Opec's deliberations on Thursday in Vienna took more than five hours, prompting the cancellation of a news conference and a gala dinner for delegates aboard a boat on the Danube.
The oil market faces a tricky patch early next year.
On Wednesday, Ghadhban said he would support at least extending existing cuts to end-2020 from March: "We have to give a positive signal to the market and to me at least we should roll-over present agreement". "We always have an agreement among OPEC members", he added.
Any price gains from the OPEC+ output cut are likely to benefit American producers not party to any supply curbing agreement. "Nobody said collective supply management was easy or fun, especially after four years of struggle against surpluses".
Oil prices surged following the announcement, with U.S. benchmark WTI and its European counterpart Brent both almost two per cent higher shortly after 1500 GMT.
West Texas Intermediate for January delivery fell 15 cents to $58.28 a barrel on the New York Mercantile Exchange as of 7:55 a.m. local time.
In any event, OPEC's policy decision was not due to be officially ratified until yesterday, when the OPEC+ meeting was scheduled. But unresolved questions around Iraq's contribution could be a bigger obstacle to a deal than usual.
The additional cuts by the Organization of the Petroleum Exporting Countries and allies including Russian Federation - a grouping known as OPEC+ - will last throughout the first quarter next year.
OPEC's second largest oil producer Iraq said on Tuesday Saudi Arabia was supporting deeper cuts for OPEC+ to 1.6 million barrels per day, or 1.6% of global demand, from the current level of 1.2 million bpd. But it achieves what Prince Abdulaziz wanted "and I think he's going to put more pressure than anyone has before to get real cuts from these guys".
The group's Joint Technical Committee said that supply reductions exceeded that target by about 40 percent in October, equivalent to about 500,000 barrels a day of additional curbs.
OPEC countries have reached a consensus on a deal on cutting oil production at the ministerial meeting on Thursday and plan to continue negotiations on Friday, Saudi Arabia's Energy Minister Abdulaziz bin Salman Al Saud told reporters. Russia, which has complied with its production quota for just three months of this year, had its proposal to exclude a very light oil called condensate approved on Thursday, potentially giving it space to keep pumping above its target.