The source told the IRIB News in Vienna that the draft text, along with its parentheses and remaining sticking points, is to be put to discussion at the level of deputy foreign ministers of the sextet.
The text will be then handed to the foreign ministers of the six countries to make the final decision on the long-awaited comprehensive nuclear agreement.
Iran and the Group 5+1 (alternatively known as P5+1 or E3+3) are in talks in the Austrian capital to hammer out a lasting agreement to end more than a decade of impasse over Tehran's civilian nuclear program.
The two sides on Tuesday extended the nuclear negotiations for seven days after the June 30 deadline expired.
On Thursday, Iranian Foreign Minister Mohammad Javad Zarif, who is also Iran's chief negotiator, is scheduled to separately meet with his German and British counterparts Frank-Walter Steinmeier and Philip Hammond for nuclear talks.
The foreign ministers of France and China as well as European Union Foreign Policy Chief Federica Mogherini are also expected to join the talks tomorrow.