WASHINGTON (AFP) - Multibillionaire American oilman Harold Hamm has been ordered to pay almost US$1 billion (S$1.3 billion) to his ex-wife to settle one of the most expensive divorce cases in US history.

According to a ruling filed in Oklahoma on Monday, the chief executive of Continental Resources owes his former wife Sue Ann Hamm US$995,481,842 for "property division alimony".

Hamm, 68, is one of America's richest people. His 68 per cent stake in Continental Resources is worth nearly US$14 billion, according to Forbes.

Time magazine included him in its 2012 list of the 100 most influential people, noting how he had grown his business through the use of hydraulic fracturing (fracking).

The couple were married in 1988.

Hamm must pay Sue Ann, 58, nearly US$323 million by the end of the year, then the rest must be paid off in monthly installments of at least US$7 million, plus interest.

He had already paid her more than US$22 million.

The ruling is subject to appeal.