The ex-wife of a US oil magnate plans to appeal against the $1bn she received from their divorce – because she believes the payout is insufficient.

An Oklahoma County court judge on Monday awarded Sue Ann Hamm $995m, along with assets worth tens of millions of dollars, in her divorce from Continental Resources CEO Harold Hamm. Her lawyer, Rob Barber, said on Thursday that they plan to contest the ruling.

“Sue Ann is disappointed in the outcome of this case,” Barber told The Oklahoman. “She dedicated 25 years as Harold’s faithful partner in family and business.”

The couple married in 1988, and Sue Ann Hamm filed for divorce in May 2012. She had worked as an Continental Resources executive and has testified before Congress on behalf of the company. Divorce proceedings were mostly closed to the public.

“She feels that an award of less than 6% of the couple’s wealth, which had grown during the marriage up to the date of the trial by more than $18bn, is not equitable,” Barber said.

The couple did not sign a pre-nuptial agreement, and the divorce award is one of the largest ever in the United States.

Per Monday’s ruling, Hamm is to pay his ex-wife $322m by the end of the year, then at least $7m each month until the remaining $650m is paid out.

Harold Hamm owns more than 68% of Continental Resource’s stock, which dropped from being worth more than $18bn before the divorce trial to $13.5bn.

Hamm founded Continental in 1967 and built up the company in the 1990s by drilling in North Dakota’s Bakken field. He served as Mitt Romney’s energy adviser when he was running for president in 2012.

It could take several years for the appeal to be reviewed in the state court of appeals, state supreme court or another higher court.