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This paper synthesizes and interprets literature data relating to the chemical and physical characteristics of kerogen (solid, insoluble sedimentary organic matter) representing more than a dozen p...

Five asphaltene samples from three different source types – immature source rock (ISA), petroleum (PA), and coal (CDA) – are studied using integrated chemical and spectroscopic techniques (elementa...

Hydrocarbon storage and transport in unconventional shale resources occurs predominantly within pores hosted by kerogen (solid and insoluble organic matter in sedimentary rocks). Kerogen-hosted pores are small, so physiochemical interactions between pore fluids and pore surfaces (e.g., adsorption) are particularly important in shale. The understanding and prediction of hydrocarbon storage and transport in shale is dependent, therefore, upon the correct understanding of both chemical composition and pore geometry of kerogen. Several recent studies have attempted to construct molecular models of kerogen physical and/or chemical structure. Developing these models is challenging, in part because of the lack of laboratory samples of kerogen for experimental characterization that preserve both its chemical and microstructural properties representative of those in the subsurface. This study presents an integrated kerogen-isolation procedure that combines closed-system chemical demineralization with pyrite removal and critical point drying. The method produces a kerogen that is not only high in chemical purity but more representative of kerogen microstructure that occurs in the subsurface. Characterization of kerogen microstructure, which can be performed by direct measurement of the isolate obtained by this procedure, is essential to better understand and predict the storage, transport, and production hydrocarbons from unconventional resources.

Previous studies on the distribution of bulk sulfur species in bitumen before and after artificial thermal maturation using various pyrolysis methods have indicated that the quantities of reactive (sulfide, sulfoxide) and thermally stable (thiophene) sulfur moieties change following consistent trends under increasing thermal stress. These trends show that sulfur distributions change during maturation in ways that are similar to those of carbon, most clearly illustrated by the increase in aromatic sulfur (thiophenic) as a function of thermal maturity. In this study, we have examined the sulfur moiety distributions of retained bitumen from a set of pre- and post-pyrolysis rock samples in an organic sulfur-rich, calcareous oil shale from the Upper Cretaceous Ghareb Formation. Samples collected from outcrop in Jordan were subjected to hydrous pyrolysis (HP). Sulfur speciation in extracted bitumens was examined using K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The most substantial changes in sulfur distribution occurred at temperatures up to the point of maximum bitumen generation (∼300 °C) as determined from comparison of the total organic carbon content for samples before and after extraction. Organic sulfide in bitumen decreased with increasing temperature at relatively low thermal stress (200–300 °C) and was not detected in extracts from rocks subjected to HP at temperatures above around 300 °C. Sulfoxide content increased between 200 and 280 °C, but decreased at higher temperatures. The concentration of thiophenic sulfur increased up to 300 °C, and remained essentially stable under increasing thermal stress (mg-S/g-bitumen basis). The ratio of stable-to-reactive+stable sulfur moieties ([thiophene/(sulfide+sulfoxide+thiophene)], T/SST) followed a sigmoidal trend with HP temperature, increasing slightly up to 240 °C, followed by a substantial increase between 240 and 320 °C, and approaching a constant value (∼0.95) at temperatures above 320 °C. This sulfur moiety ratio appears to provide complementary thermal maturity information to geochemical parameters derived from other analyses of extracted source rocks.

Integrated elemental, spectroscopic (infrared spectroscopy and X-ray absorption near-edge structure), and gas intrusion (helium pycnometry and nitrogen adsorption) analyses are used to characterize the bulk chemical, molecular, and physical microstructures of kerogen spanning a thermal maturity transect (vitrinite reflectance, Ro, from 0.5% to 2.6%) across the Woodford Shale of the Anadarko Basin, Oklahoma. The integration takes advantage of novel procedures to prepare kerogen isolates that preserve both the chemical and physical properties of the organic matter in the bulk shale. The Woodford kerogens follow the expected trends in H/C and O/C coordinates during thermal maturation for type II kerogen. Infrared spectra show that loss of hydrogen from kerogen is related to cracking of hydrogen-rich aliphatic (alkyl) carbon structures from aromatic carbons. Within the range of Ro values < 1.5%, peripheral aromatic carbons remain highly substituted with alkyl (methyl and methylene) and probably heteroatom fun...

The chemical composition of hydrocarbon fractions of artificially matured (pyrolyzed) Green River oil shale were studied by optical (visible–near-infrared, vis–NIR) spectroscopy. The shale samples were pyrolyzed to several maturities, allowing the compositions of the hydrocarbon fractions to be analyzed as a function of maturity. Oil (the hydrocarbon fraction volatile at pyrolysis conditions but liquid at room temperature), bitumen (the hydrocarbon fraction nonvolatile at pyrolysis conditions and soluble in organic solvent), and asphaltene (the fraction of bitumen that is insoluble in heptane) were all found to show an exponential increase in optical absorption with increasing optical frequency. A similar exponential increase has been observed in other hydrocarbon mixtures and is described by the Urbach tail formulation. The optical properties of the oil are found not to change with maturity, likely because oil is released by volatilization throughout the maturation process, thus producing a distilled pro...

Here, kerogens of differing heat treatments are subjected to extremely high dissociation energies by sample bombardment by 25 keV Bi3+ primary ions during analysis by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Positive and negative secondary ions are produced from this decomposition and fragment ion distributions of model compounds and kerogens are compared and starkly different results are obtained for cations versus anions. Cations exhibit a large range of C/H ratios and include highly unsaturated linear chain ions and aromatic ions. Cations of kerogens possess predominantly no heteroatoms. Positive fragment ion distributions depend on the source material being bombarded. Mature, more aromatic kerogens produce higher yields of fragment ions of highly unsaturated carbon chains while immature, more aliphatic kerogens produce more aromatic fragment ions, particularly at the higher carbon numbers. This is consistent with the observation that aromatic model compounds produce a greater fraction of hydrogen-deficient, carbon chain fragment ions, as compared to a purely aliphatic model compound. There is substantial suppression of free radical fragment cations, except for large fragments. In contrast, there is little free radical suppression of the anions. The anions tend to be very hydrogen deficient, spanning a small range of C/H ratios. Highly unsaturated to pure carbon chain fragment anions dominate while aromatic anions are not found. In both positive and negative ion spectra, the yields of fragment ions corresponding to derivatives of the carbon chain molecules, polyynes and allenes, are substantial. Some heteroatom-containing fragment anions are produced, all of which are very hydrogen deficient.

Analysis of spatial gradients of the concentration of asphaltenes in reservoir crude oils has become an important tool for oilfield reservoir characterization. Modeling these gradients requires knowledge of asphaltene properties. For example, the commonly employed Flory–Huggins–Zuo (FHZ) equation requires the asphaltene particle size, solubility parameter, and density. Asphaltenes from various sources, particularly including sources beyond conventional crude oil, can have widely varying compositions such as the H/C ratio. Here, we measured the solubility parameters and densities of five asphaltenes that span a large range of H/C ratios, including asphaltenes from coal, petroleum, and shale, and then performed a sensitivity analysis to examine how that range of compositions impacts the magnitude of asphaltene gradients as predicted by the FHZ equation. Two case studies are included in the sensitivity analysis, one in which the asphaltene gradient is driven primarily by gravity, and another in which the asp...

Using solid-state 13C NMR spectroscopy of two different asphaltenes, one derived from petroleum and the other from coal liquids, it was shown that the asphaltene molecular architecture consists of a spectrum of sizes, ranging from smaller polyaromatic hydrocarbons (PAHs; 9 condensed rings), but their distribution varies between the two. It is shown that smaller PAHs are likely more abundant in the coal-derived asphaltenes, while the largest PAH cores of the two different asphaltenes are similar in size. These observations are reinforced by optical absorption. The coal-derived asphaltenes were found to contain a small fraction of archipelago-type structures, where a small PAH is tethered to the larger PAH core via an aryl linkage, which are less evident, and likely less abundant, in the petroleum asphaltenes. An important difference between the two asphaltenes lies in their alkyl fraction, with the petroleum asphaltenes possessing significantly longer and more mobi...

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 c...

The sulfur speciation of source rock bitumen (chloroform-extractable organic matter in sedimentary rocks) was examined using sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy for a suite of 11 source rocks from around the world. Sulfur speciation was determined for both the native bitumen in thermally immature rocks and the bitumen produced by thermal maturation of kerogen via hydrous pyrolysis (360 °C for 72 h) and retained within the rock matrix. In this study, the immature bitumens had higher sulfur concentrations than those extracted from samples after hydrous pyrolysis. In addition, dramatic and systematic evolution of the bitumen sulfur moiety distributions following artificial thermal maturation was observed consistently for all samples. Specifically, sulfoxide sulfur (sulfur double bonded to oxygen) is abundant in all immature bitumen samples but decreases substantially following hydrous pyrolysis. The loss in sulfoxide sulfur is associated with a relative increase in the fra...

Sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy is used to measure how the speciation of sulfur compounds evolves within a warm-mix asphalt as a consequence of the Rolling Thin-Film Oven (RTFO) and Pressure Aging Vessel (PAV) oxidative aging procedures. Identifying the types of sulfur compounds present is important for quantifying the growth in polar sulfur-containing species that can alter the asphalt’s mechanical properties over time. Elemental analysis indicates that the sulfur content of the asphalt holds constant at 5 wt% during aging. XANES analysis indicates that thiophenic sulfur compounds are most prevalent (62%), followed by sulfide and elemental sulfur compounds. RTFO and PAV aging cause smaller and larger shifts, respectively, from sulfide to sulfoxide. The amount of unreacted sulfide remains larger than the amount of sulfoxide, even with PAV aging. The XANES spectra lack features that would be expected if engine oil additives indicative of recycled engine oil bottoms were present. The results indicate the importance of including thiophene, sulfide, and sulfoxide chemistries within molecular asphalt models.

The evolution of the electronic absorption edge of type I, II and III kerogen is studied by diffuse reflectance UV–Visible absorption spectroscopy. The functional form of the electronic absorption edge for all kerogens measured is in excellent agreement with the “Urbach tail” phenomenology. The Urbach decay width extracted from the exponential fit within the visible range is strongly correlated with the aliphatic/aromatic ratio in isolated kerogen, regardless of the kerogen type. No correlation is found between the decay width and the average size of aromatic clusters, which is explained in terms of a non-linear increase in optical absorption with increasing size of the aromatic clusters determined by 13 C NMR. Further, absorption spectra calculated with density functional theory calculations on proxy ensemble models of kerogen are in excellent agreement with the experimental results. The correlation of the decay width with conventional maturity indicators such as vitrinite reflectance is found to be good within a particular kerogen type, but not consistent across different kerogen types, reflecting systematic variations in bulk composition for different type kerogen types with the same vitrinite reflectance. Thus, diffuse reflectance visible absorption spectroscopy is presented as a rapid, calibrated and non-destructive method to monitor both the maturity and the chemical composition of kerogen. The chemical insight of kerogen in relation to its optical absorption provided by this methodology may serve for rapid screening of kerogen for electronics and optical devices in place of functionalized produced carbon.

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...

The geometry of kerogen-hosted pores partially controls storage and transport of hydrocarbons in organic-rich mudstones (commonly referred to as shales). Although these pores are considerably smaller than those encountered in conventional reservoir rocks, several laboratory techniques are available that provide some description of kerogen’s microstructure. Many of those techniques perform optimally if the kerogen sample to be studied is first isolated from the mineral matter in shale. It has been observed previously that while typical procedures for kerogen isolation result in a kerogen sample that is chemically similar to native kerogen in intact shale, these procedures result in a kerogen sample whose microstructure is considerably different from native kerogen. Here we describe a novel procedure designed to produce an isolated kerogen with more representative microstructure. The procedure involves the same solvent extraction and acid demineralization as in traditional procedures, but the final step of water removal, which is performed by heating in the traditional procedure, instead is performed here by critical point drying. Critical point drying is characterized by a liquid–gas phase transition without crossing a phase boundary. As a result, drying is not accompanied by capillary stress being exerted on the pore walls, which is responsible for drying damage in the conventional procedure. We present SEM images and surface area measurements suggesting that kerogen isolated by acid demineralization with critical point drying is microstructurally similar to kerogen in shale. We therefore propose that acid demineralization with critical point drying is an effective technique for preparing representative kerogen samples suitable for microstructural characterization.

The chemical and physical structure of immobile organic matter partially controls both the thermal evolution of organic rich shales and hydrocarbon production from these unconventional fossil fuel resources. This organic matter is typically classified into two fractions: kerogen, which is defined as insoluble in organic solvent and bitumen, which is defined as soluble. Kerogen and bitumen are complex materials that are not yet completely characterized and often considered to be compositionally similar except for molecular weight. Here we present a novel method for measuring sulfur speciation in kerogen and we report measured sulfur speciations of kerogen and bitumen from three shales. We observe a general trend of dissimilarity between kerogen and bitumen, with kerogen being dominated by non-polar sulfur forms (such as elemental, sulfide and thiophene) while bitumen is more abundant in polar sulfur forms (sulfoxide). We propose that this difference in sulfur speciation results from a mechanism involving oxidation of non-polar sulfur forms in kerogen during bitumen generation. Additionally, the measured chemical composition of bitumen suggests that it could act as a naturally occurring surfactant, impacting fluid flow and therefore the feasibility of economic hydrocarbon recovery from shales.

Hydrocarbons in subsurface reservoirs are generally found to be compositionally graded, with fluids deeper in connected and equilibrated reservoirs being relatively enriched in asphaltenes. These gradients result from effects such as gravity, entropy, and solubility. However, it is unclear if those same effects lead to gradients in the detailed molecular composition of asphaltenes. Here, we investigate the sulfur chemistry of asphaltenes from a reservoir with a large gradient in asphaltene content. Measurements of the sulfur content from combustion as well as measurements of sulfur speciation from K-edge X-ray absorption near edge structure (XANES) spectroscopy find no significant difference in the composition of the asphaltenes. Thus, different locations within this reservoir contain oils with different asphaltene concentrations, but the asphaltenes from throughout the reservoir all have the same sulfur chemistry. This result suggests that gradients in asphaltene content can be successfully modeled with ...

This review describes various platforms used for multiplexed spectroscopic analysis. We highlight the use of different types of spectroscopy for multiplexed detections, including Raman spectroscopy, surface-enhanced Raman spectroscopy, surface plasmon resonance, and fluorescence. This review also explores the use of cross-reactive sensors in combination with pattern-recognition algorithms to monitor multiple analytes in aqueous and vapor matrices. It also discusses applications of these techniques, paying special attention to their use in the detection of biologically relevant analytes.

[structure: see text] Dynamic combinatorial libraries (DCLs) containing water-soluble disulfide-linked cages (alongside macrocyclic structures) have been generated and characterized. Unlike most other strategies for generating molecular cages, the structures are held together by covalent bonds, which are formed under thermodynamic control. The diversity of the cages generated opens new possibilities for a generalized combinatorial strategy toward molecular encapsulation.

Heavy oil molecular mixtures were investigated on the basis of single molecules resolved by atomic force microscopy. The eight different samples analyzed include asphaltenes and other heavy oil fractions of different geographic/geologic origin and processing steps applied. The collected AFM data of individual molecules provide information about the molecular geometry, aromaticity, the content of nonhexagonal rings, typical types and locations of heterocycles, occurrence, length and connectivity of alkyl side chains, and ratio of archipelago- vs island-type architectures. Common and distinguishing structural motifs for the different samples could be identified. The measured size distributions and the degree of unsaturation by scanning probe microscopy is consistent with mass spectrometry data presented herein. The results obtained reveal the complexity, properties and specifics of heavy oil fractions with implications for upstream oil production and downstream oil processing. Moreover, the identified molec...