For several decades, US has been the dominant consumer for Saudi Arabia. So Saudi Arabia kept expanding their oil extraction capacities to 12.5 million barrels a day on the back of sustained increase in demand from US till the year 2008.
But the year 2009 saw the financial crisis pushing down the demand for oil from the largest customer i.e, US crashing by 10%. That pushed down the overall crude oil production by Saudi Arabia going down drastically. Also Saudi lost more in value terms as crude oil prices crashed from 110 dollars average to less than 60 dollars average.
So Saudi Arabia started looking for more dependable customer across the globe and found a good one in China.

In the year 2009, Saudi Arabia exported more oil to China than to US. Now Saudi Arabia is also planning to forge joint ventures with China in developing new oil fields wherein lifting commitments can be extracted apart from sharing of capital investments.
Already China has been having a decent investment in another Gulf major i.e, Iran and it is only a matter of time before China calls the shots in the Gulf Region. That does not augur well for the US and the Western world because China has not been truthful in arms and nuclear deals with the developing world.