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.