Kerogen

Kerogen is solid, insoluble organic matter in sedimentary rocks. It consists of a variety of organic materials, including dead plants, algae, and other microorganisms, that have been compressed and heated by geological processes. All the kerogen on earth is estimated to contain 10¹⁶ tons of carbon. This makes it the most abundant source of organic compounds on earth, exceeding the total organic content of living matter 10,000-fold.

The type of kerogen present in a particular rock formation depends on the type of organic material that was originally present. Kerogen can be classified by these origins: lacustrine (e.g., algal), marine (e.g., planktonic), and terrestrial (e.g., pollen and spores). The type of kerogen also depends on the degree of heat and pressure it has been subjected to, and the length of time the geological processes ran. This results in a complex mixture of organic compounds residing in sedimentary rocks which serve as the precursors for the formation of hydrocarbons such as oil and gas. In short, kerogen amounts to fossilized organic matter that has been buried and subjected to high temperatures and pressures over millions of years, resulting in various chemical reactions and transformations.

Kerogen is insoluble in normal organic solvents and it does not have a specific chemical formula. Upon heating, kerogen converts in part to liquid and gaseous hydrocarbons. Petroleum and natural gas form from kerogen. The name kerogen was introduced by the Scottish organic chemist Alexander Crum Brown in 1906, derived from the Greek words for wax and origin (Greek: κηρός 'wax' and -gen, γένεσις 'origin').

The increased production of hydrocarbons from shale has motivated a revival of research into the composition, structure, and properties of kerogen. Many studies have documented dramatic and systematic changes in kerogen composition across the range of thermal maturity relevant to the oil and gas industry. Analyses of kerogen are generally performed on samples prepared by acid demineralization with critical point drying, which isolates kerogen from the rock matrix without altering its chemical composition or microstructure.

Kerogen is formed during sedimentary diagenesis from the degradation of living matter. The original organic matter can comprise lacustrine and marine algae and plankton and terrestrial higher-order plants. During diagenesis, large biopolymers from, e.g., proteins, lipids, and carbohydrates in the original organic matter, decompose partially or completely. This breakdown process can be viewed as the reverse of photosynthesis. These resulting units can then polycondense to form geopolymers. The formation of geopolymers 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 acids, and the largest units are the humins. This polymerization usually happens alongside the formation and/or sedimentation of one or more mineral components resulting in a sedimentary rock like oil shale.

When kerogen is contemporaneously deposited with geologic material, subsequent sedimentation and progressive burial or overburden provide elevated pressures and temperatures owing to lithostatic and geothermal gradients in the Earth's crust. Resulting changes in the burial temperatures and pressures lead to further changes in kerogen composition, including the losses of hydrogen, oxygen, nitrogen, sulfur, and their associated functional groups, along with subsequent isomerization and aromatization. Such changes are indicative of the thermal maturity state of kerogen. Aromatization allows for molecular stacking in sheets, which in turn drives changes in physical characteristics of kerogen, such as increasing molecular density, vitrinite reflectance, and spore coloration (yellow to orange to brown to black with increasing depth/thermal maturity).

During the process of thermal maturation, kerogen breaks down in high-temperature pyrolysis reactions to form lower-molecular-weight products including bitumen, oil, and gas. The extent of thermal maturation controls the nature of the product, with lower thermal maturities yielding mainly bitumen/oil and higher thermal maturities yielding gas. These generated species are partially expelled from the kerogen-rich source rock and in some cases can charge into a reservoir rock.

Kerogen takes on additional importance in unconventional resources, particularly shale. In these formations, oil and gas are produced directly from the kerogen-rich source rock (i.e. the source rock is also the reservoir rock). Much of the porosity in these shales is found to be hosted within the kerogen, rather than between the mineral grains that occur in conventional reservoir rocks. Thus, kerogen controls much of the storage and transport of oil and gas in shale.