 The resulting high temperature and pressure caused the organic matter to chemically alter, first into a waxy material known as kerogen, which is found in oil shales, and then with more heat into liquid and gaseous hydrocarbons in a process known as catagenesis.Despite these heat driven transformations (which increase the energy density compared to typical organic matter by removal of oxygen atoms), the energy released in combustion is still photosynthetic in origin.Terrestrial plants, on the other hand, tended to form coal and methane.