LONDON — Saudi Arabia’s oil company, Saudi Aramco, said on Wednesday that it would buy a 70 percent stake in a state-controlled petrochemical giant for $69.1 billion, in what appears to be an alternative to a much-delayed initial public offering.

The deal, which involves three entities controlled by the Saudi government, largely accomplishes the primary goal for Aramco’s share offering — helping to finance an ambitious campaign to modernize the kingdom.

Aramco will buy the stake in the petrochemical company, the Saudi Basic Industries Corporation, known as Sabic, currently held by the country’s sovereign wealth fund. The remaining 30 percent of Sabic will continue to be listed on Saudi Arabia’s Tadawul stock exchange.

The transaction would give Crown Prince Mohammed bin Salman, who had been the main proponent of selling a portion of Aramco to public investors, a chunk of the cash he needs to diversify the Saudi economy, which is heavily dependent on oil. The prince hopes both to buffer the economy from oil price fluctuations and invest in industries that will provide income for the kingdom if demand for its oil wanes.