Apologies if this is too off topic but to answer your...

Apologies if this is too off topic but to answer your question:

• Africa Oil Corp has a small stake in Venus via their ownership in Impact Oil and Gas (about 3.1% effective interest atm which may grow to around 3.8% shortly). They are now fully carried to production on Venus and have cash flowing assets in Nigeria as well. The stock is really cheap imo and is my largest holding. They are aggressively buying back shares.
• Sintana has small interests in multiple blocks in the Orange Basin including Galp's recent discovery at Mopane and the block immediately north of Venus. I don't hold any shares but it's one I'm interested in.

Neither is as cheap as Invictus but they are a less risky alternative with much smaller upside imo. I don't know of any other small companies with notable stakes but there may be more. Neither trades on the ASX; not sure if that's a limitation for you. Woodside has a stake in the Saturn prospect but they're pretty big so likely not material for them.