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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Fool contributor Matt Koppenheffer does not have a financial interest in any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or Facebook. The Fool’s disclosure policy prefers dividends over a sharp stick in the eye.