I first became interested in this subject indirectly. I became interested in the subject of Peak Oil - the point at which worldwide oil production reaches an absolute peak and then begins to decline. But one relevant side question has to do with the origins of fossil fuels. Mainly because I was curious about the types of unique conditions on the planet that were present that allowed these deposits to form.

This also becomes related to the question of global climate change as well. There were times in the history of the Earth when CO2 levels were far higher than they are today, and these tended to coincide with the times at which the organisms which became fossil fuels lived and died.

There isn't a really simple answer as to when the deposits were formed - there were several such periods in the history of the Earth. For example, many coal deposits were first laid down during the aptly name Carboniferous period (~360 to 299 million years ago). Oilfields were laid down somewhat later during the Jurassic period (200 to 145 million years ago) or the Cretaceous period (145 to 65 million years ago).