(FORTUNE Magazine) – The violent strike cutting oil production in Venezuela has prompted a shudder, not a shock, in world markets, raising prices 20%.

Venezuela is the world's fifth-largest oil exporter and accounts for almost 10% of the U.S. crude supply. But thanks in part to the scramble for new oil sources after the 1973 crisis, world markets can largely absorb the hit when a producer dries up. Other suppliers kick in before prices get too outrageous--as OPEC has just done. Still, if a combination of events turned off more than one tap--the strike in Venezuela, say, plus a shutdown in Iraq or a coup in Russia--all bets would be off. And here's another cause for worry. The Mideast accounts for 30% of oil production but two-thirds of proven reserves. Given the troubles in that region, the prospect of ever greater dependency is, well, shocking. -- Cait Murphy