The problem of illegal aliens has been studied more intensively than any other facing the Carter administration, "with the possible exception of energy," Secretary of Labor Ray Marshall told a symposium on immigration here today.

"There is a need for a little more studying on this issue but not a great deal more" before President Carter presents his legislative proposals to Congress, he said.

The statement put Marshall at odds with a number of participants at this largely Hispanic conference -- including Houston City Controller Leonel Castillo, President Carter's nominee to head the Immigration and Naturalization Service -- who argue that not enough is known about the impact of aliens on U.S. unemployment to justify new laws now.

Castillo has called for more study to develop data and has said that in the meantime the alien worker problem can be best dealt with by rigorously enforcing laws now on the books, such as wage-and-hour laws.

Despite that disagreement, Marshall seemed to be doing his best to try to reassure those here that their worst fears about possible administration plans are groundless.

"We've never considered closing down the border with Mexico or mass roundups of suspected undocumented aliens," he said, speaking for a Cabinet-level committee that is scheduled to present its recommendations to Carter in about two weeks.

Marshall suggested that the committee, which also includes Attorney General Griffin Bell, Secretary of State Cyrus Vance and Secretary of Health, Education and Welfare Joseph A. Califano Jr., is likely to recommend illegal aliens, "unforgeable" work cards, amnesty for many long-term aliens who have been in the United States without proper permission, and a better system to admit alien workers to do jobs Americans aren't available for. He would give no details of those proposals, however.

Ann Gutierrez, associate director of the White House Domestic Policy Council, stressed that in two weeks the President will receive only a memorandum of tentative recommendations from the committee. Proposed legislation is "months away," she said, and will not come befor extensive discussions both inside and outside the government.

In his press conference Friday, Carter seemed to suggest that the legislative proposals on undocumented aliens would be forthcoming in "a couple of weeks."

Civil libertarians are especially wary of the proposed work card, which Marshall described to the press here as being solely for job applicants to show to an employer. He said it would have to be as reliable as an American Express charge card. Civil libertarians at the symposium said they fear such a card soon could become a national identity card.

Marshall acknowledged that "we don't know the extent to which undocumented workers displace the U.S. worker or take jobs that cannot be filled by people in the U.S." But he estimated the population of alien workers at from four to eight million people, and called them an "underclass" that works "scared and hard" and is easily exploited.

He said their employment thus depresses wages and working conditions and tends to "set up a self-fulfilling prophecy that-you can't get U.S. workers to fill those jobs."

The symposium here was the culmination of six conferences in Texas sponsored by the Chicano Training Center of Houston and funded by the Texas Committee for the Humanties and the National Endowment for the Humanities.