* Gazprom Neft mulls resumption of output at mature field
* OPEC upbeat on outlook for 2018 demand
* U.S. stockpiles fell last week (New throughout, updates prices, market activity and comments; new byline, changes dateline, previous LONDON)
NEW YORK, Aug 10 (Reuters) - Oil prices fell on Thursday, on concerns of lingering global oversupply as Russia considered a future output resumption and OPEC boosted its July production numbers.
Russian oil producer Gazprom Neft considers it "economically feasible" to resume production in mature fields after a global agreement among OPEC and non-OPEC expires, a representative of the company said.
U.S. West Texas Intermediate crude was down 35 cents or 0.7 percent to $49.21 a barrel. Brent crude futures were down 16 cents or 0.3 percent to $52.54 a barrel by 12:12 p.m. ET (1612 GMT).
The Organization of the Petroleum Exporting Countries raised its outlook for oil demand in 2018 and cut its forecasts for output from rivals next year, yet another increase in the group's production suggested the market will remain in surplus despite efforts to limit supply.
OPEC said its oil output rose by 173,000 bpd in July to 32.87 million bpd, led by the exempt producers plus top exporter Saudi Arabia, citing figures it collects from secondary sources.
Crude prices are down nearly 7 percent so far this year, pressured by concern that output cuts by OPEC and its partners may not eliminate the global crude glut.
Saudi Arabia said on Tuesday it would cut supplies by up to 10 percent in September to most buyers in Asia, the world's biggest oil-consuming region.
In a sign that investors are turning more optimistic about the pace at which oil supply and demand are rebalancing, prices for crude for prompt delivery are trading above those for delivery further in the future.
"This is the march toward the flattening of the curve," said SEB chief commodity strategist Bjarne Schieldrop.
Global stocks remain above their longer-term averages and with the U.S. summer driving season nearly at an end, investors are well aware that the attempts by OPEC, Russia and other producers to boost prices may bring unwanted side-effects.
Inventories in the United States are at their lowest since October, having fallen for 10 of the last 12 weeks.
While prices rose on Wednesday after the lower U.S. inventory numbers, Gene McGillian, manager of market research at Tradition Energy, said that information was not enough to sustain a rally.
"It seems like the market wants to go higher," he said, "The market is searching for it, the question is will it get it."
(Additional reporting by Amanda Cooper in London, Aaron Sheldrick; Editing by David Evans and David Gregorio)