Algae = Hydrogen rich = Oil-prone Wood = Hydrogen poor = Gas-prone Types of Petroleum Oil and gas are formed by the thermal cracking of organic compounds buried in fine-grained rocks. Algae = Hydrogen rich = Oil-prone Wood = Hydrogen poor = Gas-prone Marine biological productivity produces algal organic matter that is rich in hydrogen atoms and yield oil under thermal cracking, while much of the terrestrial organic matter is woody with low hydrogen content and thus is gas prone.  Accurate prediction of probable hydrocarbon type and volume, based on paleogeographic and productivity models, is critical to exploration success.