ConocoPhillips Inc., which is North Dakota's largest oil producer, intends to step up exploration of the promising Bakken geologic formation this summer, a company executive says.

ConocoPhillips already has three rigs working in the western North Dakota formation, said Philip Frederickson, an executive vice president of the Houston company. It expects to add a fourth this summer, Frederickson said.

Oil industry officials say drilling in the formation is complex and expensive. "We're trying to continue to understand" the intricacies of the formation and the oil production rates from its wells, Frederickson said.

Frederickson and other ConocoPhillips officials held a public forum in Bismarck on Wednesday on energy issues. It was hosted by the company and Bismarck State College.

The company plans to have 35 such events across the nation this year, which are billed as chances for members of the public to discuss oil and gas exploration, alternative fuels and energy conservation.

Gov. John Hoeven also met Wednesday with a group of ConocoPhillips officials to discuss ways to boost production, and the possibility of ConocoPhillips partnerships with other companies to develop such alternative fuels such as ethanol, biodiesel fuel and liquid fuel from coal.

"What we were interested in talking about is not only how we get them to do more in terms of oil and gas in the state, but how we can get them partnered with other companies we're working with to do things, like coal and alternative fuels," Hoeven said.

Of North Dakota's 119,000 barrels of daily oil production, ConocoPhillips accounts for about 40,000 barrels. Most of the oil comes from the Cedar Hills operating unit in North Dakota's southwestern corner, which the company operates with Continental Resources Inc. of Enid, Okla.

ConocoPhillips acquired its Cedar Hills interest when it bought Burlington Resources Inc. of Houston. The $33.6 billion purchase was completed in March 2006.

"We are very pleased with the production. The growth in production has been more rapid than we expected," Frederickson said. "We are very pleased with what we see in North Dakota."