Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of independent oil and gas company Petrohawk Energy (NYSE: HK) were shooting up like a geyser today, gaining as much as 63% in intraday trading on massive volume.

So what: This is an easy one. The big gains for Petrohawk were fueled by BHP Billiton's (NYSE: BHP) offer to buy the company for $12.1 billion, or $38.75 per share. According to Bloomberg, the offer is a 61% premium over Petrohawk's 20-day average price, and is well above the 25% average premium for similar deals.

Now what: As of this writing, Petrohawk's stock is trading at $38.17, just 1.5% below BHP's offer. That means that there probably isn't a huge opportunity for arbitrage here. So should Petrohawk shareholders take the money and run? I couldn't blame them -- the 60% premium is pretty sweet and likely high enough to prevent a higher offer from coming in.

For shareholders that were hot-to-trot for Petrohawk's shale plays, it may be worth taking a look at BHP. For BHP, the Petrohawk deal follows a $4.8 billion deal for shale assets from Chesapeake (NYSE: CHK) earlier this year.

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