* Oil falls 6.5 percent, Exxon tumbles in premarket
* Dow on track to turn negative for the week
* Dow down 34 pts, S&P down 4.5 pts, Nasdaq up 7.75 pts
By Ryan Vlastelica
NEW YORK, Nov 28 (Reuters) - U.S. stock index futures were lower on Friday, with energy shares likely to be under heavy pressure in a holiday-shortened session as crude oil tumbled to a four-year low.
* Wall Street was closed for the U.S. Thanksgiving holiday on Thursday and will shut three hours earlier on Friday. Trading volatility could spike since many market participants were extending the holiday.
* U.S. crude futures slumped 6.5 percent to $68.93 per barrel. It was the biggest one-day drop for crude since May 2011, and brought prices to their lowest since 2010. On Thursday, OPEC decided not to cut output, which could leave oil markets heavily oversupplied.
* Energy shares have been under pressure throughout 2014 as crude prices has slid almost 36 percent from a recent closing high. The S&P energy sector is the only S&P 500 industry group to be lower for the year, down 4.3 percent.
* The Energy Select Sector SPDR exchange-traded fund fell 4.7 percent to $81.31 in premarket trading while Dow component Exxon Mobil Corp lost 4.5 percent to $90.20, on track for its biggest one-day drop since August 2011. Chevron Corp fell 4.3 percent to $100.20 while Halliburton Co lost 6 percent to $44.50.
* The weakness in oil could be a boon for the holiday shopping season with lower gas prices possibly sparking increased consumer spending.
* The decline in oil boosted airline shares premarket. American Airlines Group Inc rose 4.9 percent to $47.18 before the bell while JetBlue Airways Corp was up 6.6 percent to $14.50. Delta Air Lines Inc jumped 5.1 percent to $46.50.
* The premarket decline suggested major indexes may not post a sixth straight weekly advance. For the week so far, the Dow is up 0.1 percent and the S&P is up 0.5 percent. The Nasdaq is up 1.6 percent, boosted by strength in technology shares, a trend that continued on Friday and lifted Nasdaq futures. Cisco Systems Inc rose 0.6 percent to $27.59.
* Equities have been strong, with the Dow and S&P 500 both closing at records on Wednesday.
Futures snapshot at 6:46:
* S&P 500 e-minis were down 4.5 points, or 0.22 percent, with 149,545 contracts changing hands.
* Nasdaq 100 e-minis added 7.75 points, or 0.18 percent, in volume of 25,684 contracts.
* Dow e-minis were down 34 points, or 0.19 percent, with 27,490 contracts changing hands. (Editing by Jeffrey Benkoe)