Jethro-1 spud early last month and was drilled using the Stena Forth drillship to a total depth of 4,400 metres in water depths of 1,350m.
Tullow is the operator of the Orinduik license with a 60% interest alongside Total (25%) and Eco Guyana (15%).
Evaluation of logging data confirms that Jethro-1 is the first discovery on the Orinduik licence and comprises high quality oil bearing sandstone reservoirs of Lower Tertiary age.
The 55m of net oil pay supports a recoverable oil resource estimate which exceeded Tullow's pre-drill forecast.
The joint venture is now evaluating data from the discovery to determine its forward appraisal plan.
"This substantial and high value oil discovery in Guyana is an outcome of the significant technical and commercial focus which has underpinned the reset of our exploration portfolio," Tullow CEO Paul McDade said.
"It is an excellent start to our drilling campaign in the highly prolific Guyana oil province."
Tullow said the discovery de-risked other prospects on the Orinduik license, including its planned Upper Tertiary Joe prospect which is planned to be spud later this month.
A third well, Carpoa-1, will be drilled following that on the adjacent Kanuku license.
"We look forward to drilling both the Joe and Carapa prospects in our 2019 drilling campaign and the material follow-up exploration potential in both the Orinduik and Kanuku licences," McDade said.
The Guyanese Department of Energy said the discovery was a "major development" that demonstrated the country's vast resources.
"The Department of Energy is encouraged by the prolific rate of discovery in the CRG and will continue to work assiduously and conscientiously to extract optimum value from these resources for all the peoples of our country," Guyana Department of Energy director Dr Mark Bynoe said.
Tullow also has a 37.5% interest in the Kanuku license in Guyana alongside Repsol.