Raman spectroscopy was studied as a thermal maturity probe in a series of Upper Devonian Ohio Shale samples from the Appalachian Basin spanning from immature to dry gas conditions. Raman spectroscopy also was applied to samples spanning a similar thermal range created from 72-hour hydrous pyrolysis (HP) experiments of the Ohio Shale at temperatures from 300 to 360°C and isothermal HP experiments lasting up to 100 days of similar Devonian-Mississippian New Albany Shale. Raman spectra were treated by an automated evaluation software based on iterative and simultaneous modeling of signal and baseline functions to decrease subjectivity. Spectra show robust correlation to measured solid bitumen reflectance (BRo) values and were therefore used to construct logarithmic regression relationships for calculation of BRo equivalent values. Raman spectra show considerable differences between natural samples and HP. residues with similar measured BRo values, indicating as-yet undetermined differences in carbon chemistry. We speculate this result may be due to differences in the sampling interactions of Raman vs. reflectance measurements, and the incomplete nature of maturation reactions in the time-limited hydrous pyrolysis residues. Samples used in this study are similar in organic assemblage (dominantly solid bitumen) to other commonly exploited North American shale petroleum systems, i.e., Bakken, Barnett, Duvernay, Fayetteville and Woodford shales. Therefore, results presented herein may be broadly applicable to other important shale plays. However, caution is suggested and Raman spectroscopy as a thermal probe may need individual calibration in each shale play due to differences in solid bitumen carbon chemistry. Samples were collected and tested between 2013 and 2018, in studies preformed by Ryder et al., 2013; Hackley and Lewan, 2018; Hackley et al., 2017; Yang et al., 2017; Hackley and Lundsdorf, 2018.

Here the spatial variation in Raman estimates of thermal maturity within individual organic domains from several shale geologic reference materials originating from the Boquillas, Marcellus, Niobrara, and Woodford Formations are assessed from the respective Raman response. We show that for all four shales the thermal maturity parameters extracted from Raman spectra by iterative peak fitting can vary widely across distances of ≤5 µm within the same organic domain.

The dataset covers X-ray diffraction (XRD) applied for mineral determination in shales from the Utica, Excello, Niobrara, and Monterey formations. The XRD was performed prior to modified Rock-Eval pyrolysis, reflectance, organic petrology, and Fourier-transform infrared spectroscopy (FTIR) being employed to analyze geochemical properties; gas adsorption (CO2 and N2) was used to characterize pore structures.

The dataset covers X-ray diffraction (XRD) applied for mineral determination in shales from the Utica, Excello, Niobrara, and Monterey formations. The XRD was performed prior to modified Rock-Eval pyrolysis, reflectance, organic petrology, and Fourier-transform infrared spectroscopy (FTIR) being employed to analyze geochemical properties; gas adsorption (CO2 and N2) was used to characterize pore structures.

This data release is a compilation of published and unpublished thermal maturity and source rock geochemical data (Rock-Eval, pyrolysis) from subsurface wells in the Permian Basin, west Texas and southeast New Mexico. These data include 67 newly collected samples and analyses from Delaware Basin wells (identified as Cicero_2022), as well as 1028 previously unpublished USGS analyses from the entire province (identified as LIMS). Data were also synthesized from publicly available sources, such as theses and dissertations, state agencies and databases, as well as from the body of published literature.

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.

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 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.