BP plc (NYSE:BP) made some major headway in its $10 billion divestiture program to raise money for its Gulf of Mexico oil spill, having announced Tuesday it agreed to sell oil and gas properties in West Texas' and New Mexico's Permian Basin, western Canada and Egypt to Apache Corp. (NYSE:APA) for $7 billion. But what happened to its rumored sale of half of its stake in the Prudhoe Bay oilfield in Alaska to Apache?

BP, feeling good that it capped the leak from its Macondo well last week, is probably keeping the Prudhoe Bay assets as its trump card. The asset is arguably the most valuable in its North American portfolio -- it's the biggest oil and gas field in North America -- and BP's 26% stake in the field is worth around $16 billion. BP has other assets to sell to reach its $10 billion goal, including properties in Pakistan and Vietnam that analysts say could fetch $1.7 billion, as well as a 60% stake in Argentina's Pan American Energy LLC and fields in Venezuela and Colombia. It's already sold some pipeline operations to Magellan Midstream Partners LP, including its crude oil storage facilities in Cushing, Okla., for $289 million.

It may also have been easier to sell the other assets rather than deal with preferential rights by its fellow Prudhoe Bay owners, Exxon Mobil Corp. (NYSE:XOM), ConocoPhillips (NYSE:COP) and Chevron Corp. (NYSE:CVX).Valuation may have also been a sticking point, as well as whether Apache would serve as operator (which it typically likes to do).

Apache is also known for taking over mature properties the majors discard and squeezing every last drop of oil and gas out of them with technology. The assets it's picking up from BP fit that bill but also happen to already be in its operating areas and come with some development upside. "We believe [the assets] fit better with [Apache's] overall strategy and portfolio," Pritchard Capital Partners LLC wrote in a Wednesday morning note.

Tudor, Pickering, Holt & Co. Securities Inc. said it was surprising that Apache would do a dilutive deal, including paying a premium for the Permian assets ($17 per barrel of oil equivalent, versus $15.60 on average). But it said Apache has a track record of "scratching out additional value from acquisitions."

Not buying the Prudhoe Bay stake would also give Apache more financial wiggle room if it wanted to boost its interest in the Lucius field in the deepwater Gulf of Mexico, despite the government moratorium on drilling there. It's already picking up 17% of the field with its acquisition of Mariner Energy Inc. (NYSE:ME), which is set to close in September. The question is whether Anadarko Petroleum Corp. (NYSE:APC), which owns 50% of the field, and Plains Exploration & Production Co. (NYSE:PXP), which owns 33%, would be eager sellers.

The bond ratings services are nervous enough about Apache as it is. Moody's Investors Service and Fitch Inc. both announced Wednesday a possible downgrade of Apache out of concern for the company's debt load, which could double to $10 billion.

In the end, selling these three sets of assets saved BP from selling off one of its crown jewels -- unless, of course, the plug doesn't hold or the liabilities prove to be greater than anticipated.