Merger and acquisition (M&A) activity in the U.S. oil and gas industry hit a decade high in 2014, with investors still clamoring for U.S. assets, but a marked shift was evident in November as commodity prices declined.

Last year’s record-breaking pace to buy up property primarily was driven by mega deals, those valued at $1 billion-plus, a trend that began early last year and continued, according to PwC’s Oil & Gas M&A analysis. Overall, 49 transactions were worth $266.1 billion in 2014, compared with 24 deals valued at $71 billion in 2013.

The quarterly report analyzes announced domestic transactions valued at more than $50 million using transaction data from IHS Herold.

“While 2014 was a very strong year for oil and gas deal activity, we saw a steady decline in November and December as the drop in oil prices accelerated, contributing to a marked shift in deal sentiment from playing offense to playing defense as companies focused on maintaining liquidity,” said PwC’s Doug Meier, U.S. energy sector deals leader. “That downward trajectory in oil prices, coupled with the impact of leverage, drove a number of deals related to corporate restructurings and portfolio right sizing activities.”

In the final quarter, 57 deals were announced valued at more than $50 million, which accounted for $128.7 billion total, or 200% growth year/year. That compares with 4Q2013, when 56 deals were tallied at $43 billion. Mega deals in 4Q2014 represented nearly all — 91% — of activity. Also last year, 252 deals were worth $321.5 billion, an increase from the 187 deals worth $117.2 billion in 2013.

“In today’s low price environment, the effects of debt could drive additional deal activity as leveraged companies look to strengthen their balance sheets by focusing on cash flow optimization and operational efficiencies,” Meier said.

According to PwC, 24 deals with values higher than $50 million related to unconventional/shale plays in the final three months jumped 139% from a year earlier to total $57 billion. In 2014, 107 unconventional deals contributed $110.3 billion, 107% growth in deal value when compared to 2013. In the upstream sector, unconventional deals represented 19 transactions and accounted for $14.9 billion, or 76% of total upstream deal volume. Five midstream shale-related deals accounted for $42.1 billion, or a 230% from the final three months of 2013.

“Overall 2014 shale deal value and volume surpassed 2013 highlighting the continued interest from investors in U.S. shale plays, especially in the upstream space, which contributed 79% of total shale deal activity,” said PwC’s John Brady, a Houston-based partner. “However, a sustained low oil price environment is driving an intense focus on returns and the deployment of assets to the most efficient shale plays.”

The most active shale plays for M&A with values higher than $50 million in 4Q2014 include $3.1 billion total for four deals in the Bakken Shale and $2.4 billion total on four deals in the Permian Basin. The Marcellus Shale contributed three worth $5.7 billion, and the Eagle Ford Shale also contributed three worth $484 million. The Niobrara formation in Colorado and the Haynesville Shale each generated one deal.

For deals valued at more than $50 million, corporate transactions in 4Q2014 represented 16 totaling $103.7 billion. For the full year of 2014, there were 59 corporate transactions that contributed $227.5 billion. Asset transactions continued to dominate total M&A deal volume in the final three months, with 41 deals representing 72% of total volume. Deal value for asset transactions reached $25 billion, accounting for 19% of the total values in the final period. For all of 2014, there were 193 asset deals worth $94 billion.

Between October and December, 15 mega deals represented $117.5 billion, versus eight deals worth $26.4 billion in the year-ago period. In all of 2014, 49 mega deals were worth $266.1 billion, accounting for 83% of total values.

Foreign investors continued to be drawn to the United States, contributing a total of 56 deals worth $71.2 billion in 2014. In final quarter, foreign buyers announced 15 deals, accounting for $25.4 billion in value, a 25% increase in deal volume and a 426% jump in values year/year. Among foreign investors, overall deal volume increased 75% and value was up 468% from 2013.

In the midstream, PwC tallied 19 deals worth $53 billion, an 111% increase in volume and 276% growth in values. Upstream deals accounted for 25 transactions representing $32.5 billion. Downstream transactions fell to six from nine, but values rose to $7 billion from $4.2 billion. The number of oilfield services deals remained the same at seven, while total deal value increased 619% to $36 billion – a 10-year high, compared with $5 billion in 4Q2013. The mega tie-up between Halliburton Co. and Baker Hughes Inc. made the difference.

Master limited partnerships (MLPs) were involved in 48 transactions total last year, representing about 19% of total activity, consistent with recent historical levels. Although financial investor deal activity dropped, there were four total transactions accounting for $2 billion in 4Q2014, compared with nine deals worth $10 billion during the same period in 2013.

“In the second half of 2014, we saw financial investors remain active in assessing deal activity across the value chain,” said PwC’s Rob McCeney, U.S. energy and infrastructure deals partner. “If the price of oil continues to drop or remains at its current levels for a sustained period, we may see financial investors look to actively manage portfolio investments and search for new opportunities with distressed assets.”