The potential environmental risks of fracking, the effective but controversial method of extracting natural gas by pumping water and chemicals into deep beds of shale, were explored in Josh Fox’s hard-hitting but one-sided 2010 documentary “GasLand.” Now “Promised Land” deal with them in a dramatized form that will certainly reach a wider audience but is just as much an assault on corporate greed and insensitivity. Directed in a leisurely, entirely conventional fashion by Gus Van Sant from a script by Matt Damon and John Krasinski, who also star, it’s an old-fashioned, painfully earnest film that conjures up a Capra-like populist fable of the little people taking on the powerful, only to switch suddenly into cynical mode in the last act. It’s unlikely to enjoy the same measure of appeal that Damon’s first outing with Van Sant (and Ben Affleck), “Good Will Hunting,” did, though like it, the picture is essentially a big-screen version of what might have been a Hallmark Hall of Fame special.

The linchpin of the story is Damon’s Steve Butler, a roving salesman for Global Energy, which sends him into the field to purchase drilling leases from the locals. He’s in line for a promotion to the executive suite, but after an interview is sent to a small Pennsylvania farming town along with his usual partner, Sue Thomason (Frances McDormand), to convince the farmers in the area to sign up. It’s fertile territory, not only because the geological evidence is favorable but because at a time of economic recession, the landowners are desperate for some financial hope.

Steve is an unusual company man—a small-town Iowa boy who saw his own community die when the Caterpillar plant closed and can relate to hard-pressed locals in a way that makes them receptive to his pitch. And at heart he’s an idealistic fellow, honestly believing that his company’s offer can save their depressed area. But despite the support of the city fathers, who see the project as a boon to the treasury (and their own pockets), he’s confronted by formidable opposition led by a high-school science teacher (Hal Holbrook), who uses a town-hall meeting to arrange a vote on whether or not to go ahead with it. The opposition is bolstered by the arrival of a glad-handing, charismatic environmental activist named Dustin Noble (Krasinski), who brings with him tales of his own town’s destruction at Global’s hands as a result of the dire environmental effects that fracking had there.

Steve’s confidence is shaken by Noble’s success in converting many of the locals to his cause, but also by his own growing misgivings about what he’s doing—and by the fact that schoolteacher Alice (Rosemarie DeWitt), in whom he has an incipient dramatic interest, gravitates toward Dustin as well. That’s why he’s so overjoyed when the company sends him evidence that Noble is a fraud, proof that he can use to turn the tide in his favor and rid himself of a rival in one fell swoop.

The strength of “Promised Land” lies primarily in two elements. One is the portrait it draws of small-town America in the throes of economic distress, which is only occasionally heavy-handed. (That town meeting, held in the high school gym, that has to be cut short when the coach intervenes because his team has to practice, is a typically nice touch.) It’s aided by the locations, Daniel B. Clancy’s spot-on production design, Greg Weimerskirch’s art direction, Rebecca Brown’s set decoration, Juliet Poicsa’s costumes and, especially Linus Sandgren’s evocative cinematography.

The other is the performance of Damon, who persuasively depicts a man torn between his principles and his suspicions that he’s been abusing them for his own profit. (It doesn’t seem plausible, however, that such a naïve fellow—and one so successful in the field—would be seriously considered as executive material in a company that appears to be without scruple.) McDormand is also fine as a no-nonsense career woman who seems more concerned with her baseball-playing son back home than her current assignment and is much less capable than her partner of blending into an unfamiliar environment. (A subplot involving her relationship with a local convenience-store owner played by Titus Welliver is a stretch, though.) And Krasinski obviously has a good time playing a jovial, yet calculating type.

Some of the supporting characters, unfortunately, are less well drawn. DeWitt is wasted in a trite role, and though Holbrook brings his accustomed authority to Frank Yates, the science teacher, the script rather stacks the deck by giving him such a distinguished past in terms of degrees and university teaching posts (as well as a hobby of raising miniature horses). On the other hand, Scoot McNairy, Terry Kinney, Tim Guinee and Lucas Black add nice touches of local color in their brief scenes.

The biggest miscalculation in the film, though, is the turn it suddenly takes toward the close. The revelation certainly acts as a prod to Steve’s final declaration of principle—he becomes the Mr. Smith of the Pennsylvania farm country—but also transforms the picture into a message about corporate manipulation that’s much less clever than it’s meant to be. And ironically it opens the film—which, as oil company critics have already pointed out, was partially funded by interests from Abu Dhabi that might be interested in restricting US drilling for natural gas—to charges that in this instance art imitates life, and vice versa. That disconcerting feeling adds to the general impression that the promise of this “Land” isn’t entirely fulfilled.