By Shariq Khan

NEW YORK, May 20 (Reuters) - NextEra Energy NEE.N has agreed to buy U.S. oil and gas investment firm Caliber Resource Partners for $1.3 billion and has separately set up a joint venture with Caliber's private equity backer, Quantum Capital Group, to manage the power company's U.S. shale assets, four sources told Reuters on Wednesday.

The deal gives NextEra greater access to U.S. natural gas supplies just days after it announced a $67 billion deal to merge with Dominion Energy D.N that would make it one of the world's largest electric utilities and suppliers of power to data centers. The data center boom is widely expected to raise demand for natural gas as a fuel for power generation.

A subsidiary of NextEra will take over Caliber's assets, which include passive stakes in oil-and-gas producing assets spanning multiple onshore U.S. shale basins, the sources said. Known in the industry as 'non-operated' interests, such stakes allow holders to earn a share of revenue from the sale of hydrocarbons by contributing a portion of drilling costs, without active involvement in the drilling process.

The Caliber assets, alongside other natural-gas producing assets NextEra already owns through its Trinity Operating arm, will be transferred to a new joint venture company that NextEra and Quantum have agreed to form, one of the sources said. The joint venture, to be called NEQ Operating, will be equally owned by NextEra and Quantum, the source said.

The sources requested anonymity to discuss confidential details. NextEra, based in Juno Beach, Florida, did not immediately respond to a request for comment. Quantum declined to comment.

The joint venture with Quantum will focus on further investments aimed at expanding NextEra's portfolio of natural-gas producing assets, one of the sources said. Alan Smith, managing director at Quantum Energy Partners, will be appointed executive chairman on a temporary basis until Quantum finalizes a full management team to oversee the JV's operations, the source said.

Industry publication Hart Energy first reported on Wednesday that NextEra was in talks to form a JV with Quantum.

Quantum has also agreed to fund the next iteration of Caliber, which will be led by the company's current management team, two sources told Reuters.