The field's gas reserves are estimated at about 140 billion cubic metres, it said.
The size of the deposit could turn out to be bigger, since the field's exact contours have not yet been fully determined and exploratory drilling is still underway, Statoil spokesman Bård Glad Pedersen told AFP.
"The well has encountered 120 metres of excellent quality reservoir with high porosity and high permeability. The gas-water contact has not been established and drilling operations are on-going," the company said in a statement.
This is Statoil's fifth major oil or gas discovery in the past year, after large fields were found in the North and Barents seas and off of Brazil.
Statoil, which holds the operating licence off Tanzania, owns 65 percent of the field and ExxonMobil 35 percent. Tanzania Petroleum Development Corporation will be entitled to 10 percent of the project once it enters into operation, Statoil said.