The slowdown is happening because drillers are waiting for pipelines to expand, new markets to develop and wholesale prices to rise.
"The hiring has tapered off. What we see is a holding pattern," said Kathryn Klaber, the president of the Marcellus Shale Coalition, an industry group.
That's a big difference from the last four years, when production doubled or tripled every 12 months and companies spent tens of billions of dollars on leases, well drilling, and related infrastructure.
Klaber said companies are still confident there's money to be made, since independent analysts say that the Marcellus is the most economical place to drill for shale gas in the nation.
The Marcellus Shale is a gas-rich formation deep underground that extends across Pennsylvania, West Virginia, New York, Ohio and Maryland, but most of the production is in Pennsylvania and West Virginia.
Bentek, a Colorado company that analyzes energy trends, said figures from the pipelines that take gas out of the Marcellus show that Pennsylvania production rose to about 2 trillion cubic feet in 2012 — roughly double the prior year. Production from West Virginia was about 700 billion cubic feet in 2012, bringing the total Marcellus output to about 2.8 trillion cubic feet.
That's about 10 percent of the nation's output of natural gas. Bentek estimates that Marcellus production will grow by about 30 percent this year, though numerous factors could affect the final number. One billion cubic feet of gas is equivalent to about 180,000 barrels of crude oil.
The official 2012 production figures for Pennsylvania and West Virginia haven't yet been released by those states, but Bentek figures are considered very reliable by government and industry sources.
Wall Street analyst Manuj Nikhanj, the head of energy research for ITG Investments, agreed with Klaber's assessments.
"I do think we're going to see growth in 2013, but the rate of growth will slow," Nikhanj said. But he added that drillers are getting "better and better" at improving the output for each well.
In 2011 and 2012, there was a highly publicized debate over the potential of the Marcellus Shale, with some claiming that the industry had exaggerated the reserves. But actual production figures have mostly put that debate to rest. When serious well drilling started in Pennsylvania in 2008, output barely registered on a national level. Now, it's grown to be the nation's most productive gas field.
Now, Klaber and Nikhanj said the bigger questions are over how fast pipelines and new markets for Marcellus gas can expand.
Since the price of natural gas has been relatively low, many companies operating in the Marcellus began to drill wells but have delayed putting them into production for now, Klaber said. "There's a staggering number of wells waiting to go online," she said.
Nikhanj estimated that 700 wells have been drilled but not hooked into production and thousands of wells have been issued permits but haven't begun drilling.