PITTSBURGH — Shell Chemical LP has extended its option to buy an industrial site that could become a multibillion-dollar petrochemical plant, the site’s current owner announced Thursday.

Horsehead Corp. said the new agreement for the 300-acre site in Monaca, about 35 miles northwest of Pittsburgh, includes plans to start demolishing an existing zinc plant in early 2014 at Shell’s expense.

Shell spokesman Kimberly Windon confirmed the agreement. She wrote in an e-mail that the demolition work will begin preparing the site for potential construction, but she stressed that Shell “is continuing its evaluation and has yet to make a final decision to proceed” with the project.

Shell chose the site, which is now a zinc plant, in March 2012 but has cautioned that it would be several years before a final decision to build was made.

The so-called ethane cracking plant would convert ethane from bountiful Marcellus Shale natural gas liquids into more profitable chemicals such as ethylene, which are used to produce everything from plastics to tires to antifreeze. A cracker plant looks very similar to a gasoline refinery, with miles of pipes and large storage tanks.

The Pittsburgh Regional Alliance, a business development group, welcomed the news. Alliance president Dewitt Peart said the planned demolition activity “is an important step forward in making the proposed petrochemical facility a reality.”

Ohio, West Virginia and Pennsylvania had all sought the plant and offered Shell major tax incentives. Monaca is about 15 miles from both the Ohio and West Virginia borders, so workers in all three states are likely to benefit.

Chemical Week, an industry publication, reported in October that Shell has chosen the multinational engineering firms Bechtel Corp. and Linde AG of Germany to do feasibility and pre-project planning for a possible Monaca plant.

Shell has estimated that a new cracker plant could employ several hundred people and create up to 10,000 construction jobs.