This is specific to drilling wells in the Green River Basin in
Colorado. There is an occurrence of a mineral called Nahcolite
(NaHCo3) or Sodium Bicarbonate. This mineral is suspect in causing
problems while trying to telemeter (transmit via signal) from a
downhole MWD (measurement while drilling) tool.  In particular the
signal is a EM (electromagnetic).  These wells must be drill using
Air/Mist instead of using a conventional drilling mud.  Hence the need
for EM telemetry.
Question is: What are the problems trying to transmit and receive
through NaHCO3 using EM Telemtry?  ie. what are the depth/range
limitations and what is the expected attenuation?

You could use a repeater higher up in the drill string to overcome the
attenuation.  How deep and thick is the NaHCO3?

Thanks for your comment.  The repeater is a possibility.  We will have
to evaluate the usefulness in terms of efficient implementation and
operation.
The NaHCl3 is seen on a log as shallow strings of about 1-10 feet,
with a percentage ranging from 10%-100%.  Vugular porosity is also
apparent with some occurrences of NaHCl3.  Which may present
additional impedence.

The occurrence of NaHCl3 begins at around 1400 feet and continues to
around 1700 feet.

http://pubs.usgs.gov/of/1991/ofr-91-0011-a/ofr-91-0011a.pdf

I can't find anything suggesting that nahcolite would attenuate the
signal.  It is  an evaporite and magnetically neutral.

There is a suggestion by an industry expert that the impedance of
NaHCO3 may be an issue.  I am skeptical about the impedance.  Do you
have any opinion on this?Thank you.

http://webmineral.com/data/Nahcolite.shtml