Just to be specific, the East Falkland Oil Fields, at that time, were rumoured to be on a similar scale to the North Sea oil fields, and the Falkland Islands were the nearest lump of land where oil terminals could be built.
Later it came out that the oil fields were nowhere near as high quality as they originally thought, but that was not yet publically known.
Whether this was known to insiders is a question that has never been answered.