HOUSTON, June 14 (Reuters) -

Marathon Oil's ( MRO ) 68,000 barrel per day crude oil

refinery went under maintenance earlier this month, putting

downward pressure on Bakken oil prices, a state regulator said

during a press conference on Friday.

All units at the Mandan refinery are currently under

maintenance, according to Justin J. Kringstad, director of the

North Dakota pipeline authority.

Marathon Petroleum ( MPC ) did not immediately respond for comment.

The Mandan refinery is North Dakota's only crude processing

facility and the dip in demand has pressured Bakken oil prices,

according to Kringstad.

Clearbrook prices as of mid-June are at a 16 cents discount

to WTI, compared with a 40 cent premium to WTI in mid-May,

according to the North Dakota Department of Mineral Resources.