This week, I've spotted at least three billion-dollar oil deals that should be of interest to Foolish investors. And it's only Tuesday! Combined with other activity in recent weeks, this suggests that there's plenty more M&A mayhem to come as we approach the end of the year.

A big splash in the Gulf
First off, Energy XXI (Nasdaq: EXXI) agreed to pick up a bunch of ExxonMobil's shallow-water Gulf of Mexico properties for $1 billion. This follows similar moves by Apache -- which grabbed Devon Energy's shallow Gulf assets for an identical sum -- and McMoRan Exploration (NYSE: MMR), which picked up the pieces from Gulf dropout Plains Exploration & Production.

Energy XXI is picking up 66 million barrels of oil equivalent (Boe) of proved and probable reserves, and 20,000 Boe per day of production. The cash flow multiple paid is 3.2. Apache got more reserves with its purchase (83 million boe), slightly less production (19,000 boe/d), and paid 3.7 times estimated cash flow. In both cases, oil and natural gas production is split roughly 50/50, so I assume the lower Energy XXI cash flow multiple is largely a reflection of higher oil prices. Any way you slice it, the purchase price looks reasonable.

With this purchase, Energy XXI becomes the third-largest oil producer on the Gulf of Mexico shelf, leapfrogging W&T Offshore in terms of reserves, and both McMoRan and Stone Energy in terms of production. The assets acquired have the potential to deliver around 720 million Boe at a development cost of around $15 per barrel.

That would be a really compelling figure, if a large component of that total resource potential was oil. The potential oil mix, surprisingly, is only around 10%, however, alongside 3.9 trillion cubic feet of gas. So the big upside appears to be in deep and ultradeep gas prospects, such as the ones Energy XXI is exploring in partnership with McMoRan elsewhere on the Gulf of Mexico shelf.

Incidentally, Exxon walked away from one of these ultradeep drilling projects a few years ago. This week's sale confirms that the company lacks an appetite for this activity. Given the likely difficulties in securing future permits to drill these extremely high-pressure wells, I can't really blame it. I'm a decided fan of wildcat drilling in the Gulf, but my preference is for companies sizing their bets more conservatively.

Yet another Bakken buy
Last week we saw Williams (NYSE: WMB) make a $925 million purchase in prime Bakken territory up in North Dakota. This week, Hess (NYSE: HES) edges it out with a $1.05 billion buy of privately held TRZ Energy. This follows closely on the heels of the company's acquisition of American Oil & Gas earlier this year.

The 167,000 acres acquired in this latest deal bring 4,400 barrels of daily production to the table. That's a pretty massive $238,600 per flowing barrel purchase price. At under $6,300 per acre, though, this purchase comes at a discount to those executed by Williams and Enerplus Resources Fund (NYSE: ERF). From what I can piece together, TRZ is active in Dunn and Williams County. You may recall that Dunn County is the location of Kodiak Oil & Gas' (AMEX: KOG) core development area. This should be very prospective acreage, suggesting that Hess may have gotten a great deal here.

Another Asian oil sands suitor
Over the past year or two, one of the most active players in the Canadian oil sands has been China. PetroChina took a big stake in a pair of Athabasca Oil Sands' projects last September. More recently, Sinopec snapped up ConocoPhillips' 9% stake in Syncrude, and a Chinese sovereign wealth fund snapped up a stake in some Penn West Energy Trust (NYSE: PWE) properties.

Showing that China isn't the only one salivating over the security of long-term oil sands supply, Thai energy company PTTEP has also stepped forward. The company is picking up 40% of Statoil's Kai Kos Dehseh oil sands project for $2.3 billion. The entry looks low-risk, as first production is slated for early 2011.

Thanks to heady oil prices, the oil sands have made a roaring comeback since the dark days of 2008. As long as you believe that the world economy will continue to support $70-plus oil prices, the oil sands are indeed an interesting place for your money. Cenovus Energy continues to be my favorite operator in that realm.

W&T Offshore is a Motley Fool Hidden Gems recommendation. Statoil ASA is a Motley Fool Income Investor selection. Try any of our Foolish newsletter services free for 30 days.

Fool contributor Toby Shute doesn't have a position in any company mentioned. Check out his CAPS profile or follow his articles using Twitter or RSS. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Fool owns shares of Devon Energy and ExxonMobil. The Motley Fool has a disclosure policy.