Jurassic Coal Research Articles

The role of water during gas generation

Resources of coal-derived gas in the deep strata are found abundant in China. Recent advances in petroleum geochemistry suggest water may contribute hydrogen and oxygen to the formation of hydrocarbons and oxygenated alteration production. However, the role of water during gas generation has been in debate due to conflicting research results. In this study, a Jurassic coal sample from the Kuqa depression, Tarim Basin, China was artificially matured by nonhydrous pyrolysis and hydrous pyrolysis at isothermal (330 °C, 350 °C, 370 °C for 72h) and non-isothermal (440 °C, 470 °C and 500 °C at heating rate of 2 °C/h) temperatures. Both isothermal and non-isothermal experiments generated more methane but less C3-5 hydrocarbon gases and CO2 non-hydrocarbon gas in the nonhydrous system relative to hydrous conditions. In the isothermal experiment, the nonhydrous pyrolysis generated at most 24% more methane relative to the hydrous pyrolysis, but the hydrous pyrolysis generated 5%∼87% more C3-5 hydrocarbon gas and at most 28% more CO2 relative to the nonhydrous pyrolysis. In the non-isothermal experiment, the nonhydrous pyrolysis generated 25%∼94% more methane relative to the hydrous pyrolysis, but the hydrous pyrolysis generated at most 2.56 times more C3-5 hydrocarbon gas and 1.04 times more CO2 relative to the nonhydrous pyrolysis. In general, the compositional variation trend of hydrocarbon and non-hydrocarbon gases is similar between the isothermal and non-isothermal experiments. This indicates that contribution from the supercritical water in the non-isothermal pyrolysis is not essential. With the presence of water, the secondary cracking of C3-5 alkanes in the hydrous experiments was significantly suppressed, as shown by the relatively higher yield of C3-5 alkanes in the hydrous systems. The availability of exogenous hydrogen from water in the hydrous pyrolysis inhibits the carbon-carbon bond cross linking, thus the reaction pathway of hydrocarbon generation is dominated by the thermal cracking of carbon-carbon bonds. The significant increase in CO2 in the hydrous pyrolysis indicates water is not only the source for exogenous hydrogen but also the source for the excess oxygen accounted for the formation of CO2. The overall geochemical implication of these experimental results is that it is important to consider the role of water in the hydrocarbon generation, especially in the deep strata where both geological and geochemical conditions are complex.

Gas generation from Jurassic coal measures at low mature stage and potential gas accumulation in the eastern Junggar Basin, China

The increasing global demand for clean energy has made it imperative to exploit unconventional oil and gas resources. Early thermogenic gas is one of the research target of unconventional gas. The Jurassic coal measures were widely distributed in the eastern Junggar Basin with high abundance of humic organic matter and low thermal degrees, showing potential for early thermogenic gas exploration. The burial depth of Jurassic source rocks ranges between 2200 m and 4500 m, with vitrinite reflectance ranging from 0.5% to 0.9%. To study the potential and possibility of gas generation from Jurassic coal measures at low mature stage in the eastern Junggar Basin, mudstone and coal samples were selected for pyrolysis experiments. The pyrolysed temperatures setting for this study are 330 °C, 350 °C, and 370 °C. The heating times at each desired temperature are 3, 7, 14, 21, and 30 days. Parameters such as vitrinite reflectance and certain biomarker ratios indicate all the samples are still at the low mature stage when the heating process finished. Genetic correlation between δ13C1 and C1/(C2+C3) ratio reveals that the generated gas in this study is mainly sourced from Type III kerogen and at the stage of low maturity. Hydrocarbon gas yield results suggest that 370 °C is the most favorable temperature for early thermogenic gas generation, especially heating for 14 days or longer. Pyrolysis results indicate time effect plays an important role in the formation of early thermogenic gas. The results of fourier transformed infrared spectra suggest aliphatic components in macromolecular structure of kerogen provide gas generating material during low mature stage. The evolution history of the Jurassic source rocks and the pyrolysed results indicate the Jurassic coal measure strata in the eastern Junggar Basin has favorable geothermal condition for accumulation of early thermogenic gas.

Study on the Liberation of Organic Macerals in Coal by Liquid Nitrogen Quenching Pretreatment

In order to study the liberation characteristics of different macerals in coal, one must improve the liberation degree of macerals in coal and promote the utilization of macerals based on their properties. Based on the idea of quick cooling to change the brittleness and toughness of different macerals, the characteristics of macerals in coal are studied here by liquid nitrogen quenching pretreatment. In particular, coarse-grained samples (with a particle size of 1–3 mm) were. For this study, coal samples were sourced from the Yan’an Formation (J2y), a middle Jurassic coal formation in the Huangling no. 1 coal mine, located in northern Shaanxi Province, China. Firstly, we analyzed the coal properties and coal petrographic characteristics by the Chinese national standard method. Secondly, the distribution characteristics of macerals in different particle sizes (>0.9, 0.5–0.9, 0.1–0.5, and <0.1) were studied. Then, the samples with different particle sizes were quenched with or without liquid nitrogen to obtain the experimental group and blank group products. Finally, the differences in the liberation characteristics between the experimental group and the blank group products were studied via analyzing the micromorphology, specific surface area, pore volume, pore size and liberation degree. Our results for the particles size and liberation degree analysis indicate that inertinite and vitrinite were enriched in the coarse particles (>0.5 mm) and fine particles (<0.1 mm) here, respectively. Moreover, quenching pretreatment could contribute to the liberation of different macerals from coal, mainly because of the different effects of stress on the different components when they suddenly encounter cold, and this kind of liberation is mainly arc-shaped liberation between different macerals. In addition, along with the above results, this paper presents an optimized model for the liberation of macerals based on a combination of screening, liquid nitrogen quenching pretreatment and re-crushing.

Thermokinetic Characteristics of Jurassic Coal Spontaneous Combustion Based on Thermogravimetric Analysis

ABSTRACT To prevent coal fire disaster, a precision grading warning method was previously proposed for coal seams that are susceptible to spontaneous combustion. The uncertainty in the reaction mechanism of the spontaneous combustion of coal greatly restrains the in-depth understanding of the fine division theory in the course of coal combustion. In this paper, further research has been conducted to study the mechanism of this method. In this study, thermogravimetric analysis was carried out on the Jurassic coal of three different metamorphic grades. The variations of the TG/DTG curves of the coal samples at four heating rates were compared to examine their kinetic model and kinetic compensation effect. For the three samples of the Jurassic coal, a similar trend was obtained for the TG and DTG curves at the different heating rates. With an increase in the heating rate, the hysteresis phenomenon appeared in the TG/DTG curves. In the low-temperature coal oxidation stage, the most probable mechanism function of Coal A and Coal B was obtained to be No.9 (G(α) = [1-(1-α)1/2]2, f(α) = (1-α)1/2[1-(1-α)1/2]−1), and that of Coal C was No.14 (G(α) = [-ln(1-α)]2, f(α) = (1-α)[-ln(1-α)]−1/2). A significant kinetic compensation effect was also observed in the oxidation stage. Despite the difference in the metamorphic grade, the reaction process of the low-temperature oxidation, as well as the rate-determining step was the same for the three samples.

Isokinetic analysis on the oxidation of Jurassic coal: a case study of samples from Xinjiang, China

Jurassic coal samples from Liuhuanggou (LHG) and Wudong (WD) coal mines, Xinjiang Province, China, were selected for oxidative thermal energy experiments. A temperature-programmed experimental system measured the variation of gaseous products and the changes in thermal energy release with temperature. Thermokinetic parameters, namely the apparent activation energy (Ea) and pre-exponential factor (A), of the coal samples for four heating rates were determined using a thermogravimetric analyser. During the low-temperature oxidation stage, two samples had temperature curves characterised by their production levels of the index gas (CO); the temperature changes were approximately equal, but the critical temperatures differed. The WD sample produced less CO gas. However, the maximum and minimum exothermic strengths of the two samples had some similarities to the temperature change curves. The characteristic temperatures for the coal samples were discovered using different heating rates. For the WD sample, the characteristic temperature varied according to the heating rate, and the thermogravimetry (TG) and derivative TG curves of the samples lagged with an increase in the heating rate. The characteristic temperatures of the sample increased with the increase in the heating rate. Isokinetic analysis (the Flynn–Wall–Ozawa and the Kissinger–Akahira–Sunose models) was used to calculate the thermokinetic parameters of the sample. The range of the heating rates was 2 to 15 °C min−1 for oxygen concentration of 21 vol%, and the Ea of the samples decreased with the increase in the heating rate.

Study on the correlation between trace elements in coal and coal-forming plants: a case study

ABSTRACT For this study, coal samples were sourced from the Yan’an Formation (J2y), a middle Jurassic coal formation in the Northern Shaanxi province in China. We investigated the correlations between trace elements in coal and occurrence of coal-forming plants (CFPs) in an effort to determine the relationship between the abundance of CFPs and trace elements in the coal formation. Twenty-one trace elements (Be, B, P, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, As, Se, Sr, Mo, Ag, Cd, Ba, Hg, Tl, and Pb) in coal were determined using inductively coupled plasma-atomic emission spectrometry (ICP-AES). The enrichment characteristics of trace elements in coal were performed by using concentration coefficient value. CFPs were identified and measured by analyzing spore-pollen in the coal samples. Correlation analyses between trace elements in coal and CFPs were conducted using the SPSS 21 software package. Our results indicated that the coal formation contains an enriched concentration of Cd, slightly enriched concentrations of Zn and Ag, low concentrations of V and Mo, normal concentrations of the other 16 trace elements. CFPs identified in the samples included 17 families (Cupressaceae, Cyatheaceae, Cycadaceae, Dicksoniaceae, Dipteridaceae, Ephedraceae, Equisetaceae, Ginkgoaceae, Gleicheniaceae, Lycopodiaceae, Lygodiaceae, Osmundaceae, Pinaceae, Pteridaceae, Podocarpaceae, Polypodiaceae, and Taxodiaceae). Polypodiaceae is the richest in Pteridophyta. Ginkgoaceae, Cycadaceae, and Podocarpaceae were richer in Spemaiophyla. Those with a correlation coefficient more than 0.5 were found between the following: (i) Be, Sr, and Equisetaceae; (ii) V, Ni, and Cu, and Polypodiaceae; (iii) P and Ginkgoaceae; and (iv) Ba, Pb, and Cupressaceae. Significant negative correlations were found between the following: (i) Ni, Cu, Ba, Pb, and Osmundaceae; (ii) Sr and Polypodiaceae; and (iii) Sr and Ginkgoaceae.

The Influence of Hydrogeology to Generation of Hydrogen Sulfide of Low-Rank Coal in the Southeast Margin of Junggar Basin, China

The salinity, chemical properties, and migration characteristics of groundwater in coal measures are the key factors that affect the generation, migration, and reservoir of hydrogen sulfide (H2S) in low-rank coal seams. Taking the Jurassic coal and rock strata in the southeastern margin of the Junggar basin as the research object, according to the hydrogeological characteristics of the coal measures, the region is divided into 4 hydrogeological units. The coalbed methane contains a large number of secondary biogas. Along the direction of groundwater runoff, the salinity and the pH value increase gradually. The salinity in the hydrogeological units is low; it is not conducive to the propagation of sulfate-reducing bacteria and the formation of hydrogen sulfide of the Houxia, the south of Manasi River, and Hutubi and Liuhuangou area, the western region of the Miquan. The high salinity center and depressions of low water level (hydrodynamic stagnation zone) in the hydrogeological unit of the Liuhuanggou and the Miquan are the main areas for the production and enrichment of H2S in the low-rank coal. The high salinity in water is formed by infiltration, runoff, and drought evaporation. At the same time, the deep confined water environment closed well; in conditions of hydrocarbon-rich, under the action of sulfate-reducing bacteria, bacterial sulfate reduction will occur and hydrogen sulfide formed. According to the circulation characteristics of water bearing H2S in the region, imbricate and single bevel two kind generation and enrichment mode of hydrogen sulfide under the action of hydrodynamic control. The solubility of hydrogen sulfide in pure water and solutions of NaCl and Na2SO4 with different molar concentrations was calculated. The H2S solubility of groundwater in coal measures of 4 hydrogeological units was estimated.

Division of Methane Weathering Zones in the Middle and Lower Jurassic Coal Seams of Santanghu Basin

Division of Methane Weathering Zones in the Middle and Lower Jurassic Coal Seams of Santanghu Basin

Gas generation from coal: taking Jurassic coal in the Minhe Basin as an example

The gas generation features of coals at different maturities were studied by the anhydrous pyrolysis of Jurassic coal from the Minhe Basin in sealed gold tubes at 50 MPa. The gas component yields (C1, C2, C3, i-C4, n-C4, i-C5, n-C5, and CO2); the δ13C of C1, C2, C3, and CO2; and the mass of the liquid hydrocarbons (C6+) were measured. On the basis of these data, the stage changes of δ13C1, δ13C2, δ13C3, and δ13CO2 were calculated. The diagrams of δ13C1–δ13C2 vs ln (C1/C2) and δ13C2–δ13C1 vs δ13C3–δ13C2 were used to evaluate the gas generation features of the coal maturity stages. At the high maturity evolution stage (T > 527.6 °C at 2 °C/h), the stage change of δ13C1 and the CH4 yield are much higher than that of CO2, suggesting that high maturity coal could still generate methane. When T < 455 °C, CO2 is generated by breaking bonds between carbons and heteroatoms. The reaction between different sources of coke and water may be the reason for the complicated stage change in delta^{{{13}}} {text{C}}_{{{text{CO}}_{{2}} }} when the temperature was higher than 455 °C. With increasing pyrolysis temperature, δ13C1–δ13C2 vs ln (C1/C2) has four evolution stages corresponding to the early stage of breaking bonds between carbon and hetero atoms, the later stage of breaking bonds between carbon and hetero atoms, the cracking of C6+ and coal demethylation, and the cracking of C2–5. The δ13C2–δ13C1 vs δ13C3–δ13C2 has three evolution stages corresponding to the breaking bonds between carbon and hetero atoms, demethylation and cracking of C6+, and cracking of C2–5.

Geological Controls on Mineralogy and Geochemistry of the Permian and Jurassic Coals in the Shanbei Coalfield, Shaanxi Province, North China

Coal as the source of critical elements has attracted much attention and the enrichment mechanisms are of significant importance. This paper has an opportunity to investigate the mineralogical and geochemical characteristics of the Permian and Jurassic bituminous coals and associated non-coals from two underground coal mines in the Shanbei Coalfield (Northeast Ordos basin), Shaanxi Province, North China, based on the analysis of X-ray diffraction (XRD), inductively coupled plasma atomic-emission spectrometry (ICP-AES/MS), and scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS). The Jurassic and Permian coals have similar chemical features excluding ash yield, which is significantly higher in the Permian coals. Major mineral matters in the Jurassic coals are quartz, kaolinite, and calcite. By contrast, mineral assemblages of the Permian coals are dominated by kaolinite; and apatite occurring in the middle section’s partings. The Jurassic coals are only enriched in B, whereas the Permian coals are enriched in some trace elements (e.g., Nb, Ta, Th, and REY). Boron has a mixed inorganic and organic association which may be absorbed by organic matter from fluid (or groundwater) or inherited from coal-forming plants. Additionally, climatic variation also plays an important role. As for the Permian coals, kaolinite and apatite as the major carriers of elevated elements; the former were derived from the sedimentary source region (the Yinshan Oldland and the Benxi formation) and later precipitated from Ca-, and P-rich solutions. We deduced that those elevated elements may be controlled by the source rock and diagenetic fluid input. The findings of this work offered new data to figure out the mechanism of trace element enrichment of coal in the Ordos basin.

Study on the Height of the Mining-Induced Water-Conducting Fracture Zone Under the Q2l Loess Cover of the Jurassic Coal Seam in Northern Shaanxi, China

A method to calculate the height of a water-conducting fractured zone (HWCFZ) was developed based on the plate and shell theory, and the development of the HWCFZ in bedrock and Q2l loess strata is discussed in detail. First, the subsidence-deflection curve equation of the overlying stratum is theoretically derived, and then the ultimate deflection and free space height of rock strata are calculated. Moreover, the strata tensile strain is calculated by using integral calculus. In addition, the failure state of the rock is analyzed by comparing the theoretically calculated tensile strain with the experimentally measured yield tensile strain, allowing one to attain the maximum value of HWCFZ. This approach was tested at the Jinjitan coal mine; the theoretically predicted, experimentally measured, and numerically computed maximum HWCFZ values were 189.5, 187.3, and 188.5 m, respectively, demonstrating the accuracy of the proposed method. These results are highly significant for safe and environment-friendly coal mining in northwest Shaanxi, China.

Geochemical Evaluation of Enrichment of Rare-Earth and Critical Elements in Coal Wastes from Jurassic and Permo-Carboniferous Coals in Ordos Basin, China

Coal waste is a potential source of rare-earth elements (REEs) and some economically critical elements recovery. The present study reports the abundance and enrichment of REEs and critical elements in the Hancheng Permo-Carboniferous (Weibei coalfield) and Binxian Jurassic coals (Huanglong coalfield) of Shaanxi, China. The Binxian coal is distinctly enriched in all REEs. The Hancheng coal is enriched in Y, Ce, Sc, La, Yb, Nd, Pr, Gd, Er, Sm and Dy and depleted in Lu, Eu, Ho, Tb and Tm, compared with that of the average earth’s crust abundance (ECA), world coal and the US coal. In the Binxian raw coal, the REEs contents, namely Sc 13.8, Y 13.1, La 27.8, Ce 48.49, Sm 4.1, Nd 22, Eu 0.8, Gd 3.8, Pr 5.43, Dy 2.5, Er 1.4 and Yb 1.3 in mg kg−1. The contents (mg kg−1) of REEs in the Hancheng raw coal were Sc 8.8, Y 18.6, La 34.8, Ce 60.2, Nd 26, Yb 1.8, Eu 01, Pr 6.71, Sm 5.3, Dy 3.4, Tb 0.66, Er 1.9 and Gd 5.16. The contents (mg kg−1) of critical elements in the Binxian raw coal were Cr 30.8, Pb 41.5, Ni 49.7, Cu 35.7, Ba 257.9, V 51.7, Zn 63.1, Li 135, Ga 20.6, U 2.9, Th 10.2 As 12.7, Al 98,887, Fe 23,916 and Ti 4289. The contents (mg kg−1) of critical elements in the Hancheng coal were Cr 384, Pb 56.1, Ni 93.9, Cu 49.5, Ba 371, V 90.7, Zn 7653, Li 183, Ga 35.9, U 4.9, Th 17.7, As 10.1, Al 108,344, Fe 20,433 and Ti 2873. The contents of REEs in the Binxian and Hancheng coals were not in a promising range, whereas some of the critical elements were highly abundant. The Al2O3/TiO2 ratio indicated that the Ordos Basin sediment was derived from intermediate-felsic rocks with a slight variation (r = 0.98). The Eu and Gd show positive anomalies with negative Ce anomalies almost in all the samples. The positive anomalies of Gd represent the intrusion of hydrothermal fluid with possibly Ba interference in the Binxian coal. The high volatile matter in the Binxian coal reflects bituminous to anthracite coal, whereas the low volatility of the Hancheng coal reflects peat to lignite coal. The recovery of the REEs and critical elements as a by-product from these coals may not only increase the revenue but will also lead to an improvement in the environmental quality.

Mineralization zoning of coal-metal deposits in China

Substantive progress has been made in extracting germanium, gallium, rare earth elements (REEs), lithium, and other metals from coal. Coal deposits are becoming important sources of many strategic metal resources. However, ore deposit theory study for coal-metal resources has not been systematic. This paper summarizes the mineralization characteristics of germanium, gallium, lithium, and REEs in coal on the basis of previous research results and field investigations, including those on seven metallogenic zones of coal metal deposits in China. Coal-germanium deposits, products of magmatic hydrothermal activity, mainly occur in low rank coal in the areas of Erlian, the Hailaer Basin, Inner Mongolia, and the Sanjiang district in the Yunnan Province. Occasionally, rich-germanium-coal can be found in other areas. Coal-gallium deposits are mainly in the Permian coal of North China, Jurassic coal of Northwest China, and late-Permian coal of South China. The accumulation of gallium in coal is closely related to aluminum-containing minerals. Coal-lithium deposits mainly occur in the Zhungeer coal field, Inner Mongolia, the Pingshuo coal field, and the Jingcheng coal field in the Shanxi Province, and some coal fields in South China. Similarly to gallium, the accumulation of lithium in coal is also closely related to aluminum-containing minerals. Coal-REE deposits mainly occur in the Early Jurassic coal of the Erlian Basin and Inner Mongolia, and Late Permian coal in South China. REEs of Erlian basin coal likely entered in ionic forms in early stages of coal or peat bog formation. REEs of South China coal are likely associated with REE-rich trass volcanic ash which deposited synchronously with peat bog accumulation. Abundant abnormal metal elements in coal are important for classification of metallogenic zones. Regional tectonic background and evolution, characteristics of stratigraphic sedimentary environment, and coal quality are also important factors. (1) Delineation of germanium metallogenic zones in the coal follows the principles that: coal in the coal area should be low-metamorphic bituminous (long-flame) coal; the basement of the coal-bearing basin was broken and faults developed in the region; and there are obvious traces of hydrothermal intrusion in or around the coal measures in the zone. (2) Delineation of gallium metallogenic zones in coal is based on these traits: an obvious gallium source should exist in the region; the provenance area has favorable weathering and denudation conditions; and an aluminum-gallium formation under strong leaching is contained within. (3) Delineation of lithium metallogenic zones in coal requires: relatively clear sources of lithium in the zones; favorable weathering and denudation conditions in the provenance area; and aluminum-lithium containing rocks under strong leaching. (4) Delineation of rare earth mineralization zones in coal is based on: large scale volcanic eruptions in or near peat bogs during the accumulation period, with tuff in the coal-bearing strata; and hydrothermal intrusion into coal-measure strata during the peat bog period or early coal-forming period. According to the principles above, seven coal-metal metallogenic zones were identified, including: (1) the Erlian Basin-Hailaer Basin coal-germanium and REE deposit zone; (2) the Tianshan coal-gallium deposit zone; (3) the South-Yinshan coal-gallium-lithium deposit zone; (4) the East-Taihang Mountain coal-gallium deposit zone; (5) the Qilian-Qinling coal-gallium-lithium deposit zone; (6) the Sichuan-Yunnan-Guangxi coal-gallium-lithium and REE deposit zone; and (7) the Sanjiang (west Yunnan Province) germanium deposit zone.

Organic petrology and geochemistry of Triassic and Jurassic coals of the Tabas Basin, Northeastern/Central Iran

Jurassic and Triassic coals and organic matter-rich shales of the Tabas Basin, Iran, were investigated with respect to their thermal maturation and petrographic composition. The Triassic coals of the Parvadeh coal field range in vitrinite reflectance between 1.17% and 1.37% and the Jurassic coals of the Mazino coal field between 2.08% and 2.29% VRr. Maceral analysis revealed a predominance of vitrinite in all samples, with slightly higher percentages of inertinite in the Jurassic samples. Rock–Eval analyses confirm the presence of type III kerogen in the Triassic coals, while the Jurassic coals contain strongly carbonized residual kerogen. The range of sulfur contents for coals from the Parvadeh coal field (0.37% to 4.64%) and Mazino coal field (0.45% to 2.92%) is related to the effect of marine water in peat. The studied samples are characterized by the predominance of short- over long-chained n-alkanes. The relatively high Pr/Ph ratios indicate predominance of terrestrial organic matter whereas Pr/n-C17 and Ph/n-C18 ratios prove oxic conditions during deposition. In addition, the DBT/Phen ratio shows that these coals formed in fluvial/deltaic environments. Molecular geochemical parameters such as carbon preference index (CPI, 0.99–1.04), methyl phenanthrene index (MPI, 1.20–1.60), methyl naphthalene ratio (MNR, 1.61–3.45), and ethyl naphthalene ratio (ENR, 4–6.78) confirm the high maturity of the samples. Burial and thermal history reconstruction indicates necessity of an erosional thickness of about 4000 m in the Parvadeh area. Towards the Mazino area, a higher basal heat flow up to 80 mW m−2 is assumed for the Paleogene leading to higher maturities.

Genetic type and source of natural gas in the southern margin of Junggar Basin, NW China

Abstract Natural gas has been discovered in many anticlines in the southern margin of the Junggar Basin. However, the geochemical characteristics of natural gas in different anticlines haven't been compared systematically, particularly, the type and source of natural gas discovered recently in Well Gaotan-1 at the Gaoquan anticline remain unclear. The gas composition characteristics and carbon and hydrogen isotope compositions in different anticlines were compared and sorted systematically to identify genetic types and source of the natural gas. The results show that most of the gas samples are wet gas, and a few are dry gas; the gas samples from the western and middle parts have relatively heavier carbon isotope composition and lighter hydrogen isotope composition, while the gas samples from the eastern part of southern basin have lighter carbon and hydrogen isotope compositions. The natural gas in the southern margin is thermogenic gas generated by freshwater-brackish water sedimentary organic matter, which can be divided into three types, coal-derived gas, mixed gas and oil-associated gas, in which coal-derived gas and mixed gas take dominance. The Jurassic coal measures is the main natural gas source rock in the southern margin, and the Permian lacustrine and the Upper Triassic lacustrine-limnetic facies source rocks are also important natural gas source rocks. The natural gas in the western part of the southern margin is derived from the Jurassic coal measures and the Permian lacustrine source rock, while the natural gas in the middle part of the southern margin is mainly derived from the Jurassic coal measures, partly from the Permian and/or the Upper Triassic source rocks, and the natural gas in the eastern part of the southern margin is originated from the Permian lacustrine source rock. The natural gas in the Qingshuihe oil and gas reservoir of Well Gaotan-1 is a mixture of coal-derived gas and oil-associated gas, of which the Jurassic and Permian source rocks contribute about half each.

Post-250 Ma thermal evolution of the central Cathaysia Block (SE China) in response to flat-slab subduction at the proto-Western Pacific margin

The flat-slab subduction model has been proposed to explain the complex Mesozoic–Cenozoic geotectonic record of SE China as part of the Western Pacific margin. Here we investigate the post-250 Ma thermal history of the Shaoguan-Longnan region to evaluate the time-temperature evolution of local sedimentary and magmatic rocks in the context of the flat-subduction model. Our multi-chronological approach includes zircon U-Pb geochronology, 40Ar/39Ar dating of K-feldspar and biotite, (U-Th)/He dating of zircon and apatite, and fission-track dating of apatite from basement and/or sedimentary rocks, as well as vitrinite reflectance analysis of coal seams. Zircon U-Pb dating revealed Early Silurian and Late Jurassic crystallization ages for the Fuxi (ca. 438 Ma) and Liyuan (ca. 162 Ma) granites, respectively. Syn-/pre-depositional single-grain zircon (U-Th)/He ages from Lower Jurassic sandstones record a Middle Triassic exhumational cooling, interpreted to reflect the Indosinian Orogeny, possibly caused by a flat-slab subduction event. Late Jurassic biotite 40Ar/39Ar age (ca. 156 Ma) obtained from the Late Triassic Mengdong granite, together with maximum vitrinite reflectance values of 1.7%–2.3% measured from a Lower Jurassic coal sample, document a thermal maximum in the Late Jurassic, likely related to the prior regional magmatism arising from delamination of the subducting Paleo-Pacific plate. K-feldspar 40Ar/39Ar plateau age from the Fuxi granite (ca. 125 Ma), and zircon (U-Th)/He mean ages (152–115 Ma) record a protracted cooling through the K-feldspar Ar and the zircon He partial retention zones during the Cretaceous. The Cretaceous cooling is in accord with the prediction of regional uplift and erosion in response to foundering of the flat-slab and consequent lithospheric rebound. The rapid cooling during the Paleocene–Eocene, recorded by apatite fission-track central ages (72–39 Ma) and apatite (U-Th)/He mean ages (38–33 Ma), is interpreted to reflect Cenozoic rifting in the leadup to the opening of the South China Sea.

Application of channel-belt scaling relationship to Middle Jurassic source-to-sink system in the Saishiteng area of the northern Qaidam Basin, NW China

Palaeodrainage basin, as an important component of the source-to-sink system, contains critical information on provenance and palaeoenvironment. Previous studies indicate that the scaling relationships of source-to-sink system components generally follow power laws, and channel-belt thickness represents a reliable first-order proxy for the drainage area. In this study, a database of borehole cores and geophysical well logs of the Jurassic coal measures from Saishiteng area in the northern Qaidam Basin was used to reconstruct the palaeogeography, and to identify single-story channel-belts. Three palaeochannels, namely, River A, River B and River C, were identified which were persistent throughout the Dameigou and Shimengou Formations during the Middle Jurassic. The mean channel-belt thicknesses of River A, River B and River C in the Dameigou Formation were 9.8 m, 8.9 m and 7.9 m, respectively, and those in the Shimengou Formation were 7.4 m, 6.2 m and 5.4 m, respectively. We estimate the drainage area of three major rivers by using scaling relationships between drainage area and channel-belt thickness. The drainage areas of River A, River B and River C in the Dameigou Formation were 63.0 × 103 km2, 50.1 × 103 km2 and 37.7 × 103 km2, respectively, and those in the Shimengou Formation were 32.3 × 103 km2, 21.2 × 103 km2 and 15.3 × 103 km2, respectively. The drainage basin lengths of River A, River B and River C in the Dameigou Formation were 300.4 km, 239 km and 180.2 km, respectively, and those in the Shimengou Formation were 154.3 km, 101.3 km and 73.1 km, respectively. For both the Dameigou and Shimengou Formations, River A showed the largest scale, followed by River B and River C succeedingly, which was mainly determined by the stretch direction of provenance in the southern Qilian Mountains. The variations of channel-belt thickness, drainage area and drainage basin length between Dameigou and Shimengou Formations are the response of source-to-sink system to the transformation from extension to compression depression during the Middle Jurassic in the northern Qaidam Basin.

The differences of element geochemical characteristics of the main coal seams in the Ningdong coalfield, Ordos Basin

In this study, we investigate the vertical variation of the geochemical characteristics of the Jurassic coal in the Ningdong coalfield, Ordos Basin. The major and trace element compositions (including rare earth elements) of three of the main coal seams were determined using optical microscopy, X-ray fluorescence spectrometry, inductively coupled plasma mass spectrometry, and scanning electron microscopy with energy dispersive X-ray. The results indicate that the three coal seams have similar properties with low-ash yields (ash yield < 10%) and low-sulfur contents (total sulfur < 1%), and the maceral composition of the three coal seams is generally dominated by inertinite (>50% on average). The geochemical characteristics of the three coal seams vary. Mn is enriched in the three coal seams, but their degrees of enrichment differ. The degree of Sr and Ba enrichment gradually increases from coal 18 (85 μg/g for Sr and 80 μg/g for Ba) to coal 6 (230 μg/g for Sr and 421 μg/g for Ba) and then to coal 4-2 (1667.06 μg/g for Sr and 741.69 μg/g for Ba). In addition, the Sr and Ba occurrence of the three coals differs. The REE contents and distribution patterns of the three seams reflects a stable source supply during sedimentation. The variation in the sedimentary environment of the coal seams is the main factor controlling the differences in their geochemical characteristics.

Geochemical and Mineralogical Characteristics of the Middle Jurassic Coals from the Tongjialiang Mine in the Northern Datong Coalfield, Shanxi Province, China

There is limited information available on the minerals and elements present in the Jurassic coals from Datong Coalfield. This paper investigates the geochemical and mineralogical characteristics of the Middle Jurassic coals from the Tongjialiang Mine using X-ray powder diffraction (XRD), X-ray fluorescence spectrometry (XRF), inductively coupled plasma mass spectrometry (ICP-MS), and scanning electron microscopy in combination with energy-dispersive X-ray spectrometry (SEM-EDS). No.12 coal is a low-medium volatile bituminous coal and is characterized by low ash yield content, low moisture content, and ultra-low sulfur content. Compared with Chinese coals, the Tongjialiang coals have slightly higher average percentages of MgO and P2O5, and lower average percentages of the other major oxides, including SiO2, TiO2, Al2O3, Fe2O3, CaO, MnO, Na2O, and K2O. Compared with the World hard coals, Be, Cr, Co, Ni, Ge, Sn, Ta, and W are slightly enriched in the Tongjialiang coals. The concentrations of Li, F, Sc, V, Cu, Ga, Se, Sr, Zr, Nb, Hf, Pb, Th, and U are close to the average values of the world’s hard coals. The minerals in No.12 coal mainly include quartz, kaolinite, siderite, and ankerite, along with smaller amounts of pyrite, illite, calcite, and rutile. The formation of syngenetic siderite in No.12 coal is related to the weathering of biotite in the gneiss of the Yinshan Upland. The modes of occurrence of ankerite indicate that the coals may be affected by the injection of low temperature hydrothermal fluids. It is noteworthy that a portion of epigenetic ankerite may be a product of metasomatism between syngenetic siderite and the epigenetic Fe-Mg-Ca rich hydrothermal fluids. The ratios of Al2O3/TiO2, REY (rare earth elements and yittrium) enrichment patterns, the modes of occurrence of siderite and ankerite, as well as the enriched lithophile and siderophile elements indicate that the No.12 coal may have originated from the Yinshan Upland and may also have been influenced by low temperature hydrothermal fluids that might have circulated in the coal basin.

Impact of the crystallite parameters and coal ranks on oxidation and combustion properties of Carboniferous coals and Jurassic coals

Coal fires have become a global issue that poses the immense threat to energy resources, environment, the economy, and human health. Given that the chemical structure substantially affects the propensity and behavior of coal fire, two raw Xinjiang Jurassic coals (collected from Piliqing and Yuxing collieries, PLQ and YX) and two Shanxi Carboniferous coals (collected from Dongqu and Fenghuangshan collieries, DQ and FHS) were selected to explore the impact of crystallite parameters and coal ranks on the oxidation and combustion properties of Carboniferous and Jurassic coals. X-ray diffraction (XRD) was used to determine the partial ordering and crystal parameters, crossing-point temperature (CPT) was used to evaluate the spontaneous combustion tendency, and temperature-programmed system (TPS) was used to estimate the coal spontaneous combustion tendency and forecast the coal self-ignition property. The experimental data and the existing literature were used to compare and discuss the variations. Results show that Jurassic coals were characterized by higher volatiles, moisture and oxygen, lower coal ranks, and degree of order than the Carboniferous coals. The values of CPT are PLQ < YX < DQ < FHS, thereby showing the same variation trend as the coal ranks and interlayer spacing (d002). The CPT value has good positive linear with random reflectance of vitrinite (Ro) and a perfectly negative linear with interlayer spacing (d002), indicating that the coal ranks and interlayer spacing (d002) have a significant impact on the combustion properties. The oxygen consumption also is impacted by coal ranks and interlayer spacing (d002). The concentration of index gases tends to increase exponentially with an increase in coal temperature. The Jurassic coals easily and rapidly produce the index gases.