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.