NEW YORK, May 2 (Reuters) - Planned month-long maintenance at a major BP gas processing plant in Pascagoula, Mississippi, will shut in a "tremendous" amount of Gulf of Mexico oil production, the chief executive of Plains Exploration & Production said on Thursday. Plains said it had been notified by BP that the plant would be shut for 36 days beginning on May 3; the planned maintenance had been announced in 2012. The plant processes production from wells in the eastern corridor of the deepwater Gulf of Mexico. "It is shutting in a tremendous amount of oil production in the Gulf of Mexico," Jim Flores, chairman, president and CEO at Plains, told analysts on a conference call following earnings. "It's shutting in Thunder Horse, Ni Kika, Horn Mountain, Marlin and so forth for 36 days. So you'll probably see the HLS (Heavy Louisiana Sweet) crude oil tighten." He did not provide an estimate for the total shut-in, but BP-operated Thunder Horse is the biggest platform in the world with the capacity to produce 250,000 barrels per day (bpd) and 200 million cubic feet per day. Na Kika, also operated by BP, can pump 110,000 bpd. Asked about the maintenance and its impact on upstream oil production, a BP spokesman said the company typically does not comment on day-to-day activities. Net average daily sales volumes from the Horn Mountain platform and the Marlin Hub, both of which Plains bought from BP last year, was 44,000 barrels of oil equivalent per day in the first quarter of 2013, Plains said.