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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.

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.

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 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.