The price of the deal was not disclosed.
The dropdown is the partnership's first acquisition since its initial public offering in August 2013.
Eagle Rock’s midstream assets include approximately 8,100 miles of gathering pipeline and more than 800 million cubic feet per day (MMcf/d) of processing plants.
Williams now owns 100% of the general partner and about 50% of the limited partner units in Access Midstream Partners.
Total purchase price was $5.8 million.
The purchase of Morgan Stanley’s energy business related to TransMontaigne was completed on a debt-free basis for a cash purchase price of $200 million, including working capital, plus $347 million for inventory transferred at the closing.
Meagher Energy Advisors has been retained to assist with the sale.
The deal includes Houston-based Apache’s interests in the Lucius and Heidelberg oil production development projects.
The acquisition adds about 60 MMboe of proved reserves to Houston's Sanchez.
The Oil & Gas Asset Clearinghouse will conduct the July 16 auction.
Pittsburgh's Atlas acquired about 47 MMboe reserves.