NEW YORK -- Global mergers and acquisitions activity in upstream oil and natural gas posted $232 billion in total spending in 2012 and was paced by three massive transactions, Wood Mackenzie said in a market analysis.

The exploration and production sectors were led in terms of market impact by Russian energy giant Rosneft's purchase of a smaller Russian oil producer, TNK-BP, for $58 billion. That deal potentially makes Rosneft the largest crude supplier on the planet in terms of liquids output.

Russia's Federal Anti-monopoly Service still has to clear the merger, but news reports yesterday indicated authorities had approved the sale. Dow Jones Newswires said the merger stands to hand BP PLC a 19.8 percent stake in Rosneft as part of the British oil giant's plan for exiting TNK-BP.

The other big M&A deals were the CNOOC-Nexen deal and Freeport-Plains, Wood Mackenzie said. China National Offshore Oil Corp., China's largest offshore upstream producer, inked a $15.1 billion takeover of Canada's Nexen Inc., though approval is still pending from U.S. regulators because of Nexen's assets in the Gulf of Mexico.

Freeport-McMoRan Copper and Gold Inc., a copper miner, struck a deal to purchase Plains Exploration and Production Co. for upward of $9 billion. Plains is focused on plays in California, Texas and the Gulf.

Overall, the manager of Wood Mackenzie's M&A analysis service, Luke Parker, said 2012 was a strong year with 456 transactions, though it failed to eclipse 2010, when M&A upstream deals were counted at 466 worldwide.

"The majors have very strong balance sheets and cash flow, so we can expect an uptick in activity," Parker added of the coming year. "The Asian NOCs [national oil companies] will continue to play their part, with a second wave of players looking to make up ground on their Chinese counterparts. For both groups, small to mid-size asset acquisitions focused on long-life resource themes will remain the focus."

The report says key areas to watch for are U.S. tight oil, Canada unconventional gas, exploration-focused corporate M&A and big liquefied natural gas projects, in east Africa in particular. Wood Mackenzie forecasts deal valuations to continue a steady upward trend, given what the firm called "growing industry confidence in the long-term sustainability of high oil prices."

Wood Mackenzie noted the emergence of Chinese energy firms specifically, reporting $46.9 billion spent by Asian national oil companies.

CNOOC's acquisition of Nexen "was the headline deal, but in total, CNOOC, Sinopec and PetroChina spent over $31 billion in 2012," Parker said, adding that he expects the trend to continue.

Major Western oil companies, meanwhile, shifted from the seller class to the buyer. BP was the notable instigator, with its exit from TNK-BP, and Exxon Mobil Corp. and Royal Dutch Shell PLC were the only majors to be considered active buyers in 2012.

Parker also noted that LNG has seen more than $60 billion in M&A action over the last five years. The sector is one to watch, he said.

"LNG-focused M&A has boomed," he said. "We fully expect the market for LNG assets will remain buoyant, with activity again concentrated on pre-development projects during 2013."