While people have used naturally seeping petroleum for centuries, the discovery of commercially viable oil deposits is generally attributed to 1847 in Pennsylvania, USA.

Here's a breakdown:

* Ancient Times: People have been using naturally occurring petroleum for centuries, mainly for medicinal purposes and as a sealant.

* 1847: Edwin Drake drilled the first successful oil well in Titusville, Pennsylvania, marking the beginning of commercial oil production. This discovery revolutionized the oil industry and led to the development of modern oil extraction techniques.

It's important to remember that the discovery of oil itself is much older than 1847. However, the discovery of commercially viable oil deposits, which led to the modern oil industry, is widely acknowledged to have occurred in 1847.