Kansas Geological Survey, Open File Report 96-50
Detection of a moderately thick upper Morrow sandstone in the channel, as opposed to a channel which is filled with just shale or other non-producing lithologies, is more difficult. Interpretation of the single well model suggests that in a relatively small area in which not much varies laterally in the stratigraphic column from the top of the Cherokee to the Mississippian besides the presence or absence of moderately thick upper Morrow sandstone bodies in a relatively thin channel, lower resolution seismic data might be able to detect the presence or absence of the sandstone provided random noise levels are below 10%. The seismic anomalies associated with this difference is an increase in amplitude of the Mississippian reflection and trough above it, and a slight isochron increase as the moderately thick to thick sandstone body is replaced laterally by a lower velocity shale. These same seismic anomalies should also occur and be more obvious on higher resolution seismic data, even with random noise levels at 10% and possibly higher.
Over a wider area, and as lithologic or thickness variations in the Cherokee to Mississippian interval become more complex, detection of moderately thick upper Morrow sandstones in a thin to moderately thick channel becomes more difficult. Lateral changes in stratigraphy above or below the interval of interest can result in a confusing seismic signature and possibly lead to an erroneous interpretation. This is particularly a problem for lower resolution seismic data, particularly where noise levels are at or above 10%. This problem is not as bad for higher resolution seismic data because the wavelett is affected by a thinner region due to its smaller dominant wavelength. Even so, relatively large variations in stratigraphy within the Morrow section or slightly above it can affect the reflectivity and complicate the interpretation. Considering this, it would be wise to use the seismic method for detecting moderately thick (8-10 ft.; 2.4-3.0 m.) upper Morrow sandstones only in areas where relatively good well ties (including sonic and possibly density logs) are available to help with the identification of useful seismic anomalies. Of course the thicker the sandstone is, or the greater change in velocity between it and the adjacent shale, the easier it will be to detect it. However, the maximum thickness of an upper Morrow sandstone penetrated in a well is not much more than 15 ft. (4.6 m.), so even in the best circumstances, detection will not be easy.
The lower resolution multi-well model suggests that identification of moderately thick upper Morrow sandstones in an environment where the stratigraphy outside the area of interest varies laterally is unlikely using lower frequency seismic data. However, given suitable well control, interpretation of the higher resolution multi-well model suggests that detection of moderately thick upper Morrow sandstone bodies should be possible with higher resolution seismic data. Although tuning effects due to channel thickening may complicate amplitude interpretation, these effects may be taken into account by knowing if the Morrow-Mississippian interval in question is above, below, or near tuning thickness. This can be done with the aid of some well control. Modeling suggests that in general, on higher resolution seismic data, amplitudes of the Mississippian peak and overlying trough will increase less when a moderately thick to thick upper Morrow sandstone is present than when the channel is filled with shale, as long as channel thickness at or below maximum tuning thickness. This observation may also hold for channel thicknesses above the maximum tuning thickness but it is not clear. Along with this anomaly, channels containing a moderately thick to thick upper Morrow sandstone should show a slightly thinner isochron anomaly than an equivalent thickness channel containing only shale. The magnitude of the difference would depend on the thickness of the sandstone and the difference in velocity between it and the adjacent shale. Interpretation of the higher-resolution multi-well seismic model also suggests that for moderately thick (~66 ft.; 20 m.) channel, the appearance of an extra peak nearly midway between the middle Cherokee and Mississippian peaks and below the major trough between the middle Cherokee and Mississippian reflections, suggests that a moderately thick upper Morrow sandstone may not be present.
In conclusion, the seismic well log models described in this report suggest that both lower frequency and higher frequency seismic data should be able to detect relatively thin, to thick Morrow channels in the Minneola complex. However, detection of moderately thick (8-10 ft.; 2.4-3.0 m.) upper Morrow sandstone bodies in thin (21 ft.; 6.4 m.) to moderately thick (66 ft.; 20 m.) Morrow-Mississippian channel fill sequences in the Minneola Complex would be extremely difficult if not impossible except under the most ideal conditions using the lower resolution seismic data that currently exists within the complex. Higher resolution seismic data on the other hand may be able to detect moderately thick or thicker upper Morrow sandstones provided sufficient well control is available to determine the difference in seismic signature between a channel which contains the sandstone, and one in that is filled with just shale or other non producing lithologies. Well control is also needed to identify lateral changes in stratigraphy outside the interval of interest that may effect the seismic signature within the interval of interest.