Chinese investment in Afghanistan is nothing new, and they have been involved with the Afghan owned conglomerate, the Watan Group most notably to extract steel.
http://www.watan-group.com/steel?php
The Chinese have large amounts of money to invest (we haven't) and Afghanistan has resources which need large amounts of money to exploit.
And there may be no spoils to take. No sizable oil fields currently exist in Afghanistan, and non may exist. The Chinese are desperate for oil to fuel their industry, and this is a punt on a possible new source, but could quite easily be a futile quest.
British and American companies are more interested in piping oil through Afghanistan from neighbouring landlocked companies. That is a better bet than hoping there may be oil there.