E.I. du Pont de Nemours & Co. took an early lead in the three-company tournament to acquire Conoco Inc., the nation's ninth-largest oil company.

Du Pont, the nation's biggest chemical producer, said today that at the close of business Tuesday it had 48 million shares of Conoco on hand, about 56 percent of the company's outstanding stock.

But Conoco shareholders who have sent (or tendered) their stock to Du Pont have until midnight Aug. 4 to decide whether to offer their holdings to the other contenders in the Conoco takeover battle. Mobil Corp. and Seagram Co.

Separately, House energy oversight and investigations subcommittee chairman John Dingell (D-Mich.) revealed that his panel will conduct an investigation into the role oil company profits play in what he called escalating bidding wars for U.S. oil concerns. Dingell expressed "deep concern" about a possible Mobil takeover of Conoco in a letter to Attorney General William French Smith. A similar letter was sent to Smith by Sen. Patrick Leahy (D-Vt.).

Du Pont's offer -- $95 a share for 38.7 million shares (about 45 percent of the outstanding stock) and 1.7 shares of Du Pont stock for each of the remaining shares -- has been made with the approval of Conoco's board. At today's closing price of $44.50 for Du Pont (down $1.25 on the day), the total value of the Du Pont offer is bout $7.3 billion.

Mobil, the nation's second-largest corporation, has offered the richest package, worth almost $8.2 billion. Mobil said it will pay $105 a share for 43.5 million shares and exchange Mobil securities worth $85 a share for the rest.

Because investors and professional Wall Street traders are concerned that a merger of the second and ninth oil companies will not survive antitrust scrutiny, there has been a reluctance to tender shares to Mobil, Wall Street sources said