VANCOUVER, June 18 (Reuters) - Pacific Rubiales Energy's largest group of shareholders has begun a proxy fight to block the roughly C$2 billion ($1.6 billion) takeover of the Canadian oil producer by Alfa SAB de CV and Harbour Energy Ltd.
O'Hara Administration, speaking for a group of shareholders holding about 20 percent of Pacific Rubiales shares, said on Thursday it had filed a proxy circular and letter urging other minority investors to vote against the deal.
The group said it was not attempting to unseat the Pacific Rubiales board at this time.
A Pacific Rubiales spokesman did not have an immediate comment on the proxy move, but said a statement would be issued shortly.
Mexican conglomerate Alfa and Harbour Energy, a joint venture between Asia's Noble Group Ltd and U.S. private equity firm EIG Global Energy Partners, in May offered to buy Toronto-based Pacific Rubiales for C$6.50 per share, or about C$2.05 billion.
Shares of Pacific Rubiales, which operates major oil fields in Colombia, were up 0.91 percent at C$5.56 in afternoon Toronto Stock Exchange trading.
O'Hara has long opposed the deal, which it says undervalues the company and "inappropriately incentives management at the expense of the minority shareholders."
Pacific Rubiales has said the all-cash deal provides certainty and liquidity in a period of volatility for the oil market and that management "negotiated the offer with a view as to maximizing value for all shareholders."
The company is the second-biggest oil producer in Colombia, with output of about 145,000 barrels per day. However, it is set to lose about 65,000 barrels per day of that in mid-2016 when its contract to operate state-run Ecopetrol's Rubiales field expires.
The offer from Alfa and Harbour Energy came during difficult times for Pacific Rubiales, whose shares have plunged 70 percent over the last year as crude prices fell and the company struggled under its hefty debt-load.
The stock jumped more than 25 percent after the takeover was announced but is still a fraction of its 52-week high of C$23.79 reached a year ago.
Besides O'Hara, the group includes IPC Investments Corp; Telmaven Overseas Inc; Volbor Trading Ltd; Memphis Investment Limited; Agency Partner Corp; Orlando Alvarado; Fundacion Nemone; and the Adar Macro Fund.
$1 = 1.2224 Canadian dollars Reporting by Julie Gordon; Editing by Lisa Von Ahn