The US does indeed consume over 20% of the world's oil. But that is down from over a third in 1969. China used to consume 1% of the world's oil that same year. Last year they were about to 11%. Those figures are going to continue to converge. Infact, by 2030, China may consume more oil than the US.
Oil supply is not falling as you suggest. Oil supply is growing, as is Saudia Arabia's production:

The problem is two fold. One, as Lore said, worldwide demand continues to grow despite the US and EU using less oil. In other words, the rest of the world's oil consumption is growing faster than our's is shrinking. And consumption is growing faster than supply is growing(much of the spare capacity in the world is gone). Oil supply growth has slowed to a trickle compared to what it has been for the last several decades.
The other issue is the one Planty was talking about: oil production costs are growing. We picked the low hanging fruit first when it came to oil. We went straight for the super giant oil fields that would spout oil into the air just by poking a hole in the ground. And for decades, those fields provided us with plentiful oil with little effort. But these fields are starting to age. They are requiring more effort to get the oil out such as gas injection. These production methods have higher costs. Even worse, we stopped finding new super giant oil fields. The last super giant oil field discovered was Cantarell in 1976. Now that the super giants are ageing, we need to replace them. Unfortunetly the new batch of oil fields we are finding fall far short of the ones we found decades ago. They are smaller in size, located in more difficult terran(deep ocean, arctic, etc) are not really oil(tar sands, oil shale, etc), and are generally much more expensive to produce oil from. And if oil prices ever fall below what is profitable for these expensive oil fields to produce from, they shut in production until prices rise again.
$this->bbcode_second_pass_quote('', 'd')espite declining demand in some countries that historically were heavy users of oil, the world demand for oil seems likely to continue to rise. The I.E.A. forecast that global energy demand — including demand for energy produced by other sources — is likely to rise by 35 percent by 2035, with a large part of the increase coming from China and India.
In 1969, the United States consumed a third of the oil used in the world, while China used less than 1 percent. Last year the United States’ share was less than 22 percent, while the Chinese accounted for 11 percent. The I.E.A. forecasts that by 2030, the American share could be less than the Chinese one.
By 2035, American consumption of oil is expected to be as much as one-third less than it was last year. In China, oil consumption is expected to be up as much as two-thirds from the 2011 level, and India’s is predicted to more than double.
$this->bbcode_second_pass_quote('', 'G')hawar is one of only six super-giant oil fields that have produced more than 1 million barrels per day at their peak. Discovered in 1948, and just 174 miles long by no more than 31 miles wide, Ghawar is an extraordinary structure. “It is unlikely that any new oilfield will ever rival the bounteous production Ghawar has delivered to Saudi Arabia and the international petroleum markets” No other super-giant has been discovered in the last 35 years (the last was Cantarell in 1976). Conventional super-giants such as Ghawar may never be discovered again, although exploration is pushing into new areas offshore and in the Arctic.