Oil is, today, a fungible commodity. The notion of a dawning mercantilism suggests that its fungibility may be short-lived. The whole point of gaining an equity stake--as with the other current trend toward long-term, bilateral supply contracts--is to remove your supply chain from the uncertainties of a global marketplace. Fungibility defines what is on the market, but to a far lesser extent supplies that are locked down into equity stakes or long-term contracts.

Remember, in a market environment, the value to a buyer of an essential commodity is not just in the average price (lower is better), but also in reducing variance (lower is better) of this price.

Classic option theory.

Consider a parallel situation with electricity demand. It's even worse than oil since it can't be stored at all (as opposed to having capacity to store a very small fraction of yearly consumption).

If alot needs to be purchased on the spot market then the variance will be very high and the consumer is subject to intermitted insane spikes which will hurt them economically.

China is interested in reducing the variance of its costs, not just optimizing the mean.