Elucidation of the Alum Shale kerogen structure using a multi-disciplinary approach

The significance and validity of integrating data obtained from a variety of analytical techniques to understand, elucidate and model kerogen's complex chemical structure is reported here using degradative (open and closed system pyrolysis, chemical oxidation), non-degradative (¹³C CP/MAS NMR) and optical (incident white light and blue light) methods. Seven Cambrian Alum Shale samples, ranging in maturity from immature to post-mature with respect to petroleum generation, were studied and were chosen for their simple geological history, uniform organic matter type and high organic carbon content. The Alum Shale kerogens, which primarily consist of algal organic matter, liberate low molecular weight gaseous and aromatic compounds on pyrolysis and give mostly branched dicarboxylic acids on chemical oxidation. ¹³C NMR spectroscopy shows that the Alum Shale kerogens are anomalously rich in oxygen-bearing functional groups (such as C = O, ArCO, CHO, CH_xO), most of which apparently remain intact within the kerogen macro-molecule (KMM) through the diagenetic and catagenetic stages. Fragments released by different degradative techniques are quantified and the aromaticity (f_a), O/C and relative proportions of various carbon types estimated by ¹³C NMR. A synthesis of these data has allowed us to better understand the chemistry of the Alum Shale kerogen.     

Environmental control of carbon isotope variations in Pennsylvania black-shale sequences, Midcontinent, U.S.A.

A reversal of the conventional carbon isotope relationship, “terrestrial-lighter-than-marine” organic matter, has been documented for two Pennsylvanian (Desmoinesian) cyclothemic sequence cores from the Midcontinent craton of the central United States. “Deep” water organic-rich phosphatic black shales contain a significant proportion of algal-derived marine organic matter (as indicated by organic petrography, Rock-Eval hydrogen index and ratios) and display the lightest δ¹³C-values (max −27.80‰ for kerogen) while shallower water, more oxic facies (e.g. fossiliferous shales and limestones) contain dominantly terrestrial organic matter and have heavier δ¹³C_kerogen-values (to −22.87‰ for a stratigraphically adjacent coal). δ¹³C-values for extract fractions were relatively homogeneous for the organic-rich black shales with the lightest fraction (often the aromatics) being only 1‰, or less, more negative than the kerogen. Differences between extract fractions and kerogens were much greater for oxic facies and coals (e.g. saturates nearly 5‰ lighter than the kerogen).A proposed depositional model for the black shales calls upon a large influx of nutrients and humic detritus to the marine environment from the laterally adjacent, extremely widespread Pennsylvanian (peat) swamps which were rapidly submerged by transgression of the epicontinental seas. In this setting marine organisms drew upon a CO₂-reservoir which was in a state of disequilibrium with the atmosphere, being affected by isotopically light “recycled-CO₂” derived from the decomposition of peaty material in the water column and possibly from the anoxic diagenesis of organic matter in the sediments.     

Characterization of Estonian Kukersite by spectroscopy and pyrolysis: Evidence for abundant alkyl phenolic moieties in an Ordovician, marine, type II/I kerogen

The kerogen of a sample of Estonian Kukersite (Ordovician) was examined by spectroscopic (solid state ¹³C NMR, FTIR) and pyrolytic (“off-line”, flash) methods. This revealed an important contribution of long, linear alkyl chains in Kukersite kerogen. The hydrocarbons formed upon pyrolysis are dominated by n-alkanes and n-alk-1-enes and probably reflect a major contribution of selectivity preserved, highly aliphatic, resistant biomacromolecules from the outer cell walls of Gloeocapsomorpha prisca. This is consistent with the abundant presence of this fossilized organism in Kukersite kerogen. In addition high amounts of phenolic compounds were identified in the pyrolysates. Series of non-methylated, mono-, di- and trimethylated 3-n-alkylphenols, 5-n-alkyl-1,3-benzenediols and n-alkylhydroxybenzofurans were identified. All series of phenolic compounds contain long (up to C₁₉), linear alkyl side-chains. Kukersite kerogen is, therefore, an aliphatic type II/I kerogen, despite the abundance of free phenolic moieties. This study shows that phenol-derived moieties are not necessarily associated with higher plant-derived organic matter.The flash pyrolysate of Kukersite kerogen was also compared with that of the kerogen of the Guttenberg Oil Rock (Ordovician) which is also composed of accumulations of fossilized G. prisca. Similarities in the distributions of hydrocarbons and sulphur compounds were noted, especially for the C₁–C₆ alkylbenzene and alkylthiophene distributions. However, no phenolic compounds were detected in the flash pyrolysate of the Guttenberg kerogen. Possible explanations for the observed similarities and differences are discussed.     

Primary cracking of kerogens. Experimenting and modelling C1, C2–C5, C6–C15 and C15+ classes of hydrocarbons formed

Mathematical models of hydrocarbon formation can be used to simulate the natural evolution of different types of organic matter and to make an overall calculation of the amounts of oil and/or gas produced during this evolution. However, such models do not provide any information on the composition of the hydrocarbons formed or on how they evolve during catagenesis.From the kinetic standpoint, the composition of the hydrocarbons formed can be considered to result from the effect of “primary cracking” reactions having a direct effect on kerogen during its evolution as well as from the effect of “secondary cracking” acting on the hydrocarbons formed.This report gives experimental results concerning the “primary cracking” of Types II and III kerogens and their modelling. For this, the hydrocarbons produced have been grouped into four classes (C₁, C₂–C₅, C₆–C₁₅ and C₁₅₊). Experimental data corresponding to these different classes were obtained by the pyrolysis of kerogens with temperature programming of 4°C/min with continuous analysis, during heating, of the amount of hydrocarbons corresponding to each of these classes.The kinetic parameters of the model were optimized on the basis of the results obtained. This model represents the first step in the creation of a more sophisticated mathematical model to be capable of simulating the formation of different hydrocarbon classes during the thermal history of sediments. The second step being the adjustment of the kinetic parameters of “secondary cracking”.     

Calcareous nannoplankton, planktonic foraminiferal, and carbonate carbon isotope stratigraphy of the Cenomanian–Turonian boundary section in the Ultrahelvetic Zone (Eastern Alps, Upper Austria)

Ultrahelvetic units of the Eastern Alps were deposited on the distal European continental margin of the (Alpine) Tethys. The Rehkogelgraben section (“Buntmergelserie”, Ultrahelvetic unit, Upper Austria) comprises a 5 m thick succession of upper Cenomanian marl-limestone cycles overlain by a black shale interval composed of three black shale layers and carbonate-free claystones, followed by lower Turonian white to light grey marly limestones with thin marl layers. The main biostratigraphic events in the section are the last occurrence of Rotalipora and the first occurrences of Helvetoglobotruncana helvetica and Quadrum gartneri. The thickest black shale horizon has a TOC content of about 5%, with predominantly marine organic matter of kerogen type II. Vitrinite reflectance and Rock-Eval parameter T_max (<424 °C) indicate low maturity. HI values range from 261 to 362 mg HC/g TOC. δ¹³C values of bulk rock carbonates display the well documented positive shift around the black shale interval, allowing correlation of the Rehkogelgraben section with other sections such as the Global Boundary Stratotype Section and Point (GSSP) succession at Pueblo, USA, and reference sections at Eastbourne, UK, and Gubbio, Italy. Sediment accumulation rates at Rehkogelgraben (average 2.5 mm/ka) are significantly lower than those at Pueblo and Eastbourne.     

Application of the pyrolysis-carbon isotope method for determining the maturity of kerogen in the Bakken shale

A maturity indexing procedure based on the isotopic difference between the total accumulated methane produced by exhaustive pyrolysis and the kerogen (Δ¹³C) and the mole ratio of methane to kerogen carbon (CMR), has been tested by applying a standardized technique, i.e. exhaustive pyrolysis (600°C for 120 hr) of extracted-powdered samples and measurement of the amounts and isotopic composition of the methane and kerogen carbon, on a suite of 15 Bakken shale samples.A linear relation was found between the carbon mole ratio of pyrolysis-derived methane and total organic carbon and the δ¹³C difference between the pyrolysis-derived methane and total organic carbon (r = −0.79); and between the amount of CH₄ generated from exhaustive pyrolysis and H/C atomic ratios (r = +0.91).     

Organic matter in a low productivity anoxic intraplatform basin in the Triassic Tethys

This paper presents isotopic, bulk geochemical and biomarker data measured on organic matter accumulated in a narrow extensional basin developed at the oceanward margin of the huge Triassic carbonate platform in the Alps–Appenines domain. The integration of isotope signatures, organic petrographical and biomarker evidence together with the composition of kerogen pyrolysates suggests immature organic matter predominantly of algal origin with a minor, but not negligible, higher plant derived and moderate bacterial contribution for the entire sequence. The mineral sources are dominated by platform-derived subtidal Dachstein limestone with a minor palaeosol input and a moderate contribution of autochtonous quartz. Nevertheless, parallel variations observed in the mineral content, as well as in the amount and the quality of the organic matter reflect variations in the palaeoenvironment. The increased humidity, existing in the period of the accumulation of the upper section of the sequence, led to the restriction of dolomitization. A slightly greater higher plant derived contribution, in this section, is evidenced by the composition of bitumen and the results on GC and GC/MS analyses on the non-aromatic hydrocarbon fraction of bitumen. Moreover, the climate-induced weathering enchanced the primary productivity and resulted in a pronounced increase in the TOC content. The average estimated value of the planktonic productivity is about three times higher for the calcite-rich sequence than the dolomite-rich one, being 44.2 and 15.3 tC_org/m²/Ma, respectively. The low to moderate planktonic productivity shows that anoxic conditions, observed for the entire succession, are a consequence of the stagnant water stratification rather than high planktonic productivity. Depth trends in the data measured on kerogens (HI, OI, δ¹³Corg values, composition of pyrolysate) together with the δ¹⁸O excursions and Δδ¹³C values appear to be controlled by sea-level fluctuations. Consistent with the high abundance of alkyltiophenes in kerogen pyrolysates, the high S_org/C ratios (ranging between 0.05 and 0.10) suggest the importance of natural sulfurization in the formation of the sulfur-rich type-II-S kerogen occurring in all of the samples.     

Effects of thermal maturation on stable organic carbon isotopes as determined by hydrous pyrolysis of Woodford Shale

Acquiring crude oils that have been expelled from the same rock unit at different levels of thermal maturation is currently not feasible in the natural system. This prevents direct correlation of compositional changes between the organic matter retained in a source rock and its expelled crude oil at different levels of thermal maturation. Alleviation of this deficiency in studying the natural system requires the use of laboratory experiments. Natural generation of petroleum from amorphous type-II kerogen in the Woodford Shale may be simulated by hydrous pyrolysis, which involves heating crushed rock in contact with water at subcritical temperatures (<374°C). Four distinct stages of petroleum generation are observed from this type of pyrolysis; (1) pre-oil generation, (2) incipient-oil generation, (3) primary-oil generation, and (4) post-oil generation.The effects of thermal maturation on the δ¹³C values of kerogen, bitumen, and expelled oil-like pyrolysate from the Woodford Shale have been studied through these four stages of petroleum generation. Similar to the natural system, the kerogens isolated from the pyrolyzed rock showed no significant change in δ¹³C. This suggests that the δ¹³C value of kerogens may be useful in kerogen typing and oil-to-source rock correlations. δ¹³C values of bitumens extracted from the pyrolyzed rock showed an initial decrease during the incipient-oil generation stage, followed by depletion during the primary- and post-oil generation stages. This reversal is not favorable for geochemical correlation or maturity evaluation. Saturated and polar components of the bitumen show the greatest δ¹³C variations with increasing thermal maturation. The difference between the δ¹³C of these two components gives a unidirectional trend that serves as a general indicator of thermal maturation and is referred to as the bitumen isotope index (BII).δ¹³C values of the expelled pyrolysates show a unidirectional increase with increasing thermal maturation. The constancy and similarity of δ¹³C values of the aromatic components in the expelled pyrolysates and bitumens, with increasing thermal maturation, encourages their use in oil-to-oil and oil-to-source rock correlations. Isotopic type-curves for expelled pyrolysates indicate that they may be useful in oil-to- oil correlations, but have a limited use in oil-to-source rock correlations.     

A molecular dynamics study of natural organic matter: 1. Lignin, kerogen and soot

Molecular dynamics (MD) simulations were performed on molecular models of a spectrum of natural organic matter (NOM) samples represented by two lignin samples (a softwood lignin and a hardwood lignin), a kerogen (Green River Shale kerogen) and a soot sample (n-hexane soot). Simulated thermodynamic properties of each model, including glass transition temperature (T_g), thermal expansion coefficient (α), density (ρ) and solubility parameter (δ) were compared against experimental data for corresponding samples. Results revealed relatively good agreement for glass transition temperature and solubility parameter for softwood lignin, Green River Shale kerogen and n-hexane soot models. An unexpectedly low solubility parameter for a hardwood lignin model suggests, however, certain model deficiencies in terms of intermolecular interactions. In addition, a lower density for a n-hexane soot model relative to the sample was attributed to the small cluster size and poor parallel stacking of aromatic clusters in the model. Discussion of the results is provided in the context of utilizing thermodynamic properties as constraints for improved structural modeling of NOM.     

Uranium mineralization in the Alum Shale Formation (Sweden): Evolution of a U-rich marine black shale from sedimentation to metamorphism

The Alum Shale Formation is a metal-rich black shale, deposited on the Baltoscandian platform between Middle Cambrian and Early Ordovician. These black shales may be of particular economic interest for their relatively high uranium content (100–300 ppm) and their wide distribution from Norway to Estonia. Scandinavian Alum Shale may thus constitute a great potential resource of uranium, as a low grade ore. The Alum Shale Formation is particularly interesting to study the mineralogical expression and content of uranium in series submitted to progressive burial and metamorphism. For this purpose, the behavior of U, P, Ti and organic matter was studied on a series of representative samples from most Alum Shale prospection zones. In southern Sweden, where Alum Shale underwent fairly shallow burial, uranium concentrations have no mineralogical expression except a rather high U content of biogenic phosphates. Calcite concretions (beefs) and fractures recorded the migration of hot overpressured hydrocarbons and brines from thermally mature areas to immature Alum Shale. However, thermal maturation and fluid migration did not allow remobilization of uranium and metals. At the opposite, in northern Sweden, where the series were folded, duplicated and submitted to low grade Greenschist metamorphism during Caledonian orogeny, phospho-silicates U-Si-Ca-P (±Ti ±Zr ±Y) and minor amounts of uraninite are identified and indicate that U, P, and Ti were mobile and precipitated as new phases. The effect of metamorphism is therefore important to consider as the leachability of U, especially during (bio)-hydrometallurgical processes, which will be by far different between the two considered areas.     

Comparison of organic geochemistry and metal enrichment in two black shales: Cambrian Alum Shale of Sweden and Devonian Chattanooga Shale of United States

In most black shales, such as the Chattanooga Shale and related shales of the eastern interior United States, increased metal and metalloid contents are generally related to increased organic carbon content, decreased sedimentation rate, organic matter type, or position in the basin. In areas where the stratigraphic equivalents of the Chattanooga Shale are deeply buried and and the organic material is thermally mature, metal contents are essentially the same as in unheated areas and correlate with organic C or S contents. This paradigm does not hold for the Cambrian Alum Shale Formation of Sweden where increased metal content does not necessarily correlate with organic matter content nor is metal enrichment necessarily related to land derived humic material because this organic matter is all of marine source. In southcentral Sweden the elements U, Mo, V, Ni, Zn, Cd and Pb are all enriched relative to average black shales but only U and Mo correlate to organic matter content. Tectonically disturbed and metamorphosed allochthonous samples of Alum Shale on the Caledonian front in western Sweden have even higher amounts for some metals (V, Ni, Zn and Ba) relative to the autochthonous shales in this area and those in southern Sweden.     

Comparative studies of the kinetic parameters of various algaenans and kerogens via open-system pyrolyses

Kinetic parameters were determined for the first time, via open-system pyrolyses, on algaenans (highly resistant biomacromolecules that are selectively preserved during kerogen formation) isolated from extant microalgae. Parallel studies were also carried out on 10 kerogens exhibiting, with one exception, a low level of maturity. These kerogens included samples chiefly derived from the selective preservation of the above algaenans and samples mainly, or almost exclusively, derived from the “natural vulcanization” pathway. Important differences in activation energy (E_a) distributions were observed between the four algaenans investigated and correlated with their chemical structures. The kerogens predominantly derived from algaenan-selective preservation (Pula alginite, NE 70 and BJ 248 Torbanites, Rundle Oil Shale) also exhibited pronounced differences in E_a distributions. These distributions provided: (i) information on the diversity of the source materials; and (ii) reflected the occurrence of important differences in chemical structures and thermal behaviour between three of the tested kerogens, even though they are all classified as low maturity type I. The Kimmeridge Clay samples and the Lorca Oil Shale showed broad E_a distributions shifted to low energies when compared with the above algaenans and kerogens. Such shifts reflect an important (or even almost exclusive for some of these kerogens) contribution of materials originating from sulphur incorporation into various lipids during early diagenesis. Finally, the kinetic data derived for the nine low maturity fossil samples were extrapolated to a very low, geological heating rate of 3°C Ma⁻¹ and the generation rate curves and cumulative yield curves thus obtained were compared.     

Primary alteration—oxidation of marine algal organic matter from oil source rocks of the North Sea and Norwegian Arctic: new findings

The presence of partially oxidized algal organic matter in oil-prone marine source rocks, is the rule rather than the exception. Partially oxidized, algal kerogen can still act as a significant source of liquid hydrocarbons. However, the corresponding peak of C₁₂ + hydrocarbon generation is shifted to a considerably lower maturity level compared with that of the classical Type II kerogen. The extent of primary alteration-oxidation of marine algal kerogen is monitored by means of solid state microfluorescence spectroscopy. A new parameter, the Primary Alteration Factor (PAF) is established, and the relationships between PAF and H/C, O/C, HI, TOC and between PAF and %₀δ¹³C are determined. The present data show large variations in the bulk chemistry of immature marine algal kerogens, and reveal evidence for gradational dehydrogenation/oxidation of the source organic matter. This contrasts with the recently proposed mechanism for kerogen formation. SEM analysis reveals a relationship between the physical breakdown of algal organic matter and the formation of liptodetrinite. FTIR analysis shows that the incorporation of primary oxygen in the kerogen macromolecules is not in the form of carbonyl or carboxyl functionalities. The presence of highly unreactive, stable oxygen, associated with aromatic structures in partially oxidized algal kerogen, is suggested by resistance of the kerogen to graphitization. The FTIR data also suggest the presence of aryl ether oxygen. The present findings raise fundamental questions regarding the mechanisms of kerogen cracking and kerogen formation, and have important implications for petroleum exploration.     

Pyrolysis-GC characterization of whole rock and kerogen-concentrate samples of immature Jurassic source rocks from NW-Germany

Nine rock samples from three Jurassic stratigraphic units of a shallow core from NW Germany were analyzed by pyrolysis-gas chromatography. The units contain a mixed Type-II/III kerogen (Dogger-α), a hydrogen-rich Type-II kerogen (Lias-), and a hydrogen-poor Type-III kerogen (Lias-δ). All of the kerogen was immature (R_o = 0.5%). Two sets of kerogen concentrates (“AD”: HCl/HF followed by a density separation, and “A”: only acid treatment) prepared from the rock samples were also analyzed to make a detailed comparison of the pyrolysates of rock and corresponding kerogen-concentrates.Hydrogen-index (HI) values of the kerogen concentrates prepared from organic-carbon poor rock were nearly 200% higher than HI values of the rock samples. Changes in HI were minimal for the samples containing Type-II kerogen. The A and AD samples from the C_org-poor rock yielded pyrolysates with n-alkane series of very different molecular lengths. Pyrograms of the rock samples had n-alkane series extending to n-C₁₄; the chromatograms of the A samples reached the n-C₁₄-nC₂₀ range. The AD samples from C_org-poor rock and all three sample types from the C_org-rich rock had n-alkane series up to n-C₂₉. The benzene/hexane and toluene/heptane ratios for the C_org-poor rock and A samples were far higher than for the AD samples, which had ratios similar to those of all three sample types from the C_org-rich rocks. These results indicate that choice of kerogen preparation method is critical when C_org-poor samples are analyzed.     

The shale gas potential of Tournaisian, Visean, and Namurian black shales in North Germany: baseline parameters in a geological context

Carboniferous black mudrocks with known petroleum potential occur throughout Northern Germany. However, despite numerous boreholes exploring for conventional hydrocarbons, the potential for shale gas resources remains uncertain. Therefore, an integrated investigation was conducted to elucidate the shale gas potential for three different Carboniferous facies incorporating baseline parameters from sedimentological and organic-geochemical analyses. Tournaisian–Namurian fine-grained rocks of the Culm-facies, with Type II + III kerogen were deposited in the basin center. TOC contents of up to 7 % occur in the Lower Alum Shale (3.6 % VRr) and up to 6 % in the Upper Alum Shale (4.4 % VRr). Bands of organic-rich black shales, reflecting sea-level variations controlled by global eustatic cycles, occur within the Tournaisian–Visean “Kohlenkalk”-facies north of the Rhenish Slate Mountains and in the Rügen island area. In both areas the organic matter is characterized by a kerogen Type II + III with TOC contents of up to 7 % and maturities of up to 4.2 and 1.8 % VRr, respectively. Black hemipelagites intercalated with coarser-grained silt- and sandstones occur in the Synorogenic Flysch Formation of the Namurian A along the southern basin margin. TOC contents vary from 0.5 to 2.0 % with Type III kerogen dominated organic matter and maturities of up to 2.5 % VRr. The baseline parameters presented in this paper indicate a shale gas potential for the sediments of the Culm-facies on the southern basin margin and of the “Kohlenkalk”-facies in the Rügen area.     

Comparison of natural gases accumulated in Oligocene strata with hydrous pyrolysis gases from Menilite Shales of the Polish Outer Carpathians

This study examined the molecular and isotopic compositions of gases generated from different kerogen types (i.e., Types I/II, II, IIS and III) in Menilite Shales by sequential hydrous pyrolysis experiments. The experiments were designed to simulate gas generation from source rocks at pre-oil-cracking thermal maturities. Initially, rock samples were heated in the presence of liquid water at 330 °C for 72 h to simulate early gas generation dominated by the overall reaction of kerogen decomposition to bitumen. Generated gas and oil were quantitatively collected at the completion of the experiments and the reactor with its rock and water was resealed and heated at 355 °C for 72 h. This condition simulates late petroleum generation in which the dominant overall reaction is bitumen decomposition to oil. This final heating equates to a cumulative thermal maturity of 1.6% R_r, which represents pre-oil-cracking conditions. In addition to the generated gases from these two experiments being characterized individually, they are also summed to characterize a cumulative gas product. These results are compared with natural gases produced from sandstone reservoirs within or directly overlying the Menilite Shales. The experimentally generated gases show no molecular compositions that are distinct for the different kerogen types, but on a total organic carbon (TOC) basis, oil prone kerogens (i.e., Types I/II, II and IIS) generate more hydrocarbon gas than gas prone Type III kerogen. Although the proportionality of methane to ethane in the experimental gases is lower than that observed in the natural gases, the proportionality of ethane to propane and i-butane to n-butane are similar to those observed for the natural gases. δ¹³C values of the experimentally generated methane, ethane and propane show distinctions among the kerogen types. This distinction is related to the δ¹³C of the original kerogen, with ¹³C enriched kerogen generating more ¹³C enriched hydrocarbon gases than kerogen less enriched in ¹³C. The typically assumed linear trend for δ¹³C of methane, ethane and propane versus their reciprocal carbon number for a single sourced natural gas is not observed in the experimental gases. Instead, the so-called “dogleg” trend, exemplified by relatively ¹³C depleted methane and enriched propane as compared to ethane, is observed for all the kerogen types and at both experimental conditions. Three of the natural gases from the same thrust unit had similar “dogleg” trends indicative of Menilite source rocks with Type III kerogen. These natural gases also contained varying amounts of a microbial gas component that was approximated using the Δδ¹³C for methane and propane determined from the experiments. These approximations gave microbial methane components that ranged from 13–84%. The high input of microbial gas was reflected in the higher gas:oil ratios for Outer Carpathian production (115–1568 Nm³/t) compared with those determined from the experiments (65–302 Nm³/t). Two natural gas samples in the far western part of the study area had more linear trends that suggest a different organic facies of the Menilite Shales or a completely different source. This situation emphasizes the importance of conducting hydrous pyrolysis on samples representing the complete stratigraphic and lateral extent of potential source rocks in determining specific genetic gas correlations.     

Fingerprinting Marcellus Shale waste products from Pb isotope and trace metal perspectives

Drill cuttings generated during unconventional natural gas extraction from the Marcellus Shale, Appalachian Basin, U.S.A., generally contain a very large component of organic-rich black shale because of extensive lateral drilling into this target unit. In this study, element concentrations and Pb isotope ratios obtained from leached drill cuttings spanning 600 m of stratigraphic section were used to assess the potential for short and long term environmental impacts from Marcellus Shale waste materials, in comparison with material from surrounding formations. Leachates of the units above, below and within the Marcellus Shale yielded Cl/Br ratios of 100–150, similar to produced water values. Leachates from oxidized and unoxidized drill cuttings from the Marcellus Shale contain distinct suites of elevated trace metal concentrations, including Cd, Cu, Mo, Ni, Sb, U, V and Zn. The most elevated Mo, Ni, Sb, U, and V concentrations are found in leachates from the lower portion of the Marcellus Shale, the section typically exploited for natural gas production. In addition, lower ²⁰⁷Pb/²⁰⁶Pb ratios within the lower Marcellus Shale (0.661–0.733) provide a distinctive fingerprint from formations above (0.822–0.846) and below (0.796–0.810), reflecting ²⁰⁶Pb produced as a result of in situ ²³⁸U decay within this organic rich black shale. Trace metal concentrations from the Marcellus Shale leachates are similar to total metal concentrations from other black shales. These metal concentrations can exceed screening levels recommended by the EPA, and thus have the potential to impact soil and water quality depending on cuttings disposal methods.     

Effect of maturity on carbon isotope ratios of oils and condensates

For modelling isotopic variations in oils it is convenient to differentiate the effects of oil generation ( 100–150°C) from the effects of oil to gas cracking ( 150–180°C). During generation, δ¹³C of kerogen may increase by up to 1% due to release of isotopically light oil and gas, although most kerogens show little or no chan δ¹³C of the generated oil increases by between 0 and 1% (av. 0.5%) due to mixing of isotopically heavy oil with an initial isotopically light unbound fraction, possibly of bacterial origin. The change occurs mostly over the first 20% of generation. During oil to gas cracking, kinetic isotope effects become important and the effect on δ¹³C of the remaining oil can be modelled as a Rayleigh process. δ¹³C increases by 1.5% by 50% cracking. Insufficient data are available to calibrate the effects at higher levels of cracking, and modelling these variations is hindered by a lack of understanding of the mechanism of pyrobitumen formation. However, increases greater than about 4% are unlikely to be observed. With increasing maturity, the low molecular weight fractions become isotopically heavy faster than the high molecular weight fractions. As a result, any separation of the low molecular weight fraction into a gas phase (“condensate formation”) will produce an isotopic difference between oil and condensate that depends on maturity. In the early stages of generation the condensate may be up to 1% lighter than the remaining oil. With increasing maturity, this difference at first decreases and then increases in the opposite sense. By half way through oil to gas cracking the condensate may be 1.5% heavier than the residual liquid. More subtle rearrangement reactions may result in small, but significant, changes to the shape of the isotope “type-curves” when different oil fractions are compared.     

Dike intrusions into bituminous coal, Illinois Basin: H, C, N, O isotopic responses to rapid and brief heating

Unlike long-term heating in subsiding sedimentary basins, the near-instantaneous thermal maturation of sedimentary organic matter near magmatic intrusions is comparable to artificial thermal maturation in the laboratory in terms of short duration and limited extent. This study investigates chemical and H, C, N, O isotopic changes in high volatile bituminous coal near two Illinois dike contacts and compares observed patterns and trends with data from other published studies and from artificial maturation experiments. Our study pioneers in quantifying isotopically exchangeable hydrogen and measuring the D/H (i.e., ²H/¹H) ratio of isotopically non-exchangeable organic hydrogen in kerogen near magmatic contacts. Thermal stress in coal caused a reduction of isotopically exchangeable hydrogen in kerogen from 5% to 6% in unaltered coal to 2-3% at contacts, mostly due to elimination of functional groups (e.g., OH, COOH, NH₂). In contrast to all previously published data on D/H in thermally matured organic matter, the more mature kerogen near the two dike contacts is D-depleted, which is attributed to (i) thermal elimination of D-enriched functional groups, and (ii) thermal drying of hydrologically isolated coal prior to the onset of cracking reactions, thereby precluding D-transfer from relatively D-enriched water into kerogen. Maxima in organic nitrogen concentration and in the atomic N/C ratio of kerogen at a distance of ∼2.5 to ∼3.5 m from the thicker dike indicate that reactive N-compounds had been pyrolytically liberated at high temperature closer to the contact, migrated through the coal seam, and recombined with coal kerogen in a zone of lower temperature. The same principle extends to organic carbon, because a strong δ¹³C_kerogen vs. δ¹⁵N_kerogen correlation across 5.5 m of coal adjacent to the thicker dike indicates that coal was functioning as a flow-through reactor along a dynamic thermal gradient facilitating back-reactions between mobile pyrolysis products from the hot zone as they encounter less hot kerogen. Vein and cell filling carbonate is most abundant in highest rank coals where carbonate δ¹³C_VPDB and δ¹⁸O_VSMOW values are consistent with thermal generation of ¹³C-depleted and ¹⁸O-enriched CO₂ from decarboxylation and pyrolysis of organic matter. Lower background concentrations of ¹³C-enriched carbonate in thermally unaffected coal may be linked to ¹³C-enrichment in residual CO₂ in the process of CO₂ reduction via microbial methanogenesis.Our compilation and comparison of available organic H, C, N isotopic findings on magmatic intrusions result in re-assessments of majors factors influencing isotopic shifts in kerogen during magmatic heating. (i) Thermally induced shifts in organic δD values of kerogen are primarily driven by the availability of water or steam. Hydrologic isolation (e.g., near Illinois dikes) results in organic D-depletion in kerogen, whereas more common hydrologic connectivity results in organic D-enrichment. (ii) Shifts in kerogen (or coal) δ¹³C and δ¹⁵N values are typically small and may follow sinusoidal patterns over short distances from magmatic contacts. Laterally limited sampling strategies may thus result in misleading and non-representative data. (iii) Fluid transport of chemically active, mobile carbon and nitrogen species and recombination reactions with kerogen result in isotopic changes in kerogen that are unrelated to the original, autochthonous part of kerogen.     

The average molecular weight and shape of the “polymeric acids” found in Black Trona Water from the Green River Basin

Macromolecular organic material, called “polymeric acids”, has been isolated from Black Trona Water by exhaustive dialysis and characterized as the sodium salt in 0.10 M sodium carbonate, pH 10, by several physico-chemical methods. Analysis by gel filtration chromatography on Sepharose-CL 6B indicates that the “polymeric acids” are polydisperse and composed of species of relatively high molecular weight ( 4 × 10⁵, using proteins as standards). With this method, the range of molecular weights appears to be rather narrow. If “polymeric acids” are transferred from sodium carbonate, pH 10, into distilled water, selfassociation occurs and all species elute in the void volume. The weight-average molecular weight determined in 0.10 M sodium carbonate, pH 10, by the light scattering method is 1.7 × 10⁵. Sedimentation velocity analysis at 20°C with the analytical ultracentrifuge gives a value for S_20,w of 5.4 and the shape of the Schlieren patterns suggest a polydisperse sample with a relatively narrow range of sizes. Analysis of the molecular weight distribution by a sedimentation equilibrium method indicates that the range of molecular weights is 8 × 10⁴ to 2.1 × 10⁵. The partial specific volume ( ) of “polymeric acids” is 0.874 ml/g. Viscosity measurements yield a value for [η] of 2.5 ml/g, which indicates that the “polymeric acids” are compact (spherical or ellipsoidal) in shape.