From my understanding you have 3 levels of fuel within a barrel of oil. You're highest grade (WTI), Brent blend and Opec basket oil, the last of which is the heavy fuel oil, meaning the oil breaks down much slower and retains many more chemicals as a result than lighter oils.
The UN resolution states that as of Jan 1, 2020, cargo, industrial and cruise ships found to be using heavy Opec basket oil will be impounded. Operating costs for companies to convert to medium Brent crude grade oil will rise by at least 4%, which, from a investor viewpoint means choppy times and opportunities ahead, because guaranteed some random say Iranian ship pulls into port in a western country, gets tested and be found to be in breach of fuel emittance laws.
A ship that's out of action indefinitely means companies will inevitably have to go bankrupt (due to huge exposure to debt levels that most global companies are exposed to already), meaning opportunities.
Already have my eye on one such global shipping company. Now we sit back and wait...