Angola's decision to join the Organization of Petroleum Exporting Countries hasn't affected Exxon Mobil Corp.'s (XOM) plans to develop reserves in the African country, the energy major's chief executive said Thursday.

"Angola joining OPEC has not influenced our forward planning," ExxonMobil CEO Rex Tillerson.

Angola's decision to become a member of the oil-producing group as of January, potentially further restricting energy companies' access to reserves. However, it's unclear how and when Angola will become subject to a production quota.

Angola officials say they intend to boost the country's oil output 25% to 2 million barrels a day next year. OPEC Secretary-General Abdalla Salem el-Badri said earlier this month that Angola would certainly have a quota assigned either this year or in early 2008.

With Angola switching sides from the non-OPEC to the OPEC camp, the prospect for crude-oil output growth in countries outside of OPEC is looking increasingly dim. Production is declining in mature provinces such as the North Sea, while non-OPEC countries such as Russia are limiting access to reserves.

"In the next two to three years, non-OPEC growth is seen modest to flat unless there's dramatic policy change to access (the non-OPEC regions)," Tillerson said. The ability to continue non-OPEC growth is "substantially challenging."

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