Mergers and acquisitions have heated up in the refining sector with more activity to come, according to analysts at Tudor, Pickering, Holt & Co.

The energy-centric investment bank said there have been five transactions announced so far this year, versus six between 2013 and 2015 and 16 between 2010 and 2012.

The transactions have included private equity firm One Rock Capital Partners acquiring the 58,000 barrel-per-day Kapolei, Hawaii refinery from Chevron (CVX) - Get Report for undisclosed terms; Par Pacific Holdings (PARR) - Get Report picking up the 18,000 barrel-per-day Newcastle, Wyo., refinery from Black Elk Refining LLC for $271 million; and Western Refining Inc. (WNR) completing the merger of the remaining 62% of Northern Tier Energy (NTI) and its 92,000 barrel-per-day refinery in St. Paul Park, Minn., for $1.5 billion.

The others are Tesoro (TSO) acquiring the 20,000 barrel-per-day Dakota Prairie, ND., refinery from Calumet Specialty Productions Partners (CLMT) - Get Report and MDU Resources (MDU) - Get Report for an undisclosed sum; and PBF Energy (PBF) - Get Report finally closing on the 155,000 barrel-per-day Torrance, Calif., refinery previously owned by Exxon Mobil (XOM) - Get Report for $537.5 million.

The reason for the uptick? U.S. crack spreads -- the difference between the price of crude oil and what products derived from them can fetch -- are down 29% year-to-date, allowing buyers and sellers to close the gap on price, TPH said. And $45 crude oil has crimped cash flow generation for the integrated oil companies, potentially pushing them to monetize assets, including non-core refineries.  

TPH said it's continuing to wait on incremental moves from Delek US Holdings (DK) - Get Report after its standstill agreement regarding its acquisition of 48% of Alon USA Energy (ALJ) expired in May. It also said Phillips 66 (PSX) - Get Report appears to be close to a sale for its 71,000 barrel-per-day Whitegate plant in Ireland.

If recent reports are to be believed, other refineries on the block include the 150,000 barrel-per-day Martinez refinery owned by Royal Dutch Shell (RDS.A) , the 55,000 barrel-per-day Burnaby refinery owned by Chevron and the 330,000 barrel-per-day Philadelphia plants owned by Philadelphia Energy Solutions (PESC) . LyondellBasell Industries (LYB) - Get Report management has also noted that its 287,000 Houston refinery might be worth more to another company, TPH said.

Some assets have been pulled from the block. Total (TOT) - Get Report and Suncor Energy (SU) - Get Report were reportedly unable to come to a price on the 50% stake in the 233,000 barrel-per-day Port Arthur refinery in Texas and Total is thought to be no longer marketing this asset, TPH said.

The investment bank expects to see Par Pacific -- which has very large net operating losses -- and PBF as likely buyers going forward as well as ongoing strong interest from private equity firms. Among the larger independents, both Valero Energy (VLO) - Get Report and Tesoro carry strong balance sheets and have been mentioned as potential buyers in earlier transactions, TPH says.