** Shares of oil and gas producers and oilfield services providers down 7-12 pct
** Brent crude oil fell towards $60/barrel, reversing gains after Saudi Arabia indicated could increase output.
** U.S. crude down 2.6 pct at $55.62/barrel
** Oil and gas producers Ultra Petroleum Corp shares down 12 pct, Swift Energy Co shares down 10 pct
** Oilfield services provider Basic Energy Services Inc shares down 8.7 pct, Precision Drilling Corp's U.S.-listed shares down 8 pct, Nabors Industries Ltd shares down 7 pct