U.S. upstream activity has reacted sharply to lower crude oil prices by reducing drilling activity, with January’s new onshore well count down 20% from December. However, new production capacity (NPC) followed the trend but not the magnitude, off 9% for oil and down 6% for natural gas, an assessment by consultant Drillinginfo has found.

“The effect of the falling well count was mitigated by the fact that wells eliminated were some of the least productive,” the Austin, TX-based researcher said in its February global assessment. “This can be seen in the increase of 13% in average per well NPC for oil and 17% for gas from the previous month.”

The information was derived using the firm’s recently launched index, which tracks newly drilled wells only; increased production because of recompletions is not included. Not all new well locations are tracked, with coverage in the selected states at about 80%.

The impacts from falling oil prices “are being felt unevenly” in onshore basins across the United States, according to the firm. Less productive plays and noncore areas are being hit the most, “while top-tier assets continue almost unaffected.”

The brunt of the impacts, according to Drillinginfo, is in the Eagle Ford Shale’s easternmost formation, the Eaglebine, which extends into East Texas north of Houston, as well as the Mississippian Lime, Granite Wash formations and “other,” which consists of “all areas outside the defined play boundaries.” As a group, the neglected plays saw a 34% decline in activity in January from December in oil NPC, with a 40% drop in new wells.

“The rest of the country, exclusive of these four areas, was essentially flat on NPC and down 10% on well count,” Drillinginfo noted.

In the oily Bakken Shale, where producers have been toiling with high-priced drilling techniques, “the same trend of core versus noncore is seen,” said researchers. North Dakota’s “four core counties (McKenzie, Mountrail, Dunn and Williams) were flat from the previous month on NPC and wells. However, the rest of the Bakken play, excluding these four counties, is down dramatically. Oil NPC for January was down 51% from the previous month and down 65% compared to October 2014.”

Meanwhile, the Permian Basin, also a revitalized oil-driven region, saw its new well count fall off 26% in January from December, according to Drillinginfo. “However, oil NPC was down by only 10%. The primary reason for this is that the drop in new wells came almost entirely from vertical wells, which are much less productive than horizontal wells.”

The big natural gas shales — Marcellus, Fayetteville, Haynesville and Barnett — as a group have remained “steady on new wells and NPC for the last several months, although the Barnett has seen a decline that has been offset by the other three plays,” said researchers.

Drillinginfo’s index uses “actual drilling locations,” as well as permit and production data to track industry activity. The index identifies newly drilled wells for the last month, based on proprietary rig and permit databases; selects appropriate analog wells for each newly drilled well from its production database; predicts peak production volumes of the newly drilled wells, based on the analog wells; and aggregates the information at the national, county and operator level. The index does not include declines in existing producing wells, nor does it track new production capacity from the offshore, Alaska, Illinois or Indiana.