European shares slip; CGG tumbles 15 pct

* FTSEurofirst 300 down 0.4 pct, Euro STOXX 50 down 0.6 pct

* CGG (NYSE: CGG - news) 's profit warning sparks sell-off in oil services

* All eyes on U.S. Federal Reserve's policy meeting

* Risks seen on the upside ahead of Fed tapering decision

By Blaise Robinson

PARIS, Dec 17 (Reuters) - European shares fell on Tuesday, trimming some of the previous session's gains, before a two-day Federal Reserve meeting where the central bank may decide to start tapering down its monetary stimulus.

Oil-services shares were among the top losers after a profit warning from CGG, a French seismic surveyor. GGG shares plummeted 15 percent in massive volumes.

At 1139 GMT, the FTSEurofirst 300 index of top European shares was down 0.4 percent at 1,253.89 points.

"Tapering or not tapering? For now it looks like some kind of reduction in the quantitative easing tomorrow has been priced in already," said David Thebault, head of quantitative sales trading, at Global Equities.

Some robust U.S. economic data, combined with last week's budget deal in Washington, have sparked speculation the Fed will announce this week that it is reducing its $85 billion a month of bond purchases. That has triggered profit-taking in stocks, which benefited from the Fed's massive liquidity injection.

The FTSEurofirst 300, which was up 15 percent on the year at the end of November, has dropped about 4.2 percent since then. The benchmark index gained 1.3 percent on Monday in what traders described as a short-covering rally following the sharp two-week slide.

"But the U.S. unemployment rate is still high while inflation pressures are very low, so chances are the Fed will wait a few more months before cutting stimulus," Thebault said. "In that case, risks are on the upside for stocks ahead of the Fed, and buying call spreads is a pretty good idea."

Around Europe, the UK's FTSE 100 index was down 0.4 percent, Germany's DAX index was down 0.4 percent and France's CAC 40 was 0.9 percent lower. The euro zone's blue-chip Euro STOXX 50 index was down 0.6 percent, at 2,959.70 points.

The Euro STOXX 50 Volatility index, Europe's 'fear gauge', hit a two-year high earlier on Tuesday, signalling a rise in risk aversion.

Shares in European oil-services companies dropped after CGG lowered its profit target, citing customers pushing back major orders. Technip (Paris: FR0000131708 - news) was down 4.6 percent, Petroleum Geo (Oslo: PGS.OL - news) -Services down 4.3 percent, Petrofac (Berlin: P2F.BE - news) down 2.3 percent and Saipem (Other OTC: SAPMY - news) down 1.9 percent.

"The oil-services sector is in a tough spot, with very low visibility on capital expenditures from the big oil companies, which are not sure where oil prices are going," said Bertrand Lamielle, the head of asset management at B*Capital (Other OTC: CGHC - news) .

Hedge funds have been increasing their short selling of a several oil-services companies recently, according to data from Markit. About 9 percent of CGG shares are out on loan, up from 2.9 percent in late September.

Short selling, a strategy popular with hedge funds, involves selling borrowed shares in the hope of being able to buy them back more cheaply later and pocket the difference.