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.