When did oil become a major energy source?

Oil became a major energy source in the late 19th century, particularly after the advent of the internal combustion engine and the rise of the automotive industry in the early 20th century. The 1859 drilling of the first commercial oil well in Pennsylvania marked the beginning of the oil industry, but its significance grew rapidly during and after World War I. By the mid-20th century, oil had established itself as a dominant energy source, largely due to its efficiency and versatility in transportation, heating, and electricity generation.