U-PB DATING AND GEOCHEMICAL CONSTRAINTS TO EARLY CRETACEOUS HYDROTHERMAL DOLOMITIZATION IN THE PROVENÇAL DOMAIN (MARITIME ALPS, NW ITALY - SE FRANCE)

The study of noble gases and reactive gases (i.e., CO2, N2 and CH4) in crustal reservoirs can help better understand the origin, migration and accumulation processes of crustal fluids in the subsurface environment, which provide further insights into fluid dynamics underground. This PhD thesis develops noble gas isotopes as geochemical tools in crustal fluid studies. Study sites have been selected to cover different types of geofluids, including geothermal fluids in Krafla, Iceland and natural gases in the Sichuan Basin, China. Following a short introduction to the research background, objectives and thesis layout are presented in Chapter 1 and Chapter 2 gives a literature review on the application of noble gases as powerful geochemical tools in hydrothermal and hydrocarbon systems. Chapter 3 describes a noble gas extraction and purification system, interfaced to a multi-collector NGX noble gas mass spectrometer (Isotopx), which is constructed particularly for this study. The detailed description of each section in sample preparation system, as well as gas sampling, extraction, purification and separation protocols is provided. Temperatures of 50 K and 95 K can be used as the optimal releasing temperatures for He and Ne on the cryotrap in sample prepline. The temperature of 210 K is tested to be the optimal temperature for releasing Ar and Kr but keeping Xe being trapped onto the charcoal trap in the prepline. Chapter 4 characterizes noble gas and stable isotope data of hydrothermal fluid system in Krafla, Iceland. Stable isotope results indicate that CO2 in the samples is considered to be magmatic in origin, with δ13C (CO2) ranging between -7.99 and -3.86 ‰. Modelling results show that processes of boiling and steam separation have occurred during the circulation of geothermal fluids in the shallow crust in the Krafla field. Air addition, possibly introduced by groundwater re-injection, has had a significant effect on the geochemical signatures of Krafla geothermal fluids as well. Chapter 5 focuses on using noble gas isotopes to trace the interaction between hydrocarbon and groundwater systems in the central Sichuan Basin, China. Twenty-six natural gas samples were collected from the Anyue and Weiyuan gas fields in the central Sichuan basin, China for stable isotope and noble gas isotope determination. Noble gases together with stable carbon isotopic parameter (Δδ 13C1-2) can be an effective tool to differentiate the occurrence of two distinct genetic groups of natural gases in the central Sichuan basin. Elemental fractionations of noble gases in samples from the western area (coal-derived gas) can be well explained by solubility-controlled Rayleigh fractionation model with relatively less effect of mass-dependent fractionation process. However, noble gas elemental and isotopic compositions in samples from the eastern area can be interpreted as a mixing effect of solubility-controlled and mass-dependent fractionation processes. This thesis further develops noble gases as a versatile tool in the study of crustal fluids. It is the first-time noble gas isotopes are used in understanding fluid dynamics in natural gas reservoirs in China. This work has demonstrated a huge potential to apply the noble gas tool in basin studies in China and Asian geology in general.

A field, petrographic and geochemical study of two Triassic–Jurassic carbonate successions from the Maritime Alps, SE France, indicates that dolomitization is related to episodic fracturing and the flow of hydrothermal fluids. The mechanism governing hydrothermal fluids has been documented with the best possible spatio-temporal resolutions specifying the migration and trapping of hydrothermal fluids as a function of depth. This is rarely reported in the literature, as it requires a very wide range of disciplines from facies analysis (petrography) to very diverse and advanced chemical methods (elemental analysis, isotope geochemistry, microthermometry). In most cases, our different recognized diagenetic phases were mechanically separated on a centimetric scale and analyzed separately. The wide range of the δ18OVPDB and 87Sr/86Sr values of diagenetic carbonates reflect three main diagenetic realms, including: (1) the formation of replacive dolomites (Type I) in the eogenetic realm, (2) formation of coarse to very coarse crystalline saddle dolomites (Types II and Type III) in the shallow to deep burial mesogenetic realm, respectively, and (3) telogenetic formation of a late calcite cement (C1) in the telogenetic realm due to the uplift incursion of meteoric waters. The Triassic dolomites show a lower 87Sr/86Sr ratio (mean = 0.709125) compared to the Jurassic dolomites (mean = 0.710065). The Jurassic calcite (C1J) shows lower Sr isotopic ratios than the Triassic C1T calcite. These are probably linked to the pulses of the seafloor’s hydrothermal activity and to an increase in the continental riverine input during Late Cretaceous and Early Cenozoic times. This study adds a new insight into the burial diagenetic conditions during multiple hydrothermal flow events.

For carbon capture and storage technology to successfully contribute to climate mitigation efforts, the stored CO2 must be securely isolated from the atmosphere and oceans. Hence, there is a need to establish and verify monitoring techniques that can detect unplanned migration of injected CO2 from a storage site to the near surface. Noble gases are sensitive tracers of crustal fluid input in the subsurface due to their low concentrations and unreactive nature. Several studies have identified their potential to act as tracers of deep fluid migration to the shallow subsurface, but they have yet to be used in a contested situation. In January 2011 it was reported extensively in global media that high CO2 concentrations in soils and related groundwater pollution had been identified on a farm property belonging to the Kerr family, located near to the town of Weyburn in Saskatchewan, Canada. The origin of this CO2 pollution was cited to be the nearby Weyburn-Midale CO2 Monitoring and Storage Project. Here, as part of an investigation funded independently of the Weyburn-Midale field operators, we present δ13CDIC, 3He/4He, 4He/20Ne, 20Ne, 36Ar, 40Ar and Kr measured in waters obtained from four groundwater wells located on and surrounding the Kerr property. We aim to establish if stable carbon and noble gas natural tracers are effective at determining if migration of CO2 from the storage project was responsible for the alleged high CO2 concentrations and water pollution measured on the Kerr farm. We compare the stable carbon isotope and noble gas ‘fingerprints’ of the Kerr groundwaters to those expected in a water equilibrated with the atmosphere under local recharge conditions, the produced CO2 obtained from production wells, and the CO2 injected into the Weyburn and Midale oil fields. We find that the stable carbon isotope data do not constrain the origin of the dissolved CO2 in the Kerr groundwaters. Due to low noble gas concentrations in the captured CO2 we are unable to completely rule out the presence of 20–34% contribution from injected CO2 to the groundwaters surrounding the Kerr property. However, we find that all of the Kerr groundwater samples exhibit noble gas fingerprints that would be expected in a shallow groundwater in contact with the atmosphere and hence there is no evidence for the addition of a deep radiogenic component or dilution from the addition of a gas phase low in atmospheric derived noble gases. Our findings corroborate previous studies that indicate that elevated CO2 concentrations found on the Kerr property are almost certainly of biological origin, and not migrated from the deep subsurface. The comprehensive follow up to these CO2 leakage allegations outlined in this study provides a robust framework for responses to any future leakage allegations at CO2 storage sites and further highlights that no single technique can categorically identify the origin of CO2 in the shallow subsurface. Hence, it is essential that the full range of geochemical tracers (stable carbon and 14C isotopes, noble gases, water chemistry, process based gas ratios) are integrated with a good understanding of geological and engineering data in response to CO2 leakage allegations in the future.

Along the Ligurian coast (NW Italy), Alpine-folded and slightly metamorphosed rocks experienced fluvial to marine erosion prior to and during the base level fall associated with the Messinian salinity crisis. Following the subsequent sea-level rise at the onset of the Pliocene, valleys incised along the coastal margins during the Messinian salinity crisis were partly filled with Pliocene marine and continental deposits. One such valley-infill system is exposed near Ventimiglia (NW Italy). Using geological cross-sections and geomorphological analysis we have constrained its shape and dimensions, as well as the morphology of its hinterland. The Messinian valley was very open, ∼10 km wide and probably 500 m deep. The basal unconformity between the Pliocene sediments and the underlying substratum is characterized by a smooth surface that has on either side of the palaeo-valley a dip between 2 and 10°. The basal unconformity in the southernmost part of the palaeo-valley roughly coincides with present-day sea level. The hinterland of the middle Pliocene sea was characterized by kilometres-wide valleys surrounded by mountains with a relief gentler than at present. The shapes and dimensions of the Messinian Ventimiglia valley and the relief during Pliocene times are different from those derived from comparable structures in SE France and NW Italy. We interpret this as being due to the exhumation history that the Ventimiglia region, different from the surrounding areas, experienced over the last few million years. Copyright © 2006 John Wiley & Sons, Ltd.

Holocene climate variability in Europe: Evidence from δ 18O, textural and extension-rate variations in three speleothems

Isotope systematics in vein gold from Brusson, Val d'Ayas (NW Italy) 3. [formula omitted] and [formula omitted] in native Au and its fluid inclusions

The Weiyuan (WY) and Changning (CN) fields are the largest shale gas fields in the Sichuan Basin. Though the shale gases in both fields are sourced from the Longmaxi Formation, this study found notable differences between them in molecular composition, carbon isotopic composition, and noble gas abundance and isotopic composition. CO2 (av. 0.52%) and N2 (av. 0.94%) were higher in Weiyuan than in Changning by an average of 0.45% and 0.70%, respectively. The δ13C1 (−26.9% to −29.7%) and δ13C2 (−32.0% to −34.9%) ratios in the Changning shale gases were about 8% and 6% heavier than those in Weiyuan, respectively. Both shale gases had similar 3He/4He ratios but different 40Ar/36Ar ratios. These geochemical differences indicated complex geological conditions and shed light on the evolution of the Lonmaxi shale gas in the Sichuan Basin. In this study, we highlight the possible impacts on the geochemical characteristics of gas due to tectonic activity, thermal evolution, and migration. By combining previous gas geochemical data and the geological background of these natural gas fields, we concluded that four factors account for the differences in the Longmaxi Formation shale gas in the Sichuan Basin: a) A different ratio of oil cracking gas and kerogen cracking gas mixed in the closed system at the high over-mature stage. b) The Longmaxi shales in WY and CN have had differential geothermal histories, especially in terms of the effects from the Emeishan Large Igneous Province (LIP), which have led to the discrepancy in evolution of the shales in the two areas. c) The heterogeneity of the Lower Silurian Longmaxi shales is another important factor, according to the noble gas data. d) Although shale gas is generated in closed systems, natural gas loss throughout geological history cannot be avoided, which also accounts for gas geochemical differences. This research offers some useful information regarding the theory of shale gas generation and evolution.

We present noble gas composition and environmental isotope signature (δ18O, δ2H, δ15NNO3-, 3H) of waters sampled from Lake Ohrid. The lake is one of Europe's deepest (max. depth ~288 m) and oldest lake, situated in southeastern part of the continent at the border between Albania and North Macedonia.The sampling campaign was in the summer of 2017, when 100 water samples were taken from 19 depth profiles (at depths of 5 m, 25 m, 50 m, 100 m, 150 m, 200 m and 250 m) for water stable isotopes, noble gases, and tritium. Samples for δ15NNO3- measurement of nitrate were collected at 7 m and 27 m depth.Lake Ohrid, situated between Galičica Mt. to the east and Jablanica and Mokra Mts to the west, is a large (surface area ~350 km2), oligotrophic, and one of the most voluminous lakes (~55 km3) and together with Lake Prespa represents the biggest water system in the Balkan region. Numerous studies have been carried out on the hydraulic connection between the two lakes using stable isotopes and hydrological modelling. The water balance of Lake Ohrid is dominated by inflow from karst aquifers, direct precipitation and with slightly smaller shares from river runoff. Lake Ohrid is strongly influenced by karstic springs, adjacent to large part of the coastline, sub-aquatic as well as surface springs which are particularly cool, clean and oxygen-rich inflowing water. The springs are fed by aquifers that are recharged from precipitation and, along the eastern shoreline, also by Lake Prespa.The lake sediment covers a record of the last 1.5 million years. To better understand the link between the atmosphere and the sediment, our goal is to estimate the water turnover time of Lake Ohrid and to give an isotopic overview about the lake system. Our measured stable isotope data provide background information about hydrogen and oxygen isotope variability of the lake. The stable isotope results together with tritium data present a prospect for estimating evaporation and mixing proportions. The noble gas results detail the layers of the estimated mixing processes. Nitrogen stable isotope data provides additional information about the locality and the type of potential pollution sources.

ABSTRACTThe 1:10,000 geological map here presented extends over about 32 km2 around the Col de Braus pass in the Maritime Alps (SE France). This area has attracted the attention of geologists since the late eighteenth century due to superb exposures of the Jurassic–Cretaceous Provençal succession, and has become a classic geological locality continuously studied until the present day. In this area, Early Cretaceous synsedimentary tectonics is evidenced by important lateral thickness and facies variations. This sector is presently placed at the western termination of a large structural domain extending from the westernmost Ligurian Alps into the French–Italian Maritime Alps, thus representing a key-area for understanding the structural setting of this part of the Western Alps.

Limestone reservoirs formed by epigenetic karst and dolostone reservoirs formed by dolomitization are both developed in the Middle Permian of Central Sichuan Basin. In recent years, high-yield industrial gas flow has been obtained from these dolostone reservoirs, which makes the Middle Permian become the most realistic succeeding carbonate strata in the basin. Based on petrographic features, geochemical analyses (including C-O-Sr isotopes, trace elements, rare earth elements, and fluid-inclusions), seismic data, and previous studies, origin of dolomite in Middle Permian dolostone reservoirs in Central Sichuan Basin is studied. Three conditions of structurally controlled hydrothermal dolomitization (including two-stage strike-slip basement-rooted faults, underlying Cryogenian aquifers (or Pre-Sinian basement) and overlying Longtan Formation aquitard) are met. The Middle Permian dolostone reservoirs consist of saddle dolomites, fine-medium crystalline dolomites, and hydrothermal minerals, which are formed by hydrothermal dolomitization. The hydrothermal fluid replaces matrix limestone to form fine-medium crystalline dolomite, while saddle dolomite and other hydrothermal minerals are directly precipitated from hydrothermal fluid. Hydrothermal dolomitization fluid reforms matrix limestone in three ways, including dissolution and filling, replacement, and hydraulic fracturing, resulting to various types of reservoir spaces including dissolution vugs (or pores). Hydrothermal dolomitization of the Middle Permian in Central Sichuan Basin mainly occurs at the intersection of high-angle basement-rooted strike-slip faults, and the seismic section shows negative flower structure style.

بررسی منشأ و شرایط شکل گیری آگات های خور و بیابانک (استان اصفهان)

Tracing the migration of mantle CO2 in gas fields and mineral water springs in south-east Australia using noble gas and stable isotopes

Hydrothermal dolomitization in Dengying Formation, Gaoshiti-Moxi area, Sichuan Basin, SW China

Hydrothermal Dolomite (HTD) is present in the Upper Sinian (Upper Proterozoic) Dengying Formation, east Sichuan Basin, China. The strata are comprised by primary dolomite. The HTD has various textures, including zebra dolomite, subhorizontal sheet-like cavities filled by saddle dolomite and breccias cemented by saddle dolomites as well occur as a fill of veins and fractures. Also co-occur MVT type lead-zinc ores in the study area. The δ13C and δ18O isotopes of HTD in the Upper Sinian Dengying Formation are lighter than those of the host rocks, while 87Sr/87Sr is higher. The apparent difference in carbon, oxygen and strontium isotopes, especially the large difference in 87Sr/86Sr isotopes ratio indicate crystallization from hot basinal and/or hydrothermal fluids. Saddle dolomite was precipitated at temperatures of 270–320° C. The diagenetic parasequences of mineral assemblage deposited in the Dengying Formation are: (1) dolomite host rock sphalerite-galena-barite-fluorite; (2) dolomite host rock saddle dolomite quartz; (3) dolomite host rock saddle dolomite bitumen; (4) dolomite host rock saddle dolomite barite. The mean chemical composition of the host dolomite matrix and HTD didn't change much during hydrothermal process. The fluids forming the HTDs in the Dengying Formation were mixtures of freshwater from the unconformity at the top of Sinian, fluids from diagenetic compaction and hydrocarbon generation & expulsion from the Lower Cambrian Niutitang Formation mudstones or the Doushantuo Formation silty mudstones, and hydrothermal fluids from the basement The hydrocarbon reservoirs associated with the HTD were mostly controlled by the basement faults and fractures and karsting processes at the unconformity separating Sinian and Cambrian strata. The hydrocarbon storage spaces of HTD included dissolved cavities and intercrystalline pores. Dissolution cavities are extensive at the top of Dengying Formation, up to about 46m below the unconformity between Sinian and Cambrian and were generated mainly during karstification. Hydrothermal alteration enhanced the reservoir property of the Dengying Formation dolomites with 3%–5% increase in porosity. No agreement has been reached why zebra dolomite occurs only in the Upper Sinian strata, which would indicate that HTD mineralization occurred during two different periods, each of them related to major extensional tectonic event. The early one related to the Xingkai taphrogenesis (Z2−€1) and the later one to the Emei taphrogenesis (D2-T2). But, all the data from saddle dolomite suggest that the predominant crystallization occurred during the latter event.

The molecular and isotopic compositions of shale gases exhibit substantial differences under different storage conditions. Gas geochemistry is widely used when evaluating gas accumulation and expulsion in petroleum systems. Gas geochemical characteristics can provide important references for determining the enrichment mechanism of shale gas reservoirs and predicting shale gas production capacity in different regions. In tectonically stable regions with similar reservoir formation and evolution histories, shale gas reservoirs are expected to exhibit favorable storage conditions with only relatively small variations in gas geochemical characteristics. In tectonically active regions, shale gas preservation conditions are expected to be more variable. In this study, we systematically analyzed the stable isotope signatures (δ13C and δD) of alkane gases (CH4, C2H6, and C3H8), along with noble gas compositions and isotopic signatures, of normally pressured Wufeng‒Longmaxi marine shale gas samples comprising a continuous pressure coefficient series from a structurally active region at the transition between an orogenic belt and the southeastern (SE) Sichuan Basin, China. The relationships between noble gas contents, isotopic signatures, and shale gas yields were evaluated, and a mechanism for normally pressured shale gas accumulation and expulsion was presented. The δ13C and δD data suggest that the normally pressured shale gas originated from late-mature thermogenic generation, equivalent to shale gas from other production areas in the inner Sichuan Basin. Gas dryness ratios [C1/(C2 + C3)] exhibit negative relationships with δ13C1 and δ13C2. Normally pressured shale gas yields exhibit a negative correlation with δ13C and a positive correlation with [C1/(C2 + C3)], suggesting differences in shale gas accumulation and expulsion across the studied region related to changes in the pressure coefficient. Noble gas isotope data suggest that the normally pressured Longmaxi shale gas received a substantial contribution of crust-derived He. Coupling noble gas and stable C/H isotope data reveals that the abundance of He and Ar, along with the δ13C signatures of alkane gases, is affected by the abundance of shale gas during the accumulation and expulsion process. The noble gas and stable isotope distribution trends presented herein can be used to evaluate Wufeng‒Longmaxi’s normally pressured shale gas accumulation and expulsion in complex structural areas of the southeastern Sichuan Basin. Better preservation conditions accompanying lower tectonic activity will normally result in higher shale gas production and a lower concentration of noble gases. The above findings show that gas geochemical characteristics could be used as effective evaluation indicators for determining shale gas accumulation mechanisms in tectonically active regions.