I added the mod after about 15 hours of play on my Space Exploration save. I now have about 30 hours and have launched my first satellite, but despite going many chunks away, no offshore oil sources have been generated. Why could this be?

Not sure, could be a map-gen/autoplace issue, could be a collision mask issue preventing oil spawning on water (SE does a lot of collision mask shenanigans). Do you definitely have the Cargo Ships mod settings related to offshore oil set correctly?

Offshore oil can be extremely rare on maps without large oceans. It only generates when the default oil autoplacer tries to make oil in a 16x16 chunk that is entirely water. Also, chunks generate when pollution reaches them, not just when you explore them, so you might not have reached any actual new chunks yet.

To completely regenerate all the oil deposits in the game, including in chunks that have already generated, run the /regenerate-oil command. This resets all deposits to their starting richness too.