Wumishan Formation Research Articles

Feasibility Study on Geothermal Dolomite Reservoir Reinjection with Surface Water in Tianjin, China

Reinjection is thought to be the most effective way to maintain reservoir pressure and production capacity for hydrothermal resources. The use of external water injection to replenish deep geothermal reservoirs is a new approach in China to addressing the problems of declining groundwater levels and energy depletion caused by the excessive and uneven exploitation of geothermal resources. However, the key challenge and focus of the feasibility assessment of this method lies in the chemical compatibility of the external water with the native geothermal reservoir water and surrounding rocks. In this paper, we discuss the geochemical response of a dolomite reservoir to lake water injection based on experiments on water–rock interaction in the Wumishan formation in the Dongli Lake area of Tianjin. The results show that after reactions with dolomite, the TDS of the reacted water decreases, indicating the occurrence of precipitation. According to the calculation results obtained using the PHQREEC program, the precipitation amount is found to be quite limited. Geochemical analysis indicates that at the initial stage of the reactions, plagioclase dissolves and releases alkaline metals like Ca-, Na-, SiO2- and Al-bearing compositions, leading to the oversaturation and precipitation of dolomite and calcite. As the reaction progresses, a portion of the dolomite dissolves, while the calcite continues to precipitate at a later stage. Illite precipitates and its effects on reservoir structure depend on its shape. Based on the experimental data, it can be concluded that the dolomite reservoir will be slightly affected by the reinjection of lake water; however, it is still a good method for the sustainable development of geothermal resources.

Hydrothermal overprinting of the Li-rich strata deposited in the Mesoproterozoic Wumishan Formation, Hebei Province, North China

Lithium (Li), as a strategic critical metal, plays a pivotal role in the emerging energy landscape, particularly in the context of Li-ion batteries driving the new energy economy. Recently, Li-rich strata (with Li2O > 0.3 wt%) have been discovered in the Mesoproterozoic Wumishan Formation in Hebei Province, North China, suggesting a prospective Li reservoir. This study investigates these Li-rich strata using geochemical and in-situ micro-analytical techniques to explore the occurrence of Li and the formation mechanism of Li-host minerals, aiming for a comprehensive understanding of the supernormal enrichment of Li. The Li-rich samples are predominantly composed of dolomite and quartz, followed by clay minerals such as illite, interstratified illite–smectite (I/S), and chlorite, with minor amounts of K-feldspar, albite, biotite, calcite, baryte, fluorite and fluorapatite. In-situ analysis and 7Li NMR spectroscopy reveal that Li predominantly occupies the octahedral sites within the structures of authigenic illite and I/S, while its absence in clastic illite, clastic chlorite, unaltered K-feldspar, and dolomite. The presence of veined minerals (e.g., fluorite, baryte, and calcite) and a strong positive correlation between Li and F imply that post-depositional hydrothermal fluids have significantly contributed to the formation of Li-host minerals. The paragenesis of these minerals suggests that Li-bearing illite has formed through the hydrothermal alteration of K-feldspar. These Li-bearing illites subsequently transformed into Li-bearing I/S, consisting of illite-rich I/S and smectite-rich I/S, under continuous hydrothermal alteration. Lithium could have been leached from the surrounding carbonate rock and tuff through water–rock interaction and subsequently enriched by post-depositional hydrothermal fluids in specific regions, leading to mineralization. These findings provide valuable insights for targeting exploration of this promising Li resource.

Earth-Moon dynamics from cyclostratigraphy reveals possible ocean tide resonance in the Mesoproterozoic era.

Cyclostratigraphy is an important observational window into the history of the Earth-Moon system. However, there is limited information from the Mesoproterozoic era (1.0 to 1.6 billion years ago); accordingly, only weak constraints on Earth-Moon separation and tidal dissipation are available for this time. To close this knowledge gap, we analyze cyclostratigraphy from the Yemahe Formation (~1.2 billion years ago), Wumishan Formation (~1.5 billion years ago), and Chuanlinggou Formation (~1.6 billion years ago) in China. We use a Bayesian inversion method to analyze the three cyclostratigraphic sections. We combine previous results with these three estimates to construct an updated Earth-Moon system evolution and tidal dissipation history after 2.5 billion years ago. The results show a tidal dissipation peak that is consistent with the model predictions within the error range but also that there may be an additional resonance fluctuation in the Mesoproterozoic era.

The rise of biogenic silicon cycling and microbial silicification promoted Mesoproterozoic chert deposition

The silicon (Si) cycle is intimately interlinked with other biogeochemical cycles (e.g., carbon and nitrogen) and plays an important role in regulating long-term global climate change and biotic evolution in Earth's history. The abundance of cherts in Mesoproterozoic shallow-marine carbonates implies unique environmental conditions for silica precipitation, but the driving mechanism behind the Si cycle in Mesoproterozoic oceans remains unclear. In this paper, we report an integrated study of the cherts from the ∼1.48 Ga Wumishan Formation of North China. The Wumishan cherts predominantly consist of microquartz (∼90%), with some silica-replaced carbonate (∼5%) and minor pyrite (∼1%) grains, indicating that the cherts were largely formed through primary silica precipitation. High germanium/silicon molar ratios (Ge/Si = 0.71–19.1 μmol/mol; mean = 8.83) and positive europium anomalies (Eu/Eu* = 0.41–2.42; mean = 1.41) of the cherts suggest a considerable contribution from hydrothermally derived Si. Diverse microbial components, including organic-rich filaments, EPS (extracellular polymeric substances) relics, mat fragments and picocyanobacteria fossils, are observed in the Wumishan cherts. These components, together with abundant organominerals resulted from the interactions of microbes and dSi in ambient environments, imply that microbial activities played critical roles in silica deposition. The Si liberated from degraded EPS or EPS-Si complex may have locally increased the dSi concentrations and changed chemical conditions in the substrate and pore-waters, promoting silica precipitation. Given that picocyanobacteria and some other prokaryotes can accumulate significant amounts of silica in their cells and EPS, biogenic silica from decomposed microbial biomass may have exerted an important influence on silica precipitation in shallow-marine environments of the Mesoproterozoic ocean.

Soft-sediment deformation structures or microbially induced sedimentary structures: the description and possible origin of the "loopites" in the Mesoproterozoic Wumishan Formation, North China

Soft-sediment deformation structures or microbially induced sedimentary structures: the description and possible origin of the "loopites" in the Mesoproterozoic Wumishan Formation, North China

Enrichment Mechanism of Lithium in Geothermal Waters from a Bedrock Reservoir in Xiong’an New Area, China

The lithium concentrations in the geothermal waters of the Wumishan Formation carbonate reservoir in China Xiong’an New Area are over 1 mg/L and are even higher than those in the geothermal waters of granite reservoirs in some areas of China. It is still unknown which are the most important factors controlling the lithium concentrations in the geothermal waters in the study area. This article selected the analysis and test data of 32 geothermal water samples obtained in recent years from the study area and combined them with hydrochemical analysis and test data from granite reservoirs in other regions of China to study the enrichment mechanism of lithium in the geothermal waters in the study area. The results of the hydrochemical data analysis indicate that the lithology, pH, and water–rock interaction between geothermal water and carbonate rocks are not the main factors affecting the lithium concentrations in the study area. The mixing of paleo-seawater and the leaching of the evaporated rocks formed by it are the most important factors controlling the lithium concentrations in the study area, and temperature is also an important factor affecting the lithium concentrations. The research results are of great significance to the study of the enrichment mechanism of lithium in geothermal waters and the formation mechanism of geothermal waters in similar areas around the world.

Spatial and Temporal Redox Heterogeneity Controlled by a Fe(II), Anoxic Upwelling System in the Early Mesoproterozoic Ocean

AbstractThe availability of oxygen and nutrients during the Mesoproterozoic (1.6–1.0 Ga) is thought to influence the rate of eukaryote evolution. The cause of the transition from low productivity in the upper Wumishan Formation to organic‐rich sediments in the Hongshuizhuang Formation remains unknown. We report FeHR/FeT, Fepy/FeHR, MoEF, UEF, VEF, and [Ce/Ce*]SN in one core of the Yanliao Basin to study the redox evolution and compare it with other sections in different depths of the Yanliao Basin to get clues of the spatial and temporal redox heterogeneity. Mo‐U covariation, low Co × Mn and CoEF × MnEF, and distribution patterns of trace metals indicate the upwelling events. An upwelling system could fuel nutrients to promote productivity levels in surface water and contribute to anoxic deep waters. The uniformly positive δ15N values are interpreted as sufficient NO3− reduction via denitrification and dissimilatory nitrate reduction to ammonium in a ferruginous upwelling condition.

Dolomite porosity related to peritidal microbialites textures and facies succession in Mesoproterozoic Wumishan Formation, North China

There are three main types of microbial dolostones in Mesoproterozoic Wumishan Formation in Jixian, North China. They are stromatolite, thrombolite and granular microbialite. The microbialite lithofacies are intensely environmentally associated. Several kinds of stromatolite can be found in the peritidal environments. Mostly planar or wavy stromatolites show in intertidal and supratidal zones while domy and conic stromatolites occur in subtidal or lower intertidal zones. In more agitated subtidal zone or tidal channels, microbial grains deposited, such as thrombolite, oncolite, and sand- gravel sized microbial-coated grains. The intraclasts related laminite facies mostly deposited in shallow subtidal zone.Representative four outcrop sections of Wumishan Formation in Jixian are measured and documented. The petrophysical and petrological study from the outcrop and well core samples reveals that the reservoir rocks are closely related to lithofacies and their depositional environments. The more agitated the depositional environment is, the better the reservoir quality, even so that after the diagenesis (surface and burial). The dolostone porosity of planar stromatolite in supratidal and intertidal are much worse than the subtidal high-agitated microbialite rocks, whether samples from the outcrops or drilling cores.The effective reservoir interval is normally located in the lower part of a peritidal microbial facies succession, mainly consists of subtidal microbialite, such as thrombolite, thrombo-stromatolite, oncolite, columnar stromatolite and coated intraclastic grainstone and rudstone, commonly deposited in agitated environment. The thickness of reservoir rocks in microbialite para-sequences ranges commonly from 4 to 14 m, deeply related to depositional cycles or sequence stratigraphy in vertical lithofacies texture and their areal extent to the paleogeography.

Present status and sustainable utilization of hydrothermal geothermal resources in Tianjin, China: a critical review

<abstract> <p>Tianjin, as one of the pioneering and most prominent cities in China, has a long history of harnessing geothermal energy. The geothermal resource available in Tianjin is primarily characterized as a low- to medium-temperature hydrothermal geothermal resource. This manuscript introduces the ongoing status and potential of geothermal utilization in China, with a particular focus on the characteristics and utilization status of geothermal resources in Tianjin, China. Moreover, the relevant strategies and challenges for cost-efficient sustainable utilization of Tianjin geothermal resources are identified. The formation parameters of heat storage characteristics of Tianjin geothermal resources are also discussed. In addition, the key paths, guidelines and challenges on how to solve the obstacles related to the geothermal resources development in Tianjin are also suggested. The summarized results indicate that the geothermal reservoirs exploited in Tianjin vary greatly, which include sandstone of Neogene Minghuazhen formation, Guantao formation, Ordovician and Cambrian and carbonate of Proterozoic Wumishan formation. Most of the exploitative geothermal resources (146 geothermal wells) in Tianjin have mainly been produced from the Wumishan formation of the Jixian system and the Guantao formation of the Neogene system. The current production capacity has been doubled, and a two-stage cascade utilization system has been established, incorporating geothermal power generation and geothermal heating. The geothermal utilization share in Tianjin is estimated to be 81.66% for heating, 16.6% for domestic hot water and 1.35% for bathing. In conclusion, notwithstanding the diversity of geothermal resources in Tianjin, it is difficult to guarantee the sustainable development and utilization of geothermal resources in Tianjin due to the unreasonable layout of geothermal wells, imbalance of production and reinjection. Hence, the integration of distributed temperature sensing and distributed strain sensing monitoring demonstrates significant promise and effectiveness in tracking water circulation and detecting flow localization problems as dynamic monitoring processes and smart thermal response tests should be recommended and established as a substantial feature required in the future utilization and development of geothermal resources in Tianjin.</p> </abstract>

High-Precision Geochronology of the Xiaojiayingzi Mo Skarn Deposit: Implications for Prolonged and Episodic Hydrothermal Pulses

Abstract The timescales and duration of ore-forming processes in skarn systems are not well constrained. To better understand this, we present high-precision chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon and isotope dilution-negative-thermal ionization mass spectrometry (ID-N-TIMS) Re-Os molybdenite geochronology of the Xiaojiayingzi Mo skarn deposit (0.13 Mt Mo at 0.22 wt %), northeastern China. The Xiaojiayingzi deposit is related to an intrusive complex composed of gabbroic diorite, monzodiorite, and granite porphyry. Molybdenite mineralization occurred in two ore blocks, Xiaojiayingzi (0.11 Mt Mo) and Kangzhangzi (0.02 Mt Mo). In the Kangzhangzi ore block, Mo mineralization is concentrated in skarn adjacent to a deep-seated granite porphyry, with minor disseminated and quartz veinlet mineralization within the granite porphyry. In contrast, economic Mo mineralization in the Xiaojiayingzi ore block is concentrated in skarns located between the contact of steeply dipping monzodiorite and the Mesoproterozoic Wumishan Formation, with minor Mo mineralization found in quartz and endoskarn veins hosted in the monzodiorite. Skarn mineralization in both ore blocks converges downward into the mineralized granite porphyry. In the Kangzhangzi ore block, skarn is zoned from deep proximal dark red-brown garnet to shallow distal dark-green pyroxene. In the Xiaojiayingzi ore block, proximal skarn is garnet rich, whereas pyroxene increases away from the monzodiorite-Wumishan Formation contact. In addition, pyroxene becomes more Fe and Mn rich with distance from the intrusions; Pb, Zn, and Ag increase toward the top of the system; and Mo and Fe increase with depth. High-precision CA-ID-TIMS U-Pb zircon geochronology indicates the gabbroic diorite crystallized at 165.359 ± 0.028/0.052/0.18 Ma (uncertainties presented as analytical/+ tracer/+ decay constant uncertainties), with subsequent crystallization of the monzodiorite and granite porphyry at 165.361 ± 0.040/0.059/0.19 and 165.099 ± 0.026/0.051/0.18 Ma, respectively. High-precision ID-N-TIMS Re-Os molybdenite geochronology indicates molybdenite mineralization at Xiaojiayingzi occurred in at least three discrete magmatic-hydrothermal pulses (nominally between 165.48 ± 0.09–165.03 ± 0.13, 163.73 ± 0.09, and 163.11 ± 0.11 Ma). The first episode of molybdenite mineralization formed in exoskarns, endoskarns, and quartz veins and had a minimum duration of 450 ± 40 k.y., between 165.48 ± 0.09/0.68/0.85 and 165.03 ± 0.13/0.67/0.85 Ma. It is likely that skarn ore represents a composite series of mineralization events, more than the three events capable of identification within analytical uncertainty of these high-precision data. Finally, Re-Os dating of quartz Mo veins cutting the monzodiorite and granite porphyry indicates that some mineralization postdated the observed intrusions, between 163.73 ± 0.09/0.70/0.86 and 163.11 ± 0.11/0.70/0.86 Ma, interpreted to be the result of deeper, unobserved intrusions. Collectively, these ages indicate that protracted, pulsed Mo mineralization at the Xiaojiayingzi deposit occurred over a period of at least 2.4 m.y. These data suggest that individual magmatic and/or skarn garnet ages may significantly underestimate the full duration of mineralization. In addition, this study highlights that systematically identifying skarn deposits associated with multiphase intrusive systems may reveal targets for future exploration, as it may point to previously undiscovered mineral resources.

Microfabrics and organominerals as indicator of microbial dolomite in deep time: An example from the Mesoproterozoic of North China

Both laboratory experiment and modern environment studies show that primary dolomite can precipitate at low temperature (≤60 ℃) only when microbes were involved. Vast dolostones in mid-Proterozoic have long been suspected to be the results of microbial mediation, but direct evidence is rare. To reveal their origin, an integrated study was conducted on the Mesoproterozoic Wumishan Formation (∼1.48 Ga) of North China using multiple techniques. The results show that the Wumishan dolostone contains abundant multiscale organominerals that are closely associated with fossilized remnants of extracellular polymeric substances (EPS) and putative bacteria fossils. These microfabrics are strikingly similar to those in dolomite precipitates produced in laboratory culture and modern sabkha environment, suggesting microbial origin. Nanoglobules (60–200 nm) preferentially attach to fossilized EPS filaments, and tend to merge into polyhedrons (3–8 μm) that in turn coalesce into microspheres (20–50 μm). EDS analysis revealed a successive decrease in carbon from nanoglobules to microspheres with the increases of Mg and Ca contents, implying an increased mineralization. Nanoglobules may have derived from EPS degradation during nucleation, and served as seeds for subsequent growth of carbonate polyhedrons. Microsphere, as a basic building block in dolostone, can transform into rhombohedral crystal via neomorphogenesis during early diagenesis. Petrographic and XRD analyses show that the dolostone is dominated by ordered dolomite and was largely formed in peritidal environment with thriving microbial community. Geochemical analyses of iodine species, redox sensitive elements, Ce anomalies, and C–O isotopes in the Wumishan dolostone suggested an environment predominated by anoxic to suboxic conditions, with low P but high Si contents in seawater.Our study offered direct evidence of microbial origin for the Mesoproterozoic dolostone, providing new insights into the “Dolomite Problem”. The massive development of microbial dolostone in Mesoproterozoic points to a specific ocean chemistry, where low oxygen and active bacterial metabolisms may have played crucial roles in precipitating dolomite. The co-existence of abundant microbial components and multiscale organominerals may be taken as textural evidence for microbial dolomites and signatures of life-environment interactions in deep time.

A Joint Method Based on Geochemistry and Magnetotelluric Sounding for Exploring Geothermal Resources in Sedimentary Basins and Its Application

The precise exploration of the characteristics of geothermal fields in sedimentary basins, such as the temperature and burial depth of their deep geothermal reservoirs, is of great significance for improving the probability of penetration and reducing exploration risks and development costs. This study proposed a joint exploration method combining magnetotelluric (MT) sounding and geothermometers. Using this method, this study estimated the geothermal reservoirs’ temperature and the circulation depth of geothermal water in the Xianxian geothermal field, a typical geothermal field in a large sedimentary basin in northern China, and prepared the temperature and depth maps of the geothermal reservoirs. The main results are as follows. First, the bedrock’s geothermal reservoirs with karst fissures in the Xianxian geothermal field have great potential for development. Among them, geothermal reservoirs in the Jixianian Wumishan formation have a top depth of 1100–1500 m and a thickness of 700–1700 m, and the geothermal reservoirs in the Jixianian Gaoyuzhuang formation have a top depth of 3700–4000 m and a maximum drilled thickness of 400 m. The geothermal reservoirs of the Xianxian geothermal field mainly have medium and low temperatures of 138–160 °C and the circulation depth of the geothermal water is 5873 m.

Paleoenvironment of Mesoproterozoic Gaoyuzhuang and Wumishan Formations, North China: New Insights from Geochemistry and Carbon and Oxygen Isotopes of Dolostones

The Mesoproterozoic Gaoyuzhuang and Wumishan Formations are major geothermal reservoirs in the Hebei Province, North China. Compared to the exploration of geothermal resources and heat-controlling structures, carbon and oxygen isotopic records of the two formations are limited. Here, we present integrated field, petrological, geochemical, carbon, and oxygen isotopic data of carbonate rocks from the Gaoyuzhuang and Wumishan Formations. The Wumishan Formation is characterized by higher CaO and MgO contents and lower SiO2 contents than the Gaoyuzhuang Formation, indicating that the source of the Wumishan Formation likely contains less terrigenous clastic materials. The two formations have low total rare earth element contents, similar to marine carbonate rocks. They show different Eu and Ce anomalies, Al/(Al + Fe + Mn) and Fe/Ti ratios, and (Co + Ni + Cu) contents. They generally show similar carbon isotopic compositions, whereas the carbonate rocks of the middle-upper Gaoyuzhuang Formation show lower δ18O values than the samples from the Wumishan and lower Gaoyuzhuang Formations. These data suggest that the two formations have experienced different transgressive–regressive cycles and that their sedimentary environments varied and were unstable in different sedimentary periods. The middle-upper Gaoyuzhuang Formation was likely affected by hydrothermal fluids, whereas the Wumishan Formation is composed of normal seawater deposits. Integrated evidence reveals that both of the Mesoproterozoic Gaoyuzhuang and Wumishan Formations were deposited in rift environments caused by the breakup of the Columbia/Nuna supercontinent.

Constraining the redox landscape of Mesoproterozoic mat grounds: A possible oxygen oasis in the ‘Boring Billion’ seafloor

The Earth’s middle age or the ‘Boring Billion’ (∼1.8 to ∼ 0.8 Ga, billion years ago) represents one of the most enigmatic intervals in Earth’s history, characterized by the absence of significant carbonate carbon isotope excursions and the sluggish evolution of eukaryotes. It is widely accepted that the atmospheric O2 level was low (<1% PAL or < 10% PAL) and the ocean remained predominantly anoxic with the development of sulfidic (H2S rich) continental margins. It is suggested that 2–10% of seafloor euxinia was sufficient to deplete some micro-nutrients in the ocean inventory, such as Mo and Cu, which are essential for nitrogen-fixation for eukaryotes. However, recent studies report multicellular fossils and episodic/sporadic inception of oceanic oxidation in Mesoproterozoic. These findings imply the possible occurrences of oxygen oasis that provided habitable niches for eukaryote evolution. Such oxygen oasis likely developed at shallow marine seafloor covered with microbial mat, where O2 produced by benthic cyanobacteria resulted in the oxygenation of seafloor. Therefore, in order to constrain the habitability of Mesoproterozoic oxygen oasis, it is essential to reconstruct the O2 fugacity in the seafloor. In this study, we analyzed pyrite sulfur isotopes and pyrite contents of the Mesoproterozoic Wumishan Formation (∼1.4 Ga). The Wumishan Formation is composed of cyclic deposition of, in a shoaling upward sequence, the subtidal calcareous shale, massive thrombolitic dolostone, and microbial laminated dolostone. Both microbial laminated dolostone and thrombolytic dolostone precipitation involved with microbial activities, and thus might record the redox condition of putative oxygen oasis in the Mesoproterozoic oceans. We apply the One-Dimensional Diffusion-Advection-Reaction (1D-DAR) model to simulate diagenetic pyrite formation in sediments. Sedimentation rate can be well constrained by the depositional cycles of the Wumishan Formation. The modelling results indicate that more than 60 ∼ 80% of H2S that was generated in microbial sulfate reduction (MSR) was reoxidized, and that organic matter supply, both from surface water and seafloor, was limited. Thus, our study indicates that the seafloor could be substantially oxygenated in Mesoproterozoic, even when the atmospheric O2 level was extremely low. Shallow marine seafloor covered with microbial mat may function as the oxygen oasis, providing habitable niches for the evolution of eukaryotes.

The primary dolostone in the Meso-Neoproterozoic: Cases study on platforms in China

Meso-Neoproterozoic dolostone is abnormally well-developed in the platform areas in China, detailly, in the Mesoproterozoic Calymmian Wumishan Formation of North China Basin, Neoproterozoic Ediacaran Qigbulak Formation of Tarim Basin, and the Neoproterozoic Ediacaran Dengying Formation of Sichuan Basin. The outcrops and drilling cores from these basins show that these dolostones are mostly thick-bedded to massive, with mud-sized to silt-sized crystalline dolostone reaching 86%–97% of the total dolostone thickness. These dolostones basically suffered no secondary metasomatism, regardless of containing microorganisms (such as algae and bacteria) or not. The analyses of sedimentary palaeogeographic features indicate that: (1) the Meso-Neoproterozoic dolostone nearly covered the entire area basins, and was widely deposited in supratidal to intertidal zones, as well as in open platform environments; (2) the dolostone was subdivided according to the crystalline size, with the distribution controlled by geomorphology, i.e., the pure mud-sized crystalline dolostone developing in depression areas, whereas the grainy mud-sized crystalline dolostone or the mound stromatolitic mud-sized crystalline dolostone developing in uplift areas; (3) deep-water basins developed between the carbonate platforms, and the seismic profiles show that these basins were formed by syngenetic deep faults in the Meso-Neoproterozoic strata. These syngenetic faults increased Mg2+ concentration in the seawater by injecting rich-Mg2+-bearing hydrothermal fluids into the carbonate platforms. The analyses of geochemical and paleoclimatic indexes reveal that there is no obvious difference in carbon and oxygen isotope ratios between dolostones having different textures, with the ratios similar to those of the global seawater of the Meso-Neoproterozoic, indicating that the silt-sized crystalline dolostone has the same sedimentary origin as the mud-sized crystalline dolostone, i.e., the former is from authigenic recrystallization of the latter rather than secondary metasomatism. Therefore, whatever the crystalline sizes are, the dolostones are all originated from sedimentation. All the petrological, sedimentary and geochemical data suggest that the dolostones covering almost the whole Meso-Neoproterozoic typical platform areas of China are of relatively primary sedimentary origin.

Hydrogeochemical and Isotopic Analyses of Deep Geothermal Fluids in the Wumishan Formation in Xiong’an New Area, China

Abstract Medium-low temperature geothermal resources in the Wumishan Formation, which is the geothermal reservoir, are local enrichment resources in Xiong’an New Area, North China. In this study, 35 water samples were collected from the bedrock of Taihang Mountains and Wumishan Formation in Xiong’an New Area and display the chemical compositions of water samples as well as the stable isotope compositions for hydrogen, oxygen, carbon, sulfur, and strontium. Hydrogeochemical characteristics and isotope compositions of water samples are analyzed to understand the origin and circulation processes of these geothermal fluids. Our results of cold groundwaters in the bedrock of Taihang Mountain indicate a more open oxidation environment, and the HCO3-Ca·Mg-type groundwater also indicates a prevailing carbonate dissolution condition. The deep geothermal fluids in the Wumishan Formation beneath Xiong’an New Area indicate a closed reduction condition, and their hydrochemical types are mainly Cl·HCO3-Na type. The diagram of hydrogen vs. oxygen isotope indicates that the recharge for the deep geothermal fluids in the Wumishan Formation of Xiong’an New Area is mainly from atmospheric precipitation. The high δ13C values (-3.4‰−-4.9‰) are notably controlled by the eluviation of the carbonate rock layers. The δ34S values vary from 18.02‰ to 27.01‰; the relatively high values indicate the eluviation of sedimentary rock layers. The high 87Sr/86Sr ratios (0.70806−0.71270) and the high Sr2+ concentrations (0.69−2.92 mg/L) suggest that the Sr in the deep geothermal fluids originates from the eluviation of both silicates and carbonates. According to the multimineral equilibrium diagram, chalcedony is saturated at the measured temperature of geothermal wells; therefore, we chose chalcedony as a geothermal thermometer for the calculation of the reservoir temperature of the Wumishan Formation, and the results vary from 68.63 to 89.10°C. Our study identifies the geothermal type of the deep medium-low temperature hydrothermal systems and also recognizes their water-rock interaction processes. We get a comprehensive understanding that the geothermal resources in the Wumishan Formation beneath Xiong’an New Area is convection-conduction type, for which potential of geothermal development and utilization is enormous.

Evaluation of the Modification Potential for Carbonate Geothermal Rocks by considering Multiple Mechanics Properties and Acid Dissolution in the Jizhong Depression, China

Abstract Evaluation of geothermal reservoir modification potential is of great significance for improving the efficiency of geothermal exploitation and reducing costs. However, it is difficult to accurately evaluate the potential of geothermal reservoir modification given the limited number of parameters used in practical studies. Therefore, an evaluation method that considers the potential of geothermal development by applying multiple parameters is essential. In this study, five types of rock samples are collected in the field from the Fujunshan Formation, Gaoyuzhuang Formation, Majiagou Formation, and the second and fourth groups of the Wumishan Formation. The uniaxial compressive strength (UCS) and uniaxial tensile strength (UTS) of rock samples were obtained by uniaxial compression tests and Brazilian splitting tests, followed by the calculation of the brittleness index utilizing UCS and UTS. Then, the joint roughness coefficient and crack width were obtained by 3D scanning and postprocessing. In addition, the fracture extension degree of each lithology was obtained via acid dissolution experiments. Analytical hierarchy processes (AHP) associated with expert experience were adopted to evaluate the modification potential of rock samples under hydraulic stimulation and acid etching based on the above parameters. The results show that the carbonate rocks of the Majiagou Formation are prone to cracking, efficient for flow and heat exchange, sensitive to acid dissolution, and thus have the highest potential for modification. The proposed evaluation method and implemented case study can provide a practical reference and solid base for further applications.

Cyanobacterial spheroids and other biosignatures from microdigitate stromatolites of Mesoproterozoic Wumishan Formation in Jixian, North China

Stromatolites have been widely reported from the Archean and Paleoproterozoic successions worldwide, and they could represent one of oldest life forms on Earth. Of these, a group of small stromatolites occur as microdigitate low-relief columns, and are also conspicuous in the field. However, biogenicity of these microdigitate stromatolites (MDSs) has long been disputed due to to the abundance of radial-fibrous texture and a lack of convincing microfossils. New examples of MDS are documented from the Mesoproterozoic Wumishan Formation of the Jixian area, North China. Vertically oriented fibrous fabrics are conspicuous and penetrate laminae as well as microscopic spheroids, which point to an abiotic genesis for this specific fabric. Stromatolite laminae contain abundant spheroids, typically 15–30 μm in diameter, with single or double outlines and they occur as solitary coccoid-like microfossils or in small aggregated colonies. Spheroids show strong fluorescence under both green and purple exciting lights, consistent with their composition of organic matter. Spheroids are abundant in the Wumishan stromatolites and they are categorized into two types. The first kind comprises micrite nuclei surrounded by sparitic sheaths, without nano-particle coatings. A smooth to grainy spheroidal surface defines the first kind of spheroids that also has a distinct rounded opening, which is often broken probably due to diagenesis and silicification. The second kind of spheroids is usually covered with nano-particles and lacks circular opening on surface. These spheroids possess large nuclei of single sparitic calcite coated with thin sparry sheaths. Overall, the Wumishan spheroids resemble coccoidal microorganisms reported from other Archean-Paleoproterozoic strata worldwide, but they are also better preserved. The rounded opening on spheroid surface is interpreted as division point of unicells during reproduction of the life cycle of bacteria akin to Myxococcoides grandis. Clump-like micro-particle aggregates in nuclei could represent daughter cells released from the parental envelope, similar to the reproduction process and life cycle suggested for similar spheroidal microfossils from other similar Precambrian occurrences. The Wumishan spheroids therefore may represent fossilized prokaryotes that could have contributed to construct the MDS. Moreover, filamentous microfossils are occasionally present in the coloumns of stromatolites, and they resemble filamentous cyanobacteria, but may not be major constructors of MDS due to their rarity in the buildups. Three types of nano-particles are also conspicuous: (1) putative organic relics, such as fragmented filaments and mucuslike biofilms (purported EPS), (2) organominerals, including nanoglobules, polyhedrons, and their aggregates, and (3) dumbbell-shaped nano-particle aggregates. All of these nano-particles are interpreted to be likely biogenic in origin, and many of them were found from the radial-fibrous fabrics of carbonate precipitates in the MDS, implying that some heterotrophic bacteria may have afficliated the precipitation of radial fibers in deep-time radial-fibrous carbonate precipitates. Therefore, abundant and diverse biosignatures (spheroids, tubular filaments, and nano-particles) are identified in the Wumishan MDSs, and we conclude that diverse filamentous and coccoidal micro-organismscontributed to the formation of the Wumishan stromatolites.

Fracture Characteristics and Heat Accumulation of Jixianian Carbonate Reservoirs in the Rongcheng Geothermal Field, Xiong'an New Area

AbstractGeothermal energy plays an important role in urban construction of the Xiong'an New Area. Geothermal reservoir fracture distribution of the Mesoproterozoic Jixianian Wumishan Formation (Fm.) carbonate reservoir in the Rongcheng geothermal field are evaluated based on FMI log from Wells D19 and D21. The results show carbonate reservoir fracture density of Well D19 is 15.2/100 m, greater than that of Well D21 with a value of 9.2/100 m. Reservoir porosity and permeability of Well D19 are better than that of Well D21, and the water saturation is bimodally distributed. The movable fluid volume ratio (BVM) of Well D19 is 2% to 8% with some zones exceeding 20%, while the value of Well D21 is less than 4%. Therefore, reservoir fractures in Well D19 are more conducive to fluid flow. Reservoir fractures have a similar occurrence to normal faults, indicating that the tensile stress field controlled the formation of such fractures. Developed reservoir fractures provide a good channel for groundwater convection. The circulation of regional groundwater and the heat exchange between water and rock and the multiple heat accumulation patterns form a stable and high potential heat reservoir in the Rongcheng geothermal field.

Fossil evidence provides new insights into the origin of the Mesoproterozoic ministromatolites

Although the biogenicity of the very common millimeter-sized ministromatolites in the Proterozoic has been noted, direct biotic evidence is missing, leading to the discussion of a chemical origin that differs from that of the cyanobacteria-related stromatolites in the Phanerozoic. Distinctive ministromatolites were recently discovered in a microbial dolostone interval of the Mesoproterozoic Wumishan Formation, at Lingyuan, North China. Thin section examination of the ministromatolites reveals the presence of abundant coccoidal microfossils ranging 10–30 μm in diameter. Petrofabric analysis shows that the ministromatolites experienced three periods of diagenetic silicification, and that the microfossils are exclusively preserved within early diagenetic chert. This supports the view that silicification penecontemporaneous with mat growth is the key to unique preservation of microfossils. In contrast, the absence of microfossils in most Precambrian carbonate stromatolites may well be interpreted as the lack of hydrochemical conditions favorable for early silicification. Although chemically-formed fibrous carbonate minerals are the dominant components of these ministromatolites, the presence of abundant microfossils indicates that the role of microorganisms in the formation of ministromatolites has previously been underestimated. Therefore, this research suggests that Mesoproterozoic ministromatolites were not solely chemical products but consist of complex associations of both biotic and chemical fabrics.