Buying and selling in the oil patch heated up this week, with about $10 billion changing hands in a flurry of mergers and acquisitions. Apache Corp. (APA) led things off, buying deepwater Gulf of Mexico assets from Devon Energy (DVN) for $1.05 billion.

Halliburton (HAL) spent $232 million to acquire Boots and Coots (WEL). Kinder Morgan Energy Partners L.P. (KMP) paid $875 million for a 50% share of Petrohawk’s (HK) gas gathering and treating assets in the Haynesville shale play; and ConocoPhillips (COP) chalked up $4.65 billion for its stake in the Canadian oil sands, selling its 9.03% stake in Syncrude to China Petroleum and Chemical Corp. (SNP).

Then this morning, Apache announced that it would merge with Mariner Energy (ME) by paying about $2.7 billion in cash and stock and assuming $1.2 billion in Mariner’s debt. Apache is paying a 45% premium to Mariner’s Wednesday closing price, and Mariner’s shares are up nearly 40% in pre-open trading this morning.

Apache’s offer for Mariner extends its reach into the deepwater Gulf, which the earlier acquisition of Devon’s assets started. Apache expects the Devon assets to contribute to per-share earnings immediately. Apache notes that the acquisition of Mariner also brings “near-term production and cash flow,” but does not specify that the deal will be immediately accretive to earnings.

Mariner’s estimated proved reserves at the end of 2009 included 181 million barrels of oil equivalent and unbooked potential of 2 billion barrels of oil equivalent. The assets purchased from Devon include proved and probable reserves of 83 million barrels of oil equivalent as of December 2009. About half the proved reserves of 41 million barrels acquired from Devon are oil and natural gas, as are 47% of Mariner’s reserves. These figures are roughly equivalent to Apache’s current reserves estimates of 1.07 billion barrels of oil and 7.8 trillion cubic feet of natural gas.

Getting a significant foothold in the deepwater Gulf drove this deal. Apache is recognizing that the technologies available to extract hydrocarbons from deepwater reserves has advanced enough virtually to guarantee that the deal will be profitable on the basis of current estimates alone. New discoveries will be icing on the cake.

As Apache’s chairman/CEO noted, Mariner’s producing and prospective acreage “will jump-start our position in the deepwater Gulf.”

A side benefit of Apache’s two acquisitions this week is that it will push the company’s market cap above $40 billion. That’s the amount Exxon Mobil (XOM) is paying for natural gas producer XTO Energy (XTO), and probably represents the upper limit of a transaction in the energy sector. Even though Apache would have been costly at previous capitalization levels, one of the supermajors could have swung a deal. Such an option is much more remote now.

Apache shares are off about 3% in early trading this morning. Still, the stock is trading close to its 52-week high, closing yesterday at less than $1/share from that point. Once profit-taking is finished, the company’s share will head upward again.

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