The most likely buyer for the natural-gas assets is an energy-focused private-equity firm or Master Limited Partnership.
People familiar with the matter described the fields as "mature," meaning that large volumes of gas have already been produced from them.
Shell's 400 wells in South Texas produce about 210 million cubic feet of natural gas every day, according to the company's website. The oldest wells have been in operation since 1953.
Shell couldn't be reached for comment.
Earlier this month, Shell announced it agreed to sell its interest in six Gulf of Mexico oil and gas fields to W & T Energy VI, a subsidiary of W&T Offshore, for $450 million. The sale was part of Shell's portfolio-restructuring efforts, which include the divesture of $7 billion to $8 billion in assets in 2010 and 2011.
Shell isn't the only large oil company looking to sell its U.S. gas assets. On Nov. 21, Bermuda-based oil exploration company Energy XXI Bermuda Ltd. agreed to buy some of ExxonMobil's shallow-water and natural gas assets in the Gulf of Mexico for $1 billion. The move will help Energy XXI become the third-largest oil producer on the Gulf shelf, with interests in seven of the top 11 oil fields there, the company said.
Deals involving Gulf of Mexico oil and gas assets are expected to heat up in the wake of the BP oil spill. As a result of the disaster, U.S. lawmakers are expected to increase the maximum amount companies would have to pay for economic losses and damage resulting from similar incidents.
That means small and midsize companies operating in the Gulf region will likely have to merge with other companies to meet those anticipated increases in liability caps.
Shale oil and gas also continues to be a hot ticket for merger transactions. Earlier this month, Chevron announced it was acquiring Atlas for $3.2 billion, giving the oil company access to the Marcellus Shale resources in Pennsylvania.
In October, China's Cnooc bought a 33.3% stake in Chesapeake's Eagle Ford shale oil and gas acreage in south Texas for about $1.1 billion, marking the first acquisition by a Chinese state-run company into the U.S. shale oil and gas sector.