HOUSTON, Dec 10 (Reuters) - Exxon Mobil Corp (XOM.N) outlined a major new investment in Guyana amid government calls for delivering economic benefits to residents from deepwater oil and gas discoveries helping turn the South American nation into an oil powerhouse. A coming onshore supply base will expand jobs and boost local fabrication for future projects starting with its fourth production unit, Exxon"s president for Guyana Alistair Routledge said in remarks late Thursday. It will bring total investment in Guyana to $30 billion, he told a Georgetown Chamber of Commerce and Industry gathering. Guyana is still reviewing needed environmental permits for the production consortium"s future project, called Yellowtail, and officials have called on Exxon and partners to add financial guaranteesand local content to win needed approval. "It is difficult to overestimate the scale of what is taking place," Routledge said in Georgetown before a group that included Guyana Vice President Bharrat Jagdeo. Private sector investment offers "an enormous opportunity to transform the overall economy." Exxon is also planning a gas-to-energy project that is could reduce the cost of electricity, according to Routledge. An Exxon-led consortium with Hess Corp (HES.N) and CNOOC (0883.HK) has more than 3,200 Guyanese working on its operations and some 800 local suppliers, he said. The group discovered 10 billion barrels of oil and gas in deep waters off Guyana. A Hess spokesperson referred questions to Exxon, whose spokesperson Meghan Macdonald said Exxon issued a request for information for the shorebase, the second planned in Guyana, and is currently in the process of evaluating the submitted proposals. Exxon last month said Guyana production could hit 750,000 barrels per day (bpd) in 2026, from 120,000 bpd today. Hess separately put total output at 1 million bpd by 2027. Most of Exxon"s "entire supply chain for our offshore activities have been relocated to Guyana", moving "activities from the United States, Brazil and Trinidad and Tobago," Routledge said. (This story has been refiled to correct spelling of Exxon spokesperson to Macdonald in paragraph 7) By Sabrina Valle in Houston and Neil Mark, additional reporting by Marianna Parragua.