New York - Exxon Mobil, the world’s largest oil group by market capitalization, would be interested in investing in Mexico's oil and gas sector, but only if the Mexican government allows the company to own some energy reserves, its chief executive said.

"We're not real keen on service contracts, we're not real keen on fixed margin contracts. We’re hopeful that Mexico, will open up opportunities for greater partnerships and collaborations, and bringing technology to bear on the huge resources that the country has," Exxon Mobil CEO Rex Tillerson told reporters after a speech in New York.

Mexico's constitution bars outside exploitation of the country's oil resources, making joint ventures or profit sharing with private companies difficult.

The country has been seeking to open the door to attract investment from foreign oil and gas producers to help tap the vast reserves there. Exxon has been working with Pemex, Mexico’s national oil company, on joint studies "primarily so we can get to know one another, and see what each other’s capabilities are," said Tillerson.

Pemex awarded contracts to some foreign companies earlier this month to help develop offshore fields, but those contracts pay bonuses based on performance and do not allow for ownership of oil and gas.

Exxon's top executive said he was encouraged by the initial moves to open the Mexican energy sector, which could eventually attract financing and technology from the global industry.

Mexico is among the world’s top 10 oil producers and in addition to its offshore oil fields, it has the world's fourth-largest reserves of shale gas, according to the US Energy Information Administration.

But so far, Pemex has drilled relatively few wells in its fields near the Rio Grande because it has little capital to develop those areas.

Mr Tillerson made it clear that Exxon would be prepared to invest in Mexico only if it was allowed to share in the upside and downside of oil and gas prices.

"The US, Canada, and Mexico should work together to develop energy supplies and promote 'energy security' that would keep a flow of oil and gas to North America," he said.

Currently, North America produces about 15 million barrels of oil per day and Exxon expects that figure to grow to 18 million barrels a day by 2020. "That is a force to be dealt with in global oil markets," stated Tillerson.

"I think it's going to be a long process. And what we're advocating is just for Mexico to take the next step," he said.