A preliminary study of corporate tax sales under the controversial "leasing" provisions showed that more than 85 percent of the transactions involved property worth $10 million or more, Treasury Secretary Donald T. Regan said yesterday.

The study, which will not be released until later today, appear to suggest that the overwhelming beneficiaries of the law were big businesses, but Regan disputed this.

When the leases are viewed in terms of numbers of transactions, not dollar volume, "more than 60 percent of the number of actual lease transactions involved less than $100,000 of property, and nearly 95 percent involved less than $1 million of property," he said.

In findings that appear to conflict with a parallel study by the Joint Committee on Taxation, the Treasury said that 84 percent of the tax benefits go to the group that is supposed to benefit--companies without taxable income--and that 15 percent goes to profitable firms buying the breaks, while 1 percent goes to lawyers, investment bankers and other intermediaries.

The committee found that 76 percent, or $3.8 billion of a total 1981 revenue loss of $5 billion, went to the poor companies, while 22 percent, or $1.1 billion, went to profitable firms, and 2 percent, or $100 million, went to middlemen.

The committee made its calculations on the basis of revenues lost to the Treasury; the Treasury, on the basis of the value of the tax benefits to the poor companies.

Regan said the study shows that payments to middlemen, on a relative basis, were no larger for small leases than for large leases.

Tax leasing was created by the 1981 tax bill. Under the provision, corporations can buy and sell tax breaks on new investments through paper deals called "leases." The purpose of the provision was to give new companies and financially strapped firms the opportunity to share in the benefits of the legislation and to reduce the danger of mergers resulting from a company that has far more tax breaks than it can use.

Disclosures that General Electric Co. was able to buy enough tax credits and depreciation write-offs to get a net tax refund from the federal government and that a number of highly profitable, but nontaxpaying, companies have sold their tax breaks has created strong pressure to repeal or severely restrict the provisions.

Regan reiterated his contention that the leasing provisions are a "significant element in this administration's effort to increase growth and productivity," although on Wednesday he told a congressional committee that he is considering some proposed modifications.

The major beneficiaries of leasing were companies in mining, oil and gas drilling, lumber and paper, chemicals, airlines, primary metals, motor vehicles, railroads, shipping and utilities, Regan said, contending that the list "includes most of those industries that have been considered distressed."