Anadarko Petroleum has doubled its estimates for its gas finds offshore Mozambique.
U.S. oil company Anadarko said that the results of its Barquentine-3 appraisal well showed its fields had recoverable reserves of 15 to over 30 trillion cubic feet (Tcf) of natural gas - compared to total UK gas reserves of 9 Tcf, according to the BP Statistical Review of World Energy.
The find has boosted hopes that east Africa will become another major gas production centre.
"This could be one of the most important natural gas fields discovered in the last 10 years," said Anadarko chairman Jim Hackett.
Anadarko previously said the fields, in which Japan's Mitsui & Co and Dublin-based Cove Energy have stakes, held "at least 10 Tcf" of gas.
The results support Anadarko's plans to build a liquefied natural gas (LNG) export facility in Mozambique.
The latest drilling success should also make investors more confident that other prospects might find gas, analysts said.
"Today's announcement also significantly de-risks a further 15 prospects and leads identified on the licence," Richard Griffith at Evolution Securities said.
Explorers see Anadarko's finds as evidence of a trend of oil and gas fields extending along the East Coast of Africa - a region not seen in previous decades as having good exploration prospects.
Last month, Italian oil group Eni said it had made a giant natural gas discovery - the biggest in its history - offshore northern Mozambique, close to the Anadarko fields.
Hopes of other major finds have drawn a rash of other companies to the area including Britain's BG Group, Portugal's Galp, Norway's Statoil ASA and Texas-based Exxon Mobil.
Cove Energy is seen as a possible bid target or at least a willing seller of its Mozambique assets since it is largely a financial investor.
However, until recently, the company's assets were seen as too early in the exploration stage to be attractive to Asian state-backed companies which are acquisition hungry but which prefer to buy companies or field stakes with known reserves.