Hi Sam,
I would rewrite the piece like this:
China is adapting its oil policy as a result of the current Mid-East crisis.
Though it is the third largest oil consumer in the world after the United States and Japan, China has no oil reserves of its own. With the conflict in Iraq causing oil prices around the world to soar, Peking has responded by setting up a system of storage tanks with a 90-day supply of oil. The represents an investment of 1.5 billion Euros. Peking is also investing abroad, and has recently purchased more than 16% of an oil deposit in Kazakhstan, in an attempt to decrease China's dependance on the Middle East.
Thanks to these wide-ranging moves, China has joined the most important oil company in the world.

Reasons: the first line is quite boring, better to leap in right away with a stronger statement.
You have to say it is the third "largest" consumer - third oil consumer just suggests that it was the third country ever to consume oil.
I wanted to avoid starting two sentences with China, so I consolidated those two sentences.
I am not sure what the "with cave" means, so I just dropped that, and rewrote that sentence to make it a bit clearer.
I would rethink that "more than 16%" - it's such a precise figure that readers would wonder why you don't just give the exact figure instead of "more than". I would either change it to "more than 15%", or find out the exact percentage.
"Big" is a rather lame adjective, rather like "nice" or "good", so I would either leave it out or change to something more dramatic, like "huge" or "enormous", or even better, find out exactly how large it is and just say that.
"in order to decrease 'his' dependance": if referring to Peking, it should be "its", if referring to the Chinese people it should be "their".
To avoid repeating 'investments' I changed it to 'moves'.
Not really sure what you mean by the last phrase. Do you mean "China has become one of the most important countries in the global oil market"? I think that's what you mean but it is not clear at all. It sounds like China has joined an oil company: what oil company? You haven't mentioned one elsewhere.

Hope you find this helpful!
Carla