Geology topics

Paul C. Hackley

Publications and source records attributed to Paul C. Hackley.

At least 55 records · Page 3Linked to original sources

USGS2022

Identification of thermal maturity-relevant organic matter in Shale Working Group Report 2021

The Identification of Thermal Maturity-Relevant Organic Matter in Shale Working Group of the ICCP was established in 2008 to provide guidelines for identifying and measuring the reflectance of the population of dispersed organic matter that is relevant to thermal maturity determination. Information products published by the working group include ASTM D7708 Standard Test Method for Microscopical Determination of the Reflectance of Vitrinite Dispersed in Sedimentary Rocks (ASTM, 2014) and two interlaboratory studies (ILS) to determine precision statistics for this ASTM test method (Hackley et al., 2015, 2020). Poor reproducibility of solid bitumen and vitrinite reflectance measurements in the second ILS (Hackley et al., 2020) suggested that further work is required to standardize the identification of thermal maturity-relevant dispersed organic matter for reproducible reflectance measurements. Thus, at the 2019 ICCP meeting in The Hague, Netherlands, the working group decided to pursue additional ILS via image-based approaches to improve reproducibility.

ICCP News↗

USGS2021

New insights into organic matter accumulation from high-resolution geochemical analysis of a black shale: Middle and Upper Devonian Horn River Group, Canada

Organic matter (OM) accumulation in organic matter-rich mudstones, or black shales, is generally recognized to be controlled by combinations of bioproductivity, preservation, and dilution. However, specific triggers of OM deposition in these formations are commonly difficult to identify with geochemical proxies, in part because of feedbacks that cause geochemical proxies for these controls to vary synchronously. This apparent synchronicity is partly a function of sample spacing, commonly at decimeter to meter intervals, which may represent longer periods of time than is required for the development of feedbacks. Higher resolution data sets may be required to fully interpret OM accumulation. This study applies a novel combination of technologies to develop a high-resolution geochemical data set, integrating energy-dispersive X-ray fluorescence (EDXRF) and infrared imagery analyses, to record proxies for redox conditions, bioproductivity, and clastic and carbonate dilution in millimeter-resolution profiles of 133 core slabs from the Middle and Upper Devonian Horn River shale in the Western Canada Sedimentary Basin, which provides decadal-scale temporal resolution. A comparison to a more coarsely sampled data set from the same core results in substantially different interpretations of variations in bioproductivity, redox, and dilution proxies. Stratigraphic distributions of organic matter accumulation patterns (bioproductivity-control, siliciclastic/carbonate-dilution, and redox conditions-control) show that organic enrichment events were highly varied during deposition of the shale and were closely related to second- and third-order sea-level changes. High-resolution profiles indicate that bioproductivity was the predominant trigger for organic matter accumulation in a second-order highstand, particularly during deposition of third-order transgressive systems tracts. Organic matter accumulation was largely controlled by dilution from either carbonate or clastic sediments in a second-order lowstand. Bioproductivity-redox feedbacks developed on timescales of decades to centuries.

British Columbia, Northwest Territories↗

USGS2021

Vitrinite reflectance analysis

Vitrinite is a maceral group (a set of organic matter types with similar properties and appearance) derived from the remains of woody material from vascular plants and is composed of the thermally evolved products of lignin and cellulose. Vitrinite is the dominant component of humic coal and is found as a minor component dispersed into sedimentary rocks, especially mudrocks, the primary source rocks for petroleum. Vitrinite reflectance analysis is the microscopical determination of the reflectance of incident light from vitrinite as calibrated to standards of known reflectance. The reflectance of vitrinite increases systematically with increasing thermal maturation during burial and heating of sedimentary rocks, and this property allows vitrinite reflectance analysis to be used as a common geothermometer for source rocks in petroliferous basins. Vitrinite reflectance analysis also is used as a method to determine coal rank.

Book chapter↗

USGS2021

Organo-facies and mineral effects on sorption capacity of low-maturity Permian Barakar shales from the Auranga Basin, Jharkhand, India

Shales associated with the Lower Permian (Barakar Formation) sediments of the Auranga Coalfield, India, occur in the immature–early mature stage. The sorption capacity of Barakar shale samples has been studied through high-pressure methane (CH 4 ) adsorption and low-pressure N 2 gas adsorption (LPN 2 GA) methods, supported with proximate analyses, programmed pyrolysis, optical petrography, and with energy-dispersive spectroscopy, X-ray diffraction, and inductively coupled plasma mass spectrometry. The sorption capacity is a function of the organic and inorganic constituents present in the shale samples. The methane sorption capacity (MSC) and Langmuir volume of the shale samples vary from 0.217 to 0.314 and 0.315 to 0.429 mmol/g rock, respectively. The BET-calculated surface area of the studied shales varies from 8.12 to 30.36 m 2 /g. The sorption capacities show the importance of the total organic content (TOC) through weak but positive correlations with MSC ( r 2 = 0.45) and S 1 values (mg hydrocarbons/g rock from programmed pyrolysis; r 2 = 0.40). Moreover, apparent inverse relationships were observed between MSC and clay mineral abundances, suggesting that individual clay mineral types may influence MSC, although more work is needed. The TOC-normalized MSC (MSC*) of shale samples shows a positive trend with quartz plus clay mineral content and ash yield of r 2 = 0.64 for both. In addition, MSC* shows a negative logarithmic relationship with S 1 + S 2 ( r 2 = 0.63) and a positive linear relationship with TOC-normalized total organic matter (TOM*) ( r 2 = 0.88, when 5 low TOM* samples are excluded) indicating complex relationships possibly including bitumen retention in the sample pore spaces. The micropore study of the samples through LPN 2 GA, applying Dubinin–Radushkevich, Dubinin–Astakhov, and density functional theory models, shows the dominance of micro-mesopore concentrations in the shale matrix of ∼2 nm pore diameter. However, these pores might be present as blind or closed pores. The presence of thorium and zirconium is reflective of terrigenous detrital matter, i.e., moderately to strongly recycled sediments. The fluviatile facies of deposited shales in the Auranga Coalfield are noted by the significant presence of kaolinite (32.5–78.3%), which suggests the importance of its effect on the sorption capacity of proximal terrigenous shales.

Jharkhand↗

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

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↗

USGS2021

An integrated geochemical, spectroscopic, and petrographic approach to examining the producibility of hydrocarbons from liquids-rich unconventional formations

The geochemical and petrophysical complexity of source-rock reservoirs in liquids-rich unconventional (LRU) plays necessitates the implementation of a more expansive analytical protocol for initial play assessment. In this study, original samples from selected source-rock reservoirs in the USA and the UK were analyzed by 22 MHz nuclear magnetic resonance (HF-NMR) T1-T2 mapping, followed by hydrous pyrolysis, and a modified Rock-Eval pyrolysis method (multi-heating step method-MHS). The above methods were complemented by organic petrography under reflected white and UV light excitation of the original and pyrolyzed samples. The analytical protocol presented attempts to better qualify and quantify different petroleum fractions (mobile, heavy hydrocarbons, viscous, solid bitumen), thus provide valuable and refined information about producibility of target intervals during appraisal. Results show how the hydrocarbon fractions interpreted from peak locations and intensities on NMR T1-T2 maps are in good agreement with those from MHS pyrolysis in terms of hydrocarbon mobility/producibility. Results from HP (Hydrous Pyrolysis) experiments show that an exception to this general agreement between NMR and MHS estimates occurs for the Kimmeridge Blackstone Clay samples, where MHS shows an increase of >90% in producible hydrocarbon yields vs. minimal to no presence of mobile hydrocarbons in NMR T1-T2 maps. This study clarifies the role of pore structure and networks in these discrepancies of producible oil estimates when comparing programmed pyrolysis to NMR-based techniques. This novel, multi-step and multidisciplinary approach provides a more advanced screening protocol for identifying zones of higher oil-in-place (OIP) and predicting fluid mobility prior to drilling or completions.

Fuel↗

USGS2021

Hydrous pyrolysis of New Albany Shale: A study examining maturation changes and porosity development

The characterization of nanoscale organic structures has improved our understanding of porosity development within source-rock reservoirs, but research linking organic porosity evolution to thermal maturity has generated conflicting results. To better understand this connection, an immature (0.25% solid bitumen reflectance; BR o ) sample of the New Albany Shale was used in four isothermal hydrous pyrolysis (HP) experiment sequences at 300°, 320°, 340°, and 370°C, with residues collected periodically for a maximum of 103 days. The HP residues, along with the original immature sample and two naturally matured (1.49 and 1.56% BR o ) New Albany Shale samples were analyzed for organic petrology, total organic carbon (TOC) content, and organic porosity evaluation using correlative light and electron microscopy (CLEM). All of the HP series increased in thermal maturity with increasing duration of pyrolysis, though reflectance for each series plateaued within 25 days of maturation. Initially, TOC in the HP residues decreases (from 14.24 wt. %) with increasing thermal maturity until ∼1.0% BR o where TOC remains at ∼9–10 wt. % for all remaining residues. Qualitative CLEM observations within the 50–100 day 300° and 340°C HP sequences (0.95–1.70% BR o ), and the naturally matured samples, develop organic porosity in smaller (<5 μm in diameter), void-filling solid bitumen that occurs in spaces between clays and other fine-grained minerals. The 370°C HP residues developed significant organic porosity, relative to the other HP temperature series in all solid bitumen accumulations regardless of size. Overall, the study indicates that temperature and duration of artificial maturation play an important role in the abundance of pores in the HP residues. This work expands on our understanding of the conditions needed for the generation and development of organic porosity in the New Albany Shale and potentially in other marine source-rock petroleum systems.

Illinois, Indiana↗

USGS2021

Meter-scale lithofacies cycle and controls on variations in oil saturation, Wolfcamp A, Delaware and Midland Basins

Typical meter-scale lithofacies cycles from the Wolfcamp A in the Delaware and Midland Basins comprise basal carbonate facies overlain by calcareous or siliceous mudrocks. Siliceous mudstones are the most organic-rich facies with high total organic carbon ( TOC > 3 wt. %), whereas thin carbonate beds have the lowest organic matter (OM) content among the lithofacies present ( TOC TOC, programmed pyrolysis analysis, and residual gas analysis from rock crushing. Oil saturation index ( OSI ) (the amount of free oil normalized by TOC ; OSI = S 1 × 100/ TOC ) is used as an indicator of oil enrichment or depletion in the reservoir, where S 1 is volatile oil in programmed pyrolysis (temperature = 300°C). Both TOC -lean carbonate and TOC -rich mudstone lithofacies have high OSI in these meter-scale cycles (average OSI is 124.5 mg HC/g TOC for carbonate beds), indicating that migrated oil is present. Residual gas analyses show lower dryness values (C 1 /C 1–5 ) and higher oil indicator values (100 × C 4+5 /C 1–5 ) in TOC -lean carbonate beds compared to the TOC -rich mudstones, likely indicating a cumulative oil and gas charging effect through source rock maturation. Oil and gas generated at different stages of thermal maturation were partially expelled from OM-rich siliceous/calcareous mudstones into adjacent OM-lean carbonate beds. This study shows oil expulsion from source to adjacent carbonate beds is a key factor in variations of oil saturation in the Wolfcamp A.

AAPG Bulletin↗

USGS2021

Organic geochemistry and petrology of Devonian shale in eastern Ohio: Implications for petroleum systems assessment

Recent production of light sweet oil has prompted reevaluation of Devonian petroleum systems in the central Appalachian Basin. Upper Devonian Ohio Shale (lower Huron Member) and Middle Devonian Marcellus Shale organic-rich source rocks from eastern Ohio and nearby areas were examined using organic petrography and geochemical analysis of solvent extracts to test ideas related to organic matter sources, oil–source rock correlation, thermal maturity, and distances of petroleum migration. The data from these analyses indicate organic matter in the Ohio and Marcellus Shales primarily was derived from marine algae and its degradation products, including bacterial biomass. Absence of odd-over-even n - alkane distributions (n - C 13 to n - C 21 range) in gas chromatograms and low gammacerane index values in Devonian source rocks are similar to those of Devonian-reservoired oils in eastern Ohio, suggesting an oil–source rock correlation. Lower Paleozoic oils from eastern Ohio, in contrast, are characterized by the presence of odd-over-even n - alkane distributions (n - C 13 to n - C 21 range) and higher gammacerane values, which discriminate them from Devonian shale-derived oils. Thermal maturity estimates from equilibrium(?) biomarker isomerization ratios suggest that some of the Devonian source rock samples are at middle to peak oil window conditions (i.e., approximate vitrinite reflectance values of 0.8%–0.9%). This observation requires local to short-distance (<50 mi) lateral migration for emplacement of Devonian-sourced oils into Devonian reservoirs of eastern Ohio and may impact exploration and assessment of petroleum resources in the Upper Devonian Berea Sandstone.

Ohio↗

USGS2021

Oil–source correlation studies in the shallow Berea Sandstone petroleum system, eastern Kentucky

Shallow production of sweet high-gravity oil from the Upper Devonian Berea Sandstone in northeastern Kentucky has caused the region to become the leading oil producer in the state. Potential nearby source rocks, namely, the overlying Mississippian Sunbury Shale and underlying Ohio Shale, are immature for commercial oil generation according to vitrinite reflectance and programmed pyrolysis analyses. We used organic geochemical measurements from Berea oils and solvent extracts from potential Upper Devonian–Mississippian source rocks to better understand organic matter sources, oil–oil and oil–source rock correlations, and thermal maturity in the shallow Berea oil play. Multiple geochemical proxies suggest Berea oils are from one family and from similar source rocks. Oils and organic matter in the potential source rocks are from a marine source based on pristane-to-phytane (Pr/Ph) and terrestrial-to-aquatic ratios, carbon preference index values, n - alkane maxima, C-isotopic composition, and tricyclic terpane and hopane ratios. Any or all of the Devonian to Mississippian black shale source rocks could be potential source rocks for Berea oils based on similarities in oil and solvent extract Pr/n - C 17 and Ph/n - C 18 ratios, sterane distributions, C-isotopic values, and sterane/hopane and tricyclic terpane ratios. Multiple biomarker ratios suggest Berea oils formed at thermal maturities of approximately 0.7% –0.9% vitrinite reflectance. These data require significant updip lateral migration of 30–50 mi from a downdip Devonian black shale source kitchen to emplace low-sulfur oils in the shallow updip oil-play area and indicate that immature source rocks nearby to Berea oil production are not contributing to produced hydrocarbons.

Kentucky↗

USGS2021

Organic petrology and geochemistry of the Sunbury and Ohio Shales in eastern Kentucky and southeastern Ohio

As part of a study to determine the origin of oil and gas in the Berea Sandstone in northeastern Kentucky and southeastern Ohio, 158 samples of organic-rich shale from the Upper Devonian Olentangy and Ohio Shales and the Lower Mississippian Sunbury Shale, collectively referred to as the “black shale,” were collected and analyzed from 12 cores. The samples were analyzed for total organic carbon ( TOC ) content, organic petrography, and programmed pyrolysis. Previously acquired analytical data for 11 samples from 2 additional wells in eastern Kentucky were also used. Most of the samples were organic rich (>5 wt. % TOC ), high in sulfur (>2.0 wt. %), and dominated by liptinite macerals. The vitrinite reflectance ( VR o ) and equivalent vitrinite reflectance ( VR eq ) values, calculated from bitumen reflectance ( BR o ) measurements, were found to be in close agreement. The calculated reflectance values from programmed pyrolysis temperature at which the maximum release of hydrocarbons occurs ( T max ) showed better agreement with measured VR o after T max was corrected for excessive hydrogen index values for several samples. Thermal maturation parameters were found to increase in a northwest–southeast direction, paralleling an increase in black shale thickness and depth of burial. The thermal maturity proxies indicate the northwestern part of the study area to be more thermally mature than previously indicated. Geochemical and biomarker data from Berea oils indicate migration of oil from more thermally mature to less thermally mature areas. As such, the occurrence of petroleum liquids in the Berea Sandstone cannot be predicted directly from conventional thermal maturity proxies ( T max , VR o , and BR o ) because these methods do not account for migrated petroleum.

Ohio, Kentucky↗

USGS2021

Molecular and isotopic gas composition of the Devonian Berea Sandstone and implications for gas evolution, eastern Kentucky

Since 2011, the Devonian Berea Sandstone in northeastern Kentucky has produced oil where thermal maturity studies indicate that likely source rocks, namely, the Devonian Ohio Shale and Mississippian Sunbury Shale, are thermally immature. Downdip, where source rocks are mature for oil, the Berea Sandstone and Ohio Shale primarily produce gas. To investigate this thermal maturity discordancy, the molecular and isotopic composition of gases from the Berea Sandstone (oil associated) and Ohio Shale (nonassociated) were analyzed to understand the gas generation and migration history. Collected along a northwest-southeast transect in eastern Kentucky, samples range from 1079 to 4602 ft, respectively. All are wet gases with a thermogenic origin (δ 13 C-CH 4 = −53.5‰ to −46.1‰). This is mostly consistent with a reevaluation of thermal maturity in a companion study that expands the area mature for oil. Gas migration is required, however, in updip parts of the Berea play where vitrinite reflectance ( VR o ) values are less than 0.6%. Southeast regional dip exerts a first-order influence on thermal maturity as gases become drier and isotopically heavier downdip. Correlation of δ 13 C values for heavier hydrocarbon gases in a natural gas plot with VR o contour spacing demonstrates the influence of nearby source rock thermal maturity on gas composition. Downdip, migration of oil and the attendant increase in permeability relative to gas may account for the dominance of gas production in the Ohio Shale. Migration along with basin uplift after the Alleghany orogeny may have contributed to development of a gas phase that exsolved from oil.

Kentucky↗

USGS2021

The Confocal Laser Scanning Microscopy Working Group of the ICCP: Final report 2021

This report summarizes the activities and results of the Confocal Laser Scanning Microscopy (CLSM) working group (WG) of the International Committee for Coal and Organic Petrology (ICCP), from its inception in September, 2015, to the present day (September, 2021). The purpose of this report is to document the history of the working group and to compile and evaluate its results. The CLSM WG examined an immature, organic-rich sample of Kimmeridge Clay, which was characterized via CLSM imaging and spectroscopy. In addition, mechanically polished and broad ion beam (BIB) milled sample preparations were characterized via atomic force microscopy. Highlights of findings from the CLSM WG include: the interpreted presence of Botryococcus; incomplete blocking of laser light from highly reflective materials; surface roughening and surface flattening induced by differential BIB milling dependent on location and scale of measurement; substitution of uranium for iron in sulfides; red-shift of reflectance and auto-fluorescence from below the sample surface; positive alteration from laser-induced photo-oxidation of the sample surface including fluorescence blue-shift; blueshift associated to higher fluorescence intensity regions in amorphous organic matter; need for fluorescence spectroscopy standardization as applied via CLSM; and the suitability of CLSM to predict solid bitumen reflectance via calibration to an extant data set. Due to the inability of WG members to continue participating in a WG format, the CLSM WG is hereby finalized. This report represents the final product of WG activity, with the aim to summarize the information included herein for a future peer-reviewed manuscript.

Final Report↗

USGS2020

Photoluminescence imaging of whole zircon grains on a petrographic microscope—An underused aide for geochronologic studies

The refractory nature of zircon to temperature and pressure allows even a single zircon grain to preserve a rich history of magmatic, metamorphic, and hydrothermal processes. Isotopic dating of micro-domains exposed in cross-sections of zircon grains allows us to interrogate this history. Unfortunately, our ability to select the zircon grains in a heavy mineral concentrate that records the most geochronologic information is limited by our inability to predict internal zonation from observations of whole grains. Here we document the use of a petrographic microscope to observe and image the photoluminescence (PL) response of whole zircon grains excited under ultraviolet (UV) light, and the utility of this PL response in selecting grains for geochronology. While zircon fluorescence has long been known, there is limited documentation of its utility for and application to geochronologic studies. Our observations of zircon from an un-metamorphosed igneous rock, two meta-igneous rocks, and a placer deposit show that variations in the PL color are readily observable in real-time, both among grains in a population of zircons and within single grains. Analyses of cross-sections of the same grains demonstrate that the changes in PL correlate with zoning in backscattered electron (BSE) and cathodoluminescence (CL) images as well as with changes in U + Th concentration and spectroscopic proxies for radiation damage. In other words, the whole grain PL provides a low-resolution preview of the U + Th zoning expected in a cross-sectioned grain. We demonstrate the usefulness of this “preview” in identifying and selecting the subset of zircon grains in a heavy mineral separate that has metamorphic rims of sufficient width to date by secondary ionization mass spectrometry (SIMS). The data are also used to place preliminary constraints on the age and U + Th concentrations at which a yellow PL response is observed in natural samples. The PL response of zircon is well-known among spectroscopists, and these simple applications demonstrate several ways in which the response might be more effectively used by geochronologists.

Minerals↗

USGS2020

Nanoscale molecular composition of solid bitumen from the Eagle Ford Group across a natural thermal maturity gradient

Microscopic solid bitumen is a petrographically defined secondary organic matter residue produced during petroleum generation and subsequent oil transformation. The presence of solid bitumen impacts many reservoir properties including porosity, permeability, and hydrocarbon generation and storage, among others. Furthermore, solid bitumen reflectance is an important parameter for assessing the thermal maturity of formations with little to no vitrinite. While the molecular composition of solid bitumen will strongly impact associated parameters such as the development of organic matter porosity, hydrocarbon generation, and optical reflectance, assessing the molecular composition of solid bitumen in situ within shale reservoirs can be challenging due to the small grain sizes (often ≤1 μm in diameter) and the inherent heterogeneity of shale formations. Here we employ the recently developed atomic force microscopy based infrared spectroscopy (AFM-IR) technique to investigate solid bitumen molecular composition in situ within shale samples from the Late Cretaceous Eagle Ford Group. These samples possess sulfur-rich type II kerogens that span a natural thermal maturity gradient from early oil generation to the dry gas window. The application of AFM-IR allows for the rapid collection of thousands of compositional measurements from solid bitumen with ∼50 nm resolution. Our results indicate that (i) solid bitumen from the lower Eagle Ford displays both intra- and intergranular variation in the relative abundance of CH 2 , C═C, and C═O moieties present; (ii) this molecular variation tends to, but does not always, decrease with an increase in thermal maturity; and (iii) the solid bitumen composition between samples, from an atomic ratio perspective, is more similar than analysis of bulk kerogen isolates would indicate. These findings are discussed with perspective toward understanding the impact of thermal stress on the composition of secondary organic matter within the Eagle Ford Shale and highlight the growing awareness that organic matter heterogeneity within petroliferous mudrocks extends down to the nanoscale regime.

Energy and Fuels↗

USGS2020

Evidence of cosmic impact at Abu Hureyra, Syria at the Younger Dryas Onset (~12.8 ka): High-temperature melting at >2200 °C

At Abu Hureyra (AH), Syria, the 12,800-year-old Younger Dryas boundary layer (YDB) contains peak abundances in meltglass, nanodiamonds, microspherules, and charcoal. AH meltglass comprises 1.6 wt.% of bulk sediment, and crossed polarizers indicate that the meltglass is isotropic. High YDB concentrations of iridium, platinum, nickel, and cobalt suggest mixing of melted local sediment with small quantities of meteoritic material. Approximately 40% of AH glass display carbon-infused, siliceous plant imprints that laboratory experiments show formed at a minimum of 1200°–1300 °C; however, reflectance-inferred temperatures for the encapsulated carbon were lower by up to 1000 °C. Alternately, melted grains of quartz, chromferide, and magnetite in AH glass suggest exposure to minimum temperatures of 1720 °C ranging to >2200 °C. This argues against formation of AH meltglass in thatched hut fires at 1100°–1200 °C, and low values of remanent magnetism indicate the meltglass was not created by lightning. Low meltglass water content (0.02–0.05% H 2 O) is consistent with a formation process similar to that of tektites and inconsistent with volcanism and anthropogenesis. The wide range of evidence supports the hypothesis that a cosmic event occurred at Abu Hureyra ~12,800 years ago, coeval with impacts that deposited high-temperature meltglass, melted microspherules, and/or platinum at other YDB sites on four continents.

Abu Hureyra↗