BG Group said it encountered a 180-foot pay of natural gas in its Mzia-1 well about 25 miles off the southern coast of Tanzania.
Prior to drilling the well, the company estimated there were 7 trillion cubic feet of gross recoverable reserves in its four previous discoveries drilled off the country's coast in deep waters.
A preliminary evaluation of the Jodari-1 exploration well, announced in March by the company, found it contained a gross recoverable reserve estimate of 2.5 trillion to 4.4 trillion cubic feet of natural gas.
The World Bank announced last year it approved an International Development Association credit of $27.8 million to back Tanzania's energy expansion projects. The bank said the financial assistance would help cover costs tied to replacing or upgrading transmission lines and other equipment on the country's electrical grid.