W3D95C:
W3D95C:
Interpret seismic data across and adjacent to a proposed salt cavern based industrial waste disposal site on the north central portion of the XXX Salt Dome in XXX County, TX. The specific objective was to evaluate if there is a salt overhang that could decrease the horizontal extent of salt between the edge of the cavern and the salt-sediment interface.
The seismic data for each of the three surveys was loaded on a Landmark Graphics interpretation workstation at Interactive Interpretation & Training. Three projects were set up: SEM; Sheik; and Composite. The Top of Caprock map was gridded and the grid entered as a horizon into the Composite project. The well-based Top of Caprock horizon was generated by interpolating between points taken on a 2,500 foot grid in X and Y, triangulating a surface between these points, writing these surfaces into an horizon in 100 foot square bins in the Composite project, smoothing the results, and creating a purple horizon. This purple well-based Top of Caprock horizon was exported and then imported into the SEM project.
The size of the projected Salt Caverns was converted to seismic travel time and horizons made to show anticipated location within the SEM 3-D seismic volume. The west cavern was colored red, the south cavern blue, the north cavern orange, and the east cavern green.
In addition, location maps, the description of the proposed caverns, the paper seismic section for line S-2, and other relevant information was scanned and entered into the HyperJournal named ESF. The ESF HyperJournal is the on-line repository for interpretation results and key data examples. This HyperJournal can be delivered to anyone with a Sun or Silicon Graphics workstation for a detailed review of the interpretation process and the work flow.
A detailed interpretation was done on the SEM 3-D seismic survey. The most obvious reflector was determined to be the Top of Caprock reflector. This interpretation was confirmed by bringing the gridded well-based Top of Caprock map derived from drilling results into the SEM project and displaying the results as a purple horizon. The purple horizon tracks the strong reflector fairly closely through-out the SEM 3-D seismic survey. There are a few places where, because of variations in the Top of Caprock surface, the purple horizon is up to 50 ms from the strong reflector (150 foot vertically at the Top of Caprock), but overall the well-based Top of Caprock horizon is within about 20 ms of the strong reflector (60 foot vertically at the Top of Caprock).
The strong reflector has a strong peak-trough-peak, and the Top of Caprock was picked on the trough as a red horizon (yellow 'active horizon' on cross-sections). There was no consistent seismic reflector that could be picked as the Top of Salt. Based on drilling information a 50 ms phantom of the Top of Caprock horizon was created to define the Top of Salt as a blue horizon. The well-based and seismic interpretation of the Top of Caprock are shown on Line 36 (Figure 2), which runs through both the proposed blue and green (south and east) caverns. Notice that the caprock has been highlighted with a semi- transparent gray overlay. The Top of Caprock horizon is generally at about 300 ms. Figure 3 shows the proposed west or red cavern just northwest of the SEM 3-D survey on Trace 12. Figure 4 shows the proposed location of the south or blue cavern on Trace 18. Figure 5 shows the proposed north or orange cavern, just northwest of the SEM 3-D survey on Trace 37. Figure 6 shows the proposed location of the east or green cavern on Trace 43. The proposed blue and green caverns are also shown on seismic cube displays in Figure 7 and Figure 8 respectively.
The map of the Top of Caprock horizon dips 75 ms (~225 feet) from the south end of the survey to the north, with a general east-west strike (Figure 9). The relative spatial location of the four proposed salt caverns is shown in perspective view on Figure 10. There are a series of in-line (southwest to northeast) 100 to 500 foot offsets in the contours. These are interpreted as being due to very small displacements. Smoothing through these offsets, shows a maximum vertical offset of 10 ms (~30 feet). The displacements are probably due caprock alteration and/or volume change and the apparent strike-slip component could be related to salt growth movement. These small compaction displacements can be interpreted on the seismic sections. Figure 11 and Figure 12 show the displacement interpretations on Trace 18. The maximum displacement on any of these displacements is less than 15 ms (~45 feet). There was nothing located on the seismic to imply that any of these cracks in the sediments above the XXX Salt Dome have had any active movement during the time that the last 100+ms (~300+ feet) of sediment have been deposited in the area. This meas that these features are at least older than Pleistocene. Figure 13 and Figure 14 show the displacement interpretations on Trace 43. On both of the cube displays it is evident how the in-line seismic direction is right along the strike of the displacement.
The seismic reflectors beneath the Top of Caprock horizon is not coherent enough to show the push-down effect of low velocity zones that would indicate high porosity or cavernous intervals. However, in order to more closely evaluate any seismic evidence of variations in the internal structure of the caprock, a series of seismic amplitude extraction maps were made. Figure 15 shows a StratAmp extraction of the maximum absolute seismic amplitudes between the Top of Caprock and the Top of Salt horizons. Note the strong amplitude variations in this map. These variations are interpreted as being related to calcite distribution in the caprock.
A poster was made that shows these various caprock amplitudes in map view. This series of maps show the maximum absolute seismic amplitude for different intervals within and just beneath the caprock of the XXX salt dome. The map showing the amplitudes for the first five milliseconds (<30 feet) under the Top of Caprock horizon looks much the same as Figure 15. Of course, these amplitudes are influenced by the waveform generated at the Top Caprock interface. But it is interesting how a map of the first 10 ms under the Caprock interface smears out the sharp amplitude contrasts. An amplitude map from the interval from 10 to 20 ms (~30-60 feet) beneath the Top of Caprock shows the high amplitude contrasts missing. We interpret this to mean that this section is below the calcite zone.
The 2-D seismic section S-2 is orthogonal to the salt dome. A poster was generated that shows the tie between this seismic line and the two 3-D seismic surveys. There is a good tie to the Top of Caprock interpretation from the SEM 3-D survey. The tie to the interpreted Top of Caprock from the Sheik 3-D survey fits spatially, but there is not a good seismic reflector for this interface. Figure 16 shows the interpretation of the Top of Caprock and the Top of Salt horizons on line S-2. The identification of this interface is straight forward. The dip of the salt-sediment interface means that there would need to be very long offsets to get accurate imagery of this surface.
A letter dated August 25, 1994 from Mr. Cameron Walker of Walker Geophysical Company stated "No anomalous geological structures appear on this data set, and no evidence of late-arriving seismic events suggestive of turning waves was present." When Dave Hale of Chevron presented the Turning Wave Theory at the SEG in Houston a few years ago, he stressed that special seismic processing was needed to find these seismic events. Line S-2 is a candidate for this type of processing. As depth increases in the area of the Sheik 3-D survey, there are small increases in velocity. Seismic waves bend at these small increments of depth and velocity and refract along the acoustic interface boundaries. At the angle of incidence the head wave between the refractions and the reflections has been shown to form a reflection that can be used to map the side of a salt dome. These reflections typically come in at about 8-10 seconds depth. The fact that there are no reflections on S-2 at these depths, and the straight forward identification of the salt- sediment interface do not justify special seismic processing to identify Turning Waves.
There is not an obvious salt-sediment interface, nor Top of Caprock reflectors in the Sheik 3-D seismic survey. However, there are areas of non-reflection that imply there is salt or that salt moved through the area and disrupted the sediments. Figure 17 shows the interpretation of the Top of Caprock and the shadow Top of Salt based on projecting these reflectors from S-2 and areas of non-reflection. The horizontal reflectors in the Sheik area can be nicely correlated with the reflectors coming up against the salt on line S-2.
The structure of the Top of Caprock is simple in the area of the proposed salt caverns. The map of the Top of Caprock across the entire XXX Salt Dome (Figure 18), shows this area is a relatively simple dome. Based on the integrated interpretations described in this report, of the SEM 3-D, S-2 2-D, and Sheik 3-D seismic surveys, there does not appear to be any salt overhang on the north side of the XXX Salt Dome.
