By Jim Jelter, MarketWatch SAN FRANCISCO (MarketWatch) -- Oil and gas stocks got a modest boost at the open Friday from a buoyant stock market as investors reacted to a surprise drop in the U.S. July unemployment rate, an encouraging sign for energy demand. Early action carried the NYSE Arca Oil and Gas Index (XOI 962.61, +3.14, +0.33%) 0.5% higher to 964 points, with Anadarko Petroleum (APC 49.79, +0.69, +1.41%) taking the lead among percentage gainers, up 2.4% at $50.28 a share. At the same time, however, energy stocks' upside momentum was being slowed by falling oil prices. September crude oil futures on the New York Mercantile Exchange were last down 71 cents, or 1%, at $71.23 a barrel as the U.S. dollar gained ground against other major currencies. Recent strength in the greenback has checked the further advance of dollar-denominated global oil prices. See Futures Movers. The NYSE Arca Natural Gas Index (XNG 463.81, +7.30, +1.60%) was up 1.2% at 462 points, with 14 of its 15 components in positive territory. The one decliner in the group was EOG Resources Inc. (EOG 75.32, -2.90, -3.71%) , down 3% at $75.16 a share. EOG reported late Thursday that it swung to a second-quarter loss on revenue of $861 million, below what analysts were looking for. The Philadelphia Oil Service Sector Index (OSX 171.89, +0.28, +0.16%) was 1.1% lower at 169.7 points, extending losses in what has been a rough week for the index overall. The sector is in course for about a 5% decline from a week ago as a batch of mediocre earnings and cautious full-year outlooks put an end to the group's July rally.

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