Marathon Oil Corp., which began 2012 as a pure-play exploration and production (E&P) company, has its sights trained on two big shale plays in the United States: the Eagle Ford and the Bakken, company officials said Wednesday.

The Houston-based producer, which restructured last year and spun off its refining unit, added more than one million net acres to its North American resource base last year, up from 600,000 net acres in 2010. By themselves the “multiple” transactions in the Eagle Ford late in 2011 have given the producer more than 300,000 acres, COO David E. Roberts Jr. said. He and CEO Clarence P. Cazalot Jr. shared a microphone to discuss the company’s quarterly and year-end performance.

Cazalot’s enthusiasm for the company’s onshore resource base was evident. He highlighted its upstream production last year, which was up 7% year/year on “the highest level of rig activity in many years.”

The producer’s total Lower 48 net production rose 20% in 4Q2011 from a year earlier to 91,000 boe/d from 76,000 boe/d. For the full year Marathon replaced 266% of its liquid hydrocarbons and 116% of natural gas production. Marathon spud 126 operated wells last year, more than double the 54 wells spud in 2010. And it recorded a 96% average operational availability for all of the major company-operated E&P properties, up from 94% in 2010.

This year, big plans are on the table to boost liquids-weighted output. Capital spending for 2012 is set at $4.8 billion, well above 2011’s $3.7 billion, with most of the development money budgeted for the U.S. onshore. By this summer the company expects to ramp up about 35 operated drilling rigs in the United States. And, it’s matching its production pace with infrastructure additions.

If all goes according to plan, Marathon’s Lower 48 onshore net output by the end of 2012 is forecast to average 120,000-130,000 boe/d, which is almost one-third higher than it was in 4Q2011, said Roberts. “We have internal infrastructure designed and being constructed to match our growth plans on a go-forward basis” over the next five years.

The Eagle Ford Shale, which is considered Marathon’s top long-term E&P prospect, will see a lot of action in the coming months. About 15,000 boe/d net is flowing today from the South Texas wells, close to the exit rate for 2011. The company had some operational issues late last year but with problems resolved, it’s full steam ahead.

Look for output from the Eagle Ford to double by the end of this year to about 30,000 boe/d net, said Roberts. Over the next five years the flow rate in the play is predicted to reach 100,000 b/d.

Preparations already are in place to match the growth, he said. Fourteen rigs are now running in the trend; four more are scheduled to be added by September. Eight wells await completion. Four hydraulic fracturing crews are under contract for the year.

To the North, Marathon also is picking up the pace; in North Dakota’s Bakken Shale, where it has an estimated 406,000 acres. The play had record average production of 24,000 boe/d net in the final three months of 2011, which was 70% higher year/year. Output is 95% crude oil. Last month 19 (gross) operated wells were awaiting completion. Six rigs are running in the play, along with one rig dedicated to completions. One more rig is scheduled to be moved there this spring.

While unconventional targets are a mainstay for Marathon this year, don’t expect the company to pursue gas targets until prices strengthen, said Roberts.

Six rigs now are running in the gassy Woodford Shale but “we’re monitoring that very closely,” he said. “If I drill a dry gas well, I get in trouble,” he said laughing. “We’re trying not to do that…We’re just not directing any money to gas wells at all. We have no lease issues that we’re chasing in terms of pure, dry gas…”

Roberts didn’t indicate whether Marathon might pull down gas rigs, as some of its competitors have done. However, the company has “35 to 40 rigs on contract in the frame of reference for this year with the ability to expand or contract if we need to.”