Carboniferous Limestone Research Articles

Relevant influence of alkali carbonate doping on the thermochemical energy storage of Ca-based natural minerals during CaO/CaCO3 cycles

The calcium-Looping process is an advantageous candidate for thermochemical energy storage in Concentrated Solar Power plants. Achieving fast thermal energy storage at a moderate temperature would be highly beneficial for the heat storage process. In this study, commercially available alkali carbonates (Li2CO3, Na2CO3, K2CO3) were used as dopants for limestone/dolomite to improve their decarbonation rates and cycling stability. Results show that alkali carbonate doping reduces the decarbonation temperature of limestone and dolomite by accelerating the ion diffusion during decomposition. However, the doping of alkali carbonate also enhanced the sintering of limestone-based CaO so that the alkali carbonate doped limestones fail to maintain their heat storage capacity after multiple cycles. In contrast, the Li2CO3 doped dolomite still maintains excellent cyclic stability, attributed to the inhibition effect of the MgO skeleton on sintering. From these results, we then derived the effect scheme of Li2CO3 on the decarbonation and carbonation of limestone or dolomite. This study presents a simple yet effective method of reducing the heat storage temperature of Ca-based mineral materials and maintaining their cycling stability, simultaneously, with promising potential for industrial use.

Garnet major and trace element evidence of the alteration and mineralizing processes associated with genesis of the Qiaomaishan skarn deposit, Xuancheng ore district, eastern China

The Qiaomaishan deposit is a skarn-type Cu–W deposit located within the Xuancheng ore district of the Middle–Lower Yangtze River Metallogenic Belt of eastern China. The mineralization within the deposit is predominantly located along the contact between a porphyritic granodiorite intrusion and surrounding Carboniferous limestone units. The deposit is associated with a well-developed and zoned skarn system that includes both endoskarn and exoskarn zones. This study presents new garnet major, trace, and rare earth element (REE) geochemical data for these skarn zones, discusses the factors (e.g., crystal chemistry, oxygen fugacity and water/rock interaction) that control the incorporation of trace elements into garnet, and constrains the formation of skarn within the Qiaomaishan deposit. Grossular (Adr21–70Grs29–78; type 1) garnet is generally located within the endoskarn and is light REE (LREE) enriched but heavy REE (HREE) depleted with weakly negative Eu anomalies and higher total REE contents than the exoskarn garnet in the study area. Type 1a grossular garnet is further split from type 1b grossular garnet by the presence of positive Pr and Nd anomalies and high LREE concentrations. Exoskarn garnet has pure andradite compositions and is split into type 2 and 3 garnets, where type 2 andradite garnet has variably positive Eu anomalies and variable total REE contents that are indicative of formation under transitional or oscillating conditions (e.g., pH, oxygen fugacity). In comparison, the type 3 andradite garnet has low total REE contents and positive Eu anomalies. Endoskarn garnet Al2O3 and total REE contents positively correlate, indicating that REE is incorporated into this garnet through coupled YAG-type substitution. The differences between the total REE contents of the type 2 exoskarn and endoskarn garnets may reflect host rock buffering as well as variations in the crystal chemistry of the two garnets present within the different parts of the skarn system (grossular and andradite). In comparison, the differences in the total REE contents of the two types of exoskarn garnet may reflect variations in water–rock ratios and crystal growth rates. The different Eu anomalies and U contents present within the different types of garnets record variations in the pH and oxygen fugacity of the hydrothermal fluid over time. These changes may have promoted the crystallization of magnetite and scheelite and the formation of the tungsten mineralization within the deposit. This study demonstrates the usefulness of garnet in tracking variations in mineralizing conditions and processes and confirms the validity of these minerals as vectors or indicator minerals that can be used in combination with traditional exploration approaches to determine whether a system is mineralized or not during the early stages of exploration.

Groundwater flow velocities in karst aquifers; importance of spatial observation scale and hydraulic testing for contaminant transport prediction.

We review scale dependence of hydraulic conductivities and effective porosities for prediction of contaminant transport in four UK karst aquifers. Approaches for obtaining hydraulic parameters include core plug, slug, pumping and pulse tests, calibration of groundwater flow models and spring recession curves. Core plug and slug tests are unsuitable because they do not characterize a large enough volume to include a representative fracture network. Pumping test values match regional-scale hydraulic conductivities from flow modelling for the less intensively karstified aquifers: Magnesian Limestone, Jurassic Limestone and Cretaceous Chalks. Reliable bulk hydraulic conductivities were not available for the intensively karstified Carboniferous Limestone due to dominance of flow through pipe conduits in Mendips. Here, the only hydraulic conductivity value found from spring recession is one order of magnitude higher than that indicated by pumping tests. For all four carbonate aquifers, effective porosities assumed for transport modelling are two orders of magnitude higher than those found from tracer and hydrogeophysical tests. Thus, a combination of low hydraulic conductivities and assumed flowing porosities resulted in underestimated flow velocities. The UK karst aquifers are characterized by a range of hydraulic behaviours that fit those of karst aquifers worldwide. Indeed, underestimation of flow velocity due to inappropriate parameter selection is common to intensively karstified aquifers of southern France, north-western Germany and Italy. Similar issues arise for the Canadian Silurian carbonates where the use of high effective porosities (e.g. 5%) in transport models leads to underestimation of groundwater velocities. We recommend values in the range of 0.01-1% for such aquifers.

Lower carboniferous fractured carbonates of the Campine Basin (NE-Belgium) as potential geothermal reservoir: Age and origin of open carbonate veins

Within the Lower Carboniferous limestones in the boreholes of Poederlee, Heibaart and Turnhout (Campine Basin, NE-Belgium), open veins partially cemented by calcite and dolomite are considered important with regard to the geothermal reservoir potential of these limestones. Based on a detailed petrographic analysis, stable carbon and oxygen as well as clumped Δ47 isotope analysis and U/Pb age dating, the formation conditions of these veins have been reconstructed and were placed in the geodynamic evolution of the sedimentary basin. All host rocks are recrystallized, and the open carbonate vein cements often possess a similar depleted δ18OPDB signature as their recrystallized host rock. The latter thus indicates vein cement precipitation was likely rock-buffered. The δ13C signature of the host rock often displays a marine signature, while in the veins two isotopic clusters can be differentiated. The first cluster groups veins with cleavage twins, and are characterized by slightly to moderately depleted δ13C, reflecting a mixture of marine carbon with carbon derived from organic matter. This group typically occurs in the open veins from the microbially dominated Poederlee and Heibaart buildups characterised by on-lapping organic-rich Namurian black shales. One of these veins formed at 216 ± 6 °C, indicating that it crystallised during the Sudetic – Asturian deformation stage of late Carboniferous age, while others formed between 110 and 130 °C, indicating that they could have been formed during the late Carboniferous burial stage or after the Sudetic – Asturian deformation and uplift. The second cluster groups veins that may, but often do not, contain cleavage twins. They show host-rock buffered δ13C signatures. These veins have a Cretaceous (late Aptian/early Albian) and/or Paleocene U/Pb age and formed at temperatures dominantly between 50 and 80 °C based on clumped isotopes.

The lower Carboniferous blastoid Astrocrinus tetragonus (Austin and Austin), from Grassington, Yorkshire, and the validity of Astrocrinus benniei (Etheridge)

Despite surviving longer than any other blastozoans, blastoids were exceptionally conservative in their morphology and usually symmetrically pentaradiate. Astrocrinus is a rare exception that lacked a stem and some thecal plates, although differing interpretations of its morphology and taxonomy have been published. Astrocrinus had a flattened, tetralobate theca covered in minute spines, and with a plane of symmetry through the AB interray and the D ray. Four ambulacra are long, thin, and curved down deep sinuses between the thecal lobes to reach the basal surface. The D ambulacrum is short, broad and horizontal. Astrocrinus tetragonus was first described from the Carboniferous Limestone of Settle, Yorkshire. Here it is recorded for the first time from the Brigantian, near Grassington. The new specimens confirm that A. tetragonus had a single basal plate which is kite-shaped, entirely surrounded by four radials and separated from the D ray radial. Astrocrinus benniei was described from the Scottish Brigantian and its basal plate is elongate pentagonal with a short common suture with the D ray radial. Astrocrinus occurs in the Irish Asbian and Brigantian. To date, only A. tetragonus can be confirmed from Ireland.

A green and efficient acid system for carbonate reservoir stimulation

Matrix acidizing is a well stimulation method that is applied to enhance productivity. The success of a stimulation job depends mainly on the efficiency of the acid systems that are employed. This study investigated a new and efficient acid system that is biodegradable, not corrosive, and safe to handle. The system efficiency was compared to that of HCl and chelating agents. Acid coreflooding has been applied to chalk, dolomite, and limestone carbonate rock samples to measure the efficiency in terms of injected pore volume to breakthrough (PVBT). The rock samples were characterized in terms of porosity, permeability, composition using X-ray diffraction (XRD), and pore distribution using nuclear magnetic resonance (NMR). The effluent was collected at different times and analyzed using induced coupled plasma (ICP). Medical computed tomography (MCT) was used to visualize the interior of the rock samples after acidizing. The new acid system was more efficient in terms of acid spending than straight HCl acid and chelating agents in Indiana limestone. At the optimum injection rate, chelating agents had an optimum PVBT value of about 15 compared to 0.85 for plain HCl acid. Using the new acid system resulted in an optimum PVBT value of only 0.41, which is less than half of HCl acid. A numerical wormhole simulator was implemented to match the experimental PVBT and assess the reactivity of the acid system.

A contribution to understand the Portuguese emblematic Ançã limestone bioreceptivity to fungal colonization and biodeterioration

• Portuguese Ançã limestone bioreceptivity to fungal colonization analysis. • Penicillia growth is favored in copiotrophic environments. • Aspergilli and Aeminium ludgeri growth is only slightly superior in oligotrophic environments. • After one-year incubation period all fungal species produced alterations. • Aspergillus westerdijkiae induced calcium carbonate mineralization. Fungal deteriorative action on historical stone objects often leads to important and incalculable cultural heritage losses. The Ançã stone is a unique limestone type widely used in some of the most important and emblematic Portuguese monuments. Being a soft and porous calcium carbonate limestone, this substrate is known to have a significant bioreceptivity to microbial colonization often resulting in notorious biodeterioration. Nonetheless, fungal bioreceptivity and biodeterioration of this lithotype remains scarcely characterized. With this in mind, the main aims of this work were to evaluate these aspects in a controlled laboratory environment. Inoculation of several fungal species in stone replicas revealed that Penicillia growth occupied larger areas under rich micro-nutrient environments, while Aspergilli and the black fungal species Aeminium ludgeri only denoted a slightly more accentuated growth under depleted nutrient conditions. Moreover, all species produced noticeable aesthetic alterations. In addition, we were able to verify A. ludgeri endolithic growth, Aspergillus glaucus putative hyphae penetration into calcite layers and mineralization phenomena occurring around A. westerdijkiae hyphae. By itself, this study provides a key update regarding fungal biodeterioration of this ancient, emblematic and remarkable limestone.

Pemanfaatan Precipitated Calcium Carbonat dari Batu Kapur dalam Pembuatan β- TCP sebagai Bahan Dasar Implan Tulang

One of the biocompatible materials that are often used as a basic materials for bone and dental implants is tricalcium phosphate. This material can be produced from precipitated calcium carbonate (PCC) limestone because it has a very high calcium content. The method used to synthesize tricalcium phosphate was deposition of CaO and H3PO4 precursors in ethanol media. The results of the synthesis was characterized using FT-IR instruments, AAS, UV-Vis spectroscopy, and XRD. Based on the results of the UV-Vis spectroscopic analysis, the Ca/P ratio of the synthesis results obtained was 1.87. FT-IR test showed the presence of a Ca-O group at wave number 1400 cm-1 and PO43- group at wave numbers 561 cm-1 and 1041 cm-1. XRD test showed highest peaks of β-TCP (tricalcium phosphate) according to JCPDS no. 09-0169.

Well Log Analysis of Nahr Umr, Shuaiba and Zubair Formations in EB-4 well of East Baghdad Oil Field, Iraq, using Rock Physics Templates (RPTs)

RPT is a method used for classifying various lithologies and fluids from data of well logging or seismic inversion. Three Formations (Nahr Umr, Shuaiba, and Zubair Formations) were selected in the East Baghdad Oil field within well EB-4 to test the possibility of using this method. First, the interpretations of the well log and Density – Neutron cross plot were used for lithology identification, which showed that Nahr Umr and Zubair formations consist mainly of sandstone and shale, while the Shuaiba Formation consists of carbonate (dolomite and limestone). The study was also able to distinguish between the locations of hydrocarbon reservoirs using RPT. Finally, a polynomial equation was generated from the cross plot domain (AI versus Vp/Vs) to estimate one parameter from the other in these formations, and vice versa.

Testing the relationship between marine transgression and evolving island palaeogeography using 3D GIS: an example from the Late Triassic of SW England

The Rhaetian transgression marked a major change in landscape. The Permian and Triassic had been a time of terrestrial conditions across Europe, including much of mainland UK, as well as the North Sea and Irish Sea, represented by red bed clastic successions. Seas flooded across Europe at 205.7 Ma and the shift from terrestrial to marine environments is marked in the UK by the switch from the red beds of the Mercia Mudstone Group to the black mudstones and shelly limestones and sandstones of the Penarth Group. The area around Bristol was marked by a complex landscape in which an archipelago of islands of Carboniferous limestone was formed in the new shallow seas. The application of new methods in geographical information systems allows a detailed exploration of a number of conformable surfaces, the unconformity between the underlying Paleozoic rocks and the overlying Mesozoic strata, as well as levels within the latest Triassic sediments, marking the advance of the sea and interactions with the coeval tectonics, which caused some islands to rise and some basins to descend. The new geographical information system models show a sequence of palaeogeographical reconstructions of the archipelago and relate this to the island tetrapod faunas, which show strong evidence of the species–area effect. Supplementary material: Supplementary tables S1-S6 and 2D island map GIS files are available at https://doi.org/10.6084/m9.figshare.c.5273256

The age and formation mechanisms of Late Triassic fissure deposits, Gloucestershire, England: Comments on Mussini, G. et al. (2020). Anatomy of a Late Triassic Bristol fissure: Tytherington fissure 2

In the Late Triassic the landscape NE of present-day Bristol, SW England was dominated by Carboniferous Limestone ridges and cuestas that became progressively buried by continental Mercia Mudstones and finally inundated during the Rhaetian marine transgression. Mussini et al. (2020) adopt the assertions of earlier collaborators back to Whiteside and Marshall (2008) that terrestrial vertebrate assemblages from sediments contained within karstic fissure systems in the former limestone ridges at Cromhall, Tytherington and elsewhere are restricted to the Rhaetian. We review and reject the sedimentological, stratigraphic, geomorphological and topographic arguments for this and reassert a long pre-Rhaetian (Norian) history for the vertebrate-bearing fissure systems at both Tytherington and Cromhall. We also reject the contemporaneous Rhaetian freshwater-seawater mixing zone dissolution model for the fissure systems adopted by Mussini et al. (2020) and reaffirm that the Tytherington and Cromhall fissures developed as conduit caves with a long Triassic history. Applying a new regional study of the Rhaetian transgressive surface, we also show that whilst the fissures at Cromhall remained sealed after the Norian, those at nearby Tytherington were re-exposed in the Late Rhaetian. Already partially filled with Norian sediments, the Tytherington fissures were subject to reworking on the seabed. Internal collapses, probably triggered by well documented repeated regional seismicity, led to the chaotic state of the Tytherington fills when downward moving Rhaetian marine components came to lie amongst and mix with earlier Norian terrestrial sediments. The vertebrate associations in the Tytherington fissures therefore contain a substantial Rhaetian input whilst those at Cromhall do not.

Foam-Assisted Water–Gas Flow Parameters: From Core-Flood Experiment to Uncertainty Quantification and Sensitivity Analysis

Uncertainty quantification and sensitivity analysis are crucial tools in the development and evaluation of mathematical models. In enhanced oil recovery, the co-injection of foam in porous media has been investigated through laboratory experiments and mathematical models as a promising technique for improving sweep efficiency. In this work, we study two mathematical models of foam flow in porous media. First, we present a foam quality-scan experiment using nitrogen and low concentration of an alpha-olefin sulfonate surfactant in brine using Indiana limestone carbonate core. Second, we evaluate the models based on their ability to represent the experimental data using inverse uncertainty quantification techniques. Third, the parameters’ estimated distributions are used to perform both forward uncertainty quantification and sensitivity analysis. We also present a detailed comparison of the models, and analyses on the experimental data, model discrepancy, and sources of uncertainties. The experimental results of foam apparent viscosity in carbonate rocks are consistent with other experiments in sandstones: The foam quality transition is present; the difference in apparent viscosity values is of the same magnitude as the difference in permeability. Propagation of uncertainties from the estimated parameter distributions through the models showed a good match between experimental data and model predictions. The sensitivity analysis showed that the model’s parameters play different roles and depend on the quantity of interest, the foam quality regime, and limiting water saturation. To summarize, this study provides essential information for possible improvements in the experiments and mathematical models of foam flow in EOR processes.

A Pleistocene deposit preserved in deep karst at Coolough, County Galway, western Ireland

Buried karst features result from a complex interplay between carbonate bedrock and structural geology. The enhancement of carbonate aquifers through this interplay creates hydrogeologically complex subsurface drainage networks and highly variable surface topographies. This article reports on the sediment fill and fossil pollen record from a large enclosed negative karst landform at Coolough, near Galway City in the west of Ireland. The landform was discovered during the preliminary site investigation for the proposed N6 Galway City Ring Road. Major joint sets developed in the Carboniferous (Mississippian) Limestone bedrock govern the local hydrogeology and structural evidence suggests that a local fault has controlled the east–west propagation of the deep enclosed depression. Fluvial and lacustrine palaeoenvironments are interpreted from the sediments infilling the enclosed depression, within which three discrete diamicts are identified, potentially representing distinct phases of glaciation. Pollen assemblages, which reflect palaeoenvironments of open woods and grasslands, have been correlated with other deposits in Ireland and Europe and suggest a minimum date of the Early Pleistocene for the oldest of the diamicts, and thus an upper age limit on the development of the karst depression. Fluvial disturbance to the deposit is evident and is likely to be responsible for the chronological discrepancy between the Early Pleistocene and Middle Pleistocene sediments both recorded from the deposit infill. The findings of this multidisciplinary study contribute to an understanding of Quaternary landscape evolution in western Ireland.

HYPROP measurements of the unsaturated hydraulic properties of a carbonate rock sample

The unsaturated hydraulic properties provide important theoretical and practical information about fluid flow in soils and rocks for a range of soil, environmental and engineering applications. In this study we used the evaporation (HYPROP) and chilled-mirror dew point (WP4C) methods to estimate the water retention and unsaturated hydraulic conductivity curves of an Indiana Limestone carbonate rock sample. The obtained data were analyzed in terms of unimodal and bimodal VG (van Genuchten, 1980) and PDI (Peters et al., 2015) type functions. Bimodal functions were found to produce excellent descriptions for the unsaturated hydraulic data we measured, slightly better than the standard unimodal formulations. For our particular test using an Indiana Limestone rock sample, we did not find much improvement when accounting for film and corner flow using the PDI formulation, relative to the VG model. As far as we know, this is the first time the HYPROP methodology was applied to a rock sample.

Groundwater Hydrogeochemical Mechanisms and the Connectivity of Multilayer Aquifers in a Coal Mining Region

Multiple runoff connections for groundwater supply and water quality evolution mechanisms were disclosed using hydrochemistry, multivariate statistics, stable hydrogen and oxygen isotopes, and inverse hydrogeochemical modeling in a multi-layer groundwater system in a north China coal-mining district. Groundwater quality was mainly influenced by dissolution and weathering of carbonate, silicate, gypsum, halite, and fluorite, as well as cation exchange. Sulfate enrichment in the Carboniferous limestone aquifer may be due to pyrite oxidation, while gypsum dissolution and sewage contribute sulfate to the Quaternary alluvium. The Ordovician limestone groundwater is hydraulically connected to the other two aquifers. Incongruent dissolution of dolomite occurs when the Ordovician limestone water contacts the Carboniferous aquifer, while evaporation occurs when the Ordovician limestone water migrates upward through fractures to the Quaternary aquifer.

Features of the coarse clasts (psephites) distribution in the Upper Quaternary sediments of the northern Barents Sea

The paper presents the characteristics of the coarse clasts (psephites, larger than 1 cm) sampled in the northern part of the Franz Victoria Trough (Barents Sea) during the “Transarktika-2019” expedition. The studied sedimentary section was formed during the transition from the last deglaciation environments to the marine Holocene settings. The amount of psephites in deglacial sediments is much higher than the one in Holocene sediments. The petrographic composition of the psephites, their roundness (according to Waddell and Khabakov classifications) and shape (according to Zingg classification) were studied in detail. It is shown that the majority of psephites is represented by non-rounded or poorly rounded varieties. Isometric and disc-type shapes are predominating among coarse clasts. Isometric psephites prevail in “deglacial sediments”, while disc-shaped, bladed, and rodlike, as a rule, are current in marine Holocene sediments. The petrographic composition of psephites is mostly represented with carbonate (limestone and dolomite) and sandstones. The amount of carbonates increases from the Holocene to the deglacial part of the sediment section. Other rocks found in smaller quantities are represented by quartzites, cherts, shales, basalts, crystalline schists, gneisses, granites, pyrite. During the last deglaciation at the Late Pleistocene the iceberg rafting was the main mechanism of psephites delivery to the sampling points. The major sources of the icebergs were Franz Josef Land and the Belyi — Victoria Islands region. Rocks of Franz Josef Land are represented mainly by Cretaceous gabbro-basalt complex and Triassic sandstones and siltstones. Upper Paleozoic terrigenous and carbonate rocks, as well as Proterozoic metamorphites, occur in the west of the studied area (the area of the Belyi — Victoria Islands). The results of studying the petrographic composition of psephites allow us to conclude that during the last deglaciation icebergs from both sources were heading north towards the Nansen Basin through the Franz Victoria Trough. Both streams of icebergs failed to reach the opposite sides of the Franz Victoria Trough, mixing with each other approximately in the axial part of the trough and leaving it in northern directions.

Foam Dynamics in Limestone Carbonate Cores.

There is an increasing interest in foam applications in heterogeneous carbonate reservoirs to improve gas sweep and mitigate a high gas–oil ratio (GOR) in production wells. However, foam has been studied in sandstones more than in carbonates, and there are few experimental investigations considering matrix transport properties of foam in carbonates. Thus, this study takes a fundamental approach to improve our understanding of foam generation and transport process in the absence and presence of remaining oil in carbonates by co-injection of Alpha Olefin Sulfonate (AOS) solution and nitrogen (N2) in outcrop Indiana Limestone at high pressure and temperature after satisfying adsorption. In the oil-free core, development of the foam generation transient period and its transition into steady-state foam was rapid for all gas fractions, where the strongest foam was obtained at 90% gas fraction. Foam properties were successfully reproduced at different gas fractions. At remaining oil saturation, foam generation and propagation were significantly delayed and were observed at a high AOS surfactant concentration (5 wt %). Persistent foams were obtained both with and without remaining oil present, which withstood pressure gradients of N2 up to 0.5 bar/m for extended times. Therefore, if correctly designed, foam gas shut-off can be a low-cost low-risk technique to reduce problems with high GOR, gas-handling, and gas reinjections.

The Mid-Carboniferous boundary in the eastern slope of the Southern and Middle Urals: carbon and oxygen isotopic composition in limestones

Research subject. Limestones of the Mid-Carboniferous boundary interval of the Urals eastern slope (“Khudo-laz”, “Khudolaz-gully” and “Bolshoy Kizil” sections at the Southern Urals, as well as “Iset quarry” and “Lugovaya” sections in the Middle Urals) are considered. Methods. Carbon and oxygen isotopic composition in bulk samples was analyzed. Results. The carbon and oxygen isotopic composition in carbonate deposits of the Serpukhovian-Bashkirian boundary interval in the sections of the Southern and Middle Urals eastern slope was studied. It is shown that the value of δ 13 C in the most complete sections varies from 1.5 to 3.8‰, while δ 18 O – from 23.1 to 30.7‰. Conclusions. Variations in the carbon and oxygen isotopic composition are associated with both global and local events. In the East Uralian basins (northern periphery of Paleotethys), the shift in carbon isotopic composition of carbonate sediments appeared in the Late Serpukhovian. In some cases, this trend correlates with the positive excursion of δ 18 O, which is most likely associated with the global short-term cooling (glaciation).

Drainage feasibility of a Carboniferous thin-layer limestone aquifer based on a dewatering test: Luxi coal mine, China

Mine water damage is one of the main disasters threatening mine safety during production. The key to mine water hazard prevention and control is determining the main aquifer characteristics that threaten the mine and the hydraulic connections between aquifers and water diversion channels and proposing appropriate preventive measures. This paper focuses on the 16104 working face of the lower coal group in the Luxi coal mine of Shandong Province. The main water-filled aquifer that threatens the no. 16 coal mine is the Carboniferous no. 14 limestone thin-karst aquifer. Before adopting hydrophobic pressure-lowering measures, it is necessary to determine the water yield characteristics of the no. 14 limestone aquifer, the strength of recharging, and the hydraulic connection with the Ordovician limestone aquifer and to analyze and evaluate the feasibility of dredging. First, various geophysical exploration methods were utilized to comprehensively identify the anomalous water yield areas of the aquifer, which were verified by drilling methods. By combining these results with drilling and geophysical results, appropriate drainage and observation holes were selected. Second, a special water discharge stage and sequence were designed to carry out water discharge testing. By analyzing the change of water discharge and water pressure in dewatering test and the change of conventional ion concentration in water samples, the hydrogeological parameters were obtained, the hydrogeological situation was ascertained and the hydraulic connection with the Ordovician limestone aquifer was determined. The drainage time required for the aquifer to attain a safe water pressure and the amount of hydrophobicity were calculated. Through combination with the obtained hydrogeological parameters, the feasibility of depressurization was analyzed. The results show that the water yield property of the no. 14 limestone aquifer is not high, but the connectivity is good. There is a hydraulic connection between the no. 14 and the Ordovician limestone aquifers, but the connection is weak. The no. 14 limestone aquifer of the entire working surface was drained at a rate of 240 m3/h, indicating that the aquifer has good depressurization properties.

Paleomagnetic Constraint on the Carboniferous Paleoposition of Indochina and Its Implications for the Evolution of Eastern Paleo‐Tethys Ocean

AbstractTo better understand the rifting history and mechanism of the Indochina Block from eastern Gondwana, we report a paleomagnetic study on middle Carboniferous (~315 Ma) limestones from the southernmost Yunnan of China. Characteristic remanent magnetizations (ChRMs) were isolated from samples in 14 sites after secondary magnetizations were erased at relatively low temperature ranges, of which magnetites were interpreted to be the magnetic carriers for the former while goethites and pyrrhotites for the latter. The rock‐magnetically interpreted minerals are confirmed with a scanning electron microscope (SEM) on samples with a high concentration of magnetic minerals, of which the calcium carbonates were dissolved with a mixed solution of CH3COOH and CH3COONa. The ChRMs passed reversal and fold tests and are very likely a primary magnetization, which correspond a paleomagnetic pole of 76.1°S/181.3°E (A95 = 4.0°, N = 14) and locate the Indochina Block at 18.4 ± 4.0°S. A relatively quiet tectonic period was suggested for the North Qiangtang, Indochina, and South China blocks during the middle Carboniferous‐early Permian, and a sequential start of rapid drifting of these blocks was recognized, which is coeval with the rifting of Cimmerian terranes from Gondwana.