If you don t know much about where oil comes from, don t be ashamed. Until quite recently even professional explorers had only a rather hazy idea about the exact origin of oil. In the last Chapter, I described my experiences as a young geologist looking for oil in Trinidad and Colombia. I knew of the classic view that most of the oil in northern South America came from the La Luna Formation, a sequence of petroliferous Cretaceous shales and muddy limestones, but I had difficulty in understanding how this deep-seated source could charge the overlying Eocene reservoirs in the Middle Magdalena Valley without filling intervening sandstones. To overcome this difficulty, I proposed that the oil was generated in the shallow sequences themselves, despite their non-marine origins. It seemed a reasonable explanation but it was no more than a hypothesis. Now, geochemistry can specifically identify not only source but the date of generation. It has revolutionized the search for oil, permitting a much improved assessment of what the world s oil endowment actually is. It is of cardinal importance, and I will try to explain it in this chapter. It may be rather heavy going for the generalist, but it is important to understand at least something about the physics and chemistry of oil and gas at the outset.

Before coming to explain the origin of oil as now accepted by virtually all explorers, I should dispose of an alternative theory that has been advanced by Thomas Gold ^[1], an academic,...