The kerogen is the macromolecular organic matter that is insoluble in organic solvents (soluble portion is known as bitumen). Kerogen is present in ancient sedimentary rocks and meteorites. It is composed of smaller molecules that have covalently bound together during synthesis (including biosynthesis), diagenesis, and thermal alteration processes. In sediments, kerogen is the fossil organic matter source of oil and natural gas. The degree of hydrogen loss and aromatization is often used as an indicator of thermal maturity. The degree of structural order versus disorder for aromatic sheets is a defining characteristic of different kerogens identifiable by Raman spectroscopy. Organic-walled fossils resistant to acid maceration (such as acritarchs, spores) are observed in some kerogens.