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.