Recent high-profile deals allowing foreign drillers to tap Venezuela’s vast oil reserves have failed to quell lingering concerns about contract sanctity in a nation with a history of nationalization, said energy investment banker Dan Pickering.

Restoring daily Venezuelan oil output to the 3 million-barrel mark not seen in roughly 15 years will require major injections of foreign cash, and it’s not yet clear if overseas drillers can make a go of it in the South American nation, according to Pickering, chief investment officer at Pickering Energy Partners.

“It’s going to have to be a win for the companies that ...