UPDATE 1-Petrofac, Valiant start production from Don Southwest

* Say side-track well reveals 60 foot oil column

* Injection wells on Don Southwest to be on-stream in Oct

* Valiant shares rise 8 pct, Petrofac up 1.3 pct (Adds details)

July 1 (Reuters) - Petrofac Plc (PFC.L) and Valiant Petroleum Plc (VPP.L) said on Wednesday they started production from the Don Southwest field in the UK North Sea and that the injection wells currently being drilled were expected to be on-stream in October.

In separate statements, the companies said production from the field has commenced from two production wells, which will be supported by the water injection wells.

Oil and gas services company Petrofac, is the operator of Don Southwest with a 60 percent interest in the field, while British oil and gas company Valiant Petroleum Plc (VPP.L) holds the rest.

Valiant said a side-track well on Don Southwest had encountered a 60-foot oil column in the upper Brent formation, extending the boundary of the field to the south.

"The structure is currently being reinterpreted with the new data to better understand the implications for further prospectivity in this and the other compartments in the area," Valiant said.

Further phases to the Don Southwest appraisal and development program are currently being defined for 2010 and onwards, Valiant said. At 0815 GMT, Petrofac shares were up 1.3 percent at 678.5 pence, while Valiant shares were up 8.1 percent at 505 pence on the London Stock Exchange. (Reporting by Usman Merchant in Bangalore; Editing by Gopakumar Warrier)