What’s So Secret About Saudi Aramco?

Saudi Aramco, the giant oil company, has plans to buy a majority stake in Saudi Basic Industries Corp., the giant petrochemical manufacturing firm. But now they’re thinking twice:

Saudi Aramco no longer plans to launch what would have been one of the world’s largest-ever corporate-bond sales to fund a roughly $70 billion stake in the kingdom’s national petrochemical firm….People familiar with Aramco’s financing discussions say the oil firm is now worried about the level of disclosure required for a bond issue and whether the uncertain outlook for the oil market might damp demand for debt or increase the cost of borrowing.

Isn’t that intriguing? I wonder what Aramco is so eager to hide? Might it have anything to do with the actual size of their proven reserves?