HOUSTON (Reuters) -The U.S. upstream oil and gas M&A market is bracing for the most challenging conditions since the COVID-19 pandemic as oil prices slump and prime acreage dries up, analytics firm Enverus said on Wednesday, even though dealmaking jumped last quarter to the second-best start to the year since 2018. The expected downturn in mergers and acquisitions follows a series of blockbuster takeovers by oil and gas majors in recent years, which culminated in a record $192 billion worth of deals done in 2023. There were $17 billion worth of deals disclosed in the quarter ended March 31, but activity was disproportionately driven by Diamondback Energy, which accounted for almost half of total value, said Enverus Intelligence Research principal analyst, Andrew Dittmar.