Two-time African champions Wydad triumphed 4-2 on aggregate having won the first leg of the semi-final 3-1 in Luanda last Saturday.
Wydad will confront title-holders Al Ahly from Egypt or Entente Setif from Algeria in the May 30 final with the Cairo club defending a 4-0 first-leg lead in Algiers on Saturday.
The final is scheduled for Morocco, but Ahly have asked the Court of Arbitration for Sport (CAS) to overturn the Confederation of African Football (CAF) decision, citing the need for a neutral venue.
Playing with confidence and energy lacking in the first leg, Petro went ahead on 21 minutes through Gleison Moreira, one of three Brazilians in the starting line-up.
Amine Farhane levelled seven minutes later at the Stade Mohammed V in the Moroccan commercial capital, and while Petro had more possession thereafter, they could not score again.