Search

China produced 17.1 billion cubic meters (BCM) of methane sourced from coal seams in 2015, of which 4.43 BCM is from wells drilled from the surface. This level of production is a clear indicator that China has gone into early stage large-scale coalbed methane (CBM) development. CBM resources in China have been extensively investigated since the 1980s. Research has focused on the geological controls of reservoir character. There have been significant advances over the last 37 years that have aided China’s CBM industry. CBM resources less than 2000 m in depth in China are estimated to be 36.81 trillion cubic meters, of which more than 84% occur in nine large-scale basins, such as the Qinshui, Ordos, Junggar, Qianxi, Erenhot, and Hailar. CBM accumulation and coal reservoir characteristics are controlled by the deposition, structure, coal rank, hydrology as well as other geological factors. Each basin has its own unique geological controls that influence the character of CBM reservoirs in both subtle and obvious ways. Coal reservoir geology in some basins or regions in China are still not well understood because of the complexity of the geological settings. At present, large-scale CBM production in China only occurs within the Qinshui and Ordos basins, mostly sourced from middle-to-high rank coal reservoirs. The CBM geology in other basins needs further investigation in order to achieve large-scale commercial production. To this end, the geological research in the paper should address issues such as how to stimulate economic gas flow from deep low permeability reservoirs and how best to efficiently produce from multiple horizons simultaneously. This paper summarizes these and other key issues that are significant scientific and technical challenges for the CBM industry within China. [ABSTRACT FROM AUTHOR]

[Display omitted] • The No. 4 Coal was deposited in a relatively oxygen-rich environment. • The No.4 Coal seam was subjected to multiple stages of solutions. • Distributions of Ba and Sr display a fan-shaped trend in the Binchang mining area. • Barite and celestine in the coal are closely related to carbonate minerals. • Highly elevated Ba and Sr are the results of solutions leaching of barite deposits. 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 CO 2 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. [ABSTRACT FROM AUTHOR]

Prospecting for trace and rare earth elements will only increase in the coming decades as the world expands into sustainable energy and the use of digital technology. A key component of finding these resources is understanding how they are distributed, mobilized and emplaced. The coal beds in Shanxi Province in China have already been identified as a resource with elevated trace elements, especially lithium. This study has extended the understanding of trace elements in the region from examination of two sites, one in the Ningwu coalfield (the No. 11 coal seam) and the other in the Qinshui Basin (the No. 8 coal seam). Lithium, in particular, but most trace elements, in general, were found to be associated with the inorganic fraction of the coal seams. Specifically, trace elements are highest in the sediments overlying the coal and in the interbedded partings. The coal beds themselves were also enriched, especially in Li, but their concentrations vary in direct proportion with ash yield. Differing degrees of maturation (the Ningwu site ~0.70% Ro; the Qinshui site ~2.50% Ro) seem to have little effect on the association of trace elements with inorganics. Using the relationship between Rb vs Y + Nb, the ultimate provenance of the sediment (and thus the trace elements) may be from the weathering products of regionally proximal granites. However, two high ash coal outliers, both from the Qinshui Basin sampling site, appear to have a different trace element source, as yet unidentified, compared to all other samples. The REY (rare earth elements and Y) patterns indicate hydrothermal fluids are also responsible for REY enrichments in the coal seams. • High Li-coals of two sites in Shanxi Province in China were presented. • Differing degrees of maturation seem to have little affect on the association of trace elements with inorganics. • The relationship between Rb vs Y + Nb was employed to distinguish the provenance of the sediment. [ABSTRACT FROM AUTHOR]

Igneous intrusions into coal affect the safety, productivity, and economic viability of many coal mines. Also, the alterations in coal caused by igneous intrusions encompass a deterioration in coal quality, a decrease in permeability, an enhancement of coal rank, and geochemical and mineralogical compositions. Although there have been some studies related to the effects of igneous intrusions on the inorganic constituents in the coal, the mechanism of newly formed chemical and mineralogical associations caused by igneous intrusions has not been fully understood. This paper investigates the mineralogical and geochemical responses of coal to thermal alteration by igneous intrusion into an Early Permian coal seam in Hebei Province, North China. The Fengfeng coalfield is structurally complex, experiencing variable rates of burial and uplift, with abundant intrusions during the upper Jurassic and maximum burial in the Cretaceous resulting in an increase in rank to the north. In the search for critical minerals and elements, intrusions and hydrothermal alteration are often cited as the source for elevated concentrations. Towards the intrusion, the coal rank indicators increase from semi-anthracite to anthracite over a 500-cm distance, as does the mineral matter. The clay minerals present in coals include NH 4 -illite, kaolinite, paragonite, and chlorite (chamosite and cookeite), which were all resulted from igneous intrusions. The epigenetic minerals mainly include quartz, ankerite, calcite, pyrite, and apatite, which were derived from fluids permeating into the coal seam and chemically interacting with the original minerals or precipitating directly to form new minerals. Geochemical indicators in this coal indicate hydrothermal solutions derived from igneous intrusion. The enrichment of Li and Hg is linked to the igneous activity, potentially influencing the utilization of the coal adjacent to the intrusions. However, the concentrations of the remaining elements in the coal closest to the intrusion show no significant changes, thereby supporting the notion of a "non-universal effect" of igneous intrusions on the elemental composition and concentrations. • Towards the intrusion, the coal rank indicators increase, as does mineral matter. • Various clay minerals in coal are associated with hydrothermal fluids. • Epigenetic minerals show at least three stages of hydrothermal fluids. • Geochemical indicators are used to characterize hydrothermal fluids from intrusion. • The enrichment of Li and Hg in coal nearby intrusion is linked to igneous activity. [ABSTRACT FROM AUTHOR]

[Display omitted] • Tarim Basin has both positive and negative indicators for Zr–Nb–REY–Ga–U ores. • The Permian pyroclastics are likely the mineralization source in Tarim Basin. • Positive indications are the larger and more felsic volcaniclastics in Tarim Basin. • Negative indicators arethe relatively low organic matter and coal absence. • The Permian tuffaceous mudstones are a likely target for Zr–Nb–REY–Ga–U ores. This study is based on published data from the Tarim and Emeishan Large Igneous Provinces (TLIP and ELIP, respectively). The data are interpreted in a comparative manner, using the well-studied ELIP's mineralization as a reference. Some additional information on related U and REY-carbonate ores from Far Eastern Russia are also included in this comparison. The results show that the Tarim Basin has both positive and negative indicators for tuffaceous Zr(Hf)–Nb(Ta)–REY–Ga–U ores. Like in ELIP, the source of the mineralization in the Tarim Basin is likely to be the Permian volcaniclastic deposits. The volcaniclastic deposits in the Tarim are not well studied geochemically; however, since the composition of the parent magmatic rocks are similar between TLIP and ELIP, the volcaniclastic material is also thought to be similar. Positive indications are that the TLIP's deposits are larger (volumetrically), more felsic than the ELIP's volcaniclastics. In addition, the former are concentrated in an almost closed terrestrial basin (Tarim Basin), whereas the latter are disseminated throughout a wide continental margin. The Paleozoic strata of the Tarim Basin are deeply altered in and around fracture zones, which are widespread. The alteration is caused by acidic volcanic-related fluids that are a key factor in metal mobilization and redistribution. However, in contrast to the ELIP's area, the Tarim Basin was under influence of semi-arid climate that probably resulted in a weak activity of meteoric waters and poorly developed vegetation during deposition of the Permian volcaniclastic strata, which have a relatively low content of organic matter (<5%, commonly <2%) and contain no coal seam. These may be considered negative indicators for tuffaceous Zr(Hf)–Nb(Ta)–REY–Ga–U ores because organic acids might play a significant role in the leaching, redistribution, and the finishing processes that concentrated Zr, Hf, Nb, Ta, REY, Ga, and U within rocks with clay alteration. Finally, the Permian tuffaceous mudstones located in and around deep faults of the Tarim Basin may be a likely target for geochemical prospecting for Zr(Hf)–Nb(Ta)–REY–Ga–U ores similar to those found in southwestern China. In addition, the fracture zones in the Tarim Basin are likely to contain REY-carbonate and U ores similar to those discovered in Primorye, the Far Eastern Russia. [ABSTRACT FROM AUTHOR]