Amid a growing global reach for U.S. natural gas, and the coincidental timing of gas contract expirations, Henry Hub December basis prices are trading at a rare premium to New York Mercantile Exchange (Nymex) futures, NGI Forward Look data show.

Historically, Henry Hub basis tracks within a penny or so of the comparable Nymex contract. However, the spread began to widen in late September and has continued to expand since then. It reached as high as 3.0 cents on Monday before contracting a bit. A similar basis premium is seen through the February contract, according to NGI’s day-over-day forward basis price comparisons.

Until now, there have been only a few times since NGI launched Forward Look in 2012 that Henry Hub basis prices have moved...