Shale might be tough.

If it has natural fractures or joints, then those are typically very difficult to seal.

Are there any ponds in the immediate area surrounding your property? That would be a good sign.

Also, do you have good clay on the property? If you do encounter permeable bedrock in your pond basin, it is possible to seal it off with a clay blanket installed in a few compacted lifts.

Good luck on your project!

(If you post more questions or plans for your pond, then you may want to start a new thread.)