U.S. natural gas and oil industry dealmaking stalled in the first quarter as operators attempted to find their footing, but more executives are optimistic about opportunities to invest and expand over the coming year, new research indicates.

BDO USA LLP, PwC US and EY, a unit of Ernst & Young Global Ltd., have issued three separate reports that dig down into merger and acquisition (M&A) activity and trends, and how many global energy executives view risks to their operations.

The latest BDO Oil and Gas RiskFactor Report relied on information from the most recent U.S. Securities and Exchange Commission Form 10-K filings of 100 large, publicly traded exploration and production (E&P) companies.

For the first time since BDO launched its reports five years ago, "risks related to replacing or expanding reserves was the most frequently cited threat, with all companies indicating low prices are inhibiting their ability to make key investments in maintaining supply." The pain is widespread, according to the report, which cited IHS Inc. data, with the number of newly discovered U.S. reserves falling to a 20-year low in 2014.

The top two risks were tied at No. 1 in the BDO survey: an inability to expand reserves or find replacement reserves; and regulatory/legislative changes leading to more compliance costs. The third risk cited was volatile commodity prices.

Three risks were tied at No. 4 and were cited by 96%: operational hazards, such as blowouts/spills; natural disaster/extreme weather; and hydraulic fracturing (fracking) regulations. In the 2014 survey, operational hazards was cited by 98%, natural disasters by 96% and fracking rules were cited by 85%.

Fracking regulation has become a "near-universal concern," with nearly double the number of E&Ps citing it in their 10-Ks versus BDO's initial report in 2011.

It's no surprise that U.S. dealmaking stalled during the first quarter. PwC US, in its latest quarterly Oil & Gas M&A analysis, reviewed domestic transactions with values of more than $50 million using data supplied by GlobalData. Domestic M&A decreased from the fourth quarter in value and volume, while activity was actually "consistent" with historical trends for first quarter activity.

According to PwC, a total of 39 U.S. oil and gas deals accounted for $34.5 billion during 1Q2015. That compares with 4Q2014, where there were 70 worth $103.5 billion, and from a year earlier, when 60 transactions totaled $26.4 billion. Upstream transactions fell sharply, accounting for 12 deals worth $3.6 billion, which is down year/year by 60% in volume 71% in value. Additionally, the total number of oilfield services deals from a year ago declined 77% with only three deals; values fell 94% to $384 million.

Domestic onshore assets still remained a priority for some E&Ps during 1Q2015. Nine transactions were generated worth $50 million-plus, and together they brought in $5.3 billion, representing a year/year increase of 8% in values and 31% in volumes.

"Activity in the upstream sector related to shale plays dropped to four transactions and accounted for $588 million, or 33% of total upstream deal volume and 16% of the total upstream deal value in the first quarter of 2015," PwC said. Four midstream deals were shale-related and accounted for $4.6 billion, 160% higher than in 1Q2014.

"The drop in shale deal activity in the first quarter was highlighted by the low oil price environment as E&P companies looked to strategically realign their business objectives and investments in order to cut overall costs," said PwC energy practice partner John Brady. "As the uncertain environment has continued, E&P companies have been working to capture more value from their land organizations following previous acquisitions."

The most active area in the U.S. onshore for transactions was the Permian Basin, where four deals were valued at $1.5 billion. The Eagle Ford Shale contributed three worth $1.2 billion, while the Marcellus Shale had two worth $567 million. The Bakken and Haynesville shales each generated one transaction.

U.S. corporate transactions in 1Q2015 surpassed asset deals for the first time in five years, with 26 worth $30.4 billion. That's more than seven times higher than in 1Q2014, when there were 13 valued at $4.1 billion. Corporate transactions represented two-thirds of the volumes and 88% of total values, including two midstream deals valued at more than $1 billion.

Overall, four big transactions together represented $23 billion, or 67% of total value in 1Q2015. Also during the first three months of this year, 22 midstream transactions were done, contributing $29 billion, which was 47% higher in growth and 400% more in value from a year ago. Master limited partnership dropdowns and affiliate transactions generated 45% of the midstream deals, totaling $5 billion.

In EY's biannual Oil and Gas Global Capital Confidence Barometer, 112 global energy executives surveyed said their appetite for M&A is getting stronger, with 60% planning to complete "at least" two deals over the next year. However, they may not be mega-mergers; 74% are considering mid-cap acquisitions of up to $250 million.

"Transaction activity may have hit a five-year high in 2014, but the first quarter of 2015 was one of the quietest in recent years," said EY's Andy Brogan, global leader of oil and gas transaction advisory services. "The sudden and steep drop in oil price forced many companies, particularly those in upstream and oilfield services, to adopt an intense internal focus -- aggressively cutting spending and costs. Transaction opportunities in the form of mergers and divestments have been delayed by uncertainty over oil price outlook. Now those acquisition opportunities, coupled with increased confidence in the global economy, are setting the stage for increased M&A activity."

Nearly all (99%) of EY's respondents expect the deal market to improve or remain stable over the coming year, with 97% expressing confidence in the global economy. Most of those surveyed, 85% also now see the valuation gap between buyers and sellers remaining at "bridgeable levels," which may encourage dealmaking in the near term.