Marathon Oil Corp will buy oil and gas properties in Texas' Eagle Ford shale field $3.5 billion from private equity firm KKR and Hilcorp Resources Holdings LP, the companies said on Wednesday.

The deal for the 140,000 acres in the Eagle Ford shale is the latest in a frenzy by energy producers to snap up oil and gas properties that were too difficult for the industry to tap into only a decade ago.

Eagle Ford has emerged as the hottest of those North American shale fields, since much of its output is oil, which remains above $100 per barrel, rather than natural gas that flows from fields such as the Marcellus shale.

With the new agreement, Marathon's overall holdings in the Eagle Ford will total more than 285,000 acres by the end of the year, it said, giving it potentially 100 million barrels of oil equivalent in proved reserves.

Marathon is a currently an integrated oil company like its larger peers Exxon Mobil and Chevron Corp , owning both oil and gas production assets and refineries, but it is planning to spin off its refining assets into a separate company at the end of June.

KKR's stake in Hilcorp will be valued at $1.13 billion, nearly triple the $400 million the firm invested in the company a year ago.

The Hilcorp properties, which are located primarily in Atascosa, Karnes, Gonzales and DeWitt counties, had 36 producing wells at the beginning of May, with an output of 7,000 net barrels of oil (BOE) equivalent per day.

Year-end production is expected to be 12,000 net BOE per day, and is likely to rise to 80,000 BOE per day by 2016.

Marathon will use cash on hand to pay for the assets, which will immediately contribute to earnings and are expected to be self-funding by 2014, it said.

Marathon was advised in the deal by Barclays Capital and law firm Baker Botts, while Hilcorp was advised by Jefferies & Co and law firm Andrews Kurth LLP. KKR's law firm was Simpson, Thacher & Bartlett, LLP.

Marathon's shares fell 1.6 percent in premarket trading.

(Reporting by Matt Daily, editing by Dave Zimmerman, Maureen Bavdek and Derek Caney)