A Texas energy regulator said preliminary estimates show crude oil production in the state increased just over 13 percent in May year-on-year.

The Railroad Commission of Texas published data for May 2015, saying total crude oil production for the month averaged 2.4 million barrels per day, up from the 2.1 million bpd reported for May 2014.

May data show the second straight month for gains for the No. 1 oil producer in the nation. April crude oil production of 2.31 million bpd was a 13 percent increase year-on-year. Decreases were posted, however, in early 2015.

The momentum comes despite the steep decline in crude oil prices. The weak market leaves energy companies with less capital to invest in exploration and production, a trend reflected in declines in rig deployments in the United States.

Texas accounts for about 44 percent of all active rig deployments in the United States.

The so-called Beige Book from the Dallas Federal Reserve in early June said demand for oil field services, as well as the number of rigs actively exploring for or producing oil and natural gas, were on the decline, with losses concentrated in the Permian shale basin.

Texas depends heavily on the energy sector. In its latest statement, the Dallas bank suggested some parts of the economy were improving, notably labor market indicators.

"The employment index moved up from 5.6 in June to 10.1, suggesting faster employment growth in July," it said. "The hours worked index rose from a reading near zero to 5.3 in July, reflecting longer workweeks."

Last week, shale player Encana Corp. said about 80 percent of its new spending would target four key U.S. shale basins, including two in Texas.

Second quarter production for Encana increased more than 5 percent from the previous quarter, driven in large part by the Permian and Eagle Ford basins in Texas.