USGS Science⌕ Search

SEARCH · USGS Science

Results for “Geological Journal”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 397 records · Page 22Linked to original sources
USGS2018

Porosity of the Marcellus Shale: A contrast matching small-angle neutron scattering study

Neutron scattering techniques were used to determine the effect of mineral matter on the accessibility of water and toluene to pores in the Devonian Marcellus Shale. Three Marcellus Shale samples, representing quartz-rich, clay-rich, and carbonate-rich facies, were examined using contrast matching small-angle neutron scattering (CM-SANS) at ambient pressure and temperature. Contrast matching compositions of H 2 O, D 2 O and toluene, deuterated toluene were used to probe open and closed pores of these three shale samples. Results show that although the mean pore radius was approximately the same for all three samples, the fractal dimension of the quartz-rich sample was higher than for the clay-rich and carbonate-rich samples, indicating different pore size distributions among the samples. The number density of pores was highest in the clay-rich sample and lowest in the quartz-rich sample. Contrast matching with water and toluene mixtures shows that the accessibility of pores to water and toluene also varied among the samples. In general, water accessed approximately 70–80% of the larger pores (>80 nm radius) in all three samples. At smaller pore sizes (~5–80 nm radius), the fraction of accessible pores decreases. The lowest accessibility to both fluids is at pore throat size of ~25 nm radii with the quartz-rich sample exhibiting lower accessibility than the clay- and carbonate-rich samples. The mechanism for this behaviour is unclear, but because the mineralogy of the three samples varies, it is likely that the inaccessible pores in this size range are associated with organics and not a specific mineral within the samples. At even smaller pore sizes (~<2.5 nm radius), in all samples, the fraction of accessible pores to water increases again to approximately 70–80%. Accessibility to toluene generally follows that of water; however, in the smallest pores (~<2.5 nm radius), accessibility to toluene decreases, especially in the clay-rich sample which contains about 30% more closed pores than the quartz- and carbonate-rich samples. Results from this study show that mineralogy of producing intervals within a shale reservoir can affect accessibility of pores to water and toluene and these mineralogic differences may affect hydrocarbon storage and production and hydraulic fracturing characteristics

International Journal of Coal Geology↗
USGS2018

On the petrographic distinction of bituminite from solid bitumen in immature to early mature source rocks

The oil-prone maceral bituminite (and its equivalents: ‘amorphous organic matter’, ‘sapropelinite’, ‘amorphinite’, etc.) converts to petroleum during thermal maturation of source rocks, resulting in formation of a mobile saturate-rich hydrocarbon and a polar-rich residue of solid bitumen. Evidence of this transition is preserved in immature to early mature source rocks (e.g., Alum, Bakken, Kimmeridge, New Albany, Ohio shales, among many others), where organic petrography reveals a continuum of textures, reflectance, and fluorescence intensity occurring between bituminite and solid bitumen. Bituminite generally is characterized by high(er) intensity fluorescence, low contrast to the inorganic matrix, lower reflectance (compared to solid bitumen), and heterogeneous wispy or ‘schlieren’ textures. In comparison, solid bitumen generally shows lower intensity or no fluorescence compared to bituminite, a distinct contrast to the mineral matrix which usually includes a gray homogeneous surface of low to moderate reflectance, and may possess void-filling, embayment or groundmass textures suggesting evidence of migration or its in situ exsolution. However, these properties may manifest across a continuous spectrum in a sample or in a single microscope field, often making identification inconclusive. Unambiguous identification is further hampered by sample preparation, e.g., mechanical polish may improve homogeneity of larger accumulations, whereas smaller accumulations, or those sheltered by hard minerals, may appear more heterogeneous, leading to identification of the same organic matter as solid bitumen or bituminite, respectively. The disruptive innovation of ion milling in shale sample preparation leads to increased organic reflectance and surface homogeneity, causing bituminite and other oil-prone macerals, e.g., alginite, to develop a gray reflecting surface which is easy to confuse with solid bitumen, especially when obvious void-filling or embayment textures of the latter are absent. Herein we review distinction of solid bitumen from bituminite and alginite in immature to early mature source rocks, providing examples from nine samples illustrated by typical organic petrography images, including samples from hydrous pyrolysis experiments. Based on a review of the literature, we observe an arbitrary reflectance limit of 0.30% in mechanically polished samples seems to differentiate bituminite (<0.30%) from confident identifications of homogeneous solid bitumen (which otherwise lacks obvious void-filling or identifying embayment textures) as individual macerals on a continuous spectrum in immature and early mature source rocks. Future work conducted by governing bodies such as the International Committee for Coal and Organic Petrology (ICCP) should consider interlaboratory studies conducted on multiple immature source rock samples to develop consensus guidelines for bituminite and solid bitumen discrimination.

International Journal of Coal Geology↗
USGS2018

Gas emissions, tars, and secondary minerals at the Ruth Mullins and Tiptop coal mine fires

Both the Tiptop and Ruth Mullins coal fires, Kentucky, were reinvestigated in 2009 and 2010. The Tiptop fire was not as active in 2009 and may have been on the path to burning out at the time of the 2009 visit. The Ruth Mullins coal mine fire, Perry County, Kentucky, has been the subject of several field investigations, including November 2009–February 2010 investigations in which we measured gas emissions, collected minerals and tars, and characterized the nature of the fire. Vents exhibiting the greatest gas flux (>100,000 mg/s/m 2 ) are those with the largest amount of condensate minerals and tars. Vents with moderate gas flux (10,000–100,000 mg/s/m 2 ) are less likely to contain condensate minerals, but are collocated with tars, and vents with the lowest flux (<10,000 mg/s/m 2 ) generally lack both minerals and tars. Aliphatic hydrocarbons present in the gases include C1-C9 compounds, and aromatics include BTEX compounds. Diffuse-CO 2 emissions are concentrated along the fracture zones overlying abandoned mine works. The area of peak diffuse flux corresponds to the trend of the collapsed portal that forms vent 5. The greatest vent emissions were also recorded at vent 5. The snow-melt zone mapped in January 2010 overlies the areas of peak diffuse-CO 2 emissions measured in November; together they delineate the zone of active combustion. Comparison of greenhouse gas emissions from the two sources shows that vent emissions exceed diffuse emissions. The highly fractured, quartz-cemented roof rock funnels the majority of emissions toward the vents. Significant decreases are seen in estimates of yearly greenhouse emissions based on data gathered from November 2009 to February 2010, with estimates from November significantly exceeding any previously published estimates. For example, September 2009 estimates from vent 3 alone indicated that 19 ± 7.5 T CO 2 /yr were emitted while the November 2009 estimates were 1800 ± 690 T/yr. Barometric pressure was lower in November than September. This implies that there are many factors influencing the seasonal variations in fire emissions and that more frequent monitoring will be necessary to derive accurate estimates of coal fires' contribution to the carbon budget.

Kentucky↗
USGS2018

High microscale variability in Raman thermal maturity estimates from shale organic matter

Raman spectroscopy has recently received attention as a means to estimate thermal maturity of organic matter in petroleum generating source rocks to complement more traditional approaches such as vitrinite reflectance and programmed pyrolysis. While many studies have observed positive correlations between source rock thermal maturity and Raman spectral parameters, little attention has been given to the degree of variation in the Raman response across individual organic grains, especially for shales or mudrocks with highly dispersed organic matter. Here the spatial variation in Raman estimates of thermal maturity within individual organic grains is assessed from shales from the Boquillas, Marcellus, Niobrara, and Woodford Formations. The thermal maturity parameters extracted from Raman spectra can vary widely across distances of ≤5 μm within the same organic grain. These results illustrate the high degree of chemical heterogeneity inherent to the organic matter within these source rocks. Additionally, the spatial pattern of the Raman parameters, as revealed by 2D Raman mapping, imply that organic matter structure is influenced by associations with mineral surfaces within the surrounding rock matrix. Chemical heterogeneity and matrix effects directly impact the Raman response from these types of materials and thus the extracted thermal maturity estimate. These findings highlight the care which must be adopted when making Raman measurements of organic matter within source rock matrices, especially for samples which feature highly dispersed, heterogeneous organic matter as found in petroliferous mudrocks.

International Journal of Coal Geology↗
USGS2019

Reflectance increase from broad beam ion milling of coals and organic-rich shales due to increased surface flatness

Broad ion beam (BIB) milling is useful in organic petrology because it can yield flat sample surfaces and avert the ‘smearing’ of organic matter (OM) that results from traditional mechanical polishing. This potentially makes BIB especially useful in the study of nano-porosity, where even minor mechanical disruption of the sample surface distorts the sample characteristic of interest—the pore structure. However, several studies have observed an OM reflectance increase after BIB milling, concluding that ion milling may cause thermal alteration to OM surfaces. To better understand ion milling effects on organic matter, coal (subbituminous, high volatile bituminous, medium volatile bituminous, anthracite) and shale [Bakken Formation, Ohio Shale-Huron Member (5), Kimmeridge Clay Formation, Alum Shale, New Albany Shale] samples were prepared using traditional mechanical polishing methods. Reflectance measurements (% Ro) were gathered on all maceral types present before BIB milling, followed by re-measurement of OM reflectance at the same locations after milling. Most OM increased in reflectance after BIB milling, with some exceptions in high maturity samples. Liptinite macerals in both coal and shale samples showed the greatest percent reflectance increase on average (+133%; n=338), followed by solid bitumen (+49%; n=313), vitrinite (+26%; n=413), and inertinite (+9%; n=220). Despite the increases to OM reflectance caused by BIB milling, no evidence was found for kerogen conversion (e.g., change in maceral abundances), or for migration of newly generated petroleum (e.g., pseudomorphic replacement of kerogen by solid bitumen). Such changes occur when samples are thermally altered from immature conditions into the oil window (e.g., by hydrous pyrolysis), and, if the increases in OM reflectance were thermally driven (by BIB milling), they should have been observed in the above experiments. Herein, we also used atomic force microscopy to document a decrease in surface roughness of correlative locations of OM on pre- and post-ion milled samples. This improved surface polish caused by BIB milling appears to be the root cause of increased OM reflectance, as no other supporting evidence of thermal alteration could be found. That is, the fraction of light formerly lost to oblique scatter in diffuse reflectance from a mechanically polished surface is converted to specular reflectance after BIB ion milling. Thus the light leaves the surface at a near normal angle and returns to the detector, resulting in increased OM reflectance.

International Journal of Coal Geology↗
USGS2019

Application of electron microscopy TEM and SEM for analysis of coals, organic-rich shales and carbonaceous matter

This paper provides a brief summary of the history, development and variety of applications of electron microscopy techniques to analyze coals, organic-rich shales, and carbonaceous materials. General construction and principles of operation of transmitted (TEM) and scanning electron microscope (SEM) are outlined, along with guidance on specimen preparation, and a brief overview of published TEM and SEM applications related to coal, shales, and carbonaceous materials. This work was accepted as the chapter 18.2 of International Committee for Coal and Organic Petrology Methods Handbook at the ICCP Plenary Session on September 27, 2018 in Brisbane (Australia).

International Journal of Coal Geology↗
USGS2020

Fluorescence spectroscopy of ancient sedimentary organic matter via confocal laser scanning microscopy (CLSM)

Fluorescence spectroscopy via confocal laser scanning microscopy (CLSM) was used to analyze ancient sedimentary organic matter, including Tasmanites microfossils in Devonian shale and Gloecapsomorpha prisca ( G. prisca ) in Ordovician kukersite from North American basins. We examined fluorescence emission as a function of excitation laser wavelength, sample orientation, and with respect to location within individual organic entities and in transects across bedded organic matter. Results from spectral scans of the same field of view in Tasmanites with different laser lines showed progressive red-shift in emission maxima with longer excitation wavelengths. This result indicates steady-state Tasmanites fluorescence emission is an overlapping combination of emission from multiple fluorophore functions. Stokes shift decreased with increasing excitation wavelength, further suggesting the presence of multiple fluorophore functions with different S 1 → S 0 transition energies. This observation also indicates that at longer excitation wavelengths, less absorbed light energy is dissipated via collisional transfer than at shorter excitation wavelengths and may suggest fewer polar functions are preferentially absorbing. Confirming earlier results, emission spectra observed from high fluorescence intensity regions (fold apices) in individual Tasmanites are blue-shifted relative to emission from other locations in the same microfossil. We suggest high intensity emission is from photoselective alignment of polarized excitation with the fluorophore absorption and emission transition moment. The blue shift observed in regions of high intensity emission may be due to relative absence or realignment of polar species, e.g., bridging ether or ester functions, although variations in O abundance could not be confirmed with preliminary time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis. Tasmanites occurring in consolidated sediments are flattened from original spherical morphology and, in optical microscopy, this burial deformation results in generally parallel extinction (strain-influenced) and positive elongation. The deformation also induces fluorescence anisotropy observed as variations in emission wavelength when individual Tasmanites are measured from their long axis parallel to bedding, whereas this effect is absent in bedding-normal view. Transects from G. prisca -rich source layers into adjacent reservoir layers show decrease in fluorescence intensity and spectral red-shift (increase in full-width half-maximum with increasing red portion of the half-width). These results may suggest an increase in fluorescence quenching across the source-to-reservoir transition zone, consistent with an increase in aromaticity following petroleum expulsion and migration. These observations are supported by increasing reflectance values measured across similar micro-scale transects. Our results highlight the applicability of CLSM as a broad and under-utilized approach for the characterization of sedimentary organic matter and are discussed with perspective toward petroleum processes and thermal indices research.

International Journal of Coal Geology↗
USGS2020

A two-stage step-wise framework for fast optimization of well placement in coalbed methane reservoirs

Coalbed methane (CBM) has emerged as a clean energy resource in the global energy mix, especially in countries such as Australia, China, India and the USA. The economical and successful development of CBM requires a thorough evaluation and optimization of well placement prior to field-scale exploitation. This paper presents a two-stage, step-wise optimization framework to obtain the optimal placement of wells for large-scale development of CBM reservoirs. In the first stage, an optimal uniform well pattern is obtained by optimizing well pattern description parameters with the particle swarm optimization (PSO) algorithm. Subsequently, the location and status (active/inactive) of each well are perturbed and optimized within the patterns through the integration of the generalized pattern search (GPS) algorithm and a quality map (QM) representing the production potential. This framework was tested in a synthetic anthracite CBM reservoir in the Qinshui basin (with high gas content and low permeability) and a real field high volatile bituminous reservoir in the Illinois basin (with low gas content and high permeability). The results show that: (i) significant variations in the net present value (NPV) exist with respect to different uniform well patterns (even for cases where the total number of wells are identical), the optima of which can be efficiently determined by the PSO within 100 numerical simulation runs; (ii) the optimization of well perturbations by the GPS results in a more noticeable improvement in NPVs for the synthetic (12.3%) than for the real field model (4.6%); (iii) for the low permeable synthetic model with narrow optimal well spacings (320 m × 200 m), the contribution of the optimization of well perturbation to the NPV increment is heavily dependent on the uniform well placement solution; (iv) for the high permeable real field model with large optimal well spacings (1300 m × 1300 m), the initial uniform well placement has a very minor effect on the subsequent well perturbation solutions in terms of NPV; (v) the proposed framework significantly outperforms the conventional well-by-well concatenation procedure in terms of computational efficiency, robustness and optimal criteria set for production potential.

International Journal of Coal Geology↗
USGS2020

Examination of inertinite within immature Eagle Ford Shale at the nanometer-scale using atomic force microscopy-based infrared spectroscopy

The nanoscale molecular composition of sedimentary organic matter is challenging to characterize in situ given the limited tools available that can adequately interrogate its complex chemical structure. This is a particularly relevant issue in source rocks, as kerogen composition will strongly impact its reactivity and so is critical to understanding petroleum generation processes during catagenesis. The recent advent of tip-enhanced analytical methods, such as atomic force microscopy-based infrared spectroscopy (AFM-IR), has allowed for the major compositional features of kerogen and other types of in situ organic matter to be elucidated at spatial resolutions at or below 50 nm. Here AFM-IR was applied to examine inertinite, an important organic matter type, present in a thermally immature Eagle Ford calcareous mudstone. The data show that the nanoscale molecular composition of the examined inertinite is (i) less heterogeneous than solid bitumen in more thermally mature Eagle Ford samples and (ii) more hydrogen- and oxygen-rich than inertinite examined in the New Albany Shale.

Texas↗
USGS2021

A review of spatially resolved techniques and applications of organic petrography in shale petroleum systems

This review examines new techniques and applications of organic petrography in source-rock reservoir petroleum systems that have occurred along with development of the global ‘shale revolution’ in energy resources. The review is limited to techniques and instrumentation that provide spatially resolved information, typically at or below microscales, for dispersed organic matter occurring in situ in samples of shale and mudrock. A brief summary of ion beam sample preparation is followed by discussion of the most common analytical techniques and applications. Advantages and limitations of each technique, including requisite sample preparation, types of information generated [e.g., molecular or elemental (isotopic) abundance], sensitivity, and resolution are discussed. In a few cases, techniques not yet applied to organic petrology of shale or mudrock are described (e.g., X-ray photoelectron spectroscopy imaging), in anticipation of near-term future application. The most common in situ techniques applied for characterization of organic matter in shale and mudrock include optical (including fluorescence) and electron microscopies, Raman, fluorescence, and infrared spectroscopies, and surficial measurements via force microscopy. Techniques that show growing application to organic petrography of shale include tip-enhanced photothermal infrared spectroscopy, mass spectrometry imaging, and synchrotron-based spectroscopies, among others. It is anticipated that the future of dispersed organic matter petrography will hold continued development of integrated instrument techniques (e.g., simultaneous or sequential correlative microscopies and/or spectroscopies of the same location), increased instrumental resolution, increased use of multiscale and multimodal organic petrography investigations, and three-dimensional imaging and chemical speciation mapping applications via multiple analytical approaches.

International Journal of Coal Geology↗
USGS2021

Single-well production history matching and geostatistical modeling as proxy to multi-well reservoir simulation for evaluating dynamic reservoir properties of coal seams

Reservoir properties of coal seams such as gas and water effective permeabilities and gas content, as well as spatial distributions thereof, affect the success of gas production and CO 2 -enhanced gas recovery (EGR) with simultaneous CO 2 sequestration. These properties change during production and injection operations due to variations in reservoir pressure, matrix shrinkage/swelling, and water saturation and are therefore referred to as dynamic properties. Predicting distribution of such important reservoir properties and how they evolve during production, or injection, at unsampled locations can be particularly important for field development and project economics. In this work, dynamic properties of Black Creek coal seam of Black Warrior Basin, Alabama were mapped using pointwise results from single-well production history matching of 45 wells and classical geostatistics . It is explored if this approach can be a proxy, with its limitations, to multi-well reservoir simulation. For this purpose, a reservoir model was built using available reservoir, well and production data to compare its results with those of the geostatistical maps for the same properties. Despite the expected local discrepancies due to differences between the two approaches, the results showed similar patterns and global distributions. Specific results showed that despite long-time operation of the wells in this area, there were still areas with high gas content and low gas effective permeability within the modeled time interval that might have benefited from further development using additional wells.

Alabama↗
USGS2021

Insights on the characteristics and sources of gas from an underground coal mine using compositional data analysis

Coal mine gas originates from the gas emission zone (GEZ) of the mine, as well as the longwall face and pillars. Gas emissions are controlled directly at the sources using horizontal or vertical boreholes drilled from surface or from the entries in advance of mining, or it is captured from the fractured and caved zones (gob) using ventholes during mining. The rest of the gas, especially that gas that originates from the longwall face and caved zone, mixes with the ventilation air and travels through bleeder and return entries before being exhausted to atmosphere from air shafts. Although the gas associated with mining mostly focuses on methane, the gas is not pure methane but is a mixture (where both the components and their quantities vary depending on the sampling location). Understanding the evolution of the composition of the gas from source to different sampling and evaluation points in mines using proper statistical analysis and interpretation methods can lead to better designed degasification and ventilation systems and the selection of the most adequate utilization method for generating energy from the gas. In this work, we present the results of compositional data analysis (CoDa) of gases sampled at different locations in a longwall mine operating in Pennsylvanian coal-bearing strata in the Northern Appalachian coal basin. Sampling locations were from accessible parts of the bleeder entries, returns, bleeder evaluation points and shafts within the mine, and also from gob gas ventholes (GGV) and coal degasification boreholes drilled in the panel areas. In addition, desorbed gas samples from seams that are important for the mine were included in the analyses for comparison. The compositional data analysis showed that the gas composition shifts based on the sampling location, putting in-mine and pure coalbed gases on the opposite ends. Removal of air from the samples did not change this observation suggesting that oxygen is depleted especially for in-mine samples, which is due to oxidation. Results also suggested that desorbed gas samples may not represent the composition of the coal seam gas.

International Journal of Coal Geology↗
USGS2021

Relating Tmax and hydrogen index to vitrinite and solid bitumen reflectance in hydrous pyrolysis residues: Comparisons to natural thermal indices

Vitrinite reflectance (VR o ; %) generally is considered the most reliable technique to determine the thermal maturity of sedimentary rocks. However, it is a time-consuming process to collect reflectance (R o ; %) measurements and is subjective to the interpretation of each trained technician, who must be able to discern between vitrinite and solid bitumen and other organic matter types. Inadvertent misidentification of solid bitumen for vitrinite can lead to reports of ‘suppressed’ VR o , especially at lower thermal maturities (< 1.0% R o ). Programmed pyrolysis data, such as T max and hydrogen index (HI), are comparatively inexpensive and more time-efficient to obtain than R o data and are determined by instrument settings, rather than by operator decision, and are therefore independent of operator-based training or experience bias. This study uses hydrous pyrolysis (HP) residues from various coals and shales to relate measured VR o and solid bitumen reflectance (BR o ; %) values to their respective T max and HI values and determines whether these relationships can be used as a proxy to calculate R o in naturally matured samples. Although the estimation of R o is not always accurate, the results demonstrate that relational equations for shales and coals derived from the T max and HI data of HP residues can effectively calculate R o in natural series. Approximately 60% of calculated R o from T max and 83% of calculated R o from HI relational equations are within interlaboratory reproducibility limits (± 0.2% shale BR o ; ± 0.06% coal VR o ) when compared to their respective measured R o values from natural series. Variables that may affect accuracy of the applied relational equations include variable sedimentary organic matter composition of samples, differences of maturation reaction kinetics of the sedimentary organic matter in experimental versus natural settings, and decreasing reliability of all thermal proxy measurements at higher maturities.

International Journal of Coal Geology↗
USGS2021

Compositional evolution of organic matter in Boquillas Shale across a thermal gradient at the single particle level

The molecular composition of petroliferous organic matter and its compositional evolution throughout thermal maturation provides insight for understanding petroleum generation. This information is critical for understanding hydrocarbon resources in unconventional reservoirs, as source rock organic matter is highly dispersed, in contact with the surrounding mineral matrix, and may occur as multiple organic matter maceral types. Here, Raman spectroscopy and optical microscopy approaches were 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 h. This analytical approach allowed for correlative examination of micro-scale changes in organic matter compositional properties (e.g., aromaticity) for a variety of organic matter macerals across a thermal gradient (from marginally mature into the late oil/wet gas window) 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 maturation. 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 Raman spectra for individual organic matter macerals was performed using multivariate curve resolution (MCR) and 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, and can be used to construct thermal maturity relationships. These findings inform our understanding of how different petroliferous organic matter maceral 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.

International Journal of Coal Geology↗
USGS2022

Characterization of bituminite in Kimmeridge Clay by confocal laser scanning and atomic force microscopy

This work investigates bituminite (amorphous sedimentary organic matter) in Upper Jurassic Kimmeridge Clay source rock via confocal laser scanning microscopy (CLSM) and atomic force microscopy (AFM). These petrographic tools were used to provide better understanding of the nature of bituminite, which has been historically difficult to identify and differentiate from similar organic matter types in source rocks. As part of an International Committee for Coal and Organic Petrology (ICCP) working group, an immature (0.42% vitrinite reflectance), organic-rich (44.1 wt% total organic carbon content) sample of Kimmeridge Clay was distributed to multiple laboratories for CLSM characterization. The primary observations from CLSM imaging and spectroscopy include: 1) the interpreted presence of Botryococcus algae as a contributor to bituminite precursors; 2) color red-shift of sulfide reflectance and bituminite auto-fluorescence from below the sample surface; 3) positive alteration of bituminite from laser-induced photo-oxidation of the sample surface, including fluorescence blue-shift; 4) fluorescence blue-shift associated to higher fluorescence intensity regions in bituminite indicative of compositional (fluorophore) differences; 5) the need for fluorescence spectroscopy standardization as applied via CLSM; and 6) the suitability of CLSM fluorescence spectroscopy to predict solid bitumen reflectance from bituminite spectral emission via calibration to an extant dataset. Secondary CLSM observations include detection of reflected laser light from highly reflective inclusions in bituminite, including sulfides and fusinite, and radiolytic alteration of bituminite caused by substitution of U for Fe in sulfides. Findings from AFM include the observation that surface roughening or surface flattening of bituminite are induced by differential broad ion beam (BIB) milling and are dependent on the location and scale of AFM topology measurement. This result highlights our still limited understanding of the effects of BIB milling on sedimentary organic matter and indicates the need for further research before this technique can be advanced as a standard practice in petrographic sample preparation. Collectively, the results of this study illustrate the general applicability and versatility of AFM and CLSM as tools for organic petrology research, specifically for better understanding of the nature and properties of the bituminite maceral.

Kimmeridge↗
USGS2022

Maturation study of vitrinite in carbonaceous shales and coals: Insights from hydrous pyrolysis

The presence of vitrinite in sedimentary rocks of post-Silurian age allows its reflectance to be used to estimate the thermal maturation of organic matter in petroleum systems. Increasing reflectance of vitrinite, which is primarily driven by aromaticity, depends primarily on the time and temperature attributes of its evolutionary pathway. This study evaluated carbonaceous shales proximal to coal measures and coal samples via isothermal hydrous pyrolysis (HP) to compare differences in the maturation pathways of vitrinite. Sample residues were analysed via vitrinite reflectance (VRo), geochemical screening tests (organic carbon and programmed temperature pyrolysis), and infrared spectroscopy. The study included samples from Indian and North American basins, to observe differences in vitrinite evolution with respect to enclosing mineral matrix, starting degree of aromaticity, organic matter types, stratigraphic age, and depositional environment. The organic content of HP residues shows an intuitive response to the thermal stress of HP, e.g., a general depletion of total organic carbon (TOC) content, pyrolyzate (S2), and hydrogen index with increasing HP temperature. Infrared proxies including C-factor and CH2/CH3 generally decrease with increasing thermal maturity indicating loss of O via CO2 generation and the thermal cracking of aliphatic organic matter. Tmax, production index (PI), and VRo show intuitive increasing values with respect to HP temperature. The least mature sample (0.48 ± 0.05% VRo) generally experienced the maximum change in these parameters during maturation, whereas the most mature sample (0.99 ± 0.06% VRo) generally showed the least change. This observation is consistent with higher kinetic barriers to reaction in more aromatic vitrinite which contains higher bond dissociation energies. Devolatilization of vitrinite during HP causes formation of gas evacuation vacuoles and contraction cracks in the vitrinite grains of both coal and carbonaceous shale. Similarities in vitrinite response to HP between coal and carbonaceous shale suggest that thermal evolution of the vitrinite maceral is principally controlled by inherent rate-limiting kinetic parameters related to its molecular structure. Whereas, the stratigraphic age, sedimentary environment, surrounding organic matter, lithology, and catalysis by mineral composition have less effect. To further improve our understanding of vitrinite aromatization and kinetic parameters, future studies of vitrinite reflectance thermal evolution with temperature should include coal and carbonaceous shale from the same stratigraphic section and extant woody tissue from modern vascular plants.

International Journal of Coal Geology↗
USGS2022

Machine learning and data augmentation approach for identification of rare earth element potential in Indiana Coals, USA

Rare earth elements and yttrium (REYs) are critical elements and valuable commodities due to their limited availability and high demand in a wide range of applications and especially in high-technology products. The increased demand and geopolitical pressures motivate the search for alternative sources of REYs, and coal, coal waste, and coal ash are considered as new sources for these critical elements. This research evaluates the REY potential of coals from Indiana (USA). However, although coal data revealed REY potential, it suffered from sparse samples with complete REY measurements. Therefore, we explore the applicability of machine learning (ML) models and data augmentation techniques to demonstrate their applicability to evaluate REY potential in Indiana, and other areas in coal basins, using selected coal parameters (Al2O3, Fe2O3, C, Ash, S, P, Mo, Zn, and As contents) as covariates (indicators). Due to the relatively small sample size with complete REY data in the Indiana Coal Database, two data augmentation techniques (Random Over-Sampling Examples and Synthetic Minority Over-Sampling Technique) were used. Four machine learning algorithms (linear discriminate analysis, support vector machine, random forest, and artificial neural networks) were applied for modeling REY potential as a classification problem. The results show that application of Synthetic Minority Over-Sampling Technique prior to development of the support vector machine (SVM) models generated the best REY classification with an accuracy of 95%. The encouraging results based on Indiana coal data may suggest that a similar approach can be used for other coal basins for screening the locations with REY potential. Those locations then can be targeted for more detailed geochemical surveys to identify most promising areas and evaluate overall REY resources.

Indiana↗
USGS2022

Relating systematic compositional variability to the textural occurrence of solid bitumen in shales

This study presents Raman spectroscopic data paired with scanning electron microscopy (SEM) images to assess solid bitumen composition as a function of solid bitumen texture and association with minerals. A series of hydrous pyrolysis experiments (1–103 days, 300–370 °C) using a low maturity (0.25% solid bitumen reflectance, BR o ), high total organic carbon [(TOC), 14.0 wt%] New Albany Shale sample as the starting material yielded pyrolysis residues designed to evaluate the evolution of solid bitumen aromaticity with increasing temperature and heating duration. Solid bitumen was analyzed by Raman spectroscopy wherein point data were collected from accumulations that ranged in size and degree of association with the mineral matrix. Raman spectroscopy results show that with increasing temperature and experimental duration, solid bitumen aromaticity increases and compositional variability decreases. With regards to texture and composition, coarser-grained solid bitumen (>1.3 μm from nearest mineral grain) has consistently higher, but less variable aromaticity than thinner, wispy solid bitumen which is more intimately associated with the mineral matrix. Collocated scanning electron microscope images were used to provide qualitative assessments of porosity hosted by the organic matter. These paired data sets suggest that solid bitumen porosity development and molecular composition are linked, and these parameters are related to the textural relationships of the organic matter within the whole rock. These results are discussed with perspective towards understanding how rock fabric and texture can influence organic matter evolution during thermal maturation of organic-rich marine shales and inform our broader understanding of these important energy resources.

International Journal of Coal Geology↗