Two new names have been added to the top 10 wealthiest United States oil billionaires since the beginning of the year: Scott Duncan and Kelcy Warren, according to Wealth-X, an information resource about ultra-high-net-worth individuals and families worldwide.

At 32 years of age, Duncan, one of the heirs to the Duncan energy pipeline empire, is one of the youngest billionaires in the nation. He joined the top oil barons at number 10. Duncan has a net worth of $5.7 billion, according to the Wealth-X report released Tuesday.

The late Dan Duncan, the father of Scott and three other children, founded Enterprise Products Partners, which owns nearly 51,000 miles of natural gas, oil and petrochemical pipelines.

Warren joins the group at number seven, with $7 billion in net worth. Warren is chairman of Energy Transfer Partners, an energy pipeline company.

The two newcomers knocked Ray Hunt, the richest son of oil wildcatter H.L. Hunt, and Dannine Avara, one of the Duncan siblings, off the list.

The Koch brothers, David and Charles, retained the top two spots by a wide margin, with $47.9 billion and $47.8 billion respectively. Harold Hamm, Oklahoma oil tycoon and CEO of Continental Resources, remained in the number three spot on the list despite having to pay a $975 million divorce settlement and losses in the stock market in the past year that reduced his net worth by 36 percent, to about $11.8 billion, Wealth-X says.

Others who remained on the list are Richard Kinder, chairman of pipeline giant Kinder Morgan; George Kaiser, founder of BOK Financial Corporation; Elaine Marshall, a major Koch Industries shareholder; and Milane Frantz and Randa Williams, two other Duncan siblings.