The lure looms large: by some estimates, 20 billion barrels of oil equivalent (boe) await prospectors in Asia-Pacific's vast but largely untapped deepwater acreage, representing more than 40 percent of the region's proven oil reserves.

Yet to find hydrocarbons hidden hundreds or thousands of metres under the seas is tough and costly, and such exploration is relatively new to Asia starting barely a decade ago.

"The technical challenge for exploring the deepwater region of this area (Southeast Asia) can be geologically complex. Obviously, without a lot of previous exploration, it is a high-risk and high-cost area to explore," Norman Valentine, an analyst at Wood Mackenzie, said by telephone from Edinburgh.

It is also uncertain how much of the estimated 20 billion boe in Asia can be profitably recovered using existing technology, which is being pushed to the hilt.

"Technology has to prove itself against commercial conditions as anticipated in the future. So if you are convinced that you are going to have $40 oil, you keep on drilling," said Al Troner, managing director of US-based Asia Pacific Energy Consulting. "But a lot of people, myself included, are not."

Malaysia, Indonesia, Brunei and India have made notable deep-sea finds, raising hopes of more major discoveries. Malaysia, where Murphy Oil Corp. and Shell have made major finds in deepwater, plans to open ultra-deepwater areas to foreign investors.

But most discoveries are dominated by natural gas, which is more difficult to handle and transport than oil and requires a dedicated market for sales.

Asia has to vie with other regions for big players such as BP and Royal Dutch/Shell, which have the most advanced deepwater technology. These companies are focusing on more promising deepwater areas in West Africa, the US Gulf of Mexico and Brazil.

Most reserves found in West Africa are crude oil. The success ratio for deep-sea drilling in Angola exceeds 80 percent, above the global average of about 50 percent.