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Case Study: Covenant Field, Utah. VSP imaging sees under hopeless
surface volcanics.
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This velocity model, designed to emulate the Covenant Field in Utah (discovered by Wolverine Oil & Gas), is an imaging nightmare. The dips aren't particularly severe (50o at the most). It's the high velocity contrasts that really make things difficult. Tertiary volcanics, with a velocity of over 17,000 ft/sec overlie Tertiary sediments with a velocity of 9,000 ft/sec. This causes a critical angle of less than 30o. Thus reflected energy from the anticline structure (dipping at ~ 40o) is almost totally reflected down when it strikes the underside of the volcanics. The only viable spot where surface seismic can see through these volcanics is the small "window" shown in the Figure.
It gets worse. A steeply dipping high velocity layer (15,000 ft/sec) overlies
the much slower Arapien Shale section (8,800 ft/sec). Thus even energy from
the relatively flat basement rocks does not reach the surface. These
reflections are only imaged on the right side of the model.
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The wave equation shot record migration image of the surface seismic data
with the known velocity bears out the illumination nightmare. The center
of the section should contain a series of dipping reflectors, but it is blank.
The basement reflections visible on the lower right corner of the section are
not interpretable in the center of the section. This is more than a velocity
model building problem. The data we need to image these structures is
simply not recorded by surface seismic. We have two solutions: acquire
3D surface seismic data, looking for more "windows", or...
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...acquire and image an offset VSP. On the right, we have the
result of imaging a synthetic offset VSP. The well was placed in the middle
of the imaging problems. The aperture of the image is not very wide, but we
have successfully undershot and imaged the steeply dipping reflectors
which would have greatly reduced interpretive and model building risk.
We've also imaged the deep, flat reflectors, which are also crucial to
constrain both the interpretation and the average velocity.
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Interested in seeing more results from our Covenant Field example? We invite you to
download a PDF presentation
(4.3 MB) from our website.
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Not happy with your seismic status quo? Contact us today for a quote.
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