Jay Z should be THANKING the producer he’s accused of extortion  — or so the guy claims, telling TMZ, if it weren’t for him … the recordings  he’s allegedly holding for ransom would have been lost or destroyed … because  Jay Z was an irresponsible pothead back in the day.

TMZ broke the story  … Chauncey Mahan was detained by police Friday after Jay Z’s  people accused him of stealing the master recordings for several big songs back  in 2002 — and then trying to hold them for ransom — a $100,000 “storage  fee.”

But Chauncey insists the allegations are BS — telling TMZ, he  didn’t steal jack … Def Jam asked him to keep track of the  recordings in 2002 because no one else was doing it. According to Chauncey, Jay  Z “was a stupid kid smoking blunts” who was reckless with his intellectual  property.

What’s more, over the last 10 years, Chauncey says he’s  repeatedly notified Jay Z’s people that he was in possession of the masters, and  they blew him off. He says he finally asked for money last week when he could no  longer afford to keep the masters in storage.

Chauncey says he asked for  $100,000 to compensate him for keeping the recordings safe for 12 years. FYI,  the masters are for Jay Z’s “Volume 3” and  “Dynasty” albums — including huge songs like “Big Pimpin” and  “I Just Wanna Love U (Give It 2 Me).”

And this is interesting: Chauncey  says some of the masters sound awful … which might explain why Jay Z is  freaking out.

As we reported, police have since taken possession of the  tapes until a judge can determine rightful ownership.