Kerogen
It consists of a variety of organic materials, including dead plants, algae, and other microorganisms, that have been compressed and heated by geological processes.
The type of kerogen depends also on the degree of heat and pressure it has been subjected to, and the length of time the geological processes ran.
The result is that a complex mixture of organic compounds reside in sedimentary rocks, serving as the precursor 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.
Many studies have documented dramatic and systematic changes in kerogen composition across the range of thermal maturity relevant to the oil and gas industry.
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.
When kerogen is contemporaneously deposited with geologic material, subsequent sedimentation and progressive burial or overburden provide elevated pressure and temperature owing to lithostatic and geothermal gradients in Earth's crust.
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.
[9] Another possible method of formation is that vanabin-containing organisms cleave the core out of chlorin-based compounds such as the magnesium in chlorophyll and replace it with their vanadium center in order to attach and harvest energy via light-harvesting complexes.
For example, kerogen from the Green River Formation oil shale deposit of western North America contains elements in the proportions carbon 215 : hydrogen 330 : oxygen 12 : nitrogen 5 : sulfur 1.
The first-order Raman spectra of kerogen comprises two principal peaks;[15] a so-called G band ("graphitic") attributed to in-plane vibrational modes of well-ordered sp2 carbon and a so-called D band ("disordered") from symmetric vibrational modes of sp2 carbon associated with lattice defects and discontinuities.
IR spectroscopy is sensitive to carbon-oxygen bonds such as quinones, ketones, and esters, so the technique can also be used to investigate oxygen speciation.
[22] Similarly, sulfur speciation can be investigated with X-ray absorption near edge structure (XANES) spectroscopy, which is sensitive to sulfur-containing functional groups such as sulfides, thiophenes, and sulfoxides.
[24][25] Overall, changes in kerogen composition with respect to heteroatom chemistry occur predominantly at low thermal maturities (bitumen and oil windows), while changes with respect to carbon chemistry occur predominantly at high thermal maturities (oil and gas windows).
[9][26] Analysis by gas sorption demonstrated that the internal specific surface area of kerogen increases by an order of magnitude (~ 40 to 400 m2/g) during thermal maturation.
[29] This evolution is attributed to the formation of kerogen-hosted pores left behind as segments of the kerogen molecule are cracked off during thermal maturation.
Measurements performed with atomic force microscopy coupled to infrared spectroscopy (AFM-IR) and correlated with organic petrography have analyzed the evolution of the chemical composition and mechanical properties of individual macerals of kerogen with thermal maturation at the nanoscale.
This kerogen is rich in lipid-derived material and is commonly, but not always, from algal organic matter in lacustrine (freshwater) environments.
Hence, from the theoretical view, shales containing type I kerogen are the most promising deposits in terms of conventional oil retorting.
Type II kerogen is principally derived from marine organic materials, which are deposited in reducing sedimentary environments.
[41] The Curiosity rover has detected organic deposits similar to kerogen in mudstone samples in Gale Crater on Mars using a revised drilling technique.
