Regional geology, depositional environment and maturity of organic matter of Early to Middle Jurassic coals, coaly shales, shales and claystones from the Eastern Pontides, NE Turkey

Jurassic coals, coaly shales, shales and claystones from the Eastern Pontides in NE Turkey have been investigated using microscopical, petrophysical and detailed organic geochemical methods in order to determine their catagenetic stage, to reveal the composition of the organic matter and to discuss the depositional environment. The Liassic–Dogger period in the Eastern Pontides was characterised by the presence of a rift system which resulted in rock units of very variable lithology and facies. Coal seams, ranging from a few centimetres to several decimetres and intercalated with shales, claystones and sandstones occur within the basal part (the Aggi Formation) as well as in the uppermost part (the Hamurkesen Formation) of the rift deposits. All coal seams investigated are at a catagenetic maturation stage corresponding to subbituminous B up to low volatile bituminous ranks. They represent a depositional environment of short-lived swamp areas with intense aerobic reworking of the higher plant detritus by heterotrophic bacteria, but with possible anaerobic microenvironments at deeper levels. At least some coal seams were influenced temporarily by marine ingressions. Most samples are impure humic coals with highly variable chemical compositions as indicated by the broad range of their hydrogen contents. This variation in hydrogen content is partly attributed to variable contributions of algal material. On the other side, considering several analytical results, the hydrogen variation is primarily due to bacterial reworking that affected the composition of the organic matter to variable extent and resulted especially in an enrichment of bacterial lipids. Bacterial reworking by an active microbial community within the upper layer of the peat swamp is inferred from elevated concentrations of iso-alkanes even exceeding those of the corresponding n-alkanes in several samples which, to the best of our knowledge, have not been observed with coals before.     

Diagenetic trends of a tertiary low-rank coal series

The Mahakam delta (Kalimantan, Indonesia) coals represent all the evolution stages between freshly-deposited plant/peat material, lignites and bituminous coals. The geochemical techniques used to study this coal series included elemental analysis, extraction of humic compounds, infrared spectroscopy and ¹³C nuclear magnetic resonance of the total coal.The main mechanisms of early maturation in this series are loss of oxygenated compounds, aromatisation and condensation of the organic matter. These changes, which have already been suggested for other coal series and partially reported for sedimentary organic matter, were confirmed and described in more detail for the Mahakam coal series.     

Catagenesis of Humic Coals Inferred from the X-Ray Phase Analysis and Mathematical Modeling

Dynamics of catagenetic changes in the structure of humic coals in the Earth's interior can quantitatively be described using X-ray indices of components (phases), redistribution of which is determined by a system of differential equations in geological time. The system describes changes in the main formation parameters of coal seams in the Earth's interior (subsidence depth, temperature, pressure, and catagenesis index), on the one hand, and kinetics of catagenetic transformation of organic matter (OM), on the other. Such model makes it possible to establish regular changes in the phase composition of humic coals at different stages of coal basin formation. Using Paleozoic (Middle Carboniferous) coals of the Donets Basin as an example, it is established that main changes in the OM structure took place during the maximal subsidence of coal seams at maximal temperatures ranging from 110–15°C (for slightly metamorphosed coals of the L and G ranks) to 28°C (for anthracites) and pressures ranging from 55–74 to 146 MPa, respectively. Major processes leading to the observed X-ray phase composition of coals during the maximal subsidence of sedimentary sequences lasted approximately 40 Ma regardless of the geological age of sediments (Middle Carboniferous or Permian coal seams of the Donets or Kuznetsk coal basins, respectively).     

Evaluation of the liquid hydrocarbon potential of coal by artificial maturation techniques

In certain areas, relatively large accumulations of liquid hydrocarbons have been attributed to coals. Evaluating the source rock potential of coal requires definition of both the generative potential (quantity and composition of generated hydrocarbons), and expulsion efficiency. Hydrous pyrolysis experiments were completed using Tertiary lignites (R_o < 0.35%) from North Dakota and the Far East to evaluate the source rock potential of coal. The North Dakota lignite is vitrinite-rich (93%) and liptinite-poor (3%); the Far East lignite is liptinite-rich (32% of total maceral content). These lignites have Hydrogen Index values of 123 and 483 mg HC/g OC, respectively. Differences in oil-pyrolysate yield, composition, and temperature of maximum pyrolysate yield from hydrous pyrolysis experiments for these two lignites are related to the type and amount of liptinite and vitrinite macerals. A maximum of 48 and 158 mg oil-pyrolysate/g OC is generated and expelled from the North Dakota and Far East lignites, respectively. Although these lignites consist predominantly of gas-prone vitrinitic components, their organic-rich nature can compensate for their poor convertibility to liquid hydrocarbons. The composition of these artificially generated oil-pyrolysates are similar to some non-marine oils, suggesting that this type of organic matter can be a significant contributor to many oils. Although the overall composition of the generated products from the two lignites is similar, the distribution of these products is significantly different. Homologous series of methyl ketones and alkyl benzenes have been identified in both oil-pyrolysates. Their presence and characteristic distribution suggest that microbial degradation occurred during the formation of these lignites. Although many coals generate significate amounts of liquid hydrocarbons that are similar to naturally occurring oils, poor explusion efficiency limits their source rock potential. Significant amounts of liquid products are assimilated by the vitrinitic matrix of most coals prior to expulsion, severely limiting the amount of petroleum available for migration and reservoir accumulation. However, adequate expulsion may occur in certain liptinite-rich coals or coals occurring in unique depositional settings.     

Maturation related changes in the distribution of ester bound fatty acids and alcohols in a coal series from the New Zealand Coal Band covering diagenetic to catagenetic coalification levels

A rank series of lignites and coals of low to moderate maturation levels (vitrinite reflectance (R₀): 0.27–0.8%) from the New Zealand Coal Band were investigated using alkaline ester cleavage experiments to reveal compositional changes of ester bound components (fatty acids and alcohols) during increasing maturation. Ester bound alcohols are found to be present in highest amounts in the very immature lignite samples (R₀: 0.27–0.29%), but show a rapid decrease during early diagenesis. Ester bound fatty acids also show an initial exponential decrease during diagenesis, but reveal an intermittent increase during early catagenesis before decreasing again during main catagenesis. This intermittent increase was related to the short chain fatty acids. To obtain a maturity related signal and to eliminate facies related scattering in the amounts of fatty acids in the coal samples, the carbon preference index of fatty acids (CPI_FA) parameter is introduced here. For the long chain fatty acids (C₂₀–C₃₂) originating from terrigenous plant debris, the CPI_FA decreases with increasing maturity, showing a strong maturation related signal. During diagenesis, the same trend can be observed for the short chain fatty acids, but the intermittent increase in the amounts of short chain fatty acids is also accompanied by high CPI_FA values. This indicates less altered organic biomass at this advanced maturation level and is in contrast to the mature CPI_FA signal of the long chain fatty acids of the same samples. One possible reason for this discrepancy could be extremely different amounts of short and long chain fatty acids in the original source organic matter of these samples. However, another intriguing explanation could be the incorporation of immature bacterial biomass from deep microbial communities containing C₁₆ and C₁₈ fatty acids as main cell membrane components. Deep microbial life might be stimulated at this interval by the increasing release of thermally generated potential substrates from the organic matrix during early catagenesis. In contrast to the fatty acids, the high amounts of alcohols in the immature lignite samples are also visible in the alkene distribution from the open system pyrolysis experiments of the organic matrix before and after saponification.     

Structural investigations of sulphur-rich macromolecular oil fractions and a kerogen by sequential chemical degradation

Structural studies of a sulphur-rich kerogen and macromolecular oil fractions from the Monterey Formation were performed by selective sequential chemical degradation. The method provides low-molecular weight compounds as former building blocks of the network which allow detailed analyses on a molecular level. The degradation sequence is based on three subsequently performed reactions—a selective cleavage of sulphur bonds in the first step carried out with Ni(0)cene/LiAlD₄, an ether and ester bond cleavage (BCl₃), and an oxidation of aromatic entities by ruthenium tetroxide as a final step. Each step of this sequence afforded a considerable amount of low-molecular weight material which was separated chromatographically and studied by GC and GC/MS, while the high-molecular weight or insoluble fractions were subjected to the next reaction step.The chemical degradation products—hydrocarbons and carboxylic acids—are discussed in terms of incorporation into the macromolecular structure, distribution of heteroatomic bridges and the genetic relationships between the different macromolecular crude oil fractions and kerogen.Labelling experiments with deuterium provided evidence for a simultaneous linkage by oxygen and sulphur functionalities or by aromatic units and sulphur bonds of cross-linking macromolecular network constituents.The determination of sulphur positions in the macromolecule suggests early diagenetic sulphur incorporation into the biological precursor compounds and subsequent formation of a cross-linked network.     

Release of sulfur- and oxygen-bound components from a sulfur-rich kerogen during simulated maturation by hydrous pyrolysis

An immature sulfur-rich marl from the Gessosso-solfifera Formation of the Vena del Gesso Basin (Messinian, Italy) has been subjected to hydrous pyrolysis (160 to 330°C) to simulate maturation under natural conditions. The kerogen of the unheated and heated samples was isolated and the hydrocarbons released by selective chemical degradation (Li/EtNH₂ and HI/LiAlH₄) were analysed to allow a study of the fate of sulfur- and oxygen-bound species with increasing temperature. The residues from the chemical treatments were also subjected to pyrolysis–GC to follow structural changes in the kerogens. In general, with increasing hydrous pyrolysis temperature, the amounts of sulfide- and ether-bound components in the kerogen decreased significantly. At the temperature at which the generation of expelled oil began (260°C), almost all of the bound components initially present in the unheated sample were released from the kerogen. Comparison with an earlier study of the extractable organic matter using a similar approach and the same samples provides molecular evidence that, with increasing maturation, solvent-soluble macromolecular material was initially released from the kerogen, notably as a result of thermal cleavage of weak carbon–heteroatom bonds (sulfide, ester, ether) even at temperatures as low as 220°C. This solvent-soluble macromolecular material then underwent thermal cleavage to generate hydrocarbons at higher temperatures. This early generation of bitumen may explain the presence of unusually high amounts of extractable organic matter of macromolecular nature in very immature S-rich sediments.     

Chemical structural units of macromolecular coal components

Chemical degradations of coal resins, coal asphaltenes and coal residues have been performed with selective, non-oxidative reagents. The coal comprise samples a rank interval 0.30–1.21 %R_m. Degraded low-molecular-weight compounds were analyzed by GC-MS. The distributions of pentacyclic terpanes, isoprenoids, n-alkanes, acids and alcohols obtained as degradation products illustrate a considerable variability between the macromolecular coal fractions. A structural study on the molecular level of the degraded compounds from coal resins and coal asphaltenes show their potential to generate hydrocarbons during coalification process.     

Stable carbon isotopic fractionation of individual n-alkanes accompanying primary migration: Evidence from hydrocarbon generation–expulsion simulations of selected terrestrial source rocks

The effect of isotopic fractionation during primary migration of hydrocarbons from coals is rarely noticed because it overlaps with the isotopic effects of maturation. In this research, geological chromatography-like effects and possible physical isotopic fractionation effects on n-alkanes during primary migration from four coals and one mudstone were studied through two types of generation–expulsion simulations (generation–expulsion simulations I and II). In order to monitor the kinetic isotopic fractionation effect during primary migration and to differentiate the isotopic effects of primary migration from the isotopic effects of maturation, generation–expulsion simulation was upgraded in two aspects, source rock was separated into at least five layers, and deuterated n-C₁₅D₃₂ was added to the initial layer of the source rock (simulation II). The experimental results suggested that all terrestrial source rocks exhibit significant geological chromatography-like effects in generation–expulsion simulation. Expulsion efficiencies shown by vitrinite-rich coals are much lower than algal cannel, fusinite-rich coal and mudstone. There also exist significant physical isotopic fractionation effects in hydrocarbon primary migration processes from vitrinite-rich coals, but there is no significant isotopic fractionation effect from fusinite-rich brown coal and mudstone. Pore structure and specific surface area of source rock samples were measured by gas adsorption of both N₂ and CO₂. This indicated that vitrinite-rich coals have a higher proportion of microporosity. The differences in pore structure and adsorptive capacity of source rocks may be responsible for differences in expulsion efficiencies and isotopic fractionation effects in generation–expulsion simulations. The isotopic fractionation effect due to primary migration should be considered in making oil-source correlation when vitrinite-rich coals are concerned.     

A model of the diagenetic evolution of coaly sedimentary organic matter

Elemental composition was used to calculate the amounts of compounds produced during the diagenetic evolution of a coal series from the Mahakam delta (Kalimantan, Indonesia). These calculations were based on the following hypotheses: organic nitrogen does not take part in reactions and remains unchanged in the residual organic matter, the only compounds produced are water, carbon dioxide and hydrocarbons.This approach shows that carbon loss during diagenesis is mainly as CO₂, and hydrogen loss is mainly as H₂O. Hydrocarbon production is negligible, in accordance with absence of bacterial methane accumulations in the Mahakam delta.The δ¹³C of coals in the sequence becomes about 2 per mil more positive over the diagenetic depth range of coal evolution. Accounting for the coal δ¹³C change in terms of CO₂ loss requires that the CO₂ given off have δ¹³C of about ?40%.. Such negative CO₂ has not been observed in natural systems, except when CH₄ is undergoing oxidation. Several plausible causes for this effect are discussed.     

Stable hydrogen isotope measurement of archaeal ether-bound hydrocarbons

Compound-specific hydrogen isotope analysis of ether-bound isoprenoid hydrocarbons from archaeal membranes has been developed using chemical degradation and gas chromatography/pyrolysis/isotope ratio mass spectrometry. The ether-bound hydrocarbons are quantitatively converted to saturated hydrocarbons by cleavage of ether bonds with HI followed by H₂ reduction in the presence of PtO₂. The δD value of ether-bound hydrocarbon moieties are corrected by way of isotopic mass balance calculation for the hydrogen incorporated during the hydrogenation. The method was successfully applied to determination of the δD values of biphytane moieties in glycerol dialkyl glycerol tetraethers (GDGTs) from a Sulfolobus sp. culture and a marine sediment.     

Petrological,chemical and depositional aspects of eastern Himalayan coals from elephant flat area,Kameng district,Arunachal Pradesh,India

The eastern Himalayan coals of India associated with Permian (Lower Gondwana) sediments in the Kameng district of Arunachal Pradesh have petrographic and chemical properties differing from Peninsular Permian coals.The coals are moderately to highly crushed and have reached a semianthracitic stage. Macerals are highly reflecting and homogenized. Vitrinite and inertinite exhibit a crushing effect in the form of criss-cross fissures and cracks. Exinite is unidentifiable and has attained an inertinitic reflectivity. The Kameng coals are of high rank with average fixed carbon 88.75% and volatile matter 13.75% on d.a.f. basis. The reflectance values (R_o-max 2.02–2.31% in oil) of these coals are quite high with marked anisotropy.It is inferred that these peculiar coal properties have been attained due to prolonged tectonic disturbance in the area during the later Himalayan orogeny. The coal characteristics suggest that these coals were formed in a humid tropical climate within a deltaic regime. The depositional site experienced occasional marine influx due to tectonically controlled subsidence during peat accumulation.     

我国含铀煤矿床的某些地球化学特征

我国含铀煤矿床主要产于中-新生代陆相沉积盆地中。盆地基底为华力西至燕山期花岗岩(黑云母花岗岩、二云母花岗岩等),或酸性火山岩(火山碎屑岩),或二者均有,其含铀丰度大于8ppm,高于酸性岩克拉克值的3-10倍。基底含铀丰度高为成矿提供了丰富的铀源。铀矿区内的地下水一般为弱酸至弱碱性,重碳酸钾-钠型,硫酸、重碳酸钙(钾)一钠型等，pH值6 -7.5，含铀丰度为n.10-7n.10-6克/升，干旱地区为n。10-6-n.10-5克/升。矿体一般为层状，似层状及透镜状与煤层整合产出。     

Steranes and triterpanes generated from kerogen pyrolysis in the absence and presence of minerals

Steranes and triterpanes generated from pyrolysis of immature Monterey Formation kerogen in the presence and absence of calcite, illite and montmorillonite reveal results that are both consistent and divergent with published data that reflect the use of these biological markers as maturation indicators. The extent of isomerization of biomarkers generated from pyrolysis of kerogen at 300°C for 2 hours, at C-20 in 14α(H),17α(H)-steranes, at C-22 in 17α(H),21β(H)-hopanes and of 17β(H),21β(H)-hopanes correspond to early diagenetic stages in rock extracts from sedimentary basins. Isomerization increases with heating time and, after 1000 hours, attains values which correspond to the catagenetic stage in sedimentary basins, or equivalent to that of mature oil. Stepwise pyrolysis of the kerogen indicates faster isomerization rates for steranes and triterpanes in the bitumen than for those retained in the kerogen structure, confirming earlier studies.Presence of a mineral matrix can influence the isomerization of steranes and triterpanes considerably. Comparisons with results from kerogen heated alone, for a given maturation stage, show that calcite inhibits, illite catalyzes slightly and montmorillonite has a pronounced catalytic effect on these reactions. This effect results in early isomerization of steranes and hopanes corresponding to the catagenetic stage in the presence of montmorillonite, while kerogen or kerogen with calcite held at the same temperature (300°C) and time (10 hours) only yield isomerized products which correspond to a diagenetic stage. Further, illite and montmorillonite affect various isomerization reactions differently. The fastest reaction is the isomerization at C-20 in 14α(H),17α(H)-steranes followed by that at C-22 in 17α(H),21β(H)-hopanes and the slowest is the formation of 14β(H),17β(H) steranes.These results show that maturation measurements of rock or oil samples from sedimentary basins which use biological markers have to take into account the mineral matrix effects, which have been largely ignored until present.     

The diagenesis of carbohydrates by hydrogen sulfide

Carbohydrates react with hydrogen sulfide under low temperature (100° to 200°C) yielding a variety of organosulfur compounds including thiophenes, thiols, sulfides and sulfones. A polymer is also produced, whose elemental composition is within the range of natural coals. When reductive dehydration is carried out in the presence of hydrocarbon, organosulfur compounds are formed in the carbon number range of the hydrocarbon used. In these processes, an active hydrogen transfer catalyst is produced which facilitates the passage of hydrogen between normal paraffins and saccharide units, distributing sulfur between these two families primarily in the form of thiophene rings. The simplicity of these systems—H₂S, carbohydrates, H₂O, hydrocarbon—and the facility of the chemistry would suggest that the carbohydrates and hydrogen sulfide may be important agents in the diagenetic processes leading to petroleum and coal. Carbohydrate reduction by hydrogen sulfide may constitute an important route through which certain organosulfur compounds found in petroleum and coal entered these materials in early diagenesis.     

Hydrocarbon—Generating Characteristics of Barkinite—Rich Colas from Late Permian Longtan Formation ,South China

Leping coal (including barkinite-rich coal) is a unique kind of coal,which is widely distrbuted in the Late Permian Longan Formation,South China,In this paper,ROck-Eval,Py-GC and simulation experiment via an open-system were used to study the hydrocarbon-generating potential,hydrocarbon composition.and hydrocarbon-generating model of barkiniterich coals from the shuicheng coal field of Guizhou Province,Southwest China.The results show that barkinite-rich coals have high hydrocarbon-generating potential,with S1 S2 being 211-311mg/g,and can produce large amounts of hydrocorbon at the high-maturity stage,mostly within the temperature range of 420-450℃(corresponding to VR0 1.1-1.5%);barkinite-rich coal is one of the better oil sources and light hydrocarbon and wet gas are the major hydrocarbon components,which account for 45% and 33% of the total hydrocarbons.respectively.These characteristics are of importance for exploring oil and gas resources in the Late permian Longtan Formation coals,southwest China.     

Organic pollutants associated with macromolecular soil organic matter: Mode of binding

A study of ether-linked moieties in macromolecular bound residues of polycyclic aromatic hydrocarbons (PAH) generated in bioremediation experiments was performed using high temperature hydrolysis degradation with subsequent analysis of the products by GC-MS. This hydrolysis reaction was specifically designed to cleave ether bonds including relatively stable diarylether structures. Among the reaction products, aromatic alcohols representing typical microbiologically derived metabolites of PAH were found in addition to natural compounds. Thus, parts of the bound residues appeared to be linked within the macromolecular material by ether bonds. Model experiments with an oxidoreductase enzyme and aromatic alcohols indicate the formation of these ether bonds to be an enzyme-catalysed process.     

Occurrences and distributions of branched alkylbenzenes in the Dongsheng sedimentary uranium ore deposits,China

A series of branched alkylbenzene ranging from C₁₅ to C₁₉ with several isomers (2–5) at each carbon number were identified in sediments from the Dongsheng sedimentary uranium ore deposits, Ordos Basin, China. The distribution patterns of the branched alkylbenzenes show significant differences in the sample extracts. The branched alkylbenzenes from organic-rich argillites and coals range from C₁₅ to C₁₉ homologues, in which the C₁₇ or C₁₈ dominated. On the other hand, the C₁₉ branched alkylbenzenes dominated in the sandstone/siltstone extracts. The obvious differences of the branched alkylbenzene distributions between the uranium-host sandstones/siltstones and the interbedded barren organic-rich mudstones/coals probably indicate their potential use as biological markers associated with particular depositional environments and/or maturity diagenetic processes. Possible origins for these branched alkylbenzenes include interaction of simple aromatic compounds with, or cyclization and aromatization reactions of, these linear lipid precursors such as fatty acids, methyl alkanoates, wax esters or alkanes/alkenes that occur naturally in carbonaceous sediments. The possible simple aromatic compounds may include substituted benzenes, functionalized compounds such as phenols that are bound to kerogen at the benzyl position, and phenols that are decomposition products derived from aquatic and terrestrial sources. The distributions of methyl alkanoates and n-alkanes were found to be different between organic-rich mudstone/coal and sandstone/siltstone. From this result, it can be concluded that such differences of the alkylbenzene distributions were mainly resulting from the differences of organic precursors, although maturity effect and radiolytic alteration cannot be completely excluded.     

The structural evolution of organic matter during maturation of coals and its impact on petroleum potential and feedstock for the deep biosphere

The structural evolution of coals during coalification from peat to the end of the high volatile bituminous coal rank (VR_r = 0.22–0.81%) has been studied using a natural maturity series from New Zealand. Samples were studied using a range of standard coal analyses, Rock–Eval analysis, infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and pyrolysis gas chromatography (Py-GC). The structural evolution of coal during diagenesis and moderate catagenesis is dominated by defunctionalisation reactions leading to the release of significant amounts of oxygen and thereby to an enrichment of aromatic as well as aliphatic structures within the residual organic matter. Based on the evolution of pyrolysis yields and elemental compositions with maturity it can be demonstrated that oxygen loss is the major cause for increasing Hydrogen Index values or hydrocarbon generating potentials of coals at such maturity levels. For the first time, the loss of oxygen in form of CO₂ has been quantified. During maturation from peat to high volatile bituminous coal ranks ∼10–105 mg CO₂/g TOC has been released. This is equivalent to 2.50E−4 to 1.25E−3 mg CO₂ generated from every litre of sediment per year falling into the range of deep biosphere utilisation rates. Immature coals, here New Zealand coals, therefore manifest the potential to feed deep terrestrial microbial life, in contrast to more mature coals (VR_r > ∼0.81%) for which defunctionalisation processes become less important.     

Tracing the origins of polymethylene moieties in coal

Many coals have been shown to contain long polymethylene segments (up to C₃₀ and longer) which crack to ethylene, propylene, butadiene and other low molecular weight hydrocarbons when the coals are pyrolyzed. The polymethylene content varies in different coals and can be 10% or more in low rank coals. Cannel coals may contain up to 20% polymethylene. Polymethylene moieties are either very low or absent in low-volatile bituminous coals and anthracite.A method of analysis for polymethylene in coals, based on liquefaction in a donor solvent followed by ¹H NMR spectroscopy has been described. With this analysis, it has been shown that polymethylene moieties are present in higher concentrations in exinite macerals than in vitrinites or inertinites. Analysis of peats also show the presence of polymethylene components at levels similar to that found in coal. Analysis of various parts of present-day plants also shows the presence of polymethylene materials in concentrations varying significantly from one kind of plant to another. Within a given plant type, some parts of the plant appear to be richer in polymethylene than others. Spores and pollen are frequently especially rich in polymethylene, accounting for the corresponding high concentration in exinite macerals in coal.Samples of Texas Eocene lignites representing a depositional time interval of 15–20 million years and three distinctly different climatic and vegetational conditions were analyzed for polymethylene contents. The differences in polymethylene levels are consistent with differences in flora and support the idea that the original plants are the sources of polymethylene components in coal.