Middle Jurassic Coals Research Articles

Dual Control of Macrolithotype and Coal Structure on the Pore Parameters of Middle Jurassic Coals in the Southern Junggar Basin, NW China

Dual Control of Macrolithotype and Coal Structure on the Pore Parameters of Middle Jurassic Coals in the Southern Junggar Basin, NW China

Geochemistry of Middle Jurassic Coals from the Dananhu Mine, Xinjiang: Emphasis on Sediment Source and Control Factors of Critical Metals

In recent years, coal-type critical metal deposits have become a research hotspot in coal geology. As a major coal-accumulating basin in the Xinjiang area, the Turpan-Hami Basin contains abundant coal resources and has the potential to become a large coal-type critical metal deposit. However, previous studies on the enrichment characteristics of critical metal elements in coal are few and need further research. Based on SEM-EDS, XRF, and ICP-MS experiments, this study investigates the coal petrology, mineralogy, and geochemistry of the No. 22 coal of the Xishanyao Formation from the Dananhu Coal Mine, Xinjiang, to identify the sediment source, depositional environment, and controlling factors of the critical metal elements of the No. 22 coal. The results showed that the Dananhu coals are characterized by a low ash yield, low total sulfur content, high volatile yield, and high inertinite proportions. Quartz, kaolinite, and illite are the main minerals in the coal. Compared with the world’s low-rank coals, Ni, Co, Mo, and Ta are slightly enriched, Li, Rb, Cs, Ba, Tl, Bi, and Ge are depleted, and the concentrations of other trace elements are comparable to the average values of the world’s low-rank coals. The REY of the Dannanhu coals exhibited high fractionation, with its enrichment patterns characterized by the H-type and M-H-type. Although most of the critical metals are not enriched in the Dannanhu coals, the Ga, Zr (Hf), and Nb (Ta) concentrations in the coal ash of the Dannanhu coals have reached the economic cut-off grade and have the potential to be a substitute for rare metal resources. The terrigenous detrital sources of the Dannanhu coals mainly come from the Paleozoic dacite, andesite, and a small amount of granite from the Harik Mountain and Eastern Bogda Mountain in the Turpan-Hami Basin. The Dannanhu coals are generally in a dry and hot depositional environment, with high salinity and weak reduction-oxidation. The low source input and weak reduction-oxidation environment have resulted in low concentrations of critical metal of the No. 22 coal from the Dananhu Coal Mine.

Distribution, Occurrence and Enrichment Causes of Sodium in Middle Jurassic Coal from Zhundong Coalfield, Xinjiang

Distribution, Occurrence and Enrichment Causes of Sodium in Middle Jurassic Coal from Zhundong Coalfield, Xinjiang

Influence of high-frequency lake level fluctuations on organic matter accumulation in the northern Qaidam Basin, NW China: Insights from spectral attribute analysis and geochemistry

Assemblages of multiple organic matter-rich sedimentary energy carriers are frequently found in Jurassic nonmarine basins of China. These basins are strongly influenced by high-frequency fluctuations in lake levels. A series of coal- and oil shale-bearing successions reflecting periodic lake level variations is also present in the Shimengou Formation, northern Qaidam Basin, NW China. These Middle Jurassic coal and oil shale seams provide an excellent basis for studying the effects of high-frequency lake level fluctuations on organic matter accumulation. Moreover, additional factors influencing the primary organic matter input (e.g., wildfires) and its preservation and later diagenetic transformation (e.g., processes during coalification) must be considered. In the present study, organic matter maturity, source potential, and paleoenvironmental conditions during source rock formation are investigated based on macro- and micropetrographic data, proximate analyses, bulk geochemical parameters, biomarker analyses, stable isotope geochemistry, and spectral attribute analysis to provide a sequence stratigraphic framework for the accumulation of organic-rich intervals. According to the established depositional model, the Shimengou Formation can be subdivided into four depositional sequences, including three organic matter-rich intervals. These include the thick mixed gas- and oil-prone Sq1 coal seam with a relatively high ash yield, which represents a highstand systems tract, the gas-prone Sq3 coal seam corresponding to a transgressive systems tract, and the Sq4 oil shale layers likely corresponding to a subsequent highstand systems tract, indicating a drowned mire. The results provide insights for the optimization of high-quality sedimentary mineral resource production in similar basins worldwide.

Mineralogy and geochemistry of the Middle Jurassic coal from the Hexi Mine, Shenfu Mining Area, Ordos Basin: With an emphasis on genetic indications of siderite

Siderite is a common carbonate mineral in coal and usually contains geochemical indications (elemental associations and isotopic compositions) for sedimentary environments and regional geological history, but the origin and enrichment mechanism of geochemically-pure siderite in coal is not fully understood. A better understanding of siderite origin in coal can help to decipher the depositional environment of peat and related geological processes of rank advances and epigenesis. This paper analyzed the petrological, mineralogical, and geochemical characteristics, particularly the content, occurrence, and origins of siderite in the Middle Jurassic coal from the Hexi mine of the Shenfu mining area, Ordos Basin, using optical microscopy, field emission-scanning electron microscopy, X-ray fluorescence, X-ray diffraction, inductively coupled plasma mass spectrometry, and isotope ratio mass spectrometry. The results indicate that the No. 4 Coal is rich in inertinite and is defined as an ultra-low to low sulfur, high volatile bituminous coal. Siderite has contents varying from 5.8% to 54.4% (low temperature ash basis) and occurs in radial or concentric circle nodules in the coal and is mainly distributed within the clay matrix. Syngenetic siderite in the Hexi coals is relatively pure in chemical compositions, and was formed in a brackish water environment affected by salt lakes under arid conditions. The high content of inertinite, high Sr/Cu value, and positive Ce anomaly at the uppermost of the coal seam indicate that the escalating intensification of oxidation and evaporation. The Al2O3/TiO2 ratio and the diagrams of Al2O3/TiO2 vs. Zr/ TiO2 and Al2O3/TiO2 vs. Nb/Yb indicate that the sedimentary source rock in the study area is primarily intermediate-felsic. The δ13CPDB-δ18OSMOW diagram shows C and O for the siderite formation in this study were derived from marine carbonate rock alteration of the underlying Ordovician marine carbonate rock basement. The distribution patterns of rare earth elements and Y (Middle-type enrichment and Gd anomalies) in the coal further verify this hypothesis. Furthermore, siderite in the coal exhibits an enrichment in 13C suggest that bacterial fermentation of organic matter as another possible source of CO2. The iron of the siderite in the study coal was probably derived from the leaching of the magnetite-bearing granite.

Source of inorganic components in the Middle Jurassic inertinite-rich coals of the Southern Ordos Basin, China: With emphasis on formation of Sr- and Ba-bearing minerals

The Middle Jurassic coal seams located in the southern Ordos Basin display notably high concentrations of barium (Ba) and strontium (Sr). To acquire a comprehensive understanding of the source and occurrence of the elevated contents of Ba- and Sr-bearing minerals and their formation mechanism in the coal, a suite of petrological, mineralogical, and geochemical analyses was conducted on the Middle Jurassic coals of the Gaojiapu mine (No. 4 Coal) from the Binchang mining area, using optical microscopy, field emission-scanning electron microscopy, X-ray diffraction, X-ray fluorescence, and inductively coupled plasma mass spectrometry. The results indicate that the No. 4 Coal is a high volatile B bituminous coal, which is characterized by low ash, medium sulfur, and high inertinite contents. The modes of occurrence of minerals identified in the coal, including calcite, dolomite, pyrite, quartz, kaolinite, anatase, barite, and celestine, indicate that the No. 4 Coal may have been subjected to multiple stages of solutions injection. Barium and Sr in the investigated coals are significantly enriched, exhibiting maximum values of 19738 μg/g and 9932 μg/g, respectively, while concentrations of other trace elements are generally depleted. Barite and celestine, generally coexisting with carbonate minerals, commonly occur as cell- and fracture-fillings. Furthermore, the strong correlation coefficients (r) for Ba vs. Ca + Mg and Sr vs. Ca + Mg (0.78 and 0.61, respectively) indicate that most barite and celestine have a close association with carbonate minerals of epigenetic origin. The highly elevated concentrations of Ba and Sr are thought to be primarily derived from solutions leaching barite deposits developed in the Qinling Orogenic Belt, which is located in the south of the Ordos Basin. When mapped spatially, the distributions of Ba and Sr concentrations are fan-shaped, decreasing from the NW to the SE of the mining area. Meteoric water containing dissolved CO2 induced partial dissolution of carbonate rocks, and during this process, selected metal ions (e.g., Ca2+, Mg2+, Al3+, Si2+, Fe2+, Ba2+, and Sr2+) were redistributed and subsequently migrated along fracture zones to the coal seam. The fracture systems and abundant cavities within inertinite in the coal provided sufficient space and conduits for solutions migration.

Depositional and thermal history of a continental, coal-bearing Middle Jurassic succession from Iran: Hojedk Formation, northern Tabas Block

AbstractDuring the early Bajocian, a conspicuous coal-bearing siliciclastic succession was deposited in the northern Tabas Bock, which is important for understanding the regional geodynamics of the Central-East Iranian Microcontinent (CEIM) as well as for the Jurassic coal genesis in this part of Laurasia. Sedimentary facies analysis in a well-exposed section of the lower Bajocian Hojedk Formation (Kalshaneh area, northern Tabas Block) led to the recognition of ten characteristic sedimentary facies and three facies associations, representing channels with point bars and floodplains of a Bajocian meandering river system. Modal analysis indicates that the mature quartz arenites and quartzo-lithic sandstones of the Hojedk Formation originated from the erosion and recycling of older, supracrustal sedimentary rocks on the Yazd Block to the west. The coal petrography and maturity show an advanced maturation stage, whereas the great thickness of these continental strata points to a pronounced extension-related subsidence in the northern Tabas Block. The rapid rate of differential subsidence can be explained by accelerated normal block-faulting in the back-arc extensional basin of the CEIM, facing the Neotethys to the south. Compared to the thick Jurassic, the post-Jurassic strata are relatively thin and played a limited role in the thermal history of the coal in the northern Tabas Block. A relatively high geothermal gradient in the tectonically highly mobile area of the northern Tabas Block and/or heating by regionally widespread Palaeogene intrusions were most probably the key drivers of the thermal maturation of the Middle Jurassic coals.

Record of Middle Jurassic wildfire and its incidental mercury emissions in northern Qaidam Basin, China: Evidence from the inertinite and mercury anomalies in coal

Inertinite in coals contains important information regarding the palaeo-wildfire history and can provide insights into its triggers and relationship with the evolution of local ecosystems. However, such information in the Middle Jurassic coal is often overlooked, and the impacts of incidental mercury emission from wildfires on the mercury enrichment in coal are also underestimation. In this study, the organic petrology, geochemistry and palynology of coal seams from the coal-bearing member of Shimengou Formation in northern Qaidam Basin (NQB) were investigated. Results show that these coal seams generally contain abundant records of palaeo-wildfire: inertinite and combustion-derived polycyclic aromatic hydrocarbons (PAHs). The high consistency of these two evidences confirms the existence of widespread wildfires during the late Middle Jurassic (Bathonian) in NQB. These widespread wildfires also indicate a high level of atmospheric oxygen concentration during this period. The overall low inertinite reflectance (avg. 1.27%) and low content of coronene in coal seams indicate that these wildfires were dominated by ground and surface fire with some small-scale crown fires. These frequent wildfires under the overall humid climate are attributed to the sufficient atmospheric oxygen, fuel and intermittent dry season. The co-occurrence of wildfires records and mercury anomaly in coal seams suggests that incidental mercury emissions from wildfires enhanced mercury input in the peatlands, which is the direct trigger for mercury anomaly in the coal from the Shimengou Formation in NQB. This study not only revealed the characteristics and causes of the widespread wildfires during the late Middle Jurassic in NQB, but also provides a new insight into the abnormal enrichment of mercury in coal.

Biomarker signatures of the Middle Jurassic coals from the Zhangjialiang mine in Dongsheng coalfield, North China: Implications for palaeoenvironment and palaeovegetation

The study on the Zhangjialiang No. 2 coal of Middle Jurassic Yan'an Formation in the northeastern Ordos Basin has been analyzed by gas chromatography-mass spectrometry (GC-MS) to determine geochemical characteristics and reconstruct palaeovegetation and palaeoclimate changes. The distribution of n-alkanes is mainly unimodal, with a medium and high molecular weight. Pr/Ph ratio (2.03–10.6) is high in most samples. The values of Tm/Ts ratio are between 1.42 and 4.92, and a predominance of C29 over C27 steranes has been found for the analyzed samples. These parameters indicate that all samples were deposited in an oxidizing freshwater environment, and the organic matter was mainly derived from terrestrial higher plants, with a small amount of aquatic organisms. Thermal maturity parameters such as C3122S/(22S + 22R) homohopane, C30αβ/(αβ + βα) hopane, together with the total organic carbon (TOC) contents (43.1% - 58.6% (except floor 2-13)) and the detected of C2717β(H)-22, 29, 30-trisnorhopane (βTm), suggest that most of samples have a relatively low maturity in the early stage. Besides, gymnosperm-derived biomarkers, including labdane, norpimarane, isopimarane, phyllocladane, abietane, rimuane, ent-beyerane, ent-kaurane, suggest that the vegetation during the Middle Jurassic Yan’an Formation was characterized by conifers.

Middle Jurassic intracontinental evolution of East Asia: Insights from the Tianshifu-Dongyingfang basin of the Liaodong Peninsula, NE China

AbstractThe Middle Jurassic intracontinental shortening event in East Asia, known as episode A of the Yanshanian Movement in the eastern North China Craton and the first phase of the Daebo Orogeny in the Korean Peninsula, respectively, has been thoroughly studied during the past two decades. However, the timing of its beginning, duration, and deformational processes are still highly controversial because few studies have probed the correlations between episode A of the Yanshanian Movement and the first phase of the Daebo Orogeny. The Tianshifu-Dongyingfang basin, located in the northern Liaodong Peninsula, is filled with complete Jurassic sequences that can be compared with those in the eastern North China Craton to the west of the Tan-Lu fault zone and the Korean Peninsula. Regional stratigraphic correlations suggest that the Tianshifu-Dongyingfang basin contains not only the syntectonic conglomerates, but also the lower and upper unconformities A1 and A2 of episode A of the Yanshanian Movement, which separated the syntectonic conglomerates from the underlying Early–Middle Jurassic coal measures and the overlying Late Jurassic red beds, respectively. Detailed field survey and structural analysis show that the development of syntectonic conglomerates and unconformities A1 and A2 were controlled by the imbricate thrust fault system in the basement, which resulted from the WNW–ESE compression inversed from the fault-slip data. The youngest detrital zircon U-Pb ages obtained near unconformities A1 and A2 show that the WNW–ESE compression of episode A of the Yanshanian Movement in the Liaodong Peninsula initiated at ca. 171 Ma and ended at least by 163 Ma. Our new structural and geochronological data, combined with data from the neighboring areas, suggest that episode A of the Yanshanian Movement and the first phase of the Daebo Orogeny belong to a single intracontinental shortening event, the compressional deformation of which was derived from the westward, lowangle subduction of the Paleo-Pacific plate and transmitted to the Korean and Liaodong Peninsulas at ca. 172–170 Ma and to the eastern North China Craton to the west of the Tan-Lu fault zone at 170–165 Ma.

Geochemical characteristics and paleoclimate implication of Middle Jurassic coal in the Ordos Basin, China

Coals are highly enriched in critical elements (rare earth elements, etc.), containing much information of paleoenvironmental and paleoclimatic during deposition. The Jurassic period is a significant coal-forming period in the world. The Yan’an Formation in the Ordos Basin in north China is among the largest Middle Jurassic coal fields of the world. The purpose of this study is to investigate the geochemical characteristics of Middle Jurassic Aalenian coals of Ordos Basin and their paleoclimate significance. Sixty-two coal samples were collected from two coal mines in the Dongsheng area in the northeast Ordos Basin. The major and trace elements in the coal samples were analyzed using X-ray fluorescence (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS). The main oxides in the Yan’an coals are SiO2 and CaO. In comparison with average values for China, the coals from the Ordos Basin have a slight enrichment of CaO and MgO. Strontium and Mo values are also slightly enriched. Yttrium and rare earth elements are depleted in the coals, with a range of 8–269 μg/g (average = 48.5 μg/g; on an ash basis). The coals in the Yan’an Formation have a negative Eu anomaly, suggesting that the peat bog had a stable terrigenous material supply during the coal-forming period. The continental saline water system and oxygen-suboxygenic environment were inferred from the Sr/Ba, U/Th, Ni/Co, Sr/Cu. The Sr/Cu and Sr/Ba data of Yan’an Formation coal seams in the Ordos Basin also indicate climate change from a relatively arid condition to a more humid climate, which is consistent with change of marine environment during the Aalenian. Global cooling during the Aalenian may have triggered strengthened moisture transport by westerlies in mid-latitude inland Asia, and therefore a humid climate in the Ordos Basin.

Geological Controls on Mineralogical Characteristic Differences of Coals from the Main Coal Fields in Shaanxi, North China

Shaanxi is among the provinces with abundant coal resources in North China. These enormous coal resources (approx. 4143 Gt) are widely distributed in the Ordos Basin and its marginal fold belts. The main coal-bearing strata consist of the late Carboniferous Taiyuan Formation, the early Permain Shanxi Formation, the late Triassic Wayaobao Formation, and the middle Jurassic Yan’an Formation, which were respectively deposited in coastal plains and a lagoon environment, a continental environment, an inland open lake and a confined lake environment. The Permo-Carboniferous coals are low volatile bituminous and characterized by relatively high vitrinite content, which decreases from south to north, and from the lower coal seams upwards. By contrast, the late Triassic and middle Jurassic coals are highly volatile bituminous, but are respectively characterized by relatively high vitrinite and high inertinite content. Minerals in the Permo-Carboniferous coals, the late Triassic coals, and the middle Jurassic coals, are respectively dominated by kaolinite and calcite, quartz and kaolinite, and quartz and calcite. Furthermore, contemporary coals deposited in different coal fields or even different mines of the same coal field present different mineral characteristics. The Permain Shanxi Formation coals from the Shanbei C-P coalfield in the north of Shaanxi Province are characterized by higher kaolinite and lower carbonate contents compared to those from the Weibei C-P coalfield in the south of Shaanxi Province. The distinctive mineralogical characteristics of coals formed in different coalfields and different geological ages were ascribed to integrated influences of different terrigenous detrital input from sediment provenance, sedimentary settings (e.g., subsidence rate, sea transgression, and regression process), and hydrothermal activities.

The petrographic compositions of Chinese commercial coals: A national survey and statistical analysis

An updated survey and statistical analysis were conducted about the petrographic compositions of Chinese commercial coals nationwide. 727 commercial coal samples were systematically collected from 25 provinces in China and the spatial distributions of the samples were consistent with the resources and capacities of the coal mines in China. The petrographic characteristics of different coal-forming ages, and coal-accumulating areas were analyzed in detail. The average petrographic compositions based on reserves weight were put forward. The weighted average contents of vitrinite, inertinite, liptinite were 61.3%, 36.5%, and 2.2%, respectively. Relatively high content of inertinite and low content of liptinite were found of Chinese commercial coals. The average contents of vitrinite were the highest in Cenozoic coals (93.5%) and Late Jurassic and Early Cretaceous coals (87.1%) while that was the lowest in Early and Middle Jurassic coals (52.4%). The average contents of vitrinite were the highest in Northeast (87%) and lowest in Shanxi-Shaanxi-Inner Mongolia-Ningxia (58.8%) and Northwest (59.9%). The coals in Northwest and Shanxi-Shaanxi-Inner Mongolia-Ningxia were characterized by high content of inertinite, which were mainly Early and Middle Jurassic coals. Diverse distributions of vitrinite and inertinite were found of coals in Northwest. Considering the layout of coal production, the vitrinite content will be decreased in the western areas and increased in the east areas.

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.

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.

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

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.

Occurrence of analcime in the middle Jurassic coal from the Dongsheng Coalfield, northeastern Ordos Basin, China

This paper describes the occurrence of significant authigenic analcime (NaAlSi2O6·H2O) in the middle Jurassic coal from the northeastern Ordos Basin and discusses its origin. Twelve coal samples from borehole ZK31-19 in the Dongsheng Coalfield were collected and analyzed. The coals are characterized by a low-medium ash yield, high volatile matter and low sulfur contents. The major-element oxides in the studied coals are mostly at similar or much lower concentrations compared to the averages for Chinese coals. Similarly, most trace elements are depleted or in concentrations close to the respective averages for world low-rank coals. Based on Seredin-Dai's classification, the REY (rare earth elements and Y) in the coal mainly display N- and H-type distribution patterns, with no Ce and Eu anomalies. Coal sample ZK31-19-12 from the lower Zhiluo Formation contains analcime up to 23 wt%, with a high Na2O concentration of 1.23% (both on a whole-coal basis). The analcime occurs as lath, as subhedral granular crystals in fissures or pores and as anhedral crystals in the organic matrix. Furthermore, significant analcime has also been observed by SEM in coal from the upper Yan'an Formation (sample ZK31–19-10), which mainly occurrs as anhedral crystals. No evidence of analcime forming from precursors of volcaniclastic material, alkali zeolites or other silicates was detected, therefore, it is deduced that the analcime in the studied coals formed by authigenic precipitation. The Na2O content in the studied coal seams displays a vertical variation different from that of the ash yield (Ad) and those of the detrital-dominated elements, such as SiO2, Al2O3, TiO2, Zr, Nb and Th, indicating a different origin of Na compared with other detrital materials of terrigenous origin. Combined with the evidence of the obvious Na peak in authigenic amorphous material and gypsum mineral by SEM, it can be inferred that the source of Na may derive from the fluids. The paleoclimatic change (from subtropical warm and humid to semi-arid and arid hot, during the deposition period of the Yan'an to the Zhiluo Formation) may provide favorable conditions for analcime authigenesis, leading to its abundant precipitation in the lower Zhiluo Formation and its limited occurrence in the upper Yan'an Formation. Such paleoclimatic change is also significant for the U mineralization in the lower Zhiluo Formation.

Petrographic Controls on Pore and Fissure Characteristics of Coals from the Southern Junggar Coalfield, Northwest China

The productive potential of coalbed methane projects is controlled by pore and fissure characteristics, which are intrinsically related to coal petrology. This work attempts to identify the influence of petrographic factors on the development of pore and fissure systems in the southern Junggar Coalfield, Northwest China. Here, Middle Jurassic coal (lignite and subbituminous) petrology in coal seam No. 45 of the southern Junggar Coalfield (SJC) is studied with respect to the characteristics of pore and fissure structure with the aid of optical microscopes, scanning electron microscopy, mercury intrusion porosimetry, and nuclear magnetic resonance analysis. Maceral analysis shows coals at the SJC are dominated by vitrinite (38–87 vol %), with moderate quantities of inertinite (1–28 vol %) and liptinite (0.5–30 vol %). Decomposition of plants occurs under slightly oxic–anoxic conditions, with good tissue retention. Four types of coal facies are classified using petrographic indices, comprising (1) lower delta plain marsh, (2) lower delta plain fen, (3) upper delta plain wet forest swamp; and (4) piedmont plain moor. Pores and fissures are generally observed in telinite, collotelinite, fusinite, and semifusinite in SJC coals, indicating that the generation of pores and fissures is strongly influenced by coal macerals. Pore and fissure structures of coals in coal facies (1) appear weakly connected, whereas those in coal facies (2) reveal good connectivity. Coals in coal facies (3) and (4) show moderate connectivity between pore and fissure structure. Therefore, pore and fissure structures are significantly controlled by coal facies. This work provides practical recommendations and implementation methods for petrological studies in future coalbed methane exploration/development in the SJC. This study also serves to predict the physical properties of pores and fissures and interpret the control mechanism of coalbed methane production using coal petrology.

A REVIEW OF THE COALY SOURCE ROCKS AND GENERATED PETROLEUMS IN THE DANISH NORTH SEA: AN UNDEREXPLORED MIDDLE JURASSIC PETROLEUM SYSTEM?

This paper reviews the Middle Jurassic petroleum system in the Danish Central Graben with a focus on source rock quality, fluid compositions and distributions, and the maturation and generation history. The North Sea including the Danish Central Graben is a mature oil province where the primary source rock is composed of Upper Jurassic – lowermost Cretaceous marine shales. Most of the shale‐sourced structures have been drilled and, to accommodate continued value creation, additional exploration opportunities are increasingly considered in E&P strategies. Triassic and Jurassic sandstone plays charged from coaly Middle Jurassic source rocks have proven to be economically viable in the North Sea. In the Danish‐Norwegian Søgne Basin, coal‐derived gas/condensate is produced from the Harald and Trym fields and oil from the Lulita field; the giant Culzean gas‐condensate field is under development in the UK Central North Sea; and in the Norwegian South Viking Graben, coal‐derived gas and gas‐condensate occur in several fields. The coaly source rock of the Middle Jurassic petroleum system in the greater North Sea is included in the Bryne/Lulu Formations (in Denmark), the Pentland Formation (in the UK), and the Sleipner and Hugin Formations in Norway. In the Danish Central Graben, the coal‐bearing unit is composed of coals, coaly shales and carbonaceous shales, has a regional distribution and can be mapped seismically as the ‘Coal Marker’. The coaly source rocks are primarily gas‐prone but the coals have an average Hydrogen Index value of c. 280 mg HC/g TOC and values above 300 mg HC/g TOC are not uncommon, which underpins the coals' capacity to generate liquid hydrocarbons (condensate and oil). The coal‐sourced liquids are differentiated from the common marine‐sourced oils by characteristic biomarker and isotope compositions, and in the Danish Central Graben are grouped into specific oil families composed of coal‐sourced oil and mixed oils with a significant coaly contribution. Similarly, the coal‐sourced gases are recognized by a normally heavier isotope signature and a relatively high dryness coefficient compared to oil‐associated gas derived from marine shales. The coal‐derived and mixed coaly gases are likewise assigned to well‐defined gas families. Coal‐derived liquids and gas discoveries and shows in Middle Jurassic strata suggest that the coaly Middle Jurassic petroleum system has a regional distribution. A 3D petroleum systems model was constructed covering the Danish Central Graben. The model shows that present‐day temperatures for the Middle Jurassic coal source rock ('Coal Marker') are relatively high (>150 °C) throughout most of the Danish Central Graben, and expulsion of hydrocarbons from the ‘Coal Marker’ was initiated in Late Jurassic time in the deep Tail End Graben. In the Cretaceous, the area of mature coaly source rocks expanded, and at present day nearly the whole area is mature. Hydrocarbon expulsion rates were low in the Paleocene to Late Oligocene, followed by significant expulsion in the Miocene up to the present day. High Middle Jurassic reservoir temperatures prevent biodegradation.