Hints of potential setbacks in the U.S. natural gas refining market surfaced in earnings reports last week, an analysis published by a leading energy investment bank showed Monday.

"It was a rough second quarter earnings season for refiners as a general market downturn led to chaos regardless of beats and misses," said the report from the Houston-based investment bank Tudor, Pickering, Holt & Co.

The investment bank's analysis covers eight leading petroleum refiners who reported similar production issues during recent earnings briefings, including Marathon Petroleum Corp., Phillips 66 and Valero Energy Corporation. All of these companies are publicly traded.

The technical term for the major problem these companies encountered is the "naphtha capture rate." Naphtha is a key chemical component in natural gas production.

The chemical escapes during regular oil refining, and is captured by refiners. The chemical product is a straw-colored, mobile and flammable liquid with an aromatic odor. Naphtha is extracted from raw gas after the gasification process, and then made into high value products such as gasoline, analysts said.

Sometimes the raw product flares, reducing well output and is a concern of regulators in Texas, Oklahoma and other oil-producing states in the United States, for safety reasons.

"In terms of capture rate (of natural gas from wells), we're down about 20 percent year-over-year," said Lane Riggs, executive vice president and chief operating officer at Valero Energy Corp., during a conference call with reporters and Wall Street analysts.

This indicates that the price of natural gas may rise, as the expense and difficulty of refining it is increasing.

The oil refiners said they were simply going to work within the constraints of the system rather than increase expenses, at least for now.

Meanwhile, according to an analyst at the investment bank, the refiners are also facing other issues in the coming quarter including regulatory pressure from the U.S. Environmental Protection Agency.