The Environmental Protection Agency said Wednesday that it would weigh rules requiring disclosure of the chemicals used in hydraulic-fracturing fluids.

Companies such as Halliburton Co. and Schlumberger Ltd., which supply oil and natural gas producers, should be required to reveal substances used in the mining technology known as fracking, according to a petition filed with the EPA by the environmental group Earthjustice. In a response posted on its website Wednesday, the EPA said it would begin gathering data.

The EPA will try to provide "aggregate pictures of the chemical substances and mixtures used in hydraulic fracturing," Stephen A. Owens, an assistant administrator at the agency, said in a letter to Deborah Goldberg, an attorney for Earthjustice. "This would not duplicate, but instead complement, the well-by-well disclosure programs of states."

Fracking has led to a natural gas boom in Pennsylvania, West Virginia, and Texas, producing opposition among some residents and environmental groups who say the technology may contaminate drinking-water supplies and add to air and soil pollution. Millions of gallons of water, sand, and chemicals are injected underground, cracking rock to free trapped gas or oil.

Earthjustice, an environmental-law firm based in San Francisco, was among groups that petitioned the EPA in August under the Toxic Substances Control Act to require makers of fracturing fluid to release the ingredients in their products, data on health and environmental effects, and reports of adverse reactions.

The EPA turned down another part of the organizations' request, telling the groups on Nov. 2 that it wouldn't mandate toxicity testing for each of the chemicals. It also denied a request that other chemicals used in oil and gas exploration and production be disclosed as well.

Extraction from shale formations has grown to about 15 percent of U.S. natural gas production, and this share is expected to triple by 2035, according to the U.S. Energy Information Administration.

In its letter Wednesday, the EPA said it would issue an advance notice to the public outlining its proposal and questions for companies, local residents, and environmental groups. The agency said one key issue it would discuss was how much of the chemical information provided by companies would be made public.