The amount of natural gas traded in the United States increased in 2018 for the fourth year in a row, reaching a record high, and marketed production also reached new heights, according to an analysis of FERC Form 552 submissions by Cornerstone Research.

The records were achieved despite a slight decline in the number of Form 552 respondents compared with the previous year, according to a report released Wednesday by Cornerstone Research. In addition, aggregate exchange trading of natural gas contracts declined as trading on the Chicago Mercantile Exchange leveled off and trading on the Intercontinental Exchange continued to decrease, Cornerstone Research said.

The increase in marketed production was driven by continued development of lower-cost shale gas and tight oil resources.

A total of 146,227 TBtu of gas was traded last year, about an 11% increase from 2017, according to the report. Annual marketed production was 36,958 TBtu in 2018, a 12% increase over 2017.

While the share of next-month transactions compared to next-day transactions dropped below 50% for the first time since the Federal Energy Regulatory Agency began reporting Form 552 data in 2008, the percentage of index-priced transaction volume grew relative to fixed-price transaction volume.

"The ratio between index-priced transactions and reporting-eligible fixed-price transactions continues to widen," said Cornerstone Research's Greg Leonard, who co-authored the report. "In 2018, we saw the largest volume of index-priced transactions and the lowest volume potentially reported to indices since FERC began reporting Form 552 data."

Cornerstone reported 2018 Form 552 submissions were made by 678 respondents, with the 20 companies with the largest transaction volumes accounting for about 43% of total reported volume. Index-priced transactions comprised 80% of all Form 552 transactions in 2018, compared with 65% in 2008.

Index-priced next-day transactions increased to 37% last year from 35% in 2017, while index-priced next-month transactions declined slightly in 2018 to 43%, compared with 45% the prior year, Cornerstone Research said.