The company expects to invest £160 million in the field and believes the spending may create up to 1000 new jobs in the next three years among 30 supply chain partners in cities including Aberdeen, Newcastle, Manchester and Swansea.
The brown field allowance gives companies the opportunity to lower their tax rate on profits from existing fields where production is being increased.
Among the improvements EnQuest is making at Thistle – located 275 miles north-east of Aberdeen – are a new 30-megawatt power turbine and an enhanced process control safety system.
David Heslop, general manager for EnQuest in Aberdeen, added: "Before EnQuest acquired Thistle in 2010, production was declining and, coupled with ageing infrastructure, it was approaching the point where production may have stopped.
"As a result of our investment so far, which has included facilities and safety systems upgrades, a major rig reactivation programme and drilling of five new wells, production has significantly increased."
Alongside the Thistle extension EnQuest, which employs 1300 people in Scotland, is also budgeting for an additional $200m (£127m) to bring the Alma/Galia field back into production.
The company said the plan would see reserves in the field – formerly known as Argyll and the first to be developed in the UK North Sea – increased from 29 million barrels of oil equivalent (MMBoe) to 34 MMBoe.
Costs are now expected to be $1.2 billion with first oil still anticipated for the fourth quarter of this year.
Average daily production across EnQuest's portfolio was 22,802 barrels of oil equivalent per day in 2012 against guidance of between 20,000 and 24,000.
Mike Tholen, Oil & Gas UK's economics and commercial director, believes tax changes are helping to promote investment into the North Sea.