Gas Geochemistry Research Articles

Formation of Fe-Cu-Au deposit in basin inversion setting in NW China: A perspective from ore-fluid halogen and noble gas geochemistry

The Qiaoxiahala deposit is a significant Paleozoic Fe-Cu-Au deposit in the northern margin of East Junggar, NW China, which has been confirmed to form in a basin inversion setting (dying back-arc basin). The composition of noble gases (Ar, Kr, and Xe) and halogens (Cl, Br, and I) extracted from fluid inclusions, hosted in quartz, calcite, and epidote from various mineralization stages of Qiaoxiahala, are investigated to trace the sources of ore fluids and discuss the ore genesis. Significant multistage mineralization at Qiaoxiahala is identified as magnetite mineralization (stage III), magnetite-pyrite mineralization (stage IV), and chalcopyrite mineralization (stage V). Syn-ore fluid inclusions show salinities varying from 13.2 to 15.3 wt% (NaCl eq.) associated with elevated I/Cl ratios (214 × 10−6 to 3950 × 10−6), uniformly low Br/Cl ratios (0.78 × 10−3 to 1.30 × 10−3), and low 40Ar/36Ar ratios (<700). The low 40Ar/36Ar ratios and Br/Cl ratios preclude the involvement of magmatic or metamorphic fluids (high 40Ar/36Ar), and sub-aerial evaporation bittern brines (elevated Br/Cl). The predominance of sedimentary formation water modified by organic-rich sediments is supported by the high I/Cl ratios, low 36Ar concentrations (less than or equal to that of Air Saturated Water, ASW), and the constant 40ArE/Cl slopes (10−5 to 10−4) of fluid inclusions from various mineralization stages in Qiaoxiahala. The elevated δ18Owater values (7.2–11.1‰) of the ore-forming fluids and the common presence of organic matter (e.g. C4H6, C2H2 and CH4) in fluid inclusions further indicated the involvement of organic-rich materials. However, different δDwater values and a significant gap between halogen and noble gas data in stages III and IV-V indicate that different sedimentary formation fluids are involved during these stages, i.e., extensive water-wall rock interactions between seawater and Beitashan Formation in stage III and formation waters formed by diagenesis and compaction in stages IV-V. Multistage mineralization, i.e., Fe-oxide and chalcopyrite (-gold), and the mixing of different non-magmatic fluids with variable origins in Qiaoxiahala are similar to many iron oxide-Cu-Au deposits (IOCGs) from the Central Andes, indicating an IOCG-like type deposit for Qiaoxiahala. These finding further suggest that non-magmatic fluids dominate the formation of IOCG deposits in the basin inversion setting at the northern margin of East Junggar.

Development and status of active volcano monitoring in China

Abstract Following decades of geological surveys and studies, 14 active volcanic fields have been identified in China. Evidence for Holocene volcanism in several of these areas highlights the need to understand and monitor volcanic hazards in those regions. Six volcano observatories have thus been established in the past 40 years. This work reviews China's national capability and history of volcano monitoring, with emphasis on the Changbaishan–Tianchi Volcano Observatory and the Tengchong Volcano Observatory. The Changbaishan–Tianchi Volcano Observatory (CTVO) was constructed in 1996 and began monitoring in 1999, with limited recorded observations dating back to 1973. Currently, CTVO is the largest and most advanced observatories in China. The monitoring network of the CTVO incorporates 11 seismic and 15 GPS stations, two levelling routes and three gas geochemistry sampling points. The Changbaishan–Tianchi Volcano experienced unrest during 2002–05, evidenced in elevated levels of seismicity and ground deformation, as well as shifts in gas geochemistry. After 2006, the volcano returned to quiescence, with activities at background levels as recorded in 1973–2001. The monitoring network of Tengchong Volcano Observatory incorporates eight seismic stations, 20 GPS points, 95 levelling points and three gas geochemistry sampling points. The observations made since 1965 indicate significant seismicity, with more than 3000 events recorded in 2011, mostly related to regional tectonics. Tengchong is known for its widespread hot springs, with temperatures up to 105°C recorded at Dagunguo spring. The four other observatories are Longgang Volcano Observatory, Jingbohu Volcano Observatory, Wudalianchi Volcano Observatory and Qiongbei Volcano Observatory. They are equipped with seismic, geodetic and geochemical monitoring equipment. These areas saw only low levels of activity over the past several decades, but related fault systems are relatively active. In a relatively short time, China has gained considerable experience in observatory design and volcano monitoring and has trained up a sizeable task force, laying the foundation for sustained volcano monitoring at the national level. Future efforts must focus on maintaining and expanding observational capacity, as well as gaining better dynamic understanding to inform volcano hazard assessment.

Інноваційні технології пошуків корисних копалин, основані на дослідженнях флюїдних включень у мінералах

The innovative approach to the development of prospective technologies (methods) was substantiated on crystallogenic and physicochemical principles of the knowledge of mineral forming environments (fluids) (thermobarogeochemistry – mineralofluidology – fluid inclusions) as a new branch of geological knowledge within the framework of the new scientific direction in the geology – «thermobarometry and geochemistry of gases of veinlet-impregnated mineralization in deposits of oil- and gas-bearing areas and metallogenic provinces» as a natural phenomenon of the Earth’s lithosphere. According to him, the creation of radically new technologies and realization of prospecting for mineral deposits (first of all hydrocarbons and gold) simultaneously with the elucidation of the problem of genesis and synthesis of hydrocarbons at the atomic-molecular level fixed by such defects in the mineral crystals as fluid inclusions. Developed technologies, namely: determination of genesis of hydrocarbon gases; determination of prospects of oil and gas presence of a local area; local forecast of enriched areas of gold ore fields; express determination of potassium ions in inclusions for identification of gold-enriched and barren veinlet formations, – belong to the branch of the exploration geology and geochemistry and are used to ascertain genetic guestions, to solve tasks of the mineralogical-geochemical prediction and prospecting for mineral deposits in the local structures of oil- and gas-bearing areas and metallogenic provinces. The comparison of fluid inclusions of veinlets and host rocks based on the sections of a number of wells has shown the considerable possibilities of the developed technologies and prospects of the usage of thermobarogeochemical-mineralofluidological indicators in the complex with the geochemical and geophysical (petrophysical) methods, data of geological survey, deep-seated drilling while predicting hydrocarbon deposits in the local structures of the sedimentary strata promising for oil and gas and making necessary amendments in the directions of the following geological-prospecting works.

Геохімія флюїдів: інноваційне вирішення фундаментальної проблеми

First of all, was developed a reliable mass spectrometric method for studying fluid inclusions in minerals (on the basis of the author’s certificate of the USSR No. 454446 of 1974 for the invention of the “device for cleaning the content inclusions of solid materials”). Were created and published new scientific directions: 1. Thermobarometry and geochemistry of gases of veinlet-impregnated mineralization in sediments of oil and gas regions and metallogenic provinces. 2. Bowels of the Earth – natural physicochemical reactor. Was developed a “method (technology) for determining the prospects of oil and gas bearing of the local area” and “a method (technology) of local forecasting of enriched areas of gold-ore fields” (together with M. M. Davydenko). Was established unknown before, but objectively existing phenomenon of the material world: 1) “lack of molecular hydrogen in inclusions in minerals in the bowels of the planet Earth” (together with V. A. Kaliuzhny). Independently were established: 2) previously unknown “property of deep abiogenic methane-termobar high-temperature fluid decompose-convert natural organic residues into layers of coal with their simultaneous methane saturation and its conservation in the earth’s crust of the planet Earth”; 3) previously unknown natural “phenomenon of arbitrary formation of natural carbon methane in the coal layers of the earth’s crust of the planet Earth under the influence of abiogenic methane-containing high-temperature fluid with their conservation in them”; 4) previously unknown different chemical properties of carbon isotopes in natural processes of synthesis-formation of various carbon-containing compounds ...; 5) “previously unknown pattern of natural processes of synthesis of perfect diamond crystals from astenospheric carbon dioxide ions...”. Was justified “a new way to determine the calorification of natural gas supplied to consumers and its cubic-metre barometry”.

Partitioning of elements between high-temperature, low-density aqueous fluid and silicate melt as derived from volcanic gas geochemistry

By comparing high-quality volcanic gas and whole rock compositions, we calculated the apparent (observed) mass partition coefficients Kd* for 58 elements on six basaltic volcanoes located in arc and rift/hotspot settings. The inferred Kd* vary from ∼1100 for sulfur to 0.0001 for zirconium, i.e., within seven orders of magnitude. Only 14 elements have Kd* > 1, including highly volatile S, Se, Te and halogens, as well as Tl, Re, Os, Bi, Cd, Au, In and As. Alkali metals have Kd* in the range from 0.1 for Cs to 0.01 for Na. Partition coefficients of other rock-forming elements are <0.001. The partition coefficients for elements depend on element speciation and concentrations of ligand-forming elements in the gas such as sulfur and chlorine. Elements transported in the gas predominantly as halides have higher partition coefficients in HCl-rich arc gases, whereas elements preferably forming sulfides, hydrides and free atoms, have higher Kd* in sulfur-rich, HCl-poor and reduced rift/hotspot gases. Degassing directly from the free melt surface is negligible; deep gas passing through the erupting vent is quickly overwhelmed by the signal of low-pressure degassing. Equilibration of rising bubbles with the surrounding melt almost eliminates the difference between Kd* calculated for degassing lava flows (no connection with deep magma) and for lava lakes and open-vent volcanoes (convective mass exchange with deep magma takes place). Diffusion does not strongly affect the apparent partitioning of magmas degassing at surface. Gas bubbles growing in near-surface silicate melts at atmospheric pressure have a large density difference compared to the surrounding melt of 12–15 thousand times. This leads to the rapid expansion of such bubbles and a decrease in the thickness of the diffusion boundary layer in the melt due to its stretching around the growing bubble, which sharply decreases diffusion fractionation. As a result, the apparent partition coefficients (Kd*) for degassing basaltic volcanoes are close to the equilibrium ones (Kd) for most of the elements. The partition coefficients of volatile elements (S and Cl) calculated from the comparison of volcanic gas and rock compositions are in agreement with the values determined previously via experiments or theoretical modeling.

Geochemistry of high-maturity crude oil and gas from deep reservoirs and their geological significance: A case study on Shuntuoguole low uplift, Tarim Basin, western China

The oils of Shuntuoguole low uplift of Tarim Basin in western China have high contents of saturated hydrocarbons and a high loss of light hydrocarbon components because of volatilization. These oils are characterized by low abundance of hopanes and high abundance of C28–30 tricyclic terpanes and dibenzothiophenes and relative high abundance of C20–23 terpanes, C21–22 5α-steranes, and diasteranes in the oils. Quantitative analysis of carbon isotope and biomarkers show that the oils in the Ordovician carbonate rocks are sourced by deep Cambrian and Ordovician marine shales and marls. Geochemistry of the oils indicates that the reservoir has undergone multistage accumulations, with the late-stage oils obscuring traces of biodegradation of paleo-oils charged in the early stage. The concentration of methyl adamantanes and steranes demonstrates that oils occurred in an early stage of intense oil-to-gas cracking. Most of the gases are wet, with dryness lower than 90%, and geochemical analysis suggests that they are mainly from thermal degradation of kerogen. This study suggests that in addition to oils sourced by deeper source rocks in the Shuntuoguole low uplift in situ, there are oils that migrated from the Manjiaer depression into the Shuntuoguole low uplift.

Monitoring Network Changes during the 2018 Kīlauea Volcano Eruption

Abstract In the summer of 2018, Kīlauea Volcano underwent one of its most significant eruptions in the past few hundred years. The volcano’s summit and East Rift Zone magma system partially drained, resulting in a series of occasionally explosive partial caldera collapses, and widespread lava flows in the lower East Rift Zone. The Hawaiian Volcano Observatory (HVO) operates a robust permanent monitoring network of about 250 stations, recording a variety of real-time data streams: seismic (short-period, broadband, strong-motion), infrasound, Global Navigation Satellite Systems (GNSS), tilt, camera, laser rangefinder, and gas geochemistry. During the eruption, HVO staff quickly established 35 new temporary monitoring stations, to better constrain evolving volcanic hazards. The partial collapses of the caldera threatened to disrupt important telemetry links in the HVO monitoring network, and a major effort was undertaken in the midst of the eruption crisis to reroute radio telemetry and maintain continuity of data flow. In the process, a new data center was established in Hilo, to mitigate a long-standing potential single point of failure at the HVO facility. Over the course of the eruption from May through August, lava, ashfall, wildfire, and cliff collapse destroyed or disabled 36 stations. Thousands of earthquakes damaged the main HVO facility at Uēkahuna Bluff, causing staff to evacuate the building and relocate observatory operations in the midst of the eruption response, adding more complexity to the response effort. Throughout these events, the HVO team maintained the monitoring network, provided timely information to the public and emergency managers, and collected valuable scientific data to better understand Kīlauea Volcano.

Continuous monitoring of hydrogen and carbon dioxide at Stromboli volcano (Aeolian Islands, Italy)

The geochemistry of volcanic gases has been fruitfully applied to identify important changes in the volcanic activity. This study reviews the dataset of the volcanic gas survey performed during 2009 and 2010 at Stromboli (Aeolian Islands – Italy). Dry gas collection occurred discontinuously at the crater fumaroles for subsequent chemical and isotopic (δ13C-CO2) analyses in the laboratory.A tailor-made device enabled continuous survey of H2 molar fraction and CO2 flux on the same site. Evaluation of the raw data was performed in accordance with air temperature, atmospheric pressure, wind speed and wind direction dataset. Both MLR (Multiple Linear Regression) and FFT (Fast Fourier Transform) analyses allowed filtering the dataset from the environmental effects. The MLR analysis indicated that wind speed and air temperature affected the CO2 flux. Changes in the atmospheric pressure promoted pumping effect of the fumarole gas and caused changes in the H2 molar fraction. The power spectral analysis revealed daily cycles in both gases. A digital signal filtering procedure enabled minimizing the environmental effects.This study confirmed that gas emissions from the crater fumaroles have both chemical ad isotopic composition similar to the magmatic gas phase. The results of the continuous survey showed that changes of both H2 and CO2 correlated with changes of the volcanic activity. Therefore, H2 and CO2 resulted effective tracers of the dynamics involving the plumbing system of Stromboli. Identification of changes in the gas emissions at open conduit volcanoes offers a great advance over the ground gas survey. The results of the continuous survey at Stromboli showed that H2 could apply as an auxiliary parameter of the CO2 flux in the surveillance programs of active volcanoes.

Minoru Ozima: Autobiographical Notes

Minoru Ozima describes important influences in his scientific life, from the trauma of World War II during adolescence to studying with such giants of Earth science as J. Tuzo Wilson. He benefited from international collaborations in helping to establish noble gas geochemistry as an important discipline that reveals much about the origin and evolution of our planet Earth.

Preface

Full Conference Title: The 1st Virtual Meeting on Deep Oil and Gas (VM Deep Oil&Gas 2020) Date of the Event: 30-31 July, 2020Location: Guangzhou, Guangdong, China Preface Due to the rapid development of technology and geo-science, more and more oil and gas resources at deep areas (i.e. ∼8000m) have been discovered. However, there are still a lot of issues to be studied in such deep circumstance. This meeting aims enrolling academic and industrial scholars for the meeting. However, the CoVID-19 pandemic has influenced the academic communication, and many meetings and conferences have been canceled, postponed or changed. The Virtual Meeting on Deep Oil and Gas was finally organized in fully on-line way through internet. Regardless of the pandemic, scholars communicated their understandings on academic/technical ideas on very specific topics of deep oil and gas. Scientific topics discussed in this conference include: □ 1. Fossil fuel resources of deep basin & exploration priorities.□ 2. Geochemistry of source rocks, petroleum accumulation and alteration.□ 3. Oil and gas generation and migration in High T-P circumstance.□ 4. Geophysical and geochemical integrated exploration in deep area.□ 5. Novel geochemical technologies and basin modelling.□ 6. Geochemistry and petrophysics of unconventional oil & gas.□ 7. Deep petroleum system and modeling.□ 8. Case studies in deep and ultra-deep explorations.The VM Deep Oil&Gas 2020 was proposed in May 12, 2019 after the 10th International Conference on Petroleum Geochemistry and Exploration in the Afro-Asian Region (AAAPG2019) held in Guangzhou, China. The main reason was that some presentations in AAAPG2019 showed rapid developments of technology and theories related to deep oil and gas. This meeting aims enrolling academic and industrial scholars for discussing edge progress on evaluating potentials of deep petroleum. However, due to the CoVID-19 pandemic, many academic meetings have been canceled, postponed or changed. One month prior to the meeting, the organizing committee decided to change the meeting into an on-line one, which has been proved an effective way by some international meetings. Because the topics of the proposed meeting are firmly connected with the explorations, the organizer would not like to postpone it. Some of the presenters are from oil industry and the postponing means the delay to communicate the latest progress on deep oil exploration and discussions.

Constraints of molecular and stable isotopic compositions on the origin of natural gas from Middle Triassic reservoirs in the Chuanxi large gas field, Sichuan Basin, SW China

Abstract The Middle Triassic Leikoupo Formation in the Sichuan Basin is a gypsum-bearing stratum. The Chuanxi (CX) large gas field has been recently discovered in Middle Triassic reservoirs, with a controversial gas source. The gas geochemistry is analyzed in this study to investigate the gas origin and source. The H2S-bearing dry gas in the CX gas field has an H2S content ranging from 1.07% to 4.09%, with a gas souring index (GSI) of 0.0120–0.0438. The δ13C1, δ13C2, and δ2HCH4 values range from –31.8 to –30.4‰, from –32.9 to –29.6‰, and from –153 to –136‰, respectively. Genetic identification based on the geochemical characteristics suggests that the gas is an oil-type and derives from the secondary cracking of oil. According to the GSI and alkane carbon isotopes, the gas experienced weak TSR alteration without ethane. Potential high-quality source rocks include the Upper Permian mudstones and marlstones, as well as the Lower Cambrian mudstones, with a total organic carbon content generally higher than 2.0%. A comparison of the carbon isotopic compositions between ethane in the gas and kerogen and the hydrogen isotopic compositions of methane implies that natural gas has an affinity with the Upper Permian rather than the Lower Cambrian source rocks. The extremely low development of solid bitumen in the reservoirs suggests the inexistence of large-scale paleo-oil pools in the Leikoupo Formation. The Middle Triassic natural gas in the CX gas field originates from the Upper Permian Longtan Formation, where the gas pools are dominated by direct filling of secondary cracking gas.

CO2 favours the accumulation of excess fluids in felsic magmas

AbstractDeformation and gas emissions at active volcanoes provide insights into volcanic plumbing systems. Large discrepancies are observed between the volumes calculated from deformation data and the volumes of erupted magmas and are ascribed to the amount of excess fluids in magma reservoirs, which hinders our capacity to predict the magnitude of an imminent eruption. Here, a series of experiments demonstrates that the amount of trapped excess fluids in felsic magmas depends strongly on fluid composition. Magmas with CO2 excess fluids become permeable at 44% larger porosities with respect to the H2O‐rich counterparts at equivalent crystallinity. Available excess gas geochemistry and juvenile porosity data from felsic systems reveal that the volume discrepancy between erupted magma and syn‐eruptive SO2 increases with CO2 excess in magmas. These data highlight that CO2‐rich fluids enhance magma's capacity to store excess volatiles and shed light on the largest discrepancies between pre‐eruptive deformation and eruption intensity and magnitude.

Simulation of kerogen evolution by thermodynamic methods (on the example of Southern oil and gas region)

The article considers the methods for estimating the level of organic matter transformation, scattered in sedimentary strata, in the process of catagenetic transformations. The authors show the efficiency of kerogen property research by coal petrographic methods and palynological analysis, the purpose of which is to establish the paleotemperature that was actualized in the process of evolution. The advantages and disadvantages of establishing the organic matter generation potential by pyrolytic methods, primarily by pyrolysis by Rock-Eval, are evaluated. In the context of the organic matter evolution indirect study methods, brief information on gas geochemistry is given and the main indicators used in the analysis of the chemical composition of gases in hydrocarbon deposits are listed. It is shown that the equilibrium thermodynamics apparatus can provide information about the conditions and level of organic compounds transformation in the lithosphere. The general information on geological history of the Southern oil and gas region of Ukraine is given and the main events that influenced the modes of transformation of organic matter are shown. The most important of these were the plate collision processes and the accompanying spreading that took place during the approaching of the Arabian continent and Eurasia, which led to the formation of a faults network: submeridional and sublatitudinal. According to the chemical composition of the region's depo-sits, graphs within the coordinates C2/C3 to C2/i-C4 and ln(C2/C3) to ln(C1/C2) are plotted. According to the graphs, the authors establish the trends of organic matter evolution: development in the direction of maturation and kerogen formation without secondary oil destruction. Information on the method of calculating the kerogen/gas system composition according to the Jaynes formalism is given. According to thermodynamic simulation combined with the chemical composition of deposits, maps of equilibrium temperatures and depths for several temperature flows are constructed. The effect of the individual deposits geological structure on the evolution of its contents is shown. The map of generation depletion of the region kerogen is made. According to the results of the map analysis, it is found that the kerogen of the Kerch-Taman interpericline depression is maximally depleted, while the kerogen of the southern side of the Karkinitsky-North-Crimean depression retains the high potential for the formation of light hydrocarbons.

The control of magma crystallinity on the fluctuations in gas composition at open vent basaltic volcanoes

Basaltic open vent volcanoes are major global sources of volcanic gases. Many of these volcanoes outgas via intermittent Strombolian-type explosions separated by periods of passive degassing. The gas emitted during the explosions has high molar CO2/SO2 and SO2/HCl ratios, while during the passive degassing these ratios are lower. We present new laboratory experiments in a model volcanic conduit, which suggest that these differences in gas geochemistry are a consequence of gas migration through crystal-rich magma. We show that gas may flow along channels through the particle-laden liquid and, at a critical depth, the gas may displace an overlying crystal-rich plug en masse, producing a growing slug of gas. Owing to the friction on the walls of the conduit, this plug becomes progressively sheared and weakened until gas enriched in the least soluble volatiles breaks through, causing an explosion at the surface. When the gas slug bursts, liquid is drawn up in its wake, which exsolves the more soluble volatile components, which then vent passively at the surface until the next explosive slug-bursting event.

Bulk and position-specific isotope geochemistry of natural gases from the Late Cretaceous Eagle Ford Shale, south Texas

A dataset of bulk and position-specific isotope compositions of shale gases from the Late Cretaceous Eagle Ford Shale, south Texas is reported. The chemical and bulk isotopic compositions of the seven samples in this study and those available from the literature show that across the play, produced natural gas from the Eagle Ford Shale preserves a wide range of gas compositions (wetness, ~5 to >90%) and δ13C values of C1–C3. The depth profiles of the isotopic compositions suggest that the western region of the Eagle Ford Shale deposit had experienced as much as 700–800 m of uplift-erosion or gas migration. The timing of and lost-gas fractions by gas expulsion events very likely affected their gas compositions and bulk isotope compositions. Some deep (>3000–3500 m), matured (%Ro > 1.5) gases with heavy δ13C(C1–C3) values indicate significant loss (>50%) of the early-stage gases. For the position-specific isotope deviations of propane, the ΔC2-1 values of the five samples show small decrease with well depths, while the ΔH2-1 values have a general increasing trend. Although some samples fall very close to the equilibrium model trajectories with reasonable calculated temperatures (138–148 °C), the position-specific isotope compositions of propane from the Eagle Ford Shale gases are likely results of thermal cracking of various organic molecules within the source rocks, which have different activation energy for cracking and non-statistical distributions of C/H isotope within them.

Mechanism of ultra-deep gas accumulation at thrust fronts in the Longmenshan Mountains, lower Permian Sichuan Basin, China

Giant gas has been discovered in the lower Permian Qixia Formation of the Shuangyushi area in the Longmenshan fold-and-thrust front within dolomite reservoir at depth >6000 m. To improve the understanding of this unique ultra-deep reservoir, we address the mechanism of hydrocarbon accumulation in this area. Gas geochemistry implies that most of the gas is derived from mixing humic and sapropelic kerogens, whereas some gases are derived solely from humic kerogens. The gas reservoirs experienced two main stages of hydrocarbon charging corresponding to oil and gas generation peaks of first the lower Cambrian and second lower–middle Permian source rocks, i.e., Late Triassic and Middle Jurassic, respectively. The weak tectonism and the resistance of the ultra-deep structures to tectonic deformation at thrust fronts provided an ideal preservation environment for hydrocarbons. Our data suggest that deep to ultra-deep oil and gas accumulations in mountain front–foreland basin transition zones is a target of future exploration, providing complement to the traditional focus in shallow formations.

Dating and characterizing primary gas accumulation in Precambrian dolomite reservoirs, Central Sichuan Basin, China: Insights from pyrobitumen Re-Os and dolomite U-Pb geochronology

Primary gas accumulation with trillions of cubic meters of gas volumes was discovered recently in the Precambrian Dengying Formation of the Gaoshiti-Moxi block, Central Sichuan Basin, China. In this study we investigated the pervasive saddle dolomite cements and solid bitumens within karst vugs in the Dengying dolomite reservoirs. Using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in situ U-Pb dating, we obtained a high-precision absolute age of 259.4 ± 3.0 Ma for the saddle dolomite cementation, consistent in timing with the eruption of Emeishan flood basalts (262–251 Ma). Combined with high homogenization temperatures (Th) and salinities of primary fluid inclusion, as well as rare earth element (REE) compositions, we consider that the saddle dolomite precipitation was likely related to hydrothermal activity driven by the Emeishan mantle plume.The identification of rare surviving oil inclusions and extensive solid bitumens within the dolomite vugs is indicative of the presence of paleo-oil reservoirs. These bitumens are characterized by high equivalent vitrinite reflectance (Requ), no fluorescence, and deformed fibrous textures, typical of highly mature pyrobitumens. Gas geochemistry data indicates that only minor thermochemical sulfate reduction (TSR) has occurred. Thus the pyrobitumen was interpreted to result mainly from the thermal cracking of paleo-oils, and its associated Re-Os isochron age of 154 ± 21 Ma, representing the timing of dry gas formation. Petrographical evidence suggests that the paleo-oil charge must have taken place after sparry dolomite cementation but prior to pyrobitumen generation. This, combined with coeval aqueous inclusion Th data and published thermal-burial models, constrains the time of paleo-oil emplacement to ca. 222–205 Ma, suggesting an oil-filling event related to the Indosinian Orogeny. This study documented the first chronological data constraining the complex evolution history of a petroleum system in one of the oldest primary gas reservoirs in the world.

Microbial cycling, migration and leakage of light alkanes in the Nile Delta Tertiary fan

We present a geochemical dataset which helps unravel the complex set of processes by which light alkanes are sourced, charged, mixed, leaked and altered in the Plio-Pleistocene Nile Delta. Thermogenic gas derives from a sub-Messinian source and mixes with microbial, hydrogenotrophic, methane-rich gas in the Plio-Pleistocene section. There is strong isotopic evidence for both microbial generation and microbial degradation of ethane and propane, but these signals are only recorded where the rates of biological processes outstrip the rate of thermogenic charge. High resolution gas geochemistry profiles through four, 500–1100 m sequences of mixed channel, channel levee, mass flow deposits and hemipelagites shed light on migration and leakage pathways. We see limited geochemical evidence for migration along or across faults which cross-cut mud-rich sequences. Lateral migration along sands/silts delivers gas to structural highs, where leakage occurs vertically along focussed pathways through heterogeneous mud-rich sediments. We propose that in situ generation of microbial gas within mud-rich sequences facilitates percolation of migrating gases that replenishes C2+ components and sustains the biodegradation pathway in syntrophic microbial communities. High resolution changes in gas composition suggest an unequilibrated system which is being actively charged. In some structures, rates of charge and of microbial alteration of gas are broadly similar; in others, rates of charge exceed rates of microbial alteration. Our results have important implications for the understanding of both basin-wide fluid flow and the ecology of the deep biosphere.

Gas geochemistry of hot springs at the Tengchong field, Southwest China: Controlled by the spatial distribution of magmatic chamber

The Tengchong volcanic field, located at the collision zone between the Indian and Eurasian plates, is well known for its large-scale Holocene volcanic eruptions and extensive hydrothermal activities. Bubbling gases from eight geothermal springs at the Tengchong field were collected and analyzed for their chemical and isotopic compositions (3He/4He, 20Ne, N2, Ar, δ13CCH4 and δ13CCO2). The results show that the gas samples could be categorized into a group I dominated by N2 and a group II with CO2 as major component. The air-corrected 3He/4He ratios ranged from 0.17 Ra to 4.76 Ra and the calculated mantle contributions dramatically varied from 1.9% to 77.9% for group II. By contrast, an invariant air-corrected 3He/4He ratio of about 0.30 Ra and a relatively constant mantle contribution of 4.0% to 5.0% was obtained for group I. The sampling sites just on top of the magma chamber received significant mantle-derived helium and magmatic gas components (at the proximal site of the magma chamber: >95% magmatic gas contributions). The calculated N2/Ar ratio of the hydrothermal endmember for group II (sample sites close to the magma chamber) was 61.7–66.2, suggesting that the reaction zone temperature is between 223 °C and 252 °C. The influence of the magma chamber decreases with increasing distance of the sampling sites from the chamber apex. The sampling sites (group I) at the marginal area discharge gases with relatively low 3He/4He ratios and magmatic contributions (19–24%). The temperature of the reaction zone for this group was estimated to be 74 °C based on the N2/Ar ratio of 42. This study provided important constraints to relate hydrothermal gas geochemistry directly with the spatial distribution of the underlying magma chamber. Plain language summaryGeochemical characteristics of gases discharged from hydrothermal systems are usually affected by deep magmatic processes, shallow hot fluids circulation, and water-rock interactions in the reaction zone. In this study, we analyzed the gas abundances and isotopic compositions of hydrothermal gas discharged from the Yunnan-Tibet geothermal belt. We used a gas mixing model to quantify the contributions of mantle-derived/magmatic endmember and hydrothermal endmember. The 3He/4He ratios, magmatic contribution, and temperature of the reaction zone decrease significantly with increasing distance from the magma chamber underneath the Tengchong field, highlighting the intimate relationships between gas transport, hydrothermal circulation and the extent of the magma chamber.

New Applications in Gas Geochemistry

Gases present in the Earth crust are important in various branches of human activities. Hydrocarbons are a significant energy resource, helium is applied in many high-tech instruments, and studies of crustal gas dynamics provide insight in the geodynamic processes and help monitor seismic and volcanic hazards. Quantitative analysis of methane and CO2 migration is important for climate change studies. Some of them are toxic (H2S, CO2, CO); radon is responsible for the major part of human radiation dose. The development of analytical techniques in gas geochemistry creates opportunities of applying this science in numerous fields. Noble gases, hydrocarbons, CO2, N2, H2, CO, and Hg vapor are measured by advanced methods in various environments and matrices including fluid inclusions. Following the “Geochemical Applications of Noble Gases”(2009), “Frontiers in Gas Geochemistry” (2013), and “Progress in the Application of Gas Geochemistry to Geothermal, Tectonic and Magmatic Studies” (2017) published as special issues of Chemical Geology and “Gas geochemistry: From conventional to unconventional domains” (2018) published as a special issue of Marine and Petroleum Geology, this volume continues the tradition of publishing papers reflecting the diversity in scope and application of gas geochemistry.