So first off, ALL assets are only worth what someone is willing to pay for them. Book value on yahoo is quite old, and I do doubt it's still worth $11.41 a share, but that doesn't mean its a zero either. Not having done any sophisticated calculations, I'd say the book value has shrunk proportionally with oil/NG drop. If oil has dropped 40%, then expect book value to have dropped similarly. My guess is they are worth approx. $7.00 a share, in Book Value.
The oil/NG markets will come back, eventually. Right now I think their biggest problem is their very large debt of $692m. Servicing that debt is a tough job when your major asset's value is dropping.
Secondly, in an attempt to answer 'fraud-Street's question, no stock ever actually goes to zero. They do actually stop trading once certain rules on the exchange are violated. Those rules are complex and I can only remember a few. Each stock is in a certain category, that category requires a certain minimum capitalization. Once that is broken, letters are sent out and they have I think 60 days to get their valuation backup. Also, certain categorizations require above a dollar per share trading.
The company is free to re-list on the OTCBB, where share prices are as low as a fraction of a cent. But that doesn't help the shorts, you can't short a stock to 'zero', wait for it to de-list and expect to reap the profits. You must pay back the MMs you borrowed the shares from before it de-lists. I'm guessing the shorts get a 100% instant margin call if the MMs are told by the exchange a stock is getting de-listed.
Having said all of this, REXX is no where near such events.