Will:
IMHO, where the drilling mud comes from has nothing to do with the overall analysis of the mud and materials and its affects on your crop. Not that you don't already know this; but, the wheat crop is growing on/in the surface soil and this drilling mud is basically made up materials formulated for the drilling operation, plus the rock fillings from the hole in the ground. I believe the Texas A & M link that TennisDaze posted does a great job of making the case not to spread it on your crop fields.
Good Luck!