Oil shale is a petroleum source rock that has not undergone the natural processes required to convert its organic matter to oil and gas. However, oil shale kerogen can be converted artificially to liquid and gaseous hydrocarbons by pyrolysis. Heating oil shale in place (in situ) has a number of operational, economic, and environmental advantages over surface retorts, particularly when the shale is too deep to mine. This work describes experiments conducted at temperatures and pressures appropriate to commercially viable in situ pyrolysis. The data are needed to construct models to plan, interpret, and optimize field experiments and commercial operations. The experiments also provide insights into the chemical compositions of the native state shale and all the products of pyrolysishydrocarbon and nonhydrocarbon gases, oil, bitumen, remaining pyrolyzable kerogen, residual organic matter, and inorganic matteras functions of thermal maturation. Numerous studies of Green River oil shale pyrolysis have been published over the years. Most of these have focused on the richest interval, the Mahogany (R-7) zone and have been performed in either open (atmospheric pressure) or closed (bomb) apparatus. The new elements of this work are as follows: (1) samples were taken from the deepest of the kerogen-rich layers of the Green River Formation, the mineralogically distinct R-1 zone; (2) experiments were performed under semi-open (controlled pressure) conditions. The data generated are therefore appropriate input to models used in conjunction with in situ controlled-pressure production tests of R-1 shale. In agreement with previous work, this investigation finds that processing shale at relatively low temperatures, for longer times, and at moderately elevated pressures, reduces yields but improves product quality relative to surface retort methods. The composition of the produced oil is generally uniform over the course of artificial maturation. It has a high H/C ratio and is predominantly composed of saturates and light aromatics, which are desirable for refinery operations. The oil has little sulfur, which is mostly in thiophene-containing moieties. Extracted bitumen has a high polar content, and its H/C ratio decreases as a result of oil and gas generation during maturation. Produced gas is rich in natural gas liquids.

A series of artificial maturation (anhydrous, semi-open pyrolysis) experiments on Green River oil shale have been performed to simulate the thermal maturation of type I kerogen. The goals of this program were to develop a kinetic model of petroleum generation from oil shale and to characterize the yield and composition of petroleum as a function of artificial thermal maturity. The thermal maturity level (EASY%Ro = 0.48–1.28%) is based upon the kinetic model of kerogen degradation and is equivalent to vitrinite reflectance maturity. Here, we compare the structural characteristics of kerogen and bitumen during artificial maturation of oil shale using quantitative Fourier transform infrared (IR) spectroscopy. Quantitative comparison was enabled by a novel method for the preparation of bitumen for IR spectroscopy. Bitumen can be a reaction intermediate during maturation of kerogen, and the IR data indicate that bitumen has a structure intermediate between that of kerogen and generated petroleum. Moreover, the IR data reveal that the composition of bitumen changes with maturity, with trends that are similar in some aspects to those observed previously in kerogen, but different in others. Kerogen is characterized by the early depletion of oxygenated functional groups prior to petroleum generation (EASY%Ro < 0.9%) and then a late enrichment of oxygen at higher artificial maturity (EASY%Ro > 1.2%). In contrast, bitumen shows initial enrichment of oxygenated functional groups at low artificial maturity (EASY%Ro < 0.8%) and subsequent depletion at higher maturity. Kerogen evolution follows the previously observed trend with aliphatic carbon chains that became shorter and/or more branched as kerogen is consumed during all stages of artificial maturation. Bitumen, in contrast, appears to have aliphatic carbon chains that lengthen within the same artificial maturity range as bitumen is predominantly generated from kerogen. The aliphatic carbon content of bitumen is greater than that of kerogen at all levels of artificial maturity. Both kerogen and bitumen become more aromatic in character with increasing thermal maturity, especially above artificial levels EASY%Ro > 0.9%. This similarity likely results from loss of aliphatic chains from both organic fractions during petroleum generation, suggesting that both kerogen and bitumen can be direct sources for petroleum. The loss of aliphatic carbons from aromatic centers in both kerogen and bitumen leads to protonation of the residual aromatic rings. The IR spectra of kerogen and bitumen indicate very similar degrees of protonation of those aromatic rings.

During oil shale pyrolysis at high temperature, the conversion of kerogen to oil and gas proceeds dominantly through a mechanism involving bitumen as a reaction intermediateat low maturities bitumen is formed from decomposition of kerogen, while at high maturities bitumen is transformed primarily to oil and gas. Here, we study the chemical composition of Green River bitumens of a range of maturities by high-field 13C nuclear magnetic resonance (NMR) spectroscopy. Numerous trends in the evolution of bitumen with maturity are observed, some of which are similar to trends previously observed in kerogen while others are not. As found previously for kerogen, bitumen becomes more aromatic (less aliphatic) with increasing maturity. However, in contrast to kerogen, which is primarily consumed during maturation and was found previously to have aliphatic chains that become shorter and/or more branched with maturity, aliphatic chains in bitumen lengthen with maturity in the low maturity regime where bitumen is primarily being formed but then shorten with maturity in the high maturity regime where bitumen is primarily being consumed. The structure of aromatic rings in bitumen is essentially unchanged with maturity, as their size, alkyl substitution, and heteroatom substitution are found to be independent of maturity; in contrast, the size of aromatic rings in kerogen generally increases with maturity. These measurements of the chemical composition of the bitumen intermediate enhance understanding of petroleum generation by oil shale pyrolysis at high temperature.

The structure of asphaltenes of various maturities prepared by semiopen pyrolysis of Green River Shale is measured by elemental analysis, laser desorption laser ionization mass spectrometry (L2MS), surface assisted laser desorption ionization (SALDI) mass spectrometry, sulfur X-ray absorption near edge structure (XANES) spectroscopy, and infrared (IR) spectroscopy. These measurements demonstrate systematic changes in the composition of asphaltenes during thermal maturation. At low maturities, the evolution of asphaltene composition is dominated by changes in the heteroatoms: total sulfur as well as carbon–oxygen, sulfur–oxygen (sulfoxide), and aliphatic carbon–sulfur (sulfide) bonds are lost, while the molecular weight increases. At high maturities, the sulfur content and speciation as well as molecular weight are relatively constant while the evolution in composition is dominated by changes in the carbon backbone: the abundance of aromatic relative to aliphatic carbon increases, the length of aliphatic chains shortens, and the abundance of aromatic C–H bonds increases greatly. The distribution of different carbon–oxygen functional groups is relatively unchanged over the entire maturity range. These changes sometimes mirror and sometimes oppose compositional changes in the bitumen, suggesting that the composition of the asphaltene and maltene fractions of bitumen evolve differently. The observed changes in asphaltene structure are not fully independent of one another, as the composition of asphaltenes is constrained to maintain a balance of the strength of intermolecular forces to ensure solubility in aromatic solvent and insolubility in aliphatic solvent (the definition of asphaltenes). That constraint leads to a decrease in sulfoxide content (weakening intermolecular forces by reducing dipole interactions) concurrent with an increase in molecular weight (strengthening intermolecular forces). These trends in asphaltene composition with maturity are expected to occur in naturally occurring source rocks, such as some tight-oil formations, but not necessarily in conventional reservoir rocks where the asphaltenes escape from the source rock and enter the reservoir during maturation.