For decades, the Organization of the Petroleum Exporting Countries, better known as OPEC, has wielded extraordinary power over the global oil market. The organization, which consists of 12 member countries, accounts for roughly 40% of total global oil production and controls a little over 80% of the world's oil reserves.

But the recent surge in non-OPEC oil supplies, led by the North American shale oil boom, and the risk of a downturn in the global economy pose significant threats to the cartel's continued prosperity. Let's take a closer look.

Growth in non-OPEC supplies
Thanks to the application of advanced drilling techniques, North American oil producers have rapidly increased production of crude oil from unconventional shale plays, such as Texas' Eagle Ford shale and North Dakota's Bakken shale.

For instance, Kodiak Oil & Gas (UNKNOWN:KOG.DL), an oil producer with operations located almost exclusively in the Bakken, has grown its production by a 168% compounded annual rate since 2010, while Halcon Resources (NYSE:HK), another producer with significant operations in the Bakken, managed to grow oil production last year by a staggering 173.2%.

Meanwhile, in the Eagle Ford shale, companies such as Chesapeake Energy (NYSE:CHK) and ConocoPhillips (NYSE:COP) continue to report similarly impressive growth in output. In the second quarter, Chesapeake reported a remarkable 135% year-over-year increase in Eagle Ford production to 85,000 barrels of oil equivalent per day, while Conoco almost doubled its output from the play to 121,000 barrels per day.

Non-OPEC vs. OPEC supplies
It should come as no surprise, then, that U.S. crude oil production is expected to grow by 830,000 barrels a day, or 8.1%, to 11 million barrels a day next year, according to the International Energy Agency. Similarly, non-OPEC oil production is forecast to rise by 1.7 million barrels a day next year to 56.4 million barrels a day, an increase of 300,000 barrels a day from the agency's previous forecast.

By contrast, OPEC oil production fell by 645,000 barrels a day last month to less than 30 million barrels a day, the cartel's lowest level of production in nearly two years. Sharp decreases in output from Libya and Iraq led the overall production decline, even as Saudi Arabia -- the organization's largest producer -- ramped up production to more than 10 million barrels a day for the third month in a row.

Risk of weaker global economy
In addition to supply-side issues caused by the North American shale boom and increasing non-OPEC supply elsewhere, there is another major risk that has OPEC concerned: a weaker global economy that could reduce global demand for oil.

Though OPEC forecasts global economic growth to rise by about half a percent next year, it remains cautious because of unresolved issues in Europe's labor market, slowing growth in China and India, and the possibility that the Federal Reserve will taper its $85 billion-a-month bond-buying program.

Because of these and other concerns, the International Energy Agency recently reduced its forecast for global oil demand next year by about 100,000 barrels a day. The agency now expects global demand to rise by just 1.1 million barrels a day, or 1.2%, to 92 million barrels a day in 2014.

The bottom line
Though OPEC has recognized the threat of surging non-OPEC oil supplies, a downturn in the global economy is perhaps the biggest threat facing the cartel. If global oil demand falls, leading to a sharp decline in its price, it will almost certainly deal a blow to OPEC, as well as other large oil exporters.

Not only would it worsen their trade balances, but it could also put member countries' national budgets under tremendous stress since many of them depend on oil prices around $100 a barrel to keep their budgets balanced and to sustain large welfare payments to their citizens.