The Aneth oil field in southeastern Utah produces from a Pennsylvanian carbonate mound which forms one of the largest stratigraphic oil traps in the United States. Efforts in the past 20 years to locate this and other carbonate mounds using reflection seismology have been spectacularly unsuccessful in this area.

From very detailed isopach and structural maps made from well control, it was found that these carbonate mounds are characterized by an extremely abrupt thickening of the producing zone and a thinning of the overlying shale. Also, contrary to normal stratigraphic traps, the productive zone is actually less porous than off-mound non-productive carbonate rocks. To determine if these thickness changes would be detectable seismically, one- or two-dimensional seismic models were generated using wavelets of varying frequency content and phase. It was determined from these models that thickness changes which characterize carbonate mounds would be detectable by a decrease in amplitude of reflections from overlying shale owing to the tuning effect. However, this decrease in reflection amplitud was only found for zero-phase wavelets having a bandwidth of 10 to 70 Hz or more.

The significance of these results is that by using seismic amplitudes, we are able to determine the thickness of the shale bed even if the bed is less than 10 ft thick and surrounded immediately above and below by other thin shale beds. Since many, if not most, stratigraphic traps involve changes in bed thickness, these results suggest that with seismic modeling, zero-phase processing, and high resolution techniques many stratigraphic traps could be located with a higher degree of accuracy.

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