@porcupine87

We can make some calculations
let's see, I don't know the hash size, but say it is 256 bits, i.e. ~10^25 possible numbers.
You need aprox. 100 bytes to store a prime origin, so if you have 100TB of space, you can store up to 10^12 chains, disregarding how many years you need to find them, we will assume you already have them stored for now.

Now the chance that you have a good chain for a particular block(hash) is 10^12/10^25 = 10^-13
That's it, you will hit a suitable one every 10 trillion blocks!

Now on the time needed to calculate those chains, well if you somehow manage to get 1 million chains/day, you will need 1 million days, ~3000 years.

So, well...