Vanguard Natural Resources LLC, a U.S. oil-and-gas producer, has agreed to acquire smaller rival Eagle Rock Energy Partners L.P. for $474 million plus debt, the two Houston-based companies said Thursday.

The deal gives Vanguard a bigger presence in Oklahoma, where the company already pumps oil and gas, Vanguard chief executive Scott Smith said.

"The assets being acquired are attractive bolt-ons to our Mid-Continent, Permian, and Gulf Coast Basin operations," Mr. Smith said, referring to Vanguard's existing drilling operations in Texas, New Mexico, Colorado, Wyoming and several other states.

Terms of the deal calls for a subsidiary of Vanguard to merge into Eagle Rock for $474 million in stock and assume Eagle Rock's net debt of $140 million. Following that combination, Eagle Rock will be a subsidiary of Vanguard.

Joseph A. Mills, chief executive of Eagle Rock, said the agreement gives Eagle Rock shareholders a 24% premium over Thursday's closing price of $2.45 a share.

"We have been very impressed with the Vanguard team and look forward to working together to successfully combine our two companies," he said.

This is the second acquisition announced by Vanguard in the past month. In April, the company said it would acquire LRR Energy L.P. for $539 million in a deal similar to the one with Eagle Rock.

Write to Erin Ailworth at Erin.Ailworth@wsj.com

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