The month of September couldn't have been more different for offshore rig operator Seadrill (NYSE:SDRL) and its subsidiary Seadrill Partners LLC (OTC:SDLP). One saw its shares rise more than 10%. The other filed for bankruptcy...and still saw its shares rise 25%. 

Here's what to make of this incredibly unusual situation.

An offshore oil rig in silhouette against a sunset in open water.

Offshore rig operator Seadrill has filed for Chapter 11 bankruptcy, but shares of subsidiary Seadrill Partners are seeing an uptrend. Image source: Getty Images.

What happened

On September 13, Seadrill (the parent) took the long-anticipated step of filing for Chapter 11 bankruptcy protection, after agreeing to a restructuring plan with its creditors and bondholders. Ironically, shares "spiked" (if you can call a jump from $0.25/share to $0.475/share a "spike" for a company that three years ago commanded $45/share) to finish the month up 25%. But don't let that fool you: Existing Seadrill shareholders are expected to own less than 2% of the restructured company. 

Interestingly, this will likely have little effect on the fate of Seadrill's subsidiary, Seadrill Partners, which maintains it own set of rigs, its own ticker symbol, and -- most importantly -- its own set of finances. Shares began to rise modestly as soon as the parent's bankruptcy was announced and finished the month up 12.4%.

So what

For shareholders of Seadrill Partners, this will have surprisingly little effect, thanks to a savvy maneuver the company executed in August.

You see, Seadrill (the parent) had used some of its assets to guarantee some of Seadrill Partners' credit. If that agreement had been in effect when Seadrill filed for Chapter 11, that could have caused big problems for Seadrill Partners, because those assets would no longer have been valid as collateral. That could have thrown Seadrill Partners into an unexpected credit crisis. 

Luckily for its shareholders, Seadrill Partners altered its credit agreements so that it was guaranteeing them with its own assets and not its parent's. That insulates Seadrill Partners from Seadrill's bankruptcy and had the pleasant side effect of allowing it to resume cash distributions to shareholders.

For shareholders of Seadrill, well, the bankruptcy is officially here. If you were still holding out hope for some sort of miracle recovery, sadly, it isn't going to happen.

Now what

Well, it's too late to do anything about Seadrill, but the beaten-down Seadrill Partners may still have some upside. Despite the stock's recent rallies, it's still 89% off its 2014 high, much further down than many of its peers. However, Seadrill Partners isn't likely to see its fortunes improve until there's an oil price recovery, and it's anyone's guess when -- or if -- that will happen.

If you're dead set on buying an oil rig operator, Seadrill Partners isn't a bad choice, but be aware that the entire industry is full of potential pitfalls to recovery.