HOUSTON -- A midsized independent oil and gas company has announced a major acquisition that executives there say doubles the firm's offshore holdings and capabilities.

Plains Exploration and Production Co. told investors and analysts yesterday that it is purchasing stakes in deepwater Gulf of Mexico operations for more than $6.1 billion in two separate transactions.

Describing the deals as a "pivotal event" in the history of the company, CEO James Flores said the assets will boost his firm's offshore hydrocarbons production by around 67,000 barrels of oil equivalent (boe) per day, 87 percent of that crude oil.

And the assets are well-positioned to increase that production figure, he added during a conference call to announce the acquisitions.

"What we've contracted to is to acquire BP and Shell's interest in a tremendous, infrastructure-laden, oil-barrel-laden additional reserves in the deepwater Gulf of Mexico that will put us in a preferred position to attract new, cheaper barrels to the infrastructure," Flores said. "This will be the first opportunity for an independent to do that in the deepwater because the infrastructure has been so expensive and strictly in the hands of the majors, except for some dominant independents like Anadarko."

In the biggest transaction, Plains E&P agreed to purchase wholly and partially owned and operated assets from BP PLC for $5.55 billion. Plains will acquire the Dorado, Marlin, King and Horn Mountain deepwater offshore oil and gas fields, all four now 100 percent owned and controlled by BP.

Plains is also buying from BP working interests in the Holstein, Diana-Hoover and Ram Powell fields, where BP currently holds 50 percent, 33.3 percent and 31 percent interests, respectively. Those fields combined boost Plains' offshore production by 59,500 boe per day, primarily oil and natural gas liquids.

The company also snatched a valuable but smaller deepwater offshore property from Royal Dutch Shell PLC in a separate deal announced at the same time.

The deal will leave Plains with a 100 percent interest in the Holstein field, where Shell holds the remaining 50 percent interest in a partnership with the project operator, BP. Acquiring the other half of that field from Shell will cost Plains $560 million but will add 7,400 boe per day worth of production to the purchasing company's portfolio, again primarily crude oil and associated gas liquids.

BP says the agreement will free it up to focus on other, stronger-producing Gulf assets while spending on exploration in search of major new finds. The company aims to raise $38 billion from divestments by 2013.

Shell says the Holstein holding represented only about 2 percent of that company's total Gulf of Mexico production.

Known better for its programs in shale oil and gas drilling in places such as the Eagle Ford and Haynesville shales, Plains E&P's only other significant offshore holdings before this announced purchase were found off the coast of California and in shallower Gulf waters. But its expanding onshore shale oil presence has been a boon to the company over the past year.

Last month the company recorded second-quarter 2012 earnings of $566.7 million, a 10 percent jump from the same period last year. Plains estimated that its total hydrocarbon production grew to 98,300 boe per day, with oil and liquids production at 55,800 barrels per day.

The two deals in the Gulf of Mexico with Shell and BP will nearly double the company's liquids production once the acquisitions are closed, Flores said. In terms of size, the new assets acquired in the Gulf are very similar to four platforms Plains E&P controls in California, Flores said, and will expand his firm's payroll by a substantial amount.

Investors should also expect big gains in production stemming from these latest deals, he added.

"We have about 150 people offshore California staff. This would double our staff, adding another 165 people with these three platforms," Flores said. "And when you see the tremendous capacity, you're talking almost 250,000 barrels a day of capacity, and right now it's producing 67,000 boe a day, about 60,000 barrels of oil, so we have a lot of room to add barrels cheaply to this infrastructure."

Mindful of possible concerns over the hefty $6.1 billion price tag for this expansion, Flores insisted that cash flow growth from Plains E&P's booming onshore holdings, particularly in the south Texas Eagle Ford formation, would alone be enough to finance the deepwater acquisitions. A consortium of banks, including J.P. Morgan Chase, Bank of America and Citigroup, are underwriting the deal.

Flores said the purchases from BP and Shell represent a "hub and spoke" strategy that will underlie future deepwater Gulf of Mexico expansion. The firm is also partnering with Anadarko in another deepwater Gulf project, the Lucius project, which is expected to come online in mid-2014.