In business as in nature, a big predator’s unwanted leftovers can turn into a smaller scavenger’s feast. But only in business can some predators end up downright embarrassed by their kill. A scavenger in the right place at that moment can dine well for a long time off such a stroke of good fortune.

That’s the plausible upside for Rex Energy (NASDAQ: REXX) from its recent deal with Royal Dutch Shell (NYSE: RDS-A), which relieved Shell of some of the evidence from its regrettable — and duly regretted — US shale shopping spree of a few years ago.

In 2010, with gas prices only a little higher than they are today, Shell shelled out $4.7 billion for East Resources, an independent driller with 650,000 net Marcellus acres in its million-acre lease inventory.

The subsequent bear market in natural gas turned such deals into a liability and eventually a multi-billion write-off for the oil major, and its new CEO has been in a  hurry to dispose underdeveloped legacy assets, especially those containing mostly gas rather than the currently more lucrative crude.

On Aug. 12, Rex announced an acquisition from Shell of 207,000 Pennsylvania and Ohio acres originally leased by East Resources for a mere $120 million.

Source: Rex Energy presentation

How cheap is that? Well, Rice Energy (NYSE: RICE) recently agreed to pay Chesapeake Energy (NYSE: CHK) $336 million for 22,000 Marcellus acres, which works out to more than $15,000 per acre. Rex is paying Shell $580 per acre. Granted, the most prospective chunk of the acreage Rex is acquiring, to the north of Pittsburgh, is not as bountiful as Rice’s prize in the southwestern corner of the state. And it’s also true that Rex will initially focus on just 24,000 of the acquired acres with the best prospects and in close proximity to its current operations. Still, it appears to have landed a bargain.

The mere handful of the wells Shell has already drilled on the acreage it’s selling are expected to generate annual revenue of roughly $23 million by the end of next year at current prices. At those prices, their currently estimated proved reserves of 21 billion cubic feet equivalent of natural gas ought to eventually fetch more than $80 million. And this would just a start, because Rex estimates it’s acquiring 241 drilling locations in all, a 24% increase from its current inventory, with the potential of adding a total of up to 400 locations following smaller bolt-on acquisitions within the expanded footprint.

This is great news for a company that is already doing quite well. The most recent quarter’s results slightly exceeded expectations thanks to a 50% production increase year-over-year, a growth pace Rex is likely to maintain in the second half of the year.

Source: Rex Energy presentation

The bulk of that growth is coming from Marcellus wells in southwestern Butler County, where falling production costs have propelled the company’s internal rate of return to 38% as of June. Rex also owns leases elsewhere in Pennsylvania and in Ohio, where it has begun to drill into the Utica formation, and also has a rig drilling lower-cost conventional oil wells in the Illinois basin. The Shell acquisition will triple Rex’s Appalachian footprint to more than 300,000 net leased acres.

Source: Rex Energy presentation; excludes Shell acquisition

All that growth comes at a cost (albeit one that is falling), and Rex now has some $800 million in long-term debt (including the $140 million in convertible perpetual preffereds recently sold to finance the Shell purchase.) That’s roughly equal to the current market cap, which means that Rex has an enterprise value (debt plus market cap) of 7 to 8 times this year’s likely cash earnings ex items (EBITDAX).

The company has sufficient borrowing capacity and cash flow to finance its capital spending plans through next year, which will include one or two additional rigs and up to $125 million to develop the Shell acreage.

Despite the obvious attractions of that deal, the stock remains well off October’s peak at $25 and April’s high water mark above $21. A lot of the decline since can be chalked up to jitters over the regional discounts on Marcellus, though the takeaway capacity is improving all the time. In fact, the sizeable discount (aka basis differential) expected in the second half of this year will be largely offset by the additional value of the natural gas liquids Rex produces, so that its average realized price will be close to the NYMEX benchmark even if the differential remains wide, which is not a given.

Source: Rex Energy presentation

The near term must look promising to founder and board chairman Lance Shaner, who bought shares worth $3.3 million last week at an average price of $14.54. It was Shaner’s first major transaction since he cashed out $50 million at $20 per share a year ago.

As a reasonably priced Marcellus and Utica growth play with solid management and the potential to profit from Shell’s bumbling, Rex is worth a shot here near long-time lows. It’s another way to play the promising long-term outlook for natural gas we have been documenting for the last 18 months, with strong results. We’re adding REXX to the Aggressive Portfolio; buy below $18.