With the demise of living matter, such as diatoms, planktons, spores and pollens, the organic matter begins to undergo decomposition or degradation.[6] In this break-down process, large biopolymers from proteins and carbohydrates begin to dismantle either partially or completely. (According to Tucker (1988), this break-down process is basically the reverse of photosynthesis[7]). These dismantled components are units that can then polycondense to form polymers. This polymerization usually happens alongside the formation of a mineral component (geopolymer) resulting in a sedimentary rock like kerogen shale.
The formation of polymers in this way accounts for the large molecular weights and diverse chemical compositions associated with kerogen. The smallest units are the fulvic acids, the medium units are the humic, and the largest units are the humins. When organic matter is contemporaneously deposited with geologic material, subsequent sedimentation and progressive burial or overburden provide significant pressure and a temperature gradient. When these humic precursors are subjected to sufficient geothermal pressures for sufficient geologic time, they begin to undergo certain specific changes to become kerogen. Such changes are indicative of the maturity stage of a particular kerogen. These changes include loss of hydrogen, oxygen, nitrogen, and sulfur, which leads to loss of other functional groups that further promote isomerization and aromatization which are associated with increasing depth or burial. Aromatization then allows for neat molecular stacking in sheets, which in turn increases molecular density and vitrinite reflectance properties, as well as changes in spore coloration, characteristically from yellow to orange to brown to black with increasing depth.[8]