NEW YORK ( TheStreet) - Marathon Oil's ( MRO) $3.5 billion purchase of Eagle Ford drilling and production assets and its sale of $3 billion in shale assets signals that oil and gas players continue to become more specialized.

Marathon announced the closing of the Eagle purchase and the shale sale during its Tuesday earnings announcement, where it reported revenue of $3.8 billion in the quarter and earned 57 cents a share, which missed analysts' estimates. The Houston -based company's shares fell over 2% in afternoon trading to $25.34 a share after the earnings announcement.

Marathon closed its $3.5 billion purchase of 141 acres of Hilcorp exploration and production lands in the Eagle Ford shale of East Texas that it offered to buy from private equity giant KKR ( KKR) in June. At the beginning of 2011, Marathon was one of the first in a series of oil giants to announce a spin of its refining businesses from its oil & gas exploration operations for a more focused business model.

The refining business, which is headquartered in Findlay, Ohio and includes six oil refineries that produce approximately 7% of U.S. oil capacity was valued at $15 billion in the spin, renamed Marathon Petroleum ( MPC) and IPO'ed on the New York Stock Exchange ( NYX) in early July.

The further announcement Tuesday that the company will sell up to $3 billion in shale assets further demonstrates how energy players are becoming more focused on onshore and offshore exploration opportunities. Separately, in its earnings Tuesday, Marathon Petroleum also indicated it would look to sell assets, which are mostly refineries.

With Hilcorp, Marathon adds a big set of assets in the Eagle Ford shale to its existing Bakken and Woodford shale assets, which combined the company expects to drive overall production growth. In today's announcement Marathon chief executive Clarence P. Cazalot Jr. said of its shale business, "these assets will provide the greatest amount of the Company's production growth, enabling us to deliver 5 to 7 percent compound average production growth, 80 percent of which is estimated to be liquids, from 2010 to 2016."

Cazalot added about the company's exploration and production strategy, "Over the past five years, we have executed sales with approximately $3.5 billion in transaction value. We continue the ongoing review of our global portfolio with a goal of an additional $1.5 billion to $3 billion in divestitures over the next two to three years." Overall, Marathon, a former piece of Rockefeller's Standard Oil founded in the 19th century, now expects oil & gas production to grow by 5% in 2012, with onshore oil production like shale drilling driving growth.

Recent merger activity like Statoil's $4.4 billion purchase of Brigham Exploration ( BEXP) to own its drilling operations in shale formations are called the Bakken and Three Forks shale in the Williston Basin of North Dakota and Montana indicates of how oil players are starting to focus on regional shale drilling opportunities.

It is yet to be seen whether Marathon would dispose of the $3 billion in shale assets in a sale to another player or in a spin via a master limited partnership. In October, Quicksilver Resources ( KWK) shares fell sharply on an announcement of a MLF spin of its shale assets.

-- Written by Antoine Gara in New York