According to the EIA
In the United States, plastics are made from liquid petroleum gases (LPG), natural gas liquids (NGL), and natural gas. LPG are by-products of petroleum refining, and NGL are removed from natural gas before it enters transmission pipelines.
In 2010, about 191 million barrels of LPG and NGL were used in the United States to make plastic products in the plastic materials and resins industry, equal to about 2.7% of total U.S. petroleum consumption.
Of those 191 million barrels, 190 million barrels were used as feedstock and 1 million barrels were consumed as fuel.