From Envirowiki

Organic material generally refers to dead matter that has recently come from a living organism, such as faeces, leaves or a carcass. Over time, organic matter will rot (or compost), either aerobically, or anaerobically, to become hummus. Anaerobically decomposing plant matter in water becomes peat. It may also be covered in earth (from a flood, or landslide, and be compressed to become, over a long period, some kind of fossil fuel, such as oil, coal, or gas (depending on the type of organic matter).