Fractionation of petroleum during migration through sedimentary rock matrices has been observed across lengths of meters to kilometers. Selective adsorption of specific chemical moieties at mineral surfaces and/or the phase behavior of petroleum during pressure changes are typically invoked to explain this behavior. Given the current emphasis on unconventional (continuous) resources, there is a need to understand petroleum fractionation occurring during expulsion and migration at the nanometer to micron scale, due to the fine-grained nature of petroliferous mudrocks. Here organic matter compositional differences observed within kukersite petroleum source beds (containing acritarch Gloeocapsomorpha prisca) from the Ordovician Stonewall Formation are explored using a suite of optical and spectroscopic methods, most notably through a combined atomic force microscopy - infrared spectroscopy (AFM-IR) approach. The AFM-IR technique is capable of providing spatial resolutions approaching 50 nm and allows for an assessment of the molecular fingerprint of kukersite organic matter across transition zones from organic-rich ‘source’ layers into adjacent carbonate ‘reservoir’ layers ~150 μm away. Our results indicate that the composition of kukersite organic matter begins to vary immediately following expulsion from source layers, with loss of carbonyl groups and a concomitant increase in the CH3/CH2 ratio, indicating alkyl chain-length decrease, as migration distance increases. These chemical transitions correlate with fluorescence decrease, reflectance increase, and an increase in Raman proxies for aromaticity in the organic matter. These data are consistent with the retention of polar compounds onto mineral grains during expulsion and migration, and primacm-1ry cracking and bituminization of the Gloeocapsomorpha prisca kerogen, respectively.

The molecular composition of petroliferous organic matter and its composition evolution throughout thermal advance are key to understanding and insight into petroleum generation. This information is critical for comprehending hydrocarbon resources in unconventional reservoirs, as source rock organic matter is highly dispersed, in contact with the surrounding mineral matrix, and may be present as multiple organic matter types. Here, a combination of Raman spectroscopy and optical microscopy approaches was applied to a marginally mature (vitrinite reflectance ~0.5%) sample of the Late Cretaceous Boquillas Shale before and after hydrous pyrolysis (HP) at 300 °C and 330 °C for 72 hours. This experimental design allowed for correlative examination of micro-scale changes in organic matter compositional properties (e.g., aromaticity) for a variety of organic matter types across a thermal gradient at the single particle level. Results indicate that while the examined amorphous organic matter, solid bitumen, and vitrinite particles exhibit different aromatic signatures in the unheated shale, they effectively progress along a similar trend through composition space with thermal advance. Examined inertinite fragments were generally insensitive to the applied thermal stress, reinforcing the idea that reservoir temperature may be secondary for dictating the molecular composition of inertinite. Additional analysis of the Raman spectra for individual organic matter types was performed using multivariate curve resolution (MCR); correlation of standard Raman and reflectance-derived thermal maturity proxies against MCR parameters shows consistent trends. This trend suggests that MCR may be a fast and statistically robust method for extracting compositional information from Raman spectra of sedimentary organic matter that can be used to construct thermal maturity relationships. These findings inform the understanding of how different petroliferous organic matter types evolve throughout thermal reactions and further demonstrate that Raman spectroscopy combined with petrographic analysis can provide complementary estimates of organic matter composition and thermal maturity.

Solid organic matter (OM) in sedimentary rocks produces petroleum and solid bitumen when it undergoes thermal maturation. The solid OM is a ‘geomacromolecule’, usually representing a mixture of various organisms with distinct biogenic origins, and can have high heterogeneity in composition. Programmed pyrolysis is a common conventional method to reveal bulk geochemical characteristics of the dominant OM while detailed organic petrography is required to reveal information about the biogenic origin of contributing macerals. Despite advantages of programmed pyrolysis, it cannot provide information about the heterogeneity of chemical compositions present in the individual OM types. Therefore, other analytical techniques such as Raman spectroscopy are necessary. In this study, we compared geochemical characteristics and Raman spectra of two sets of naturally and artificially matured Bakken source rock samples. A continuous Raman spectral map on solid bitumen particles was created from the artificially matured hydrous pyrolysis residues, in particular, to show the systematic chemical modifications in microscale. Spectroscopy data was plotted for both sets against thermal maturity to compare maturation rate/path for these two separate groups. The outcome showed that artificial maturation through hydrous pyrolysis does not follow the same trend as naturally-matured samples although having similar solid bitumen reflectance values (%SBRo). Furthermore, Raman spectroscopy of solid bitumen from artificially matured samples indicated the heterogeneity of OM decreases as maturity increases. This represents an alteration in chemical structure towards more uniform compounds at higher maturity. This study may signify the potential of Raman spectroscopy as an alternative to the conventional (pseudo) Van Krevelen diagram, by revealing the underlying chemical changes. Finally, observation by Raman spectroscopy of chemical alteration of OM during artificial maturation may assist in the proposal of improved pyrolysis protocols to better resemble natural geologic processes.

Solid organic matter (OM) in sedimentary rocks produces petroleum and solid bitumen when it undergoes thermal maturation. The solid OM is a ‘geomacromolecule’, usually representing a mixture of various organisms with distinct biogenic origins, and can have high heterogeneity in composition. Programmed pyrolysis is a common conventional method to reveal bulk geochemical characteristics of the dominant OM while detailed organic petrography is required to reveal information about the biogenic origin of contributing macerals. Despite advantages of programmed pyrolysis, it cannot provide information about the heterogeneity of chemical compositions present in the individual OM types. Therefore, other analytical techniques such as Raman spectroscopy are necessary. In this study, we compared geochemical characteristics and Raman spectra of two sets of naturally and artificially matured Bakken source rock samples. A continuous Raman spectral map on solid bitumen particles was created from the artificially matured hydrous pyrolysis residues, in particular, to show the systematic chemical modifications in microscale. Spectroscopy data was plotted for both sets against thermal maturity to compare maturation rate/path for these two separate groups. The outcome showed that artificial maturation through hydrous pyrolysis does not follow the same trend as naturally-matured samples although having similar solid bitumen reflectance values (%SBRo). Furthermore, Raman spectroscopy of solid bitumen from artificially matured samples indicated the heterogeneity of OM decreases as maturity increases. This represents an alteration in chemical structure towards more uniform compounds at higher maturity. This study may signify the potential of Raman spectroscopy as an alternative to the conventional (pseudo) Van Krevelen diagram, by revealing the underlying chemical changes. Finally, observation by Raman spectroscopy of chemical alteration of OM during artificial maturation may assist in the proposal of improved pyrolysis protocols to better resemble natural geologic processes.

The organic composition of produced waters (flowback and formation waters) from the Bakken Formation and the Three Forks Formation in the Williston Basin, North Dakota were examined in this study in order to aid in the remediation of surface contamination due to spills during transport and help develop treatment methods for recycling. Twelve produced water samples were collected from wells in the Bakken and Three Forks Formations at the well head and analyzed for non-purgable dissolved organic carbon (NPDOC), acetate, and extractable hydrocarbons. NPDOC and acetate concentrations from sampled wells from ranged from 33-190 milligrams per liter (mg/L) and 16-40 mg/L, respectively. Concentrations of individual extractable hydrocarbon compounds ranged from less than 1 to greater than 450 micrograms per liter (µg/L), and included polycyclic aromatic hydrocarbons (PAHs), phenolic compounds, glycol ethers, and cyclic ketones.

Recent production of light sweet oil from shallow (~2,000 ft) horizontal wells in the Upper Devonian Berea Sandstone of eastern Kentucky and historical oil production from conventional wells in the Berea of adjoining southern Ohio has prompted re-evaluation of Devonian petroleum systems in the central Appalachian Basin. Herein, we examined Upper Devonian Ohio Shale (lower Huron Member) and Middle Devonian Marcellus Shale organic-rich source rocks from eastern Ohio and nearby areas using organic petrography and geochemical analyses of solvent extracts. The data indicate the organic matter in the Ohio and Marcellus Shales was primarily derived from marine algae and its degradation products including bacterial biomass. Absence of odd-over-even n-alkane distributions in gas chromatograms and low gammacerane index values in Devonian source rocks are similar to properties reported for Devonian-reservoired oils in eastern Ohio, suggesting a strong oil-source rock correlation. However, petrographic and geochemical parameters presented here were unable to discriminate specific shale source rocks (e.g., Ohio Shale vs. Marcellus Shale) for the Devonian oils. Lower Paleozoic oils from eastern Ohio, in contrast, are characterized by the presence of odd-over-even n-alkane distributions and higher gammacerane values which clearly discriminate them from Devonian shale-derived oils. Measurements of solid bitumen reflectance (BRo) at the thermal maturity range of the samples (immature to peak oil conditions) tend to underestimate ‘true’ thermal maturity because solid bitumen has lower reflectance than co-occurring vitrinite. Because solid bitumen dominates the organic matter in Devonian shale and vitrinite is sparse, the value of reflectance as a thermal proxy is questionable and its use may lead to reports of ‘vitrinite reflectance suppression’ in early mature to oil window mature areas. For example, thermal maturity estimates from equilibrium(?) biomarker isomerization ratios may suggest some of the Devonian source rock samples are at middle to peak oil window conditions e.g., approximate vitrinite reflectance values of 0.8-0.9%, whereas solid bitumen reflectance is approximately 0.52-0.54% in the same samples. If correct, this observation may require that the predicted onset of oil generation from Devonian shale source rocks in eastern Ohio is moved farther westward. As a consequence, only local to short-distance (30-50 mi) migration would be necessary for emplacement of Devonian-sourced oils into Devonian reservoirs of eastern Ohio, rather than long-distance migration (>50 mi) from ‘deep in the Appalachian basin’, as suggested by previous workers, potentially impacting exploration and future assessments of undiscovered petroleum resources in the Berea Sandstone. However, biomarker isomerization ratios do not show consistent relationships to other thermal maturity parameters (BRo, Tmax), thereby preventing development of robust empirical calibrations for these thermal proxies in the Devonian of eastern Ohio.

Recent production of light sweet oil from shallow (~2,000 ft) horizontal wells in the Upper Devonian Berea Sandstone of eastern Kentucky and historical oil production from conventional wells in the Berea of adjoining southern Ohio has prompted re-evaluation of Devonian petroleum systems in the central Appalachian Basin. Herein, we examined Upper Devonian Ohio Shale (lower Huron Member) and Middle Devonian Marcellus Shale organic-rich source rocks from eastern Ohio and nearby areas using organic petrography and geochemical analyses of solvent extracts. The data indicate the organic matter in the Ohio and Marcellus Shales was primarily derived from marine algae and its degradation products including bacterial biomass. Absence of odd-over-even n-alkane distributions in gas chromatograms and low gammacerane index values in Devonian source rocks are similar to properties reported for Devonian-reservoired oils in eastern Ohio, suggesting a strong oil-source rock correlation. However, petrographic and geochemical parameters presented here were unable to discriminate specific shale source rocks (e.g., Ohio Shale vs. Marcellus Shale) for the Devonian oils. Lower Paleozoic oils from eastern Ohio, in contrast, are characterized by the presence of odd-over-even n-alkane distributions and higher gammacerane values which clearly discriminate them from Devonian shale-derived oils. Measurements of solid bitumen reflectance (BRo) at the thermal maturity range of the samples (immature to peak oil conditions) tend to underestimate ‘true’ thermal maturity because solid bitumen has lower reflectance than co-occurring vitrinite. Because solid bitumen dominates the organic matter in Devonian shale and vitrinite is sparse, the value of reflectance as a thermal proxy is questionable and its use may lead to reports of ‘vitrinite reflectance suppression’ in early mature to oil window mature areas. For example, thermal maturity estimates from equilibrium(?) biomarker isomerization ratios may suggest some of the Devonian source rock samples are at middle to peak oil window conditions e.g., approximate vitrinite reflectance values of 0.8-0.9%, whereas solid bitumen reflectance is approximately 0.52-0.54% in the same samples. If correct, this observation may require that the predicted onset of oil generation from Devonian shale source rocks in eastern Ohio is moved farther westward. As a consequence, only local to short-distance (30-50 mi) migration would be necessary for emplacement of Devonian-sourced oils into Devonian reservoirs of eastern Ohio, rather than long-distance migration (>50 mi) from ‘deep in the Appalachian basin’, as suggested by previous workers, potentially impacting exploration and future assessments of undiscovered petroleum resources in the Berea Sandstone. However, biomarker isomerization ratios do not show consistent relationships to other thermal maturity parameters (BRo, Tmax), thereby preventing development of robust empirical calibrations for these thermal proxies in the Devonian of eastern Ohio.

The Bemidji crude oil spill site is a long-term USGS study site to understand the fate of crude oil in the shallow subsurface. A description of the site can be found at https://mn.water.usgs.gov/projects/bemidji. In 2014 concentrations of non-volatile dissolved organic carbon (NVDOC) were three times higher than diesel range organics (DRO) in the contaminant plume*. This is important because most of the NVDOC in the plume is composed of partial transformation products of compounds from the crude oil that are not reflected in a DRO analysis. In 2016 we conducted a campaign to determine if DRO values continue to reflect only a fraction of the NVDOC. These data are the results of that campaign. A total of 25 wells were sampled for DRO and NVDOC in August, 2016. Three wells with long term records were included in the sampling: 530B, 515B, and 9316D. Wells sampled in 2016 but not in 2014 include two wells located 14 m from a down gradient lake (1217B and 1217C) and one well in the zone sprayed by oil (956). *Bekins, B. A., Cozzarelli, I. M., Erickson, M. L., Steenson, R. A., and Thorn, K. A., 2016, Crude Oil Metabolites in Groundwater at Two Spill Sites: Groundwater, v. 54, no. 5, p. 681-691.

The Bemidji crude oil spill site is a long-term USGS study site to understand the fate of crude oil in the shallow subsurface. A description of the site can be found at https://mn.water.usgs.gov/projects/bemidji. In 2014 concentrations of non-volatile dissolved organic carbon (NVDOC) were three times higher than diesel range organics (DRO) in the contaminant plume*. This is important because most of the NVDOC in the plume is composed of partial transformation products of compounds from the crude oil that are not reflected in a DRO analysis. In 2016 we conducted a campaign to determine if DRO values continue to reflect only a fraction of the NVDOC. These data are the results of that campaign. A total of 25 wells were sampled for DRO and NVDOC in August, 2016. Three wells with long term records were included in the sampling: 530B, 515B, and 9316D. Wells sampled in 2016 but not in 2014 include two wells located 14 m from a down gradient lake (1217B and 1217C) and one well in the zone sprayed by oil (956). *Bekins, B. A., Cozzarelli, I. M., Erickson, M. L., Steenson, R. A., and Thorn, K. A., 2016, Crude Oil Metabolites in Groundwater at Two Spill Sites: Groundwater, v. 54, no. 5, p. 681-691.

Geological models for petroleum generation suggest thermal conversion of oil-prone sedimentary organic matter in the presence of water promotes increased liquid saturate yield, whereas absence of water causes formation of an aromatic, cross-linked solid bitumen residue. To test the influence of exchangeable hydrogen from water, organic-rich (22 wt.% total organic carbon, TOC) mudrock samples from the Eocene lacustrine Green River Mahogany zone oil shale were pyrolyzed under hydrous and anhydrous conditions at temperatures between 300 and 370°C for 72 hrs. Petrographic approaches including optical microscopy, reflectance, Raman spectroscopy, and scanning electron and transmission electron microscopy, supplemented by geochemical screening measurements (TOC content and programmed pyrolysis), were used to quantify differences in relative appearance, abundance and composition of solid bitumen newly generated during the pyrolysis experiments. Results show hydrous residues contain lower TOC, comprised of solid bitumen with higher aromaticity, and textures indicative of lower viscosities, than anhydrous residues from the same temperature pyrolysis conditions. These observations suggest solid bitumen forming from thermal conversion of oil-prone sedimentary organic matter under anhydrous conditions is less aromatic, although more cross-linked, than solid bitumen forming under hydrous conditions at the same time-temperature combination. A radical disproportionation mechanism favored in the presence of hydrogen radical donation from water promotes aromatization in the solid residue with concomitant expulsion of saturated hydrocarbons.