* Brent crude set for Q3 rise of nearly 16 pct
* U.S. inventories forecast to rise
* Iraqi oil minister suggests more output cuts possible (Updates with comment, refreshes prices)
LONDON, Sept 20 (Reuters) - Oil headed for its largest third-quarter gain in 13 years as prices rose on Wednesday after the Iraqi oil minister said OPEC and its partners are considering extending or deepening output cuts aimed at reducing a global supply glut.
Brent crude futures rose 64 cents to $55.78 a barrel by 1330 GMT, while U.S. West Texas Intermediate (WTI) crude futures gained 52 cents to $50.00.
The oil price is on course for a rise of nearly 16 percent this quarter, which would make this year's performance the strongest for the third quarter since 2004.
"With oil prices steadily appreciating, as investors become increasingly optimistic over OPECs effort to stabilize the saturated markets, the cartel should be encouraged to extend the current deal, which may fuel the upside," FXTM analyst Lukman Otunuga said.
The Organization of the Petroleum Exporting Countries and other producers are considering a range of options, including an extension of cuts, but it is premature to decide on what to do beyond the agreement's expiry in March, Iraqi oil minister Jabar al-Luaibi told an energy conference on Tuesday.
OPEC and non-OPEC producers including Russia have agreed to reduce output by about 1.8 million barrels per day (bpd) until March to reduce global oil inventories and support prices.
Some producers think the pact should be extended for three or four months, others want it to run until the end of 2018, while some, including Ecuador and Iraq, think there should be another round of supply cuts, al-Luaibi said.
Analysts, however, doubt that such an extension would have much of an impact on the overall oil market.
"I can't see the market tightening unless OPEC cuts output further next year," said Commerzbank strategist Carsten Fritsch.
Georgi Slavov, head of research at commodities brokerage Marex Spectron said he does not expect demand for crude oil to rise significantly in the final quarter of this year, which means supply would have to be restricted even more tightly.
U.S. crude stocks rose last week while gasoline and distillate stocks decreased, according to the American Petroleum Institute on Tuesday.
Crude inventories rose by 1.4 million barrels in the week to Sept. 15 to 470.3 million, compared with expectations of a 3.5 million barrel increase.
The U.S. Department of Energy releases official data on inventories and refinery activity later on Wednesday.
(Additional reporting by Christopher Johnson in LONDON, Aaron Sheldrick in TOKYO and Henning Gloystein in SINGAPORE; Editing by David Goodman and Louise Heavens)