The recent string of massive oil discoveries around the world are good news for major oil companies, but not necessarily the key to solving their most pressing challenge: long-term production growth.

Despite their plethora of ongoing multi-billion dollarprojects, major oil companies' producing fields are continuing to decline faster than they are able to find and develop new resources -- and new discoveries would need at least a decade to start production, analysts said. This could leave companies like Exxon Mobil Corp. (XOM), Royal Dutch Shell (RDSA), BP PLC (BP) and ConocoPhillips (COP) unable to achieve their promise of even a modest growth in the next few years and force Wall Street to change the way it values these companies' stock, says a report by investment bank Credit Suisse.

"Despite some high profile start-ups and high levels of industry activity, net production growth will remain an almost insurmountable challenge for the U.S. and European integrated oil majors out to 2020," said the report released Friday. "Growth will remain elusive," and companies won't be able to keep the production increase promises that they've made -- potentially resulting in a negative market reaction once shareholders realize the value of the companies is lower than expected, according to the report.

"The one thing that (analysts) can all take from whatever (major oil companies) tell us is that growth is going to be less that what they said," Argus Research analysts Phil Weiss said.

Major oil companies have been struggling for years to maintain, let alone raise, their output as their traditional fields dry up and rising nationalism in resource-rich countries makes it harder for them to access new deposits.

They have addressed this problem by investing billions of dollars in the exploration and development of new resources in non-conventional areas. The effort is paying off in a string of recent discoveries and the sanctioning of major capital projects. Earlier this month, BP said it found a massive field in the ultra-deep waters of the U.S. Gulf of Mexico that could contain 3 billion barrels of oil. Chevron Corp. (CVX), ExxonMobil and Shell also announced this month that they have approved the Gorgon liquefied natural gas project off Australia's northwest coast, which is expected to cost $37.1 billion and to start production in 2014.

However, oil majors would need at least 10 years to fully see the benefits of these ventures, leaving analysts worried about their ability to increase production by 2% to 5% a year, as some have offered.

"In a five year-window, it's going to be very difficult to see any growth at all," said Kenneth Medlock, an energy fellow at the James A. Baker Institute for Public Policy at Rice University.

According to Credit Suisse, the world's 13 largest private oil and gas producers' output will contract 0.7% per year to 2020, with shrinking oil output being the largest driver of the decline.Existing fields are predicted to decline 5.8% per year.

Chevron, the second largest U.S. oil company by market value after ExxonMobil, is seen as the only oil major able to hold up growth in the immediate future, driven by large projects star-up like Gorgon in Australia and the expansion of Tengiz field in Kazakhstan. However, analysts said that even the San Ramon, Calf.-based company could face production growth challenge at the end of the next decade.

But major companies maintain their optimism. ExxonMobil, which in 2008 had 7% lower production than in 2006, expects to stay flat at around 4 million oil-equivalent barrels per day in 2009. The Irving, Texas, company said it expects to have annual growth of about 2% to 3%, with the possibility that the rate could change from year to year.

"Our forecasts of future production growth are based on the summation of all ... projects within our global portfolio which we believe will be sufficiently attractive to invest in," ExxonMobil spokesman Rob Young said.

British oil giant BP said it expects to grow production by an average 1% to 2% annually over the next five years. "We believe that our already identified resource base -- and this was before recent discovery announcements -- gives us the potential to continue these growth rates beyond that out to 2020," said BP spokesman David Nicholas. These figures are unchanged from the estimates the company gave to analysts in March.

Shell said the company won't respond to analysts' speculation. ConocoPhillips declined to comment.

-By Isabel Ordonez, Dow Jones Newswires; 713-547-9207; isabel.ordonez@dowjones.com