Some deals only happen once in a blue moon. Others only come around once in a generation. But the one Wall Street bankers are eyeing in the deserts of Saudi Arabia is going to need a new set of superlatives.

That would be the planned initial public offering of state-owned oil giant Saudi Aramco, by virtually any measure one of the largest enterprises on earth. The kingdom has said that Saudi Arabian Oil Co, as it is formally known, could be worth $2 trillion to $3 trillion - roughly equal at its midpoint to the total market value of every other publicly traded oil and gas company in the world, according to S&P Global Market Intelligence.