Characteristics Of Isotopic Composition Research Articles

Understanding the hydrocarbon accumulation process in deep-ultradeep oil and gas reservoirs in complex structural areas — The Qixia Formation in the Shuangyushi structure in the Northwestern Sichuan Basin, China

Deep-ultradeep oil and gas reservoirs in complex structural areas have typically undergone multistage tectonic processes that have a considerable influence on the hydrocarbon composition, isotopic composition, and other geochemical characteristics of petroleum. By assessing the charging period of hydrocarbon accumulation during a particular age, we can determine these changes. We study the characteristics of fluid inclusions developed in the dolomite reservoir in the Middle Permian Qixia Formation in the Shuangyushi structure of the northwestern Sichuan Basin using methods such as inclusion petrography, microbeam fluorescence spectroscopy, and inclusion homogenization temperature. Furthermore, the main peak wavelength of the fluorescence spectrum, the red-green ratio/chromatic parameter, Q650/500, as well as the chromatic parameter corresponding to the emission flux at 535 nm (QF535), and their correlations with the homogenization temperature distribution are used to identify the charging period of hydrocarbons in the Qixia Formation. The results indicate that different types of fluid inclusions are developed in the Qixia Formation. The fluorescence of hydrocarbon inclusions is mainly blue, with a small amount of yellow-green and orange-yellow. The [Formula: see text] of orange-yellow, yellow-green, and blue fluorescence of the hydrocarbon inclusions ranges between 562 and 579 nm, 497 and 548 nm, and 447 and 497 nm, respectively. The distribution of the homogenization temperatures during the same period ranges from 100°C to 110°C, 120°C to 140°C, and 160°C to 170°C, respectively. By combining the burial and thermal evolution histories of the Qixia Formation, we conclude that the Qixia Formation has experienced three stages of oil and gas charging, with the time of 204, 185, and 158 Ma. This method is of great significance in determining the date and duration of oil and gas accumulation and provides technical support for the recovery of ultra-deep hydrocarbon accumulations in complex structural areas.

The Use of the Characteristics of Water Isotopic Composition for Assessing the Contribution of Winter and Summer Precipitation to the Upper Ob Runoff during the Open-Water Period

The Use of the Characteristics of Water Isotopic Composition for Assessing the Contribution of Winter and Summer Precipitation to the Upper Ob Runoff during the Open-Water Period

Differential enrichment mechanism of helium in the Jinqiu gas field of Sichuan Basin, China

Helium is extensively utilized in aerospace, healthcare, electronics, semiconductors, advanced science, and renewable energy sectors, playing a pivotal role in the advancement of human technology. Presently, helium is primarily extracted from natural gas as an associated resource. The cost of extraction decreases with an increase in the helium concentration within the gases. Therefore, understanding the mechanisms involved in helium generation, migration, and accumulation underground is vital. This article delves into the differential enrichment mechanism of helium in the Jinqiu gas field, situated in the Sichuan Basin of China, by analyzing the molecular and isotopic composition characteristics of the major gases and noble gases, employing gas dissolution and diffusion models. Compared with other Jurassic gas reservoirs in the Sichuan Basin, the Jinqiu gas field contains a significantly higher helium concentration, displaying unique enrichment patterns within its area. The majority of hydrocarbon gases originate from the local underlying Triassic Xujiahe Formation source rocks. Specifically, the western section of the Jinqiu field is primarily supplied by the higher maturity Xujiahe Formation source rocks located in the western Sichuan Basin. Notably, there is no significant contribution of mantle-derived volatile components in the Jinqiu field. Helium and argon are primarily radiogenic, whereas neon primarily originates from atmosphere. The good linear correlation between 4He and 20Ne suggests that their transfer is primarily facilitated through groundwater. The solubility calculation reveals that the helium exsolved from the in-situ pore water of the reservoir, due to stratigraphic uplift during the Himalayan movement, comprises only 0.04%–0.11% of the existing reserves. This suggests that gas desolvation plays a relatively minor role in helium enrichment in the Jinqiu field. Additionally, the calculation results of gas diffusion indicate that diffusion's contribution to helium enrichment in the Jinqiu gas field remains minimal. Finally, our analysis of methane reserve changes suggests that the combined effect of differential loss and dilution of hydrocarbon gases is the primary mechanism for helium's differential enrichment in the Jinqiu field. Notably, methane and carbon dioxide depletion may be a significant factor in helium enrichment within the crustal natural gas system.

Characteristics of carbon isotopic composition of alkane gas in large gas fields in China

Exploration and development of large gas fields is an important way for a country to rapidly develop its natural gas industry. From 1991 to 2020, China discovered 68 new large gas fields, boosting its annual gas output to 1 925×108 m3 in 2020, making it the fourth largest gas-producing country in the world. Based on 1696 molecular components and carbon isotopic composition data of alkane gas in 70 large gas fields in China, the characteristics of carbon isotopic composition of alkane gas in large gas fields in China were obtained. The lightest and average values of δ13C1, δ13C2, δ13C3 and δ13C4 become heavier with increasing carbon number, while the heaviest values of δ13C1, δ13C2, δ13C3 and δ13C4 become lighter with increasing carbon number. The δ13C1 values of large gas fields in China range from −71.2‰ to −11.4‰ (specifically, from −71.2‰ to −56.4‰ for bacterial gas, from −54.4‰ to −21.6‰ for oil-related gas, from −49.3‰ to −18.9‰ for coal-derived gas, and from −35.6‰ to −11.4‰ for abiogenic gas). Based on these data, the δ13C1 chart of large gas fields in China was plotted. Moreover, the δ13C1 values of natural gas in China range from −107.1‰ to −8.9‰, specifically, from −107.1‰ to −55.1‰ for bacterial gas, from −54.4‰ to −21.6‰ for oil-related gas, from −49.3‰ to −13.3‰ for coal-derived gas, and from −36.2‰ to −8.9‰ for abiogenic gas. Based on these data, the δ13C1 chart of natural gas in China was plotted.

The composition and differences of antimony isotopic in sediments affected by the world's largest antimony deposit zone

Antimony (Sb) isotopic fingerprinting is a novel technique for stable metal isotope analysis, but the use of this technique is still limited, especially in sediments. In this study, the world's most important Sb mineralization belt (the Xikuangshan mineralization belt) was taken as the research object and the Sb isotopic composition and Sb enrichment characteristics in the sediments of water systems from different Sb mining areas located in the Zijiang River (ZR) Basin were systematically studied. The results showed that the ε123Sb values in the sediments of the ZR and its tributaries, such as those near the Longshan Sb-Au mine, the Xikuangshan Sb mine, and the Zhazixi Sb mine, were 0.50‒3.13 ε, 2.31‒3.99 ε, 3.12‒5.63 ε and 1.14‒2.91 ε, respectively, and there were obvious changes in Sb isotopic composition. Antimony was mainly enriched in the sediments due to anthropogenic sources. Dilution of Sb along the river and adsorption of Sb to Al-Fe oxides in the sediment did not lead to obvious Sb isotopic fractionation in the sediment, indicating that the Sb isotopic signature was conserved during transport along the river. The Sb isotopic signatures measured in mine-affected streams may have differed from those in the original Sb ore, and further investigation of Sb isotopic fingerprints from other possible sources and unknown geochemical processes is needed. This study reveals that the apparent differences in ε123Sb values across regions make Sb isotopic analysis a potentially suitable tool for tracing Sb sources and biogeochemical processes in the environment.

Late Palaeozoic tectonic transformation of the North Qinling Orogenic Belt: Insight from the geochronology, geochemistry and Sr‐Nd‐Hf isotopes of Carboniferous magmatic rocks

The North Qinling Orogenic Belt (NQB) records the pivotal geological information for understanding the Palaeozoic evolution of the Qinling Orogenic Belt (QOB). Previous studies mainly focused on the subduction–collision process along the Shangdan suture zone before the Early Devonian. However, as the significant interim period between the Early Palaeozoic and Mesozoic orogeny, the Late Palaeozoic tectonic evolution of the NQB remains poorly understood. Fortunately, the Carboniferous magmatic rocks discovered this time provide substantial geological evidence for revealing the Late Palaeozoic tectonic history of the Qinling Orogenic Belt. This paper will provide a detailed analysis of zircon U–Pb ages, geochemical characteristics and Sr‐Nd‐Hf isotopic compositions of Carboniferous rocks. The Carboniferous magmatic rocks are categorized as highly differentiated S‐type granite and monzodiorite, formed at 350 ± 2.4 Ma and 353 ± 5.2 Ma, respectively. The granite shows weak peraluminous (A/CNK = 1.02–1.11) and shoshonite nature. Based on the isotopic composition and trace element characteristics, we propose that the Qinling Group paragneiss is the primary source of the Carboniferous granite. Monzodiorite is characterized by enriched LREEs and LILEs (Ba, K and Pb), depleted HFSEs (Th, Nb, Ta and Ti) and enriched Sr‐Nd isotopic composition. Monzodiorite magma source region consists of continental crustal material and lithospheric metasomatized mantle. Carboniferous magmatic rocks are the product of an extensional tectonic setting, which indicates the NQB tectonic regimes transition from compression to extension during the Carboniferous.

A Comparison Study on the δD Value of Polycyclic Aromatic Hydrocarbons in Hydrous Pyrolysate of Herbaceous Peat with Different Water Medium

AbstractTo investigate the influence of diagenetic water media on the hydrogen isotopes of individual sedimentary aromatic compounds, a series of hydrous pyrolyses were conducted on herbaceous peat. Polycyclic aromatic hydrocarbons (PAHs) in hydrous pyrolysed samples and their hydrogen isotopic composition characteristics were studied. The aqueous medium demonstrated a significant influence on the hydrogen isotopic composition of the individual PAHs generated during pyrolysis. The results showed that the PAHs formed after pyrolysis in the presence of a saltwater medium with high δD value from a salt lake had a heavy hydrogen isotopic composition. The PAHs formed after pyrolysis in the presence of a fresh water medium with low δD value from a swamp had a light hydrogen isotopic composition. The difference in the average PAH δD value between the two hydrous experiments varied from –174‰ to –109‰, suggesting that the hydrogen isotopic composition of individual sedimentary PAHs can reflect the source of the diagenetic water medium. In addition, a comparative study found that the hydrogen isotopes of PAHs were superior to those of n‐alkanes in the same sample for diagenetic water indications. The results indicated that the exchange of water‐derived inorganic hydrogen and organic hydrogen was more intensive in freshwater experiments than in saltwater experiments. With an increase in the simulation temperature, the average δD value of PAHs generated in the hydrous simulation experiments showed an increasing trend, reflecting that the δD value of sedimentary PAHs formed with the participation of diagenetic water media was still closely related to the thermal maturity of organic matter. Comparative studies showed that the δD values of different types of organic compounds produced by hydrous pyrolysis of peat were in the order, PAHs > n‐alkanes > methane.

Isotopic composition and emission characteristics of CO2 and CH4 in glacial lakes of the Tibetan Plateau

Carbon dioxide (CO2) and methane (CH4) emissions from freshwater ecosystems are predicted to increase under climate warming. However, freshwater ecosystems in glacierized regions differ critically from those in non-glacierized regions. The potential emissions of CO2 and CH4 from glacierized environments in the Tibetan Plateau (TP) were only recently recognized. Here, the first direct measurement of CO2 and CH4 emission fluxes and isotopic composition during the spring of 2022 in 13 glacial lakes of the TP revealed that glacial lakes were the previously overlooked CO2 sinks due to chemical weathering in glacierized regions. The daily average CO2 flux was −5.1 ± 4.4 mmol m−2 d−1, and the CO2 consumption could reach 38.9 Gg C-CO2 yr−1 by all glacial lakes in the TP. This consumption might be larger during summer when glaciers experience intensive melting, highlighting the importance of CO2 uptake by glacial lakes on the global carbon cycle. However, the studied glacial lakes were CH4 sources with total emission flux ranging from 4.4 ± 3.3 to 4082.5 ± 795.6 μmol m−2 d−1. The large CH4 range was attributed to ebullition found in three of the glacial lakes. Low dissolved organic carbon concentrations and CH4 oxidation might be responsible for the low CH4 diffusive fluxes of glacial lakes without ebullition. In addition, groundwater input could alter CO2 and CH4 emissions from glacial lakes. CH4 in glacial lakes probably had a thermogenic source; whereas CO2 was influenced mainly by atmospheric input, as well as organic matter remineralization and CH4 oxidation. Overall, glacial lakes in the TP play an important role in the global carbon cycle and budget, and more detailed isotopic and microbial studies are needed to constrain the contributions of different pathways to CO2 and CH4 production, consumption and emissions.

Quantifying the impacts of coal mining activities on topsoil using Hg stable isotope: A case study of Guqiao mining area, Huainan City

The Hg released from coal mining activities can endanger soil ecosystems and pose a risk to human health. Understanding the accumulation characteristics of mercury (Hg) in coal mining soil is important for effectively controlling Hg emissions and developing measures for the prevention and control of Hg contamination. To identify the potential sources of Hg in soils, the Hg concentration and isotopic composition characteristics of raw coal and different topsoil types from the areas surrounding a coal mine were determined in this study. The results showed that Hg in coal mainly exists mainly in the form of inorganic Hg, and Hg has experienced Hg2+ photoreduction prior to incorporating into coal. In addition, the composition of Hg isotopes differed significantly among different topsoil types, and the δ202Hg value of the farmland soil exhibited large negative excursions compared to the coal mining soil. The ternary mixed model further revealed the presence of substantial differences in potential Hg sources among the two regions, with the coal mining soil being greatly disturbed by anthropogenic activity, and the relative contributions of Hg from raw coal, coal gangue, and background soil to coal mining soil being 33.42%, 34.4%, and 32.19%, respectively. However, Hg from raw coal, coal gangue and background soil contributed 17.04%, 21.46%, and 61.51% of the Hg in the farmland soil, indicating that the accumulation of Hg in farmland soil was derived primarily from the background soil. Our study demonstrated that secondary pollution in soil caused by immense accumulation of solid waste (gangue) by mining activities offers a significant challenge to ecological security. These findings provide new insights into controlling soil Hg in mining areas and further highlight the urgency of strict protective measures for contaminated sites.

Carbon isotope composition characteristics of light hydrocarbon gas from typical kerogen cracking and its application for gas source identification: A case study of the Sichuan Basin

The natural gas resources in the Sichuan Basin are abundant and exhibit various phenomena, for example, mixed sources, mixed maturities and overlapping combinations, resulting in great difficulties in identifying natural gas sources. In this study, several groups of thermal simulation experiments were conducted on various types of kerogens using closed and/or semi‐open systems to investigate the characteristics of the carbon isotope compositions of light hydrocarbon gases derived from different typical kerogens. The results showed that the carbon isotope compositions and variation trends of methane and ethane are controlled by the kerogen type, thermal simulation system and thermal evolution degree. The carbon isotope compositions of light hydrocarbon gases derived from different types of kerogens exhibit some differences with increased thermal evolution degree. When the thermal simulation temperature increases from 320 to 550°C, the methane carbon isotope of typically marine source rocks, mainly type I and type II1 kerogens, is slightly higher than or equivalent to that of ethane. However, the ethane carbon isotope of terrestrial source rocks, primarily types II2 and III kerogens, is significantly heavier than methane. By comparison, the evolution trend of carbon isotope compositions of actual natural gas is well consistent with that of light hydrocarbon gases cracked from thermal simulation experiments with increased thermal evolution degree. These findings are of great importance to identify the source rock of actual natural gas with high thermal maturities, as well as to determine the sources of deep shale gas in the southern Sichuan Basin, the Yuanba gas field and the Puguang gas field. Finally, a fitting method of carbon isotope values of methane and ethane of light hydrocarbon gases can be used to more finely differentiate the sources of natural gas in complex areas with mixed sources, mixed maturities and overlapping combinations.

Origin of lithium-rich salt lakes on the western Kunlun Mountains of the Tibetan Plateau: Evidence from hydrogeochemistry and lithium isotopes

Lithium-rich salt lakes have attracted much attention as important lithium resources. The source and enrichment mechanisms of lithium in salt lakes are fundamental to uncovering the genesis of brine-type lithium deposits. However, the origin of lithium in salt lakes has been controversial, especially for lithium-rich brines associated with hard rock lithium ore has not been concerned by scholars. In this study, for the first time, we investigated the lithium isotope and hydrochemical composition characteristics of a lithium-rich salt lake (Kushui Lake) and a series of surrounding water samples in the lithium pegmatites district of West Kunlun, Xinjiang, China. Hydrochemical composition shows that different samples in the basin are rich in Li, Na, and K and poor in Mg and Ca. The water chemistry of the Kushui River, the largest river within the catchment, changes from carbonate type to sulfate subtype through evaporation. The precipitate of sulfate and chloride minerals is also gradually increasing. In addition, Li, B, Sr, Rb, Cs, U, Ti, V, Mn, Cu, Zn, Cr, Co, Ni, and REE are abnormally enriched in the rivers and lakes of the basin, especially in the spring water, which is several times higher than in the rivers. Likewise, dissolved Li and δ7Li showed ranges of 0.01–287.86 mg/L and 3.28–16.44 ‰, respectively. These data were used to determine the origin of lithium and its enrichment process in salt lakes. The results show that the source of Li in Kushui Lake is closely related to the surrounding felsic volcanic rocks, pegmatite-type lithium ore and deep fluids, whereas the water–rock interaction in the deep crust and the chemical weathering in the supergene environment play an influential role during the formation of Li-rich fluids. In addition, the spring samples in the northwestern of Kushui Lake have inherited the lithium isotopic signature of lithium-bearing source rocks from the water–rock reaction process in the deep crust, allowing us further to isolate the source of Li in the target waters. With the movement of water, lithium exhibits a trend of continuous enrichment under the influence of evaporation and mixing. At different stages of evaporation, the Li/Na ratio in water in this catchment shows a trend of increasing, decreasing, and increasing due to the adsorption of 6Li by secondary minerals and the crystallization of Na. It suggests that strong evaporation under arid conditions is the main mechanism responsible for the rapid enrichment of Li in salt lakes. Moreover, the positive relationship between Li content and the δ7Li value of rivers along the discharge path indicated that the adsorption of 6Li by secondary minerals still existed even at a minor watershed scale, resulting in the fractionation of lithium isotopes in river waters. This study may contribute to a deeper understanding of lithium sources and their enrichment processes in Li-rich salt lakes. In addition, it is also beneficial to search for new lithium resources based on the source relationships among the various lithium reservoirs.

Late Jurassic Volcanism Deduced from Geochemical, Geochronological, and Sr‐Nd‐Hf Isotopic Composition Characteristics of the Nanyuan Formation, South China

AbstractThe Nanyuan Formation contains information related to the Mesozoic tectonic transformation. In this study, three representative profiles were surveyed from the Nanyuan Formation, and multiple analyses were conducted. Zircon U‐Pb dating yielded their ages as approximately 158–146 Ma. The volcanic rocks are enriched in Rb, Th, U, K, and Pb and depleted in Nb, Ta, P, and Ti, implying their affinity for I‐type granites. The εNd(t) values (–8.3 to −6.0), (87Sr/86Sr)i values (0.7077–0.7094) of the volcanic rock, and εHf(t) values (–8.71 to 0.12) of the Mesozoic zircons suggest that the Nanyuan Formation magma originated in the lower crust with the involvement of depleted mantle materials. The parent rocks of the rhyolitic and dacitic volcanic rocks formed by partial melting of basement rocks in South China and the andesitic volcanic rocks were derived from partial melting of the metasomatites generated by slab‐mantle interaction. The fractional crystallization also played an important role in later stage. Discrimination diagrams of the volcanic rocks indicated that they formed in a volcanic arc environment. Combined with previous data, the Nanyuan Formation recorded subduction of the Paleo‐Pacific Plate before regional tectonic transformation. The compressive stress field endured until the end of the Late Jurassic.

Geochemical classification and secondary alteration of crude oil in the southern thrust belt of Junggar Basin

This study analyzed, 18 oil samples from four representative oil-bearing secondary tectonic units in the southern thrust belt of the Junggar Basin. The genetic types and the secondary alteration of oil were discussed based on the characteristics of light hydrocarbon compounds, adamantanes, sterane/terpene biomarkers, and stable carbon isotopic composition of the bulk oil. . The results show that the oil in the study area can be divided into five categories. Type I oil is characterized by 13C-depleted carbon isotopes, a low pristane to phytane ratio, and high gammacerane and low diasterane content, whereas Type II oil exhibits the opposite characteristics. Type III is very similar to Type II oil but has a higher 13C-depleted bulk carbon isotopic composition and a wider carbon number of tricyclic terpenes. Type IV and V oils are similar to Type I oils, except for relatively lower gammacerane content. Further, Type IV oils also have high ααα 20R regular sterane content, 13C-enriched bulk oil carbon isotopes, and a higher pristane to phytane ratio than Type I oil. In contrast, the relative content of C27 regular steranes of Type V oils is considerably lower than that of Type I oils. The oil in the study area has not suffered from biodegradation. Maturity information indicated by regular biomarkers (i.e., steroid, terpene, and phenanthrene) and adamantane differs significantly, with the latter exhibiting a strong cracking characteristic. This reflects the charging and mixing of oils formed at different evolution stages. The other types of oils are in the normal oil-generating window and do not suffer from intense cracking. The oil in the study area is generally subjected to evaporative fractionation, but there are obvious differences in the degree of alteration between different oil-bearing structures and different reservoirs with the same structure. This study is of great significance for understanding the origin and accumulation process of oil and gas in a complex structural zone.

Research on the characteristics of 2H and 18Ostable isotopic composition of precipitation in Hanoi

The variability in the content of environmental isotopes 2H and 18O in precipitation over time is applied to study the characteristics of rainwater and the time-varying process in the atmospheric water cycle and the variable processes in the hydrological cycle. 26 rainwater samples were collected according to rainy days from August 3 to October 29, 2020 in Ha Noi. The stable isotope composition in rainwater ranged from -108.04‰ to -40‰ and -15.87‰ to -6.53‰ for 2H and 18O, respectively. Besides, the values of 2H and 18O were located around the local meteorological waterline. The stable isotope values also clearly reflect the correlation with the daily rainfall as well as clearly reflect the influence of season and ambient temperature on the change of stable isotope value with rainwater.

Isotopic fingerprinting of dual monsoon moisture sources, evapotranspiration process and microclimate manifestation over the tropical rainforest region, western part of the Western Ghats, India

The changes in precipitation pattern provide an understanding on the hydroclimatic response to global warming during the Anthropocene. The present study investigates sources of precipitation moisture for the Indian Monsoon and the local environmental mechanisms controlling its distribution over the southwest coast of India. This is achieved by the characterization of stable isotope ratios of oxygen (δ18O) and hydrogen (δ2H) in rainwater samples collected from a high humid tropical setting (Swarna-Madisal river basin) of the Western Ghats, South India and another station further south (Bakrabail, southern edge of Nethravati river basin). This study contributes to the detailed investigation on rainwater isotopic composition and microclimate characteristics which is lacking in the humid west coast region. The rainwater isotopic composition of coast was close to that of the Arabian Sea water and reflected the first condensate of vapours which were originally formed under fast evaporation at the nearby ocean. In inland location, the higher d-excess values reflect continental moisture recycling. Evapotranspiration has led to higher kinetic fractionation effect in the inland region. The isotopic storm effect during winter monsoon season suggested the rain distribution from saturated air masses formed under deep convective effect in the Bay of Bengal. The overall local meteoric water line (LMWL) in the Swarna-Madisal basin was found to be δ2H = {(7.2 × δ18O) + 7.5}, R2 = 0.98. Further south, the LMWL of Bakrabail was, δ2H = {(8.19 × δ18O) + 16.1}, R2 = 0.98 for annual observation and displayed minimal variability for inter-seasonal slopes (7.73 for summer monsoon, 8.48 for winter monsoon and 8.36 for pre-monsoon) and intercepts (15.6 for summer monsoon, 17.8 for winter monsoon and 15.5 for pre-monsoon). In regions of vegetation dominance and humid climate, the prevailing local air mass masked the rain-out effect of marine air mass as well as the amount effect which support microclimatic settings at the local precipitation sites in southwest India. The time and space variability of regional moisture circulation in controlling atmospheric water balance has been deduced in this study. Thus, the high efficacy of stable isotopes in tracing the manifestation of microclimate in the humid tropics has been demonstrated.

Hydrogeochemistry and Isotope Hydrology of Surface Water and Groundwater in the Mountain Watersheds of Daqing River, North China

Surface water and groundwater interaction variations in time and space are crucial for effective water management, especially in low-precipitation regions. To comprehensively determine the hydrochemical characteristics and interaction processes of surface water and groundwater and to investigate the decreasing causes of water resources in semi-arid mountainous watersheds under changing environments, intensive field surveys were conducted in the Daqing River watershed, a tributary of the Haihe River basin in northern China, during two different times of the year: after the rainy season (September 2018) and before the rainy season (July 2019). Sixty surface water and groundwater samples were collected along the mountainous watershed. Using a combination method of hydrogen and oxygen stable isotope tracing and hydrochemical analysis, the hydrogen and oxygen isotopes and hydrochemical characteristics of surface water and groundwater in the mountainous watershed of the Daqing River were analyzed. Furthermore, the effect of elevation (altitude) on isotopes was discussed, and the correlation between hydrogen and oxygen isotope composition and hydrochemical characteristics was obtained. The results were processed using endmember mixing analysis to determine the amount of contribution of the surface water and groundwater interaction processes. The results show that the hydrochemical characteristics are relatively stable in the mountainous watersheds of the Daqing River, and the surface water and groundwater are mainly of the HCO3-Ca type. The slope of the local meteoric water line is smaller than the slope of the global meteoric water line, and the δD and δ18O in surface water and groundwater show a good linear relationship both before and after the rainy season. There is a decreasing trend of the value of δ18O in surface water samples with decreasing altitude, but a decreasing trend of the value of δ18O in groundwater samples is not obvious. The evaporation intensity of surface water is stronger after the rainy season than before the rainy season, and the connection between the surface water and the groundwater is stronger before the rainy season. Influenced by topographic conditions and other factors, the exchange of surface water and groundwater is frequent, and there is a large difference in the exchange ratio before and after the rainy season. The exchange ratio can be more than 50% after the rainy season. Thus, the reasons for decreasing water resources in the mountains can be implied to be due to the increasing hydraulic gradient between the mountains and the piedmont plains, and the water resources are discharged more in the form of groundwater to the downstream. The conclusions help to enhance the understanding of the water cycle in the mountainous watershed and can provide some theoretical basis for the sustainable development and utilization of water resources in the Haihe River basin and the regional water ecology of the Xiong’an New Area.

Vertical distribution characteristics and source tracing of organic carbon in sediment of the Yellow River wetland, Sanmenxia, China

To investigate the vertical distribution and sources of organic carbon in the Yellow River wetland, the carbon and nitrogen isotopic composition and hydrochemical characteristics of sediment were studied systematically. The contents of TON (0.01–0.02%) and TOC (0.07–0.2%) are low and exhibit no significant differences over the vertical distribution, illustrating the loess sediments are infertile in Yellow River wetland. NH4+ is the main form of dissolved inorganic nitrogen (DIN) in sediment, accounting for 68–98% of the total DIN. The sediment content of δ13C-TOC is − 25.17‰ ( ± 0.52‰) to − 23.90‰ ( ± 0.21‰), the poor correlation between δ13C-TOC and the C/N ratio indicates that the C/N ratio in the sediments is the result of a mixture of complex biochemical processes. The relationship between δ13C and δ15N shows that the sediment organic matter is a mixed phase of plants and suspended particulate matter (SPM). The simulation results of the three end-member mixing equations show that the sources of organic carbon in sediment are SPM, plants, and microphytobenthos, with contribution rates of 45.6% ( ± 5.80%), 27.90% ( ± 5.13%), and 26.2% ( ± 3.52%), respectively.

Carbon and Oxygen Isotopic Characteristics of Longmendong Formation in Muxu River Section of Qishan, Shanxi Province and Its Significance

The lower and upper Longmen cave formation of Late Ordovician in the Muxu river section of Qishan in the southwest margin of Ordos are Sandaogou formation of Middle Ordovician and beiguoshan formation of Upper Ordovician respectively, with continuous and complete strata. The average values of carbon and oxygen isotopes of carbonate rocks are 0.57‰ and -10.73‰ respectively, with poor correlation, which basically preserves the characteristics of original isotopic composition. The paleosalinity Z is between 120 and 130, which reflects that the late Ordovician was a marine sedimentary environment, which is consistent with the geological research results. According to the base value characteristics and drift changes of carbon isotope curve of Longmenshan formation, it can be divided into XH1-XH4 cycle segments. Generally speaking, the early carbon isotope values are mainly positive drift, with large scale and long time; The late stage is dominated by negative drift, with small scale and short time. This reflects that the sea level fluctuated greatly in the early Late Ordovician, with strong activity, and the sea level fluctuated slightly in the late Ordovician, showing a downward trend as a whole. Carbon and oxygen isotope variation curve of longmendong formation and global Ordovician δ13C variation curves shows that longmendong formation is deposited in the late Ordovician, which is equivalent to Sandbian and Katian periods. There is a positive drift event of GICE carbon isotope, which is regionally comparable.

НОВЫЙ ВЗГЛЯД НА СТРАТИГРАФИЮ СЕЛЕМДЖИНСКОГО И ТОКУРСКОГО ТЕРРЕЙНОВ МОНГОЛО-ОХОТСКОГО ПОЯСА: РЕЗУЛЬТАТЫ U-Pb, Lu-Hf, Sm-Nd ИЗОТОПНЫХ ИССЛЕДОВАНИЙ

Results of U-Pb and Lu-Hf isotopic studies on detrital zircons from metasedimentary rocks in the eastern part of the Selemdzha and Tokur terranes of the Mongol-Okhotsk belt and Sm-Nd isotopic data for whole-rocks are in complete contradiction with existing ideas about the stratigraphy of the region under consideration and indicate the need to revise the traditional principles of mapping within the belt. Two types of deposits have been established, differing in Sm-Nd isotopic characteristics of whole-rocks and Lu-Hf isotopic composition of detrital zircons. Deposits of different types are involved in the formation of the two opposing accretion systems: in front of the Siberian Craton margin (type I) and the Amur superterrane (type II). Among type I deposits, discrete groups are distinguished that differ in the lower age limit: 553–498 Ma, Late Ediacaran-Cambrian; ~373 Ma, Upper Devonian; 333–327 Ma, Upper Mississippian; ~304 Ma, Pennsylvanian. The deposits of tectonic-stratigraphic units show a general pattern of younging from north to south. Thus, we observe a structure typical of accretionary wedge-shaped terranes with a rear part in the north and a frontal part in the south. Tectonic-stratigraphic units formed by type I deposits and previously considered part of the Selemdzha and Tokur terranes, apparently, are components of the Galam terrane of the accretionary wedge.

Application of Light Hydrocarbons in Natural Gas Geochemistry of Gas Fields in China

Light hydrocarbons (LHs) are an important component of natural gas whose chemical and isotopic compositions play a vital role in identifying gas genetic type, thermal maturity, gas–gas correlation, gas–source correlation, migration direction and phase, and secondary alterations (such as evaporative fractionation, biodegradation, and thermochemical sulfate reduction) experienced by the gas pool. Through review of geochemical research into LHs over recent decades, and analysis of chemical and isotopic compositions of LHs of gases and condensates from more than 40 gas fields in China, we present an overview of the genetic mechanisms of LHs and the impacts of various factors on their geochemical compositions. The primary objectives of this review are to demonstrate the application of LH chemical and isotopic composition characteristics to gas geochemistry research and to assess the applicability and reliability of geochemical identification diagrams and parameters for determining gas genetic types, maturity, source, secondary alteration, and migration direction and phase. ▪ Three main genetic mechanisms are proposed for the formation of light hydrocarbons: thermal decomposition, catalytic decomposition of organic matter, and microbial action. ▪ Chemical and isotopic compositions of light hydrocarbons with different carbon numbers and/or structures can be used to identify the genetic types and maturity of natural gas. ▪ Content ratios and carbon isotopes of characteristic light hydrocarbons are good indicators for gas–gas and gas–source correlations. ▪ Secondary alterations (evaporative fractionation, biodegradation, thermochemical sulfate reduction) and migration of gas can be indicated by chemical and isotopic compositions of light hydrocarbons.