By Joshua Schneyer

(Reuters) - Sue Ann Hamm, the ex-wife of Oklahoma oil magnate Harold Hamm who was awarded cash and assets worth more than $1 billion in the couple's divorce this week, plans to appeal the judgment on grounds that it grossly undervalues the marital wealth she is entitled to.

Feeling shortchanged by a ruling that allows the Continental Resources chief executive officer to keep around 94 percent of the estimated $18 billion rise in his Continental shares during a 26-year marriage, Sue Ann Hamm will appeal within a few weeks, one of her lawyers, Ron Barber, told Reuters on Thursday.

She believes the decision was "not equitable," Barber said.

On Monday, Oklahoma County Court Judge Howard Haralson ordered the CEO, who is believed to own more oil than any other American, to pay his ex-wife $995 million. The ruling allows her to keep additional assets, including a California ranch and an Oklahoma home, worth tens of millions more.

The Hamm v. Hamm divorce judgment is one of the largest in U.S. history, but Sue Ann's award is a small fraction of the wealth Haralson allowed Harold Hamm to keep.

He holds more than 68 percent of Continental's stock, a stake valued at around $13.5 billion today. It was worth more than $18 billion before the 9 1/2-week divorce trial began in August. Continental shares have fallen sharply since then, in line with global oil prices.

Haralson ruled that $1.4 billion of the growth in his Continental shares during the marriage was "marital capital" to be split with Sue Ann. The rest was awarded to Harold as “separate property.”

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

A lawyer and economist, Sue Ann Hamm worked at Continental during stretches of the couple’s marriage, which began in 1988. At one point, the ruling says, she was an executive in charge of Continental's crude marketing division. She left the company in 2008. At other times she worked in the home, helping to raise the couple's two children.

In Oklahoma, a divorce appeal can be heard by a State Court of Appeals panel or the Oklahoma Supreme Court.

A higher court could review the case and affirm Haralson’s judgment, or modify the award. It could also send the case back to Haralson to be re-tried.

Family law experts say the process could take anywhere from 18 months to several years. Sue Ann Hamm has 30 days from when the ruling was filed, on Nov. 10, to appeal it.

As part of Monday's ruling, Haralson ordered Harold Hamm to pay his ex-wife more than $322 million by Dec. 31, and continue with monthly payments of at least $7 million until he covers an additional $650 million balance. She has received around $23 million from the marital estate since filing for divorce in 2012.

To secure the judgment, Haralson placed a lien on 20 million shares of the CEO’s Continental stock. Whether an appeal would alter the payment schedule remains unclear.

Craig Box, an attorney for Harold Hamm, said the CEO considers Haralson’s ruling to be “fair and equitable.” Hamm declined to comment on the prospect on an appeal.

The Hamm divorce is a "personal matter," said Continental spokeswoman Kristin Miskovsky. The company has said the case has no impact on business.

Following the news, shares of Continental extended earlier losses. They fell 3.1 percent on the day as U.S. oil prices sank below $80 per barrel for the first time in four years.

400-FOLD INCREASE

The Hamms had no prenuptial agreement. During their marriage, Continental's value soared by around 400-fold, and Hamm, the 13th child of sharecroppers, became Oklahoma's richest person.

Hamm, 68, founded Continental in 1967, more than two decades before he married Sue Ann. But Oklahoma law typically requires that the enhanced value of premarital property be split "equitably" in a divorce if it resulted from the efforts or skills of either spouse during marriage.

Haralson attributed most of Continental's growth during the marriage to passive or market factors, including rising oil prices and new drilling technologies.

Continental holds a leading position in the Bakken Shale, the largest U.S. oil discovery in decades. Harold Hamm is often credited with pioneering its development.

During the trial, his lawyers contended that passive factors and the "teamwork" of others at Continental accounted for most of the company's dazzling growth during the marriage.

"We believe that Harold should be given more credit for Continental's success," Sue Ann Hamm's lawyers said.

(Editing by Jonathan Leff and Douglas Royalty)