I'm not sure this is really about oil in the Spratley Islands anyway. Sure, the pretext is there, but this may be more about China extending its maritime defensive perimeter -- the so-called string of pearls. China's maritime claims go hand-in-hand with their activity in 2001 (EP-3 take down) and in 2009 (USNS Impeccable incident). In both cases, just like with their maritime claims, the design seems to be to have more stand-off against the US Navy.

With some exceptions, China has settled its land disputes and now turns to the sea. By effectively claiming the south China sea, China can also move a step closer to securing maritime energy transport (their real goal I think). They are building a naval base in Gwadar, Pakistan that will provide security near the Persian Gulf for their shipments. Securing the South China Sea bookends the transport route. Next comes the straight of Malacca which becomes easier to secure if you have naval might on both sides of it.

I'm not saying China can challenge US naval supremacy now, but they are taking action that could be bothersome. In a time of financial incertitude in the US, this may be the perfect time for China to act.