unconformity. The blue dip pattern drawn below the unconformity results from some type of weathering and occurs most of the time. Since this small blue pattern is identical to patterns produced by stratigraphic events, it should not be considered a diagnostic unconformity indicator.

There is independent input that a fault exists within the section of 10° east structural dip. Since there are no associated red or blue patterns, this is a middle-aged normal fault that has no distortion near the fault plane. However, a sudden structural dip change occurs when one fault block has been tilted.

There is also independent input that another fault is located just uphole. A red dip pattern, which terminates at the probable fault location, is present in this example. If the vertical extent of the red pattern is more than 200 ft, the pattern is almost certainly the result of dip into the downthrown side of a growth fault The dip direction is toward the upthrown block (example: upthrown to the northeast). If the vertical extent of the red pattern is less than 200 ft, the red pattern may result from either rollover into a growth fault or drag on the downthrown side of a later fault. When the pattern results from drag, the dip direction is toward the downthrown side and normal to the fault strike.

Continental Environment

Figure 1 illustrates some continental environment depositional features and their associated dip patterns. From bottom up, the group of tadpoles indicating east structural dip is derived from sediments deposited essentially flat in an upper delta plain environment. Sands deposited in such an environment may contain secondary porosity because some plant-produced acids are capable of dissolving sand grains.

Flood-plain sediments produce a "bag-of-nails" dip scatter. Few (if any) dips reflecting structural dip are found within such sediments.

Next is an eolian sand. The illustrated dip patterns have constant (angle of repose) dip trends underlain by blue patterns. This is a typical dip response from transverse and barchan dunes. The dip direction indicates the prevailing wind direction at the time of deposition (from west to east in this example).

 

Longitudinal dunes produce red or blue patterns whose dip directions are normal to the prevailing wind direction. Dome and parabolic dunes produce mainly red patterns dipping in the prevailing wind direction.

Swamp or marsh deposits generally produce blank zones because bedding planes have been destroyed by rooting and bioturbation.

Stream channels filled with clay plugs produce red dip patterns within shale sections. The patterns dip toward the channel thalweg.

When stream channels are filled with sand instead of clay, possibly during a marine transgression, a red dip pattern found at the base of the sand dips toward the thalweg and normal to the strike of the channel (example: thalweg to the northeast, and northwest-southeast strike). This dip pattern is overlain by a blue pattern whose dip is 90° from that of the underlying red pattern. This dip direction indicates the current flow direction within the channel (example: direction of flow to the southeast).

Point-bar sands exhibit a number of internal blue dip patterns whose dips are in the direction of current flow. A red pattern that dips toward the thalweg may also be present just above the point bar. If the beds that produce the blue dip patterns are thicker than 3 ft, the blue patterns probably result from accretion depositions that dip toward the thalweg rather than from trough crossbeds that dip down-current.

Continental Shelf, Delta-Dominated Environment

In the example in Figure 1 , delta-dominated means that some, if not all, of the stratigraphic features deposited in a deltaic environment were preserved in their original forms rather than in reworked forms. The bottom sand is channel-like and was formed by the compaction of underlying muds. All dips of the red dip pattern (faulting has been eliminated) found within the sand dip toward the axis and normal to the strike of the sand. Because of compaction of the sediments below the sand, a blue dip pattern dipping toward the channel axis is usually found beneath the sand in the underlying shales. Other logs exhibit gradients (downward-decreasing resistivity, increasing interval transit time) in the underlying shales. Sands formed by compaction may be more than 2000 ft thick.

 

Crevasse splays generate blue dip patterns pointing in the direction of current flow (example: direction of flow to the southeast). A sand deposited as a distributary mouth-bar and topped by a scour channel exhibits a red-over-blue dip pattern that dips in the same direction. The blue pattern dips in the direction of current flow (example: direction of flow to the east-southeast) and the red pattern dips toward the scour channel axis (example: axis to the east-southeast), which usually has a very limited areal extent. In general, when adjacent red-over-blue patterns dip in the same direction, the red pattern can be ignored.

Whenever a distributary mouth-bar sand undergoes shallow-water reworking, a bag-of-nails dip scatter is produced. Such sands tend to be clean with good porosities and permeabilities.

When all the original depositional features of a distributary channel are preserved, they produce a red dip pattern at the base of the sand, overlain by a blue pattern. The pattern azimuths are 90° apart. The red pattern dips toward the channel axis and normal to the channel strike (example: axis to east and north-south strike) The blue pattern dip indicates flow down the channel (example: flow from north to south). A distributary mouth-bar produces a blue dip pattern whose direction is that of current flow (example: flow from northwest to southeast) .

When the blue pattern magnitude variation is 10° or more, the distributary mouth-bar tends to be elongated in the direction of dip (inertia-dominated environment). When the dip variation is less than 10°, the distributary mouth-bar tends to be fan or crescent shaped (friction-dominated environment). Distributary mouth-bars and crevasse splays look the same on dipmeter plots.

Continental Shelf, Tide- or Wave-Dominated Environment

Figure 1 illustrates some of the stratigraphic features and associated dip patterns that are found in a continental shelf, tide- or wave-dominated environment. Many of these features are the result of reworking of previously deposited deltaic sediments.

 

At the bottom of the figure, parallel seaward-dipping cross-beds are produced by beach rock that forms in a carbonate environment at the saltwater-freshwater interface along shorelines.

An oolitic bar is identified by a red pattern immediately above the bar (assuming, of course, that it was not penetrated on the crest). The red pattern dips toward the pinch-out and normal to the strike of the bar (example: pinchout to the northeast, and northwest-southeast strike) . Dips within the oolitic bar are immaterial.

A reef also exhibits a red pattern above its top and a blue pattern in the underlying beds. Few, if any, meaningful dips are found within reefs. The overlying red pattern dips toward the pinchout and normal to the strike of the reef. The blue pattern, which results from compaction, dips toward the thicker part of the reef (example: pinchout is to the east-northeast and the reef strikes north-northwest, south-southeast) .

A buried beach ridge exhibits a red dip pattern immediately above the top of the ridge and numerous dips within the beach-ridge sand. The red pattern dips toward the shaleout and normal to the strike of the beach ridge (example: shaleout to northeast, and northwest-southeast strike) .

A sand bar that formed at the wave breakpoint also exhibits a red dip pattern above the sand but few dips within the sand (reworking increases the electrical homogeneity) . The red pattern dips toward the shaleout and normal to the strike of the bar (example: shaleout to the northeast, and northwest-southeast strike) .

In Figure 2 the bottom sand was deposited as a slip-face sand on the landward side of a beach. The internal blue dip pattern dips landward and normal to the beach strike (example: land to west, and north-south beach strike).

The next sand was deposited as beach dunes and contains varying dips resulting from festoon crossbedding. Formations on the berm crest of a beach can be deposited flat and would later indicate structural dip.

Runnel sands may exhibit blue patterns derived from mega-ripples whose dip azimuths approximate the beach strike. Small-scale ripples may produce either blank zones or random dips. The example beach strike is north-south, indicated by south-dipping blue patterns derived from megaripples.

A beach-face sand contains seaward-dipping parallel cross-beds (example: parallel crossbeds dipping 5° east indicate that seaward was to the east during deposition).

Upper shore face sands contain a few parallel seaward-dipping crossbeds (example: 1° and 2° east dips indicate that seaward was to the east during deposition). Lower shore face sands contain mainly blank zones and random dips that result from high-energy environments and extensive bioturbation.

 

Longshore current sand waves exhibit blue dip patterns dipping in the direction of transport and parallel to the nearby fossil shoreline (example: dips to south indicate transport from north to south along a north-south striking shoreline).

A tidal flood delta, or washover, fan generates landwarddipping blue dip patterns (example: west-dipping blue patterns indicate that land was to the west during deposition).

Ebb deltas produce seaward-dipping blue patterns (example: east dip indicates that seaward was to the east at the time of deposition).

Deepwater Depositional Environment

Figure 1 illustrates the sedimentary features found in deepwater (continental slope and deeper) sediments. Often, postdepositional movement occurs within sediments deposited on the continental slope. This produces a bag-of-nails dip appearance. Structural dip is extremely difficult to determine from such intervals.

 

Deposition at the distal end of submarine fans produces alternating sand-shale layers that later can become low-resistivity pay zones. Dips recorded in this environment indicate structural dip. The midfan portion of a submarine fan produces blue patterns that indicate sediment transport directions (example: transport direction was north to south) .

Debris flows produce blank zones or zones of random dips. A submarine channel penetrated near the edge exhibits a red pattern that dips toward the channel axis and normal to its strike (example: axis to the east, and north-south strike) . A near-the-axis location within a feeder channel produces only blue patterns, which indicates flow down the channel (example: south-southwest dipping blue patterns indicate flow from north-northeast to south-southwest) .

A feeder channel penetration between the axis and channel edge produces the "blue-over-red with axis 90° apart" dip pattern combination. The red pattern dips toward the channel axis and normal to the channel strike (example: axis to the east and north-south strike). The blue pattern dip direction indicates the flow direction down the channel (example: flow direction was from north-northeast to south-southwest) .

These theoretical patterns show all of the original dip patterns intact. In practice, portions of the original patterns may have been destroyed by reworking. Also, random dips that behave like noise are scattered throughout the patterns.

Exercise 1.

The upper 3 m of the log in Figure 1 are in interbedded shales and silts. The lower 4 m are mostly sand in a fluvial environment.

This exercise requires the student to study the dip curves closely, from a standpoint of similarity between adjacent side-by-side electrodes and similarity from pad to pad.

Also study the dip results from each of the three systems: CSB, MSD, LOCDIP.

Study the comparison of 5-inch correlation CSB, LOCAL DIP, and 1-foot correlation MSD. What are the bedding characteristics for each of the four intervals?

 

Solution 1:

Interval 1

Curve pairs vary from similar to unlike, and the CSB results reflect this fact. Pad-to-pad similarity is quite poor, causing the MSD dip scatter. Bedding is probably irregular and of very short lateral extension. There is some stratification, however, as indicated by the similarity of side-by-side curves.

Interval 2

The lower 2 m of this section are well-bedded with small curve contrast. Agreement between systems is fair, implying some consistency in direction. At the arrow, note that LOCDIP and MSD point north at 9°, whereas CSB shows SW crossbedding over that section. This is an excellent example of a dominant anomaly (see correlations) influencing the dips over the complete 1-ft correlation interval on the MSD, and the similar LOCDIP response.

Interval 3

Well-bedded, with good basic agreement among systems.

Interval 4

Poor bedding, with noncorrelational conductive anomalies. These are pyrite blebs, very small but very conductive.

 

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