Hi Tomek,
A reservoir is often used to refer to the body of water and is the source of our drinking water. I suppose you can use it metaphorically to refer to what the person draws on to replenish to a normal level of supplies.
A reserve is an emergency supply of something that we use when our usualy supplies run out.
A cache has a sense of a secret supply of something. You may find a cache of candy bars is in the desk of a choco-holic at work.
A stash is very similar to a cache.
A treasure trove (use "trove") is a large collection of things that are delightful.
Something that is incomprehensible is so hard to understand that you are uanable understand it, while something that is unfathomable is simply incapble of being understood. If I do a bad job of explaining something, it may be incomprehensible, but a more competent teacher can make it understood, while some mysteries of life are simply unfathomable.
I hope you will not take offense at this, but the line you're striving for sounds pompous and overblown, like typical academic writing. It actually sounds like you're tring to describe someone (a professor, perhaps?) who seems to have a limitless ability to spout incomprensible drivel. He draws from his seemingly limitless reservoir of incomprehensibility.