What oil markets bring, they can easily take away. Shares of U.S. integrated oil companies, which until recently had benefited from soaring crude, were hard hit by the International Energy Agency's massive release of oil reserves Thursday, compounding a weakening trend that had started in early May.

Those companies that produce much more oil than natural gas were particularly hurt; Chevron Corp. (CVX), which has enjoyed Wall Street's attention because of its high-profile deep-water oil projects, was down about 3% to $97.99. But ConocoPhillips (COP), which has a greater share of natural gas production, did slightly better, falling 1.96% to $71.85.

ExxonMobil Corp. (XOM), the world's largest publicly traded oil company, slid 3.03% to $71.41. Exxon is down about 11% from its May 2 valuation; Chevron's shares are down about 10% while ConocoPhillips is down about 8.4% from the same date.

Shares of Occidental Petroleum Corp, (OXY), a large California oil producer highly exposed to oil price moves, was down 2.67% to $98.29. But the stock has already lost about 12% of its value since early May due to the decline in oil prices.

Smaller oil companies, such as Anadarko Petroleum Corp. (APC) and Devon Energy Corp. (DVN), were also hit, but they represent a buying opportunity, says Shubash Chandra, an analyst with Jefferies & Co. Their current price reflects the perception that oil could go as low as $85 per barrel, Chandra said. Anadarko is down 2.60% at $70.73, while Devon stock is down 2.55% at $76.10.

Some companies that produce mainly natural gas also saw their shares drop, but less: Quicksilver Resources Inc. (KWK) was trading at $14.76, down 1.47%--that's a smaller drop than the general market, which was down 1.83%. EXCO Resources Inc. (XCO), another natural gas producer, was down 1.71% to $19.01.

The release of crude from oil reserves should have boosted refinery share prices because their operational cost goes down--but a pessimistic view of the job market kept a lid on them. However, oil under $100 per barrel should in the medium term help improve the situation. "Lower oil prices should stimulate purchasing at the pump," said Morningstar analyst Allen Good.

Valero Energy Corp. (VLO) saw shares drop 2.9% to $24.23. Sunoco Inc. (SUN) slid 2% to $39.71, and Tesoro Corp. (TSO) shares were down 1.3% at $21.78.

-By Isabel Ordonez, Ben Lefebvre and Angel Gonzalez, Dow Jones Newswires; 713-547-9214;angel.gonzalez@dowjones.com