system
Given the following post, respond as if you're commenting on Reddit:
human
We hear about how oil take millions of years to form but what about intermediaries? When does it become useful?

What does a deposit that isn't proper oil or from relatively recently deceased organisms look like and at what stage does it become useful for the petroleum industry?
gpt
There is such an intermediate stage, called *kerogen*. Kerogen is a varying mix of long-chain and complex organic molecules. It's generally not found by itself but rather mixed in with inorganic minerals in sedimentary rocks.

Rocks rich in kerogen are known as *oil shale*. They may burn directly, or by heating the kerogen will be decomposed into usable oil and gas. That's been done industrially since the 19th century, but use of oil shale has really increased in recent years as supplies of "traditional" oil dwindle.