The results of the experiment were somewhat inconclusive. Due to saturated ground conditions, possible water in the cables, a large amount of electrical interference, and possible absorption of high frequency data in the near-surface, much of the data signal was swamped by noise. Even with a 21 bit system. In either case, maximum frequencies on the final stacked and migrated sections appear to be only about 50-60 Hz before rolling off the high end at 72 dB/octave. Because of the lack of high frequencies, the chances of identifying potentially productive sandstone versus shale in the upper part of the Morrow section appears to be very small to nonexistent with this data set. However, identifying the relative thickness of the Morrow section and locating channels is possible.
Synthetic Seismogram Ties
Synthetic seismogram ties between the Harris #1 well and Line 2 are shown in Figures 18 and 19, while ties between the Tedford #1 well and Line 3 are shown in Figures 20 and 21. Figures 18 and 20 show the ties with the entire section, while Figures 19 and 21 focus the ties on the Morrow section. Note that the ties with both wells are quite good, even though there is some stretch, and the amplitudes do not always correlate. Ties between Lines 1, 2, and 3, and the previously acquired seismic data were also made. In each case, the correlation was good. Note that below the well name on the synthetic tie with the section, the numbers:
|0 ft of net sand, 6 ft. of gross sand, and 16 ft Morrow to Mississippian section (channel thickness). In the Harris #1 well though, the 6 ft. of gross sand is not well developed and may not even be a sandstone.|
Interpreted Seismic Sections
The final interpreted sections, including correlation of gamma ray logs on selected wells, are shown in Figures 22, 23, 24, 25, 26, 27. The well ties are shown in Figures 3, 4, 5, with the numbers:
|meaning the same thing as in the synthetic ties and models.|
Well control along Line 1 (Figs. 4, 6, 7, and 22) show that the line follows a channel of varying thickness. In some areas this channel contains a productive sandstone, in some places it does not. Because of the change in sandstone thickness along the channel, this is an ideal line to test if the seismic signature of the data is sensitive to the change or not. As mentioned previously, it is unfortunate the frequency content of the data makes this task difficult if not impossible with this data set. However, there are some interesting features that can be identified from the seismic data, and one possible hint of a seismic signature associated with the change from thicker sandstone to thinner sandstone.
A relatively high amplitude trough near the top of the Morrow, and relatively high amplitude peaks on either side of this trough, occur throughout most of Line 1 from station 1135 to station 1214 (Fig. 23). These high amplitudes indicate a relatively thick Morrow section and are collaborated with the well control and the lower resolution model (Fig. 15). An abrupt channel edge associated with pronounced amplitude reduction and isochron thinning can be seen northeast of station 1216 (Fig. 23). Another reduction in amplitude of this reflective cycle can be seen southwest of station 1135. This reduction is not as abrupt and appears first as an amplitude reduction of the upper peak, then the trough and lower peak farther to the southwest. The reduction in amplitude of this cycle is consistent with channel thinning indicated in the wells (Figs. 22 and 23) and low resolution model (Fig. 15). The offset of the lower peak associated with the top of the Mississippian, and offset of reflections below it, suggest faulting of the Mississippian section is likely (Fig. 23). This faulting may explain the anomalously thick Morrow clastic section in the Cannon well, and the abrupt change in channel thickness between the Cannon and Widmer wells.
The only possible indication of a seismic signature change that can be related to a change in sandstone thickness occurs between the Harris 3-9 and Norton wells (Fig 23). Channel thickness in the Norton well is 7 ft. less than in the Harris 3-9 well. With this thinning, a reduction in reflection amplitude would be expected. However, this does not seem to occur. It is possible that the lack of amplitude reduction may be related to the replacement of 6 ft. of gross sandstone in the Harris 3-9 well with shale in the Norton well. The reduction in impedance that would result from this may be enough to compensate for the overall channel thinning and maintain a relatively high amplitude in the Norton well. It would be difficult to use this change (or lack of it) in areas between well control though because one could not tell if the channel and sandstone thickness remains the same, or the sandstone and channel thickness is reduced.
Line 2 extends across the channel imaged along Line 1 and crosses the channel's margins at an angle close to perpendicular (Figs. 4, 6, 7, 24 and 25). The southern margin of the channel occurs just south of the Fager well and is quite abrupt as indicated by the drastic reduction in reflection amplitude and an isochron thinning towards the south of the reflection associated with the Morrow. Just north of the tie with Line 1, the amplitude of this reflection dies out again, indicating a fairly abrupt thinning associated with what appears to me an intrachannel high because the amplitude of the reflection increases again only 660 ft. (6 stations) north of Line 1. This intrachannel high may also be the cause of a slight reduction of reflection amplitude along the Morrow and Mississippian reflectors in the vicinity of Harris 2 well (Fig. 23). Because the margins of this high are so close to Line 1, Fresnel zone effects out of the plane of the section probably reduced the amplitude on Line 1 even though the Harris 2 well, which intersects both Lines 1 and 2, shows a relatively thick (75 ft.) Morrow clastic section.
The northern margin of the channel imaged along Line 1 occurs near the Harris 1 well, as indicated by another reduction in Morrow reflection amplitude and a decrease in the travel time to the peak associated with the Mississippian reflector. This thin Morrow section appears to thicken up again north of the Alley well as indicated by an increase in travel time to the Mississippian reflector and a slight increase in reflection amplitude. The channel margins on either side of the thin Morrow clastic section penetrated by the Harris 1 and Alley wells appears to have been controlled in part by faulting (Fig. 25). This is supported by the smaller travel times to the Mississippian reflector, and also to the peak above the top of the Morrow, and reflections below the Mississippian reflector. Alternatively this could be a velocity pull-up due to increased velocity above the Morrow reflector, however its association with a decrease in amplitude of the Morrow reflection makes this scenario less likely.
As far as a change in seismic signal related to a reduction in sandstone thickness is concerned, there is not one on Line 2. Although there is a change in sandstone thickness between 13 ft. in the Harris 2 well and 0 ft. in the Harris 1 well (Figs. 24 and 25), the changes in amplitude, isochron, and time structure values seen in the seismic data are related predominantly to changes in thickness of the entire Morrow clastic interval and not just to the change in sandstone thickness.
Line 3, like Line 2, also crosses the channel imaged along Line 1 at a perpendicular angle (Figs. 4, 6, 7, 26 and 27). Unlike Line 2 though, which crosses the thickest part of the channel in an area that contains a productive sandstone (Figs. 4, 9, 11, and 24), Line 3 crosses the thickest part of the channel in an area where the sandstone is thin to absent (Figs. 4, 9, 11, and 26). This difference provides a perfect test to compare the seismic signature of a Morrow clastic section with thin to no sand with a Morrow clastic section of similar thickness with thin to no sand.
Unfortunately, there appears to be no difference between the seismic signature of the reflection associated with the thickest part of the Morrow clastic section between Lines 2 and 3 (Figs 25 and 27). This should not be a surprise though, because the frequency content of the data is too low to show any difference as indicated by the low resolution model (Fig. 15).
Line 3 also crosses a second channel which is penetrated by the Harris 1-3 well (Figs. 26 and 27). The seismic anomaly associated with this channel contains the typical higher amplitude trough near the top of the Morrow, and higher amplitude peaks above and below this trough (with the highest amplitude peak associated with the Mississippian reflector). However, the maximum amplitude associated with this channel, is less than the amplitude of the anomaly associated with the channel penetrated by the Tedford 2-10 well (Fig 27). The Harris 1-3 well also contains a 12 ft. thick productive sandstone, whereas the Tedford 2-10 well only contains a 4 ft. non-productive sandstone. This might suggest that lower seismic amplitudes could be expected for the Morrow reflection in a channel containing a thicker potentially productive sandstone. However, the Harris 1-3 well also has about a 10 ft. thinner Morrow to Mississippian clastic interval than the Tedford 2-10 well. This suggests that the reduction in amplitude in the channel with the sandstone as compared to the channel without the sandstone, could be just due to a thinner channel as would be predicted by the lower resolution model (Fig. 15).
Although the sandstone thickness variations may not be predictable with Line 3, channel thickness are. As on Line 1, the southeastern margin of the southeastern channel crossed by Line 3 appears to be quite abrupt, and occurs just northwest of the intersection with Line 1 (Fig 27). As on Line 1 the margin is indicated by a rapid reduction in amplitude of the Morrow reflection, combined with a slight reduction in the isochron value between the two peaks adjacent to the trough associated with the top of the Morrow (Fig. 27). The northwestern margin of this channel also occurs before the Tedford 1 well, however it appears to be more gradual, with less of a reduction in amplitude indicating a thicker Morrow-Mississippian clastic section northwest of the channel than southeast of it.
The margins of the channel associated with the Harris 1-3 well are also imaged on Line 3. Unlike the southeastern channel, this channel appears to be much narrower, with its southeast margin possibly controlled by faulting in the Mississippian and older section (Fig. 27). The dip of this fault is difficult to determine, but may be towards the northwest indicating a normal fault. Evidence for the fault is the abrupt offset in peak associated with the top of the Mississippian, and several peaks and troughs below that. Offset along this fault appears to decrease with depth and become accommodated by a broader zone of flexure below 1 sec. Flexural downwarping also appears to occur in the strata above the Morrow on the downthrown side. However, this amount of downwarping is not supported by subsea values on the top of the Morrow. This suggests that although the fault may be present in the Mississippian and deeper section, most of the downwarping of reflections is probably due to statics problems or a velocity push-down somewhere above the Morrow interval. Like the northwestern margin of the southeastern channel, the northwestern margin of the channel penetrated by the Harris 1-3 well is more gradual than the southeast margin. Thinning of the Morrow-Mississippian section towards the northwest of the Harris 1-3 well is indicated mostly by an amplitude decrease of the Morrow reflection.