Orals: Fri, 2 May | Room G1
Carbonates are ubiquitous in Earth surface systems, such as sediments and cements. Contents of minor cations in carbonates can be considered as proxies of environment of formation involving stages of nucleation, growth and transformation. Thermodynamic models of carbonate solid solutions can help with this, also setting reference levels for kinetics and for interpreting the measurable variations of their composition (Rd values of metals) in time (growth rate) and in space (zoning). This message is illustrated on a few topics from my past studies facilitated by GEMS codes (https://gems.web.psi.ch).
Authigenic rhodochrosites in anoxic sediments of Baltic Sea deeps [1]. A (Mn, Ca, Mg, Sr, Ba, Fe)CO3 solid solution model was refined using the sediment profiles data and Gibbs Energy Minimization (GEM) “dual thermodynamic” (DualTG) approach to estimate all binary regular interaction parameters, consistent with the predictions in (Lippmann 1980). In the underlying thermodynamic model, porewater pH, pe, alkalinity, dissolved Mn, Fe, and S levels were controlled by equilibrium with rhodochrosite-mackinawite-greigite mineral buffer. The model matched well the observed porewater- and carbonate composition, predicting its non-linear response to variations in Mn loading, alkalinity and salinity of the sediment-porewater system.
EuIII coprecipitation in calcite under widely different conditions (Rd datasets for high pCO2; normal seawater; high-pH solutions) [2]. No binary solid solution with any of seven EuIII endmember candidates could reproduce all three datasets. This was only possible with a ternary EuH(CO3)2 – EuO(OH) – CaCO3 ideal solid solution constructed with DualTG approach, and consistent with TRLFS data.
Sr in calcite and Ca in strontianite [3]. (Ca,Sr)CO3 solid solution system with non-isostructural endmembers was investigated in a stepwise approach from atomistic to thermodynamic modelling. Binary solid solution phases with calcite- or aragonite structure have nearly symmetric moderate non-ideality. However, calculations of equilibria including both phases resulted in strongly asymmetric ‘‘miscibility gap” with ~0.3% Sr in calcite and ~3.0% Ca in strontianite. The same picture was obtained using a DQF binary solid solution model in GEM calculations of Lippmann diagrams.
Growth rate dependence of uptake of divalent ions (Rd) in calcite [4]. These facts cannot be explained by equilibrium aqueous – solid solution partitioning, and need to consider intricate relations between speciation, particle growth, adsorption, surface entrapment, and solid solution formation. Two existing Growth Surface Entrapment- (Watson 2004) and Surface Reaction Kinetics (DePaolo 2011) models could be merged into a simple Unified Uptake Kinetics equation implemented and used in GEMS.
These studies benefited from DualTG calculations that use capabilities of GEM to compute chemical potentials of elements in (meta)stable systems [5]. Aspects of DualTG “streamlining” to obtain saturation index SI of solid solutions are discussed.
References
[1] Kulik D.A., Kersten M., Heiser, U., Neumann T. (2000): Aquat. Geochem. 6, 147-199.
[2] Curti E., Kulik D.A., Tits J. (2005): Geoch. Cosmoch. Acta 69, 1721-1737.
[3] Kulik D.A., Vinograd V.L., Paulsen N., Winkler B. (2010): Phys. Chem. Earth 35, 217-232.
[4] Thien B.M.J., Kulik D.A., Curti E. (2014): Appl. Geochem. 41, 135-150.
[5] Kulik D.A. (2006): Chem. Geol. 225, 189 – 212.
How to cite: Kulik, D.: Carbonates as (meta)stable solid solutions: Thermodynamic and kinetic insights , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6064, https://doi.org/10.5194/egusphere-egu25-6064, 2025.
Calcite, the most abundant carbonate mineral in Earth’s crust, is at the heart of many environmental and technological processes. As part of the geological carbonate-silicate cycle, calcite dissolution and precipitation are central for the regulation of atmospheric carbon dioxide levels on geological timescales. Moreover, calcite is involved in buffering of sea- and freshwater pH values and biomineralization of marine organisms. Important technological applications of calcite are the use in concrete and as a storage material for anthropogenic carbon. Since most processes on calcite take place in an aqueous environment, they are governed by the structure and properties of the calcite-water interface. Consequently, there has been a large body of research establishing a detailed understanding of the interface between the most-stable calcite (10.4) surface and water. This includes the development of so-called surface complexation models describing the surface speciation and properties of the calcite-water interface in thermodynamic equilibrium with aqueous solutions of varying composition. An important part of these models is the description of the species at the interface, which heavily depends on the protonation and deprotonation of surface-bound water and interfacial carbonate groups. However, the de-/protonation of calcite is difficult to quantify experimentally due to calcite dissolution and carbonate buffering. Here, we apply interface-sensitive vibrational sum frequency generation (SFG) spectroscopy to directly assess the water species present at the calcite-water interface at high pH. With SFG spectroscopy, we can measure the vibrational spectrum of interfacial species, providing insights into the molecular organization and chemical environment at the interface. We aim to quantify the change of hydroxyl species present at the interface with increasing pH to quantify the deprotonation constant of surface-bound water contributing to the development of more-accurate surface complexation models.
How to cite: Dickbreder, T., Heberling, F., and Backus, E.: Investigating the Water Organization at the Calcite (10.4)-Water Interface at High pH, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13621, https://doi.org/10.5194/egusphere-egu25-13621, 2025.
Mediterranean sediments have registered some of the most exceptional Ba records in marine basins. Although Organic Rich Layers (ORLs) are less well studied, both sapropels and ORLs are characterized by marked increases in Ba content in response to productivity oscillations, as demonstrated by numerous data sets and sediment records. During sapropel deposition, barite was abundantly produced in the water column due to increased productivity and associated microbial processes involved in organic matter degradation. In this scenario, bacterial activity and extracellular polymeric substances (EPS) have been shown to be the main control of barite precipitation in the water column, which is further supported by experimental work and observations from microenvironments of intense organic matter mineralization in the ocean water column. Once accumulated in marine sediments, barite tends to be well preserved in both eastern and western Mediterranean basins, where the availability of sulphate in pore waters prevented dissolution. Thus, differences in barite abundance in sapropels compared to ORLs support differences in productivity rates, and also differences in primary producers and microbial processes. Indeed, differences in productivity types between modern eastern and western basins also support that such differences over time may have led to spatial differences in barite formation. The general decline in productivity, and hence microbial activity, across the Mediterranean basins during the Holocene is indicated by the remarkably low Ba content in recent sediments. Dissolution of barite through the water column is also important in modern environments and is still poorly understood in the past. Overall, a better understanding of the microbial processes involved in barite production and the factors controlling its preservation is required to further constrain the information captured by Ba proxies.
How to cite: Martinez-Ruiz, F., Monedero-Contreras, R., Monasterio-Guillot, L., Paytan, A., and Vasconcelos, C.: Controls on barite precipitation and preservation in Mediterranean sediments: from sapropel deposition to modern sedimentation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17872, https://doi.org/10.5194/egusphere-egu25-17872, 2025.
Carbonate concretions accompanied by elemental sulfur are found in many upper Miocene marine successions across the Mediterranean area (e.g. SE-Spain, Sicily, Apennine, Cyprus). Most of these rocks are characterized by molds of evaporitic minerals (mostly gypsum) suggesting an early (syngenetic) or late (epigenetic) diagenetic origin. In contrast to these findings, a case study from the Ripa dello Zolfo area in northern Italy lacks evidence of carbonate and sulfur replacement of preexisting sulfate minerals. An integrated approach including sedimentological, petrographical, stable isotope (carbon, oxygen, and multiple sulfur isotopes), and lipid biomarker analyses was used for the study of three main lithofacies: a) laminated lithofacies representing aphotic carbonate stromatolites enclosing fossils of filamentous sulfide-oxidizing bacteria; b) brecciated lithofacies deriving from the brecciation of carbonate stromatolites by mud injections; c) sulfur-bearing lithofacies deriving from the precipitation of thin laminae of elemental sulfur at or close to the sediment-water interface. The δ13C and δ18O values of authigenic carbonate minerals and δ13C of lipid biomarkers indicate that the initial formation of the laminated lithofacies was favored by organoclastic sulfate reduction in the shallow subsurface close to the sediment-water interface, producing sulfide that sustained dense microbial mats of sulfide-oxidizing bacteria at the seafloor. Calcification of the mats and consequent formation of stromatolites were possibly favored by nitrate-driven sulfide oxidation at the seafloor. The subsequent brecciation of the stromatolites was apparently the consequence of sulfate-driven anaerobic oxidation of methane (SD-AOM) in an underlying sulfate-methane transition zone (SMTZ). Focused fluid flow from a deeper zone was not only causing the brecciation of the stromatolites, but also delivered bicarbonate ions for the subsequent precipitation of additional, 13C-depleted calcite (δ13C values as low as -52‰). Along with bicarbonate, also hydrogen sulfide was produced by SD-AOM within an SMTZ in a zone below the stromatolites and was transported upwards. The oxidation of hydrogen sulfide at or close to the seafloor promoted the formation of elemental sulfur characterized by δ34S and Δ33S values close to coeval seawater sulfate. This study highlights that a multi-proxy approach has great potential for the reconstruction of spatially and temporarily separated biogeochemical processes in the shallow subsurface or at the seafloor (i.e., anaerobic oxidation of methane, sulfate reduction, sulfide oxidation) – processes that may induce the syngenetic formation of authigenic carbonate and sulfur deposits in marine sediments.
How to cite: Natalicchio, M., Birgel, D., Giunti, S., Guibourdenche, L., Pellegrino, L., Aloisi, G., Peckmann, J., and Dela Pierre, F.: Authigenic carbonate and native sulfur formation in Messinian (upper Miocene) marine sediments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17405, https://doi.org/10.5194/egusphere-egu25-17405, 2025.
Dolomite formation remains one of the most intriguing puzzles in sedimentary geology, often referred to as the “dolomite problem” . Growing evidence suggests that microbial mediation plays a critical role in overcoming kinetic barriers to dolomite precipitation. This study explores the potential of dolomite crystal morphology as a diagnostic tool for identifying microbial contributions, integrating findings from laboratory simulations and sedimentary records.
Controlled experiments reveal that microbial processes produce distinct proto-dolomite crystal morphologies under varying environmental conditions. Cyanobacterium Leptolyngbya boryana induces proto-dolomite precipitation in brackish water, forming characteristic “double-spherical” crystals with hollow interiors and organic inclusions. In contrast, the halophilic bacterium Vibrio harveyi promotes the formation of single-spherical proto-dolomite crystals with unique "pinhole" features on their surfaces, indicative of microbial residue. These results highlight the species-specific influence of microbes on crystal morphology and the critical role of environmental conditions such as Mg/Ca ratios in shaping these mineralization pathways. Sedimentary dolomites from the SG-1 borehole in the Qaidam Basin (NE Tibetan Plateau) predominantly exhibit single-spherical morphologies with surface pinholes, closely resembling those produced by Vibrio harveyi in the laboratory. Although cyanobacterial fossils are present in the sediments, the observed dolomite features strongly suggest that halophilic bacteria were the primary mediators of dolomite precipitation in this system.
This study demonstrates that dolomite crystal morphology can serve as a proxy for microbial mediation in carbonate systems. By integrating experimental and sedimentary evidence, these findings advance our understanding of biogenic dolomite genesis and provide insights into reconstructing paleoenvironmental and biogeochemical conditions.
How to cite: Zhao, Y., Han, Z., Han, C., Wang, Z., and Gao, X.: Dolomite Crystal Morphology as an Effective Indicator of Microbial Origins: Evidence from Experimental Simulations and Sedimentary Records, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1153, https://doi.org/10.5194/egusphere-egu25-1153, 2025.
The Proterozoic millimeter-sized ministromatolites have been recognized for their potential biological origin, yet concrete biotic proof has been elusive, which has sparked debates about their possible chemical origins. Recently, unique ministromatolites were identified in a microbial dolostone layer within the Mesoproterozoic Wumishan Formation at Lingyuan in Northern China. Examination of thin sections of these ministromatolites has uncovered a wealth of coccoidal microfossils, measuring 10 to 30 micrometers in size. Petrofabric analysis indicates that these ministromatolites underwent three episodes of diagenetic silicification, with the microfossils being exclusively preserved in early diagenetic chert. This finding supports the hypothesis that silicification occurring concurrently with mat development is crucial for the exceptional preservation of microfossils, which suggests that the scarcity of microfossils in most Precambrian carbonate stromatolites may be attributed to the absence of hydrochemical conditions conducive to early silicification. While chemically-formed fibrous carbonate minerals predominate in these ministromatolites, the abundance of microfossils suggests that the contribution of microorganisms to the formation of ministromatolites was previously underestimated. Consequently, this study proposes that Mesoproterozoic ministromatolites are not merely chemical byproducts but rather intricate mixtures of both biological and chemical components.
How to cite: Wang, T. and Xiong, W.: Fossil evidence provides new insights into the origin of the Mesoproterozoic ministromatolites, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-845, https://doi.org/10.5194/egusphere-egu25-845, 2025.
Within syn-rift basinal settings, the juxtaposition of rift-related clastic deposits in the hanging wall of basin-bounding normal faults against a footwall of crystalline basement is a recurrent structural setting where plays for hydrocarbon exploration or carbon storage can be found. Here, fault-controlled fluid flow can significantly influence and change the petrophysical properties of the fault zone and host rocks over time by means of mineralization and cementation, ultimately controlling fluid pathways. Investigating the timing and extent of fluid flow along major faults permits us to better understand the host rock properties and if these can potentially be favourable for subsurface extraction and storage.
Here, we present a detailed investigation of the timing and paragenesis of fluid flow along the well exposed Helmsdale Fault in NE Scotland. The Helmsdale Fault is a major tectonic feature that bounds the western side of the Inner Moray Firth Basin, which developed during rifting in the Late Jurassic. The hanging wall consists of the Late Jurassic (Kimmeridgian-Tithonian) Helmsdale Boulder Beds that are made of alternating debris flow to fault scarp deposits, whereas the footwall is composed of the Helmsdale Granite (Silurian-Devonian). There is ample evidence of paleo-fluid flow along the Helmsdale Fault in the form of calcite cementation and widespread calcite veining in both the hanging wall and in the footwall, locally making up to 5 m thick fault cores of stacked crack-seal veins. U-Pb calcite dating of fossils, veins and cements shows an initial fluid flow event that quickly follows diagenesis in the hanging wall and spans from 147 to 113 Ma, followed by a later reactivation of the fault system between 86-60 Ma. The spatial distribution of the dated calcite veins shows a clear localization over time of fluid flow along the main faults within the footwall.
Carbonate stable isotope analysis, combined with the salinity of the fluid inclusions in the calcite veins, has revealed a marine fluid composition of the calcite vein network over time, irrespective of the structural domain within the fault zone. Furthermore, clumped isotope thermometry shows a gradual temperature increase towards the footwall (35 to 65 °C), but fluid inclusion microthermometry on secondary fluid inclusions also reveals that these fluids could originally have been much hotter (up to c. 80 °C). The variability in the data suggests that two fluid pathways were active at different moments in time, with one being locally sourced in the hanging wall sediments, and the second percolating upwards along the main faults within the Helmsdale Granite. Occurrence of calcite veins derived from meteoric fluids is documented in the youngest dated vein network (60 Ma) and likely related to the later stages of regional uplift.
Our results suggest that the evolution over time of the petrophysical properties of the hanging wall with progressive mineralization and cementation exert a critical control on future fluid pathways as well as localization and style of subsequent fault deformation.
How to cite: Demurtas, M., Sharp, I., Pasqualetto, L., Krüger, Y. S., Drost, K., Meckler, A. N., and Rotevatn, A.: Fluid flow history and paragenesis along a syn-rift basin bounding fault: the Helmsdale Fault (NE Scotland), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15488, https://doi.org/10.5194/egusphere-egu25-15488, 2025.
Focusing on the shale oil reservoirs of the second member of the Kongdian Formation in Cangdong Sag, eastern China, this study explores the origin and distribution of analcime, aligns the fluid evolution stage with different analcime types, and constructs a new micron-scale water-rock reaction sequence. The study identifies six types of analcime based on occurrence characteristics (occurring as laminae, lens, fracture filling, bioshell filling, vein marginal crystal and cement). The above six types of analcime is further classified into hydrothermal fluid origin analcime (HFOA: include analcime cement, vein marginal crystal, lens, fracture filling and bioshell filling) and connate fluid origin analcime (CFOA: analcime laminae) based on major elemental indicators (Si/Al and 10K/(10K + Na)). HFOA has lower ∑REE (rare earth element) and strong positive correlation between ∑REE and LILEs (large ionic lithophilic elements); while CFOA has higher ∑REE and weak positive correlation between ∑REE and LILEs. Different analcime types correlate with varying fluid properties and transport stages. HFOA forms during magmatic hydrothermal fluid (MHF) upwelling. After the MHF entering and mixing with the lake water, thermal repulsions between the crystal particles made it move to form fine grained sedimentary layer, the connate fluid trapped in pores directly precipitate to form CFOA, or form CFOA by modifying clay minerals and feldspars. Through this study, we systematically analyzed the fluid evolution and activity characteristics of the faulted lake basin by using analcime distributed in micron-scale laminae and fractures, hoping to provide new perspectives for the study of diagenetic processes in sedimentary basins.
How to cite: Wang, J., Liang, C., Cao, Y., and Liu, K.: Fluid evolution and tracing at micron-scale of shales in faulted lake basin: A new perspective based on analcime, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-189, https://doi.org/10.5194/egusphere-egu25-189, 2025.
Introduction
Carbonate concretions form at shallow burial depths (Raiswell, 1971), with most bicarbonate ions derived from the anaerobic oxidation of organic matter (Claypool and Kaplan, 1974). Specifically, spherical carbonate concretions are proposed to grow concentrically through the diffusion-driven outward migration of the carbonate supersaturation front (Yoshida et al., 2018). Consequently, the center-to-edge isotopic and geochemical profiles of spherical carbonate concretions provide valuable records of pore-water evolution. Zoned spherical concretions, which exhibit distinct zonation, can potentially preserve traces of multiple diagenetic processes. This presents a unique opportunity to reconstruct the temporal and spatial evolution of the pore-water environment at higher resolution. This study presents a detailed analysis of zoned spherical concretions from the Miocene marine sediments of the Atsuta Formation, Japan, to elucidate their formation processes.
Results and Discussion
Well-preserved zoned spherical concretions from the Atsuta Formation are subdivided into two distinct zones: a spherical nucleus (Inner-Concretion) and an outer crust (Outer-Concretion) (Fig. 1). Its spherical shape and the presence of fossilized mud shrimp claws at the center suggest a concentric growth mode. Both the Inner- and Outer-Concretion are inferred to have formed rapidly at shallow burial depths, based on the following evidence:
- Fossilized fecal pellets and mud shrimp claws were found in both the Inner- and Outer-Concretions, exhibiting no signs of compaction.
- The carbonate content was 83.0 ± 0.5 wt% in the Inner-Concretion and 79.4 ± 0.1 wt% in the Outer-Concretion, indicating precipitation within porous sediments prior to significant compaction.
- The average stable oxygen isotope ratios (δ18O) were +0.63 ± 0.77‰ (n = 76) in the Inner-Concretion and +0.92 ± 0.48‰ (n = 44) in the Outer-Concretion, suggesting the absence of isotopic fractionation effects related to burial depth.
Detailed measurements of stable carbon isotope ratios (δ13C) (n = 129) revealed characteristic trends in each zone, with abrupt changes at their boundaries. The δ13C values in the Inner-Concretion steadily increased from -15‰ near the center to +10‰ toward the edge, reflecting isotopic fractionation associated with enhanced methanogenesis. In contrast, δ13C in the Outer-Concretion was approximately -15‰ and relatively constant. The presence of pyrite throughout the concretion suggests that at least the Outer-Concretion formed within the sulfate reduction zone.
These findings collectively indicate a shift in dominant organic matter degradation processes from methanogenesis to sulfate reduction, a trend that deviates from the classical geochemical sequence of sedimentary environments (Berner, 1981). A plausible hypothesis is that methanogenesis occurred entirely within the sulfate reduction zone, potentially as a localized microenvironment around organic matter during the formation of the Inner-Concretion.
How to cite: Praet, A., Yoshida, H., Muramiya, Y., Kano, A., Kido, T., Katsuta, N., and Umemura, A.: Zoned spherical concretions from Atsuta Formation, Japan: a record of rapid geochemical shifts in early diagenesis, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11167, https://doi.org/10.5194/egusphere-egu25-11167, 2025.
Two areas in Sweden are currently being investigated by the Swedish geological survey as possible sites for geological storage of CO2. One of them is in the Baltic Basin south of Gotland, where Cambrian sandstone from the Faludden Member (Borgholm Formation) has been recognized as a suitable storage reservoir. The Faludden reservoir is a well-sorted, fine- and medium-grained quartz arenite with high porosity and permeability. Here, we report new results obtained from scanning electron microscope analyses of the secondary mineralogy of the Faludden sandstone and implications for its CO2 storage potential. Our study shows that there are several phases of secondary mineralization in the sandstone, which to a varying degree affects the interaction with injection and storage of CO2. The results provide information on the diagenetic history of the reservoir and assessment of depth-related P-T mineralizations. The most common secondary mineralization is a patchy carbonate cement. The patches, embedding several detrital grains, are round to irregular with individual diameters of up to 5 mm. The carbonate is mostly dolomite with microscale variations towards more Mn- and Fe-rich compositions. Calcite is also present, especially in connection to occasional microfractures. In general, patchy cementation is favorable for CO2 storage since it can prevent compaction, while still allowing a relatively high porosity and permeability. Quartz cement in the Faludden sandstone is limited to scattered occurrences of fringed overgrowths at the edges of detrital quartz grains. However, we observe that the amount of secondary quartz is increasing with depth in the Baltic Basin. Subcropping lower Cambrian sandstone beds, such as the När and Viklau sandstone members are often more or less completely cemented with quartz. Thus, the process of secondary quartz mineralization is an important factor to consider when assessing the optimal depth range for CO2 storage in the basin. Other secondary minerals in the Faludden sandstone include pyrite, apatite, iron(titanium)oxides and small amounts of clay minerals (illite and kaolinite). In some samples there is evidence of dissolution of both quartz cement and dolomite, introducing a secondary microporosity. This study confirms that the sandstone from the Faludden Member is a suitable reservoir rock for CO2 storage. A thorough understanding of geological processes, including reservoir diagenesis, is essential for predicting the physiochemical interactions between the injected CO2 and the reservoir rock.
How to cite: Lindgren, P., Erlström, M., and Lindström, S.: Impact of diagenesis on the suitability of reservoir rocks for CO2 storage - Examples from a Cambrian sandstone in the Baltic Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10495, https://doi.org/10.5194/egusphere-egu25-10495, 2025.
Authigenic calcite abundantly forms during various diagenesis stages of shales. It meticulously records information on diagenetic fluid (organic/inorganic) migration and fluid-rock interactions, is important for understanding the burial diagenetic evolution, tectonic history, burial history, hydrocarbon generation and accumulation in sedimentary basins. During early burial diagenesis, the sulfate-methane transition zone maintains high porewater alkalinity through anaerobic oxidation of methane, promoting calcite nodule formation. Upon entering the hydrocarbon generation window, periodic opening and closing of fractures occur at lamina interfaces due to overpressure from hydrocarbon phase transitions and crystallization forces. In these fractures, calcite solubility decreases with fluid pressure reduction, leading to fibrous vein precipitation under strong overpressure conditions and bladed or equant crystal formation under weak overpressure conditions. Recrystallization processes controlled by the Ostwald mechanism. Additionally, authigenic calcite preserves characteristics reflecting multiple sources and distinct evolution stages, resulting in notable isotope fractionation features, the carbon isotope features represent cumulative results of various processes. Utilizing physical and numerical simulations based on burial conditions aids in analyzing authigenic calcite genesis and reconstructing the diagenetic environment in which it formed.
How to cite: Wang, Y., Cao, Y., Wang, J., and Liang, C.: Characterization and genesis of authigenic calcite in multiple diagenetic stages of shales, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2823, https://doi.org/10.5194/egusphere-egu25-2823, 2025.
Late Cretaceous Kawagarh Formation, from Hazara Basin of northern Pakistan, exhibit extensive diagenesis including dolomitization, which significantly impacts its reservoir potential. In this study we investigated the processes that drove dolomitization in Kawagarh Formation through integration of petrographic analysis and carbon-oxygen isotope geochemistry. Thin sections optical microscopy revealed various dolomite textures, including planar-euhedral and non-planar-anhedral crystals, as well as evidence of recrystallization and porosity enhancement. Multiple textural indicated the multiple phases of dolomitization associated with variable diagenetic conditions.
Carbon (δ¹³C ) and oxygen (δ¹⁸O) isotope analysis of dolomite showed the distinct geochemical signatures indicative of dolomitizing fluid sources. These isotopes suggested a mixed origin, involving marine and meteoric fluids, with potential contributions from hypersaline brines during burial diagenesis. The spatial and isotopic variability suggested that dolomitization was influenced by tectonic activity and stratigraphic controls, which resulted in highly heterogeneous in porosity and permeability.
The findings from this study provided new insights into the diagenetic history of the Kawagarh Formation and enriched our conception of dolomitization mechanisms in analogous settings. The study highlighted the critical role of integrated petrographic and isotopic approaches in unraveling complex diagenetic processes, with implications for reservoir characterization and hydrocarbon exploration.
How to cite: Rehman, S. U., Munawar, M. J., and Ahsan, N.: Geochemical and Petrographic characterization of dolomitized carbonates in Kawagarh Formation, Northern Pakistan , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3153, https://doi.org/10.5194/egusphere-egu25-3153, 2025.
Jurassic Shaximiao Formation is an important tight gas reservoir in southwest Sichuan. In this study, thin section identification, scanning electron microscopy (SEM), X-ray diffraction (XRD), Cathodolumines- cence(CL), electron probe analysis, fluid inclusions and isotopic analysis were used to investigate the characteristics of the reservoir and the influence of diagenesis on the reservoir. And the following research results are obtained.First, the reservoir rock types of Shaximiao Formation in this area are primarily feldspathic sandstone and lithic feldspathic sandstone and secondarily lithic sandstone and feldspathic lithic sandstone. Second, the reservoir performance of feldspathic sandstones is much better than that of lithic sandstones. Feldspathic sandstones are mainly vertically distributed in the first and second sub-members of the second Member of Shaximiao Formation and laterally the most developed in the eastern area.Third, the reservoir Spaces in southwest Sichuan are mainly primary intergranular pores and secondary intergranular pores. Compaction plays a dominant role in the reduction of porosity, and due to the protective effect of chlorite coating, the porosity in southwest Sichuan is reduced by 23.45%. Cementation played a secondary role in the reduction of porosity, which reduced the porosity of southwest Sichuan by 12.4% respectively. On the contrary, dissolution plays a positive role in the increase of porosity, which increases the porosity of southwest Sichuan by 4.52%.In conclusion, Compaction and cementation lead to reservoir densification in Shaximiaoformation. Chlorite cementation protects the primary porosity to a certain extent, and dissolution is the main factor to increase the porosity.
How to cite: Li, J. and Qiu, L.: The influence of sandstone diagenesis on reservoir of Shaximiao Formation in southwest Sichuan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6530, https://doi.org/10.5194/egusphere-egu25-6530, 2025.
Marine carbonates are archives of geochemical proxies, such as e.g. Sr and U-Pb isotopes, which potentially can be utilized in the reconstruction of past climate conditions, ancient seawater composition and/or their alteration during burial and fluid interaction, if interpreted accurately. The ability to confidently reconstruct environmental conditions in the past times is of great importance since they can be linked with changes in the biosphere. For example, the Ediacaran-Cambrian transition was a period where significant evolutionary change modified the biosphere towards appearance of extant animal clades and the establishment of metazoan-dominated ecosystems. Any contribution towards an intact, continuous record of environmental conditions through multiple proxies will help to understand better the timing, nature and sequence of events that preceded or accompanied such changes in biodiversity. However, carbonate rocks are susceptible to numerous post-depositional processes (such as: oxidative weathering, diagenesis, burial, lithification, deformation, dissolution and reprecipitation), which may alter the geochemical record. Additionally, detrital components may increase the complexity of the geochemical signature and the carbonate composition, particularly in shallow marine settings with variable continental run-off.
Thus, we have to understand and identify the presence or absence of such processes, before extracting meaningful geological information from these archives. Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry (LA-ICP-MS), is a tool that offers spatial resolution when performing geochemical analyses, which may help to interpret the geochemical data more confidently. In this work, we combine observations from Sr and U-Pb isotopic systematics supported by trace element abundances to identify domains that are indicative of post-depositional processes, over protracted time and variable in their extent. The sample material is taken from drill cores in Namibia, obtained within the frame of the GRIND project, which aims to investigate the late Edicaran period. Situated in the south of Namibia, the Gariep belt was geologically active during the Pan African orogenesis and possibly affected by the Atlantic Ocean opening.
How to cite: Paul, A. N., Gerdes, A., Cantine, M., and Ovtcharova, M.: Progress in the evaluation of U-Pb dates of late Ediacaran carbonate rock from drill cores through simultaneous Sr isotope analysis (Laser Ablation – Split Stream – Inductively Coupled Plasma – Mass Spectrometry), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15410, https://doi.org/10.5194/egusphere-egu25-15410, 2025.
This study investigates sedimentary and hydrothermal uranium (U) mineralization in the Neoproterozoic Cave-Temple Arenite Member of the Kerur Formation, Badami Group, Karnataka, within the South Indian Shield (SIS). The studied succession comprises three fluvio-alluvial depositional cycles, with the middle cycle recording evidence of marine sedimentation. Uranium enrichment occurs across the fluvial sediments of the 1st and 2nd cycles, as well as the marine sediments of the 2nd cycle. The higher grade of mineralization, however, is restricted to the lowermost fluvio-alluvial segment of the 1st cycle, where significant hydrothermal enrichment is evident.
Three distinct modes of U-mineralization have been identified. The first one (the primary mode) represents syn-depositional, microbially induced sedimentary uranium accumulation within organic matter (OM)-rich clay laminae of marine shales, siltstones, and fine sandstones of the 2nd cycle. This mode is characterized by microcrystalline uranium-(calcium)-phosphosilicate (UPS) phases, often intermingled with uranium titanates, and is associated with microcrystalline sedimentary pyrites, often clustered as framboidal aggregates, and kerogenous OM. Provenance analyses and petrography of U-bearing sedimentary rocks suggest U-sourcing from the Archean-Palaeoproterozoic granitic and felsic basement rock suites of the Dharwar Craton. A positive correlation (r = +0.8, ρ < 0.01; n = 26) is observed between bulk rock OM content (TOC%) and uranium enrichment in the primary mode. Micro-RAMAN spectroscopy confirms the association of UPS phases and sedimentary pyrites with OM-rich matrices and clay-rich organic laminae, while the carbon and sulfur isotopic analyses of the bulk reinforce the biogenicity of the host sediment. The enrichment of redox-sensitive trace elements like V, Mo, Cu, Co, Ni, and As within the clay- and OM-rich sedimentary rocks further indicates the presence of active redox cycling along with biogeochemical and paleoproductivity processes during the syn-to-meta-depositional phases.
The remaining two modes correspond to secondary, post-depositional hydrothermal uranium enrichment within the fluvial sandstones of the 1st cycle and the fluvio-marine sedimentary rocks of the 2nd cycle. These modes manifest as uranium phosphosilicate and uranium silicate phases, associated with hydrothermal pyrites in fractures, micro-veinlets, or intergranular patches. Unlike the primary, syn-sedimentary mode, the hydrothermal mineralization does not show any distinct correlation between uranium concentration and TOC%. Isocon mass balance further suggests that uranium, iron, and high field strength elements (HFSEs) were mobilized from these sediments hosting the primary mode, likely facilitated by organometallic ligands, such as siderophores associated with OM, during the epigenetic hydrothermal process under oxygenated hydrothermal conditions. Mobilized uranium was subsequently trapped by pyrites in hydrothermal fractures, forming the secondary modes of U-mineralization.
This dual mechanism highlights an initial microbially mediated, OM-induced uranium accumulation, acting as a vast, low-grade source for later hydrothermal remobilization and enrichment. The findings emphasize the interplay of depositional environments, microbial activity, and hydrothermal remobilization of biogenic accumulation in uranium mineralization, with implications for sediment-hosted uranium exploration.
How to cite: Sen, A., Pal, D. C., Samanta, P., Stüeken, E. E., Bose, S., Ghosh, N., and Mukhopadhyay, S.: Role of syn-sedimentary biogenic and epigenetic hydrothermal uranium enrichment in the formation of sediment-hosted uranium mineralization: evidence from the Neoproterozoic Badami Group, Southern India, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-682, https://doi.org/10.5194/egusphere-egu25-682, 2025.
The co-evolution of life and minerals has profoundly shaped Earth's biological and geological history, with skeletal biomineralization emerging in eukaryotes over 800 Ma. Chitin, a key organic scaffold in modern biomineralization, was previously identified in fossils as old as the Cambrian (~505 Ma). Here, we extend this record by demonstrating the presence of fossilized chitin in 1-Ga acritarch-like fossils from the Lakhanda Lagerstätte, Siberia. These large spherical fossils (100–200 µm) align with the characteristics of acritarchs, organic-walled microfossils interpreted as cysts of planktonic protists. Spectroscopic analyses using nano-infrared spectroscopy of acritarch-like fossils showed molecular features diagnostic of chitin, including amide I and II bands and polysaccharide-related bands. These findings push back the timeline for chitin utilization in protists by 500 Ma, demonstrating its role in early biomineralization processes. This discovery highlights the significance of chitin in early protist evolution and its role in shaping biomineralization pathways. It also underscores the potential of advanced spectroscopy techniques to reveal the chemical and biological signatures of ancient life with unprecedented precision.
How to cite: Mehta, N., Bekker, A., Waeytens, J., Podkovyrov, V., Conrad, L., Baert, K., and Bonneville, S.: Chitin detected in 1.0 Ga old acritarch-like fossils from Lakhanda Lagerstätte and its implication for skeleton biomineralization, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3102, https://doi.org/10.5194/egusphere-egu25-3102, 2025.
Posters on site: Fri, 2 May, 08:30–10:15 | Hall X2
The growing awareness of the relationships between minerals and microorganisms has deeply impacted the geosciences in the last decades. Microorganisms have been recognized to play a critical role in element cycles, resulting in the precipitation of minerals and in the formation of peculiar rocks (e.g. stromatolites and thrombolites) that punctuate the geological record. Moreover, the Earth history was characterized by episodes of massive accumulation of mineralized remains of microorganisms on the ocean floors, resulting in the formation of biogenous oozes that represent natural archives of past climatic and oceanographic variability. Finally, the rapid growth of minerals can act as a trap for microorganisms that can be perfectly preserved within the crystal lattice or fluid inclusions for millions of years. Such strict connections between the geosphere and the biosphere are mostly unknown to the general audience, although the impact of microbial (microorganisms) and nanoscopic (viruses) life on our society is emerging more and more. The project “Minerals & Microbes, a possible relationship: GEOMICROBIology for dummies” aims at showing the microbial life enclosed in minerals and rocks. Improving people awareness about the role of microorganisms in shaping the Earth will contribute to understand the importance of life as a “geological force”. The project GEOMICROBI aspires to raise such awareness throughout imaging (mostly by SEM high-definition photomicrographs) the "invisible life" hidden within minerals and rocks. Main target of the project is the creation of a photographic exhibition accompanied by informative conferences on the theme "Minerals & Microorganisms". This poster is intended to attract the attention of the international sedimentological community showing the first results of this dissemination initiative on mineral-microorganism interactions.
How to cite: Pellegrino, L., Natalicchio, M., Carnevale, G., Cavagna, S., Dela Pierre, F., Lozar, F., Nallino, E., Pastero, L., Varese, C., and Walter, J. D.: Minerals & microorganisms, a possible relationship: an awareness project of GEOMICROBIology, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20419, https://doi.org/10.5194/egusphere-egu25-20419, 2025.
Late Archean Banded Iron Formations (BIFs) serve as exceptional reservoirs of primordial aquatic precipitates, offering a valuable window into the ancient ocean water chemistry and biogeochemical cycles that operated prior to the Great Oxygenation Event (GOE) around 2.4 Ga. It is generally believed that the primordial mineralogy of these BIFs was dramatically modified to Fe-oxides (magnetite and/or hematite) during subsequent hydrothermal and metamorphic episodes. While the earlier consensus does not clearly support an authigenic to early diagenetic origin for magnetite, some experimental studies suggest its stability within microbially influenced primary authigenic to early diagenetic environments. Despite being affected by several post-diagenetic alteration events, the central part of the Chitradurga Schist Belt (CSB) in the Western Dharwar Craton (WDC), particularly around the Chitradurga district, adequately preserves a wide array of primary mineral assemblages, with locally developed dispersed magnetite grains. Detailed petrographic observations supported by SEM-EDS analysis of the cherty Banded Iron Formation (BIF), stratigraphically positioned atop the shallow-water unstable shelf association of the Vanivilas Formation within the Chitradurga Group (3.0–2.6 Ga), offer a valuable opportunity to investigate the origin of these magnetite grains, their association with primary mineral assemblages, and their diagenetic modifications.
The primary mineral assemblages are present as submicron-scale lump-like structures (10–50 µm) embedded within silica (SiO₂) matrix, intervened by a network of silica-filled shrinkage cracks. Based on mineralogy and texture, three microfacies have been identified: a) silicate-carbonate-phosphate-bearing green lumpy microfacies (greenalite + siderite + apatite ± magnetite), b) silicate-oxide-bearing red lumpy microfacies (greenalite + hematite ± siderite), and c) silicate-sulphide-bearing black lumpy microfacies (greenalite + pyrite). Magnetite occasionally appears as a primary lump-forming mineral in the first microfacies, whereas in the second variety, it develops along the periphery of associated Fe³⁺-bearing mineral phases. The coexistence of euhedral-shaped, submicron-sized magnetite (1–5 µm) within these primary lumps, along with greenalite, suggests their origin through the reduction of a primary Fe³⁺-bearing oxy-hydroxide phase, formed in near-surface `oases' of O₂-rich seawater through cyanobacterial oxidation of hydrothermally sourced Fe²⁺. The reduction of this Fe³⁺-bearing oxy-hydroxide phase to form a metastable Fe²⁺-bearing hydrous green clay (greenalite) and more stable magnetite can occur either during settling through the water column or during authigenic to early diagenetic stages via dissimilatory iron reduction (DIR) at the sediment-water interface.
The possibility of DIR is further supported by textural evidence within silicate-oxide-bearing microfacies, where subhedral to anhedral magnetite is present along the periphery of these Fe³⁺-bearing lumps. The presence of Fe³⁺-bearing phases in the core reflects the signature of an incomplete reaction involving Fe³⁺ oxy-hydroxides and organic matter to form magnetite. Our findings reevaluate the debate over the origin of magnetite in Late Archean BIFs, suggesting that magnetite can form within biologically influenced microenvironments, even during authigenesis and/or early diagenetic stages.
How to cite: Bose, S., Sen, A., Samanta, P., and Mukhopadhyay, S.: Possible origin of authigenic to early diagenetic magnetite through ‘Dissimilatory Iron Reduction’ (DIR) within Late Archean Banded Iron Formation from Chitradurga Schist Belt (CSB), Western Dharwar Craton (WDC), India, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-671, https://doi.org/10.5194/egusphere-egu25-671, 2025.
In addition to the widely acknowledged marine ferromanganese deposits, i.e., polymetallic nodules and cobalt-rich crusts, Fe-Mn (oxyhydr)oxides ubiquitously precipitate as micronodules in the oxic aquatic environments. Due to their micrometer-scale or even smaller size, ferromanganese micronodules have received limited attention until they were found to be one of the important rare earth elements and yttrium (REY)-holding phases in the deep-sea REY-rich mud, and noteworthily estimated to preserved a ~1.28-7.62 Tt Mn budget, exceeding that of nodules and crusts by at least two orders of magnitude. Combined with the increasing demands for critical elements in high-tech industries, the economic and scientific potential of micronodules, which could strongly scavenge trace elements from ambient environments, deserve to be revised.
In our study, Fe-Mn micronodules were hand-picked from surface sediments of the North-Western (NW) Pacific and the North-Eastern (NE) Pacific Clarion-Clipperton Fracture Zone (CCFZ) for morphological and in-situ geochemical analyses with field-emission scanning electron microscope (SEM), electron probe microanalyzer (EPMA) and laser ablation (LA) inductively coupled plasma mass spectrometry (ICP-MS). The characteristic microbial-like mineralization structures, including the irregular aggregates of rod microbial-like particulates, biofilm, and the phalanxes of ellipsoid microorganisms, were commonly observed in micronodules. Their Mn/Fe ratios most frequently fall in the range of < 10 (46.1%), with a maximum value (reaching 698) that is much higher than the published EMPA data of polymetallic nodules. Unlike the continuous variation of elemental contents in nodules, the concentrations of some elements in micronodules, such as Al, Ca, K, Co, Ni, Cu, Sr, Mo and REYs other than Ce, showed a mutative tendency, being slowed down or even reversed, with the increase of Mn/Fe ratio in the ranges of Mn/Fe < 10 and Mn/Fe > 10. Whereas, the declining tendencies in Ce contents and Ce anomaly remained generally stable.
Considering the loose structure of Fe-Mn (oxyhydr)oxides, and after multiple comparisons, Mn/Fe < 5 is used here to distinguish hydrogenetic and mixed hydrogenetic-early diagenetic type (grouped into hydrogenetic type hereafter) from diagenetic type. Amounts of geochemical data of hydrogenetic Fe-Mn deposits (including micronodules, (macro)nodules and crusts) were collected. Comparative analyses reveal that both the hydrogenetic crusts (n = 289) and nodules (n = 159) have weaker positive Ce anomalies (avg. 2.23 and 2.97, respectively) than our hydrogenetic micronodules (n = 204, avg. 6.27). Combined with the similar Ce content in all three hydrogenetic ferromanganese deposit types, and the lower concentrations of other REYs in hydrogenetic micronodules, the migration of REYs (except for Ce) from micronodules to porewater is indicated.
All these findings demonstrated that the precipitation and aggregation of micronodules were predominantly controlled by the redox state of ambient pore water, even in surface sediments, and were largely influenced by microbial activities. The potential of ferromanganese micronodules to serve as a buffer in the enrichment of REYs and critical elements in pelagic sediments and an archive of the ambient redox conditions enhanced their value of further comprehensive and in-depth studies.
How to cite: Zhang, H., Dong, Y., Peng, Y., Zhou, J., and Chu, F.: Characteristics of ferromanganese micronodules in surface sediments of the tropical North Pacific Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19852, https://doi.org/10.5194/egusphere-egu25-19852, 2025.
In his 1897 article, Friedrich Wilhelm Ostwald wrote that “during departure from any state, and the transition to a more stable one, not the under given circumstances most stable state is reached, but the nearest one“. The word “nearest” essentially gave rise to the widespread interpretation that during a phase transition not the thermodynamically most stable but a metastable phase forms first, which is usually referred to as Ostwald’s step rule. It is considered a general rule rather than strict physical law, although its precise physical basis seems not fully understood on a mechanistic level, despite its potential importance for mineral formation under Earth’s surface conditions.
While Ostwald’s step rule is commonly explained through the classical nucleation theory, there are several inconsistencies that are not explained by this theory. One is that a transition to the stable phase cannot be forced by strongly increasing the driving force (supersaturation), and also adding seed crystals may not help. This conundrum particularly applies to the two most abundant minerals in Earth’s sedimentary record, dolomite and quartz (Meister et al., 2014), which are observed not to precipitate directly from aqueous solution as long as the solution remains supersaturated with respect to one of their metastable polymorphs.
Here, an alternative concept is proposed that would be consistent with Ostwald’s (1897) original formulation and with several observations from natural environments and laboratory experiments. The difference lies in the translation of the word “nearest”, not in a thermodynamic sense as “having a similar Gibbs energy”, but kinetically as “having the smallest energy barrier”. In the latter case, Ostwald’s step rule would become an actual physical law, equivalent to the Arrhenius law. This goes along with the concept that not the thermodynamic barrier of nucleation but some kinetic barrier, not affected by supersaturation, is responsible for the efficient inhibition of the phase. Inhibition (giving rise to Ostwald’s step rule) would then not be a matter of nucleation but of growth.
Meister et al. (2014) Early diagenetic quartz formation at a deep iron oxidation front in the Eastern Equatorial Pacific. GCA 137, 188–207.
How to cite: Meister, P.: Ostwald’s step rule: rule of thumb or strict physical law?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6901, https://doi.org/10.5194/egusphere-egu25-6901, 2025.
The reconstruction of the burial depth experienced by sedimentary successions before their uplift is crucial for various geological applications, such as thermal history analysis, subsidence reconstruction in sedimentary basins, reservoir quality prediction and, more in general, the tectonic evolution of fold-and-thrust belts. A commonly used method for addressing this problem in clastic sequences is low-temperature thermochronology (LTT), including techniques such as (U-Th)/He and fission-track analysis (AFT) on apatite grains, or organic matter maturity indices. However, these methods have two main limitations: 1) they are T-dependent, requiring knowledge or, more commonly, assumptions about the geothermal gradient for the studied sedimentary sequence over the considered time span, which can be challenging for deep-time analysis; and 2) these techniques are most effective at temperature higher than 60°-80° for (U-Th)/He and 120° for AFT. This means that for regions with a normal geothermal gradient of 30°C/km or lower (e.g., foreland basins), low-T thermochronology is less reliable for determining burial depth of less than 2-4 km experienced by rocks before exhumation.
In this contribution we aim to address these limitations by filling the “blind window” of LTT and avoiding uncertainties related to the past geothermal gradient. We do this by using the degree of compaction in sand-sized clastic rocks (COPL-CEPL indexes analysis) as a proxy to estimate the minimum burial depth experienced by exhumed clastic sequences. We apply a compaction-driven approach coupled with diagenetic modelling to estimate the burial depth of clastic units exposed in the eastern Tertiary Piedmont Basin (TPB) which occupies an episutural position on the tectonic junction between the Alps and the Northern Apennines collisional belts. Due to its complex tectonic setting, the studied sedimentary succession has undergone a largely unknown post-depositional history, making it possible to test several regional burial/exhumation scenarios for the Eocene-lower Miocene sequence. Our results suggests that the eastern part of the TPB underwent to more burial than previously expected; this implies that it continued to subside and accumulate sediment until the end of the Miocene, with uplift and erosion likely beginning at the end of Miocene due to the combined effects of Northern Apennines contractional tectonic phase and the Messinian Salinity Crisis. Overall, this case of study demonstrates that the quantitative study of the degree of compaction coupled with diagenetic modelling can be a reliable tool for maximum burial reconstruction in the depth-temperature window where current low-T thermochronological methods hardly work.
How to cite: Stendardi, F., Tamburelli, S., and Di Giulio, A.: Reconstructing the burial history of uplifted clastic sequences using compactional indices and diagenetic modeling (a northern Italy case of study) , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3679, https://doi.org/10.5194/egusphere-egu25-3679, 2025.
Purpose: The Bonan Sag is located in the Bohai Bay Basin in eastern China, where the chlorite content is high in the lower part of the fourth member of the Shahejie Formation. In this paper, the genesis of chlorite in the study area was analyzed by means of casting thin section observation, scanning electron microscope, EPMA and LA-ICP-MS, and its influence on reservoir was discussed.
Experimental samples and analysis methods: In this paper, several samples of Well H1 and Well H2 in the lower part of the fourth member of Shahejie Formation in Bonan sag are selected. Firstly, the morphology and size of chlorite in the sample were observed and described in detail by casting thin sections and scanning electron microscopy. Then, the thin sections were polished, and the different forms of chlorite were subjected to electron probe and laser ablation with the assistance of scanning electron microscopy. The elemental composition of chlorite was measured and analyzed.
Result: Well H1 is mostly coated chlorite, attached to the edge of the particles, inhibiting the secondary enlargement and cementation of quartz and other particles, which is conducive to pore development. The H2 well is mostly rose-like chlorite, which is mainly filled in the middle of the pores, plugging the pores and reducing the porosity and permeability of the reservoir.
The Fe/(Fe + Mg) values of chlorite are generally 0.3-0.7, but they can be clearly divided into two categories: one category has an average value of 0.37, which is significantly smaller than the standard chlorite, and most of them are coated chlorite (relatively rich in Si); the average value of the other type is 0.66, which is significantly larger than that of the standard chlorite, often showing a rose-like (relatively rich in Fe).
Conclusion: The content of quartz and feldspar in well H1 is high, which is coated chlorite rich in Si. It is formed by the dissolution of feldspar and quartz during diagenesis, which has a positive effect on reservoir physical properties. The content of magmatic rock debris and metamorphic rock debris in well H2 is higher, which is Fe-rich in rose chlorite. It is formed by the alteration of magmatic rock and metamorphic rock debris during diagenesis, which has a negative effect on reservoir physical properties.
Keywords: Bonan Sag; Tight sandstone; Reservoir; Chlorite genesis;
How to cite: Zhang, H., Liu, G., Liu, K., Chen, J., and Wang, S.: The genesis of chlorite in tight sandstone reservoirs and its influence on reservoir properties: A case study of the lower fourth member of Shahejie Formation in Bonan Sag, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-25, https://doi.org/10.5194/egusphere-egu25-25, 2025.
Diagenetic processes alter petrophysical properties, such as porosity and permeability, of clastic sediments. An understanding of these processes is therefore pivotal for any reservoir quality assessments. Thermal exposure is among the crucial factors influencing diagenesis and can vary significantly in widespread formations, e.g. due to regional variations of the burial history. Especially quartz overgrowth is controlled by temperature and we here demonstrate the effect of spatial thermal exposure variability on the degree of cementation. Our field example is from the aeolian Jurassic Etjo sandstone that was buried by the Lower Cretaceous Paraná-Etendeka Large Igneous Province in Namibia and we show data from outcrops lying >100 km apart from each other – named Waterberg, Mt. Etjo, and Gamsberg.
Our petrographic analysis shows significant differences in the degree of compaction and cementation between the localities (Salomon et al., 2024). Waterberg samples have a mean quartz cement volume of 6.5 % and intergranular volume (IGV) of 23.7 %. Mt. Etjo samples have a higher quartz cement volume (15.4 %), but lower IGV (19.7 %). As Waterberg samples had an on average 72 % larger nucleation surface area available for quartz cement growth than samples from Mt. Etjo, we argue that the latter locality has experienced a higher thermal exposure. This is supported by a kaolinite-to-illite transformation that only occurs at Mt. Etjo. We attribute the temperature differences to the localities’ position underneath the volcanic cover with Mt. Etjo being closer to the volcanic center than Waterberg. Gamsberg samples have a very high mean IGV of 30.7 % and very high mean quartz cement volumes of 24.3 %. Here, two quartz growth generations are evident and separated by an exhumation period of the sandstone. The origin of the first generation is yet unclear, whereas the second one is also attributed to the Etendeka burial.
Our case study serves as a prime example for the variability in diagenetic character within a sandstone formation, depending on its geographic position. It underscores the importance of understanding spatial variabilities in the burial/temperature history when predicting the diagenetic properties of a reservoir rock.
Reference: Salomon, E., Stollhofen, H., Caracciolo, L., Bonnell, L.M., Lander, R.H., Kittel, M., 2024. Burial of a sand sea: Heterogeneous compaction and cementation of the jurassic Etjo Formation, Namibia and what it tells. Marine and Petroleum Geology, 168, 107044.
How to cite: Salomon, E., Stollhofen, H., Caracciolo, L., Bonnell, L. M., and Lander, R. H.: Heterogeneous cementation and compaction in a sandstone and the role of temperature – an example from the Etjo Formation, Namibia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16554, https://doi.org/10.5194/egusphere-egu25-16554, 2025.
The Groningen gas field, Europe’s largest onshore gas reservoir, has undergone extensive compaction due to prolonged gas extraction, leading to surface subsidence and seismicity. Understanding the diagenetic processes controlling reservoir quality is essential for managing these risks. This study investigates the role of dolomitization and associated diagenetic alterations in shaping the petrophysical and geomechanical properties of the Rotliegend sandstones. Detailed petrographic analysis, incorporating optical and scanning electron microscopy, reveals that early dolomite cementation significantly reduced primary porosity while preserving intergranular volume (IGV) and enhancing mechanical stability by inhibiting grain rearrangement and compaction during burial. Dolomite cementation was particularly prominent in zones proximal to mudstone interbeds, indicating its strong depositional and diagenetic control. Authigenic clays, including illite, smectite, and chlorite, were found to inhibit late-stage quartz cementation, preserving reservoir quality in specific zones. Burial depth and prolonged thermal exposure intensified quartz overgrowths and illite development, particularly in deeper sections of the reservoir. Clustering patterns among authigenic minerals revealed strong positive and negative associations: Illite, illite-smectite, and chlorite frequently co-occur in fine-grained sandstones, particularly in northern wells, where their abundance reduces pore space and permeability. Conversely, high dolomite content negatively correlates with clay minerals and quartz cement, reflecting the inhibitory role of early dolomitization on subsequent diagenetic mineral development. Quartz cementation plays a minor role in compaction due to shallow burial conditions and the presence of clay rims around grains, which limited growth of quartz cements. Mechanical compaction primarily affected sandstones lacking dolomite, where authigenic clay content and quartz cementation are higher, correlating with higher porosity and permeability than sandstones that are dolomitized. This study highlights the interplay between depositional setting, diagenetic timing, and burial history in shaping reservoir quality and stability. By linking petrographic trends to geomechanical behavior, these findings provide critical insights into mitigating subsurface risks associated with gas extraction in Groningen and analogous reservoirs worldwide.
How to cite: Mulder, S. and Miocic, J.: The Role of Dolomitization and Clay Rims in Shaping Sandstone Evolution: Insights from the Groningen Gas Field, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12855, https://doi.org/10.5194/egusphere-egu25-12855, 2025.
Early Eocene limestones in Tunisia exhibit notable variations in facies and thickness, indicative of a depositional environment primarily within a platform setting. Most planktonic-rich microfacies are deposited in slope-basin areas conducive to preserving organic matter. This study focuses on the Bou Dabbous Formation, a known source-rock explored in various wells across Tunisia. It aims to evaluate its potential as a tight reservoir through a comprehensive analysis of core samples from two outcrop analogs: Oued Kasseb (OK) and Ragoubet Tassera (TS) sections in northwestern Tunisia.
Rock-Eval pyrolysis of 42 samples revealed variable organic richness, with higher Total Organic Carbon (TOC) in the OK section (max TOC = 2.75 wt.%) compared to the TS section (max TOC = 1.11 wt.%). Tmax values ranged from 433°C to 454°C, indicating a predominantly mature to marginally mature stage of hydrocarbon generation. Hydrogen Index (HI) values classified the organic matter primarily as Type II, with some Type II/III, suggesting a planktonic marine origin under anoxic to suboxic conditions.
Inorganic analysis, including XRF and XRD analyses, showed significant lithological variations between massive limestone and marly limestone layers. Major oxides such as MgO and SiO2 varied notably, with silica-rich layers prominent in the TS section and magnesian limestones in the OK section. Trace elements like Sr and S highlighted diagenetic processes and variations in paleoclimate, suggesting semi-humid conditions during deposition. The presence of pyrite, influenced by anoxic conditions and diagenesis, further supports these findings. The complex diagenetic processes affecting the Bou Dabbous limestones impact their petrophysical properties, including fractures, silicification, and cementation.
The region's significant tectonic activity has resulted in a complex fracture network, as observed in field studies and thin-section analyses. These fractures, partially filled with calcite and asphaltene, are attributed to compaction and tectonic stresses. Stylolites, formed by chemical compaction and bitumen-filled fractures, indicate the circulation of acidic solutions related to petroleum generation processes. These processes altered the primary pore system by either enhancing reservoir properties (such as fracturing and oxidation) or reducing and destroying porosity through cementation, mechanical and chemical compaction, and the precipitation of asphaltene, phosphate, and pyrite. This emphasizes the complexity of diagenetic controls on the porosity evolution. The brittleness index (BI) for the Bou Dabbous Formation, determined based on carbonate fractions, detrital content, and TOC percentages, was high, ranging from 0.84 to 0.98. This suggests a high fracturing sensitivity among the studied area's rocks.
Overall, the findings indicate that the Bou Dabbous limestones have potential as a tight reservoir with favorable conditions for unconventional hydrocarbon exploration, influenced by its significant organic content, mature hydrocarbon generation, and brittleness characteristics. These results underscore the formation's importance in regional petroleum exploration and development. Further investigations should be conducted on borehole cuttings, along with rigorous fracturing simulations, to realistically evaluate the potential of the Bou Dabbous Formation in Tunisia as an unconventional hydrocarbon reservoir within the explored petroleum system.
How to cite: Arfaoui, I., Omar, H., Singh, A., Rachdi, M., Montacer, M., Baudin, F., Collin, F., and Boulvain, F.: Tight Reservoir Potential of the Early Eocene Bou Dabbous Formation in Northwestern Tunisia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18386, https://doi.org/10.5194/egusphere-egu25-18386, 2025.
This study explores the formation of anthropogenic conglomeratic rock caused by the lithification of fluvial sediment through calcium leaching from historical paper mill sludge (PMS) deposits near Penicuik, Scotland. Calcium-rich waste generated by historic paper mill activities accumulated along the stream bank, resulting in localized cementation of sediment. Field observations identified lithified stream bed deposits beneath the PMS heaps, where natural sedimentary clasts were bound by calcium carbonate (CaCO₃). Microstructural analysis using Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDX) revealed that the cementing material is predominantly calcite. This was further corroborated by X-ray Diffraction (XRD), confirming a calcite-rich mineralogical composition. Stable isotope analysis (δ¹³C and δ¹⁸O) suggested a mixed carbon source, with approximately 40% derived from atmospheric CO₂ and 60% from lithogenic origins, highlighting complex geochemical processes. The results demonstrate that calcium ions (Ca²⁺) leached from PMS dissolved into pore water, subsequently migrating into adjacent fluvial sediments. This facilitated the precipitation of calcite, binding sediment grains into a cohesive structure and forming an anthropogenic conglomerate. This process mimics natural lithification mechanisms but occurs at the Earth's surface over significantly shorter timescales, bridging natural and anthropogenic geological systems. This anthropogenic lithification challenges conventional sedimentary rock classifications and has broader implications for waste management, pollutant immobilization, and carbon sequestration. By illustrating the interaction of industrial waste with natural sedimentary environments, the study emphasizes human influence on geological processes and provides insights into the evolving role of industrial byproducts in shaping modern sedimentary systems.
How to cite: Alam, M. J. and MacDonald, J. M.: Leaching of Legacy Paper mill Sludge promotes Lithification through cementation of fluvial sediments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4473, https://doi.org/10.5194/egusphere-egu25-4473, 2025.
Sediment archives are invaluable for reconstructing past environmental conditions. Clastic sediments are formed through physical processes, such as weathering of rocks, during which minerals are released from the parent material, transported, and eventually deposited. However, both transport and depositional processes—such as abrasion, mixing, or chemical dissolution—can alter the original signals preserved in the sediment. Water plays a central role in these transformations by promoting processes like dissolution and recrystallization of minerals. This is particularly relevant for sediments deposited in stagnant waters such as lakes or marine basins. Similarly, infiltration of surface water flows into sediments, enhances chemical reactions and facilitates the transformation of minerals. Fine-grained sediments, such as sands or clays, are especially reactive due to their large specific surface area, which increases their interaction with fluids.
To determine the original composition, isotopic signatures, or depositional and source age of the sediment and the contained minerals, it is crucial to know whether, and to what extent, chemical alterations have occurred during and after the deposition. A critical factor in the analysis is whether the sediment layer represents a closed or open system. In closed systems, the original chemical signatures are preserved, as no significant element exchange occurs with the surrounding environment. In contrast, in open systems, interactions with water can lead to the loss or accumulation of elements, which can alter the sediment's original composition at the time of deposition, making it difficult to interpret its geochemical and mineralogical history.
In a comprehensive mineralogical and geochemical study combined with field observations, we analyzed solids and eluates of 250 sediment samples from the Miocene the Northern Alpine Foreland basin. Additionally, we separated and analyzed the mineral chemistry of over 30 samples (also clay fraction) using SEM. We examined the mineral textures with high-resolution microscopy. Furthermore, we performed extensive leaching experiments to study the mobilization behavior of the contained elements under “open system conditions”.
The results of this study revealed the following: 1) A significant proportion of the minerals in the sediments are newly formed. 2) Experiments showed that processes like dissolution and precipitation can proceed very quickly (within hours to days), depending on factors such as pH, Eh and concentration of dissolved elements. However, these processes can be halted when the sediment runs dry or when chemical equilibrium in the system is achieved. 3) In open-system sediment profiles, water infiltration causes the leaching of elements that are transported to deeper stratigraphic layers, where they precipitate and form new mineral phases.
A key observation is the elution behavior of arsenic: In sediments known to represent open systems, arsenate is not mobilized in the elution tests, because it is fixed in the mineral phases or is already desorbed during the water exchange. In closed systems, elevated arsenic concentrations in the eluate indicate that these sediments have not undergone significant post-depositional water exchanges. The analysis demonstrated that arsenic is predominantly adsorbed on phyllosilicates. Accordingly, an elution test is a reliable indicator of water exchange in sediments after their deposit.
How to cite: Aßbichler, D., Weichselgartner, N., Diesner, N., Kayalar, M., Otte, C., Kellner, M., Henning, H., and Tautenhahn, S.: Arsenic — an indicator for post-depositional water exchange in sediments at neutral to alkaline conditions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16523, https://doi.org/10.5194/egusphere-egu25-16523, 2025.
Circulation of silica-bearing hydrothermal fluids along faults affects petrophysical and mechanical properties of fault-related rocks by modifying their texture and mineralogy, with strong implications on geofluid storage and seismicity in the shallow crust. However, in the subsurface, it is extremely difficult to predict the geometry of silicified rock volumes along and around fault zones as well as their petrophysical properties and, therefore, outcrop analogues can provide important insights. The Kornos-Aghios Ioannis Fault (KAIF) on Lemnos Island (Greece) is a silicified extensional fault system active at shallow depth (<1 km) that is well exposed over 10 km length and juxtaposes volcanic rocks against turbidite sandstones. In this study, we investigate the distribution, petrophysics and mineralogy of silicified rocks along two across-fault transects through a multi-analytical approach that combines data from X-ray diffraction analysis, Hg-intrusion porosimeter, digital image analysis, X-ray micro-computed tomography and unsteady-state gas permeameter. The permeability of silicified fault cores (i.e. breccias, cataclasites, ultracataclasites), characterized by quartz contents >70 wt. %, decayed of 3 orders of magnitude (from 100 to 10-3 mD) with respect to pristine host rocks as pore space was occluded by silica cements. In fault damage zones, porosity of massively silicified sandstones strongly varies in the range 2-13% because of the presence of dissolution intragranular and intercrystalline pores whose formation is strongly controlled by the mineralogy (i.e. microcrystalline silica, sulphides and feldspars are preferentially dissolved). However, permeability of these massively silicified rocks remains low (<0.01 mD), regardless of their porosity, due to the low connectivity of the pore network. In the silicified volume characterized by reduced permeability, that extends 100’s of meters from the master fault plane being locally greater than the damage zone, the permeability drop produced by cementation is partially counterbalanced by higher fracture density and connectivity because of increased rock brittleness (UCS increases up to 30% compared to pristine host rocks). Moreover, all the samples analyzed show that porosity values are sensitive to pressure and strongly decrease with increasing confining pressure (up to 17 MPa). Our results show that hydrothermal silicification along faults may strongly degrade the reservoir quality in the surrounding area (100’s of meters from the master fault plane) where its effect is only locally counterbalanced by an excess permeability produced by dissolution, fractures and subsidiary faults. However, the intensity and extension of silicification are heterogeneous along-fault strike and fault segments not affected by hydrothermal silicification can interrupt the along-strike continuity of low-permeability silicified fault rocks.
How to cite: Berio, L., Balsamo, F., Storti, F., Bezerra, F. H., Brito, M. F., and Rodrigues, M. A. F.: The impact of fault-controlled hydrothermal silicification on the petrophysical properties of sandstones: insights from the Kornos-Aghios Ioannis Normal Fault (Lemnos Island, Greece), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4528, https://doi.org/10.5194/egusphere-egu25-4528, 2025.
Posters virtual: Wed, 30 Apr, 14:00–15:45 | vPoster spot 2
EGU25-2377 | Posters virtual | VPS26
Dynamic Control of Multimineral Diagenetic Processes on the Evolution of Pore-Throat Structures and Hydrocarbon Accumulation Windows in Tight Sandstone ReservoirsWed, 30 Apr, 14:00–15:45 (CEST) | vP2.4
EGU25-2655 | Posters virtual | VPS26
Diagenesis, reservoir-quality, and oil-bearing heterogeneity of the Eocene deep-lacustrine mudstone in the Qibei Sub-sag, Bohai Bay Basin, ChinaWed, 30 Apr, 14:00–15:45 (CEST) | vP2.5
Lacustrine organic-rich Eocene mudstones are well developed and demonstrates significant exploration potential for shale oil in the Qibei Sub-sag, Bohai Bay Basin, China. However, their oil content displays strong heterogeneity, which poses challenges for effective exploration and development. Diagenesis implicates compaction, cementation, dissolution/re-precipitation processes that raises critical questions regarding reservoir quality and oil-bearing heterogeneity.
Integrated high‐resolution petrologic analysis, organic geochemistry, and pore throat structure characterization provide a powerful approach to investigate the diagenesis, reservoir and oil-bearing characteristics. The 50 samples were collected from the 111.39-m-thick Eocene the first Sub-member of the third Member of the Shahejie Formation lacustrine oil-prone source rock succession penetrated by the two wells in the Qibei Sub-sag. Six typical lithofacies were identified: laminated medium-grained calcareous shale, laminated fine-grained mixed shale, thin-bedded fine-grained mixed mudstone, thin-bedded medium-grained mixed mudstone, massive medium-grained mixed mudstone, and thin-bedded coarse-grained felsic mudstone.
The micritic calcite laminae formed during the sedimentary stage underwent recrystallization during the early to middle diagenetic stages, transforming into granular sparry calcite. Potassium feldspar dissolution and clay mineral transformation resulted in the formation of authigenic albite and quartz. These diagenetic processes promoted the development and preservation of intercrystalline/interparticle pores. As a result, the laminated medium-grained calcareous and laminated fine-grained mixed shale reservoirs exhibit superior reservoir properties, primarily characterized by interparticle pores, intercrystalline pores, clay mineral-associated pores, and bedding fractures. With a median pore throat diameter of 11.6 nm and an average porosity of 6.53%, these reservoirs are classified as Type I. The thin-bedded fine-grained mixed shale primarily develops clay mineral-associated pores and interparticle pores, with some bedding fractures. Its median pore throat diameter is 9.2 nm, and the average porosity is 5.56%, classifying it as a Type II reservoir. The thin-bedded medium-grained mixed and massive medium-grained mixed mudstones mainly develop interparticle pores and clay mineral-associated pores. These have a median pore throat diameter of 12.6 nm and an average porosity of 4.3%, classifying them as Type III reservoirs. In felsic mudstone, calcite cementation significantly reduced porosity during the early diagenetic stage. This results in the poorest porosity development in the thin-beded coarse-grained felsic mudstone, which has a median pore throat diameter of 15.9 nm and an average porosity of 3.26%, classifying it as Type IV reservoir.
The laminated medium-grained calcareous shale, laminated fine-grained mixed shale, and thin-bedded fine-grained mixed mudstone exhibit relatively high oil content and OSI values. The average oil content values are 2.48 mg/g, 2.64 mg/g, and 2.30 mg/g, respectively, and the average OSI values are 144 mg HC/g TOC, 163 mg HC/g TOC, and 168 mg HC/g TOC. These lithofacies are favorable for shale oil exploration and development. We suggest that addressing the challenges of mudstone diagenesis will significantly improve understanding and prediction of reservoir quality and oil-bearing heterogeneity in unconventional shale oil plays.
How to cite: Wang, J., Zhao, J., You, Z., Pu, X., Liu, K., Zhang, W., Shi, Z., Han, W., and Wang, Z.: Diagenesis, reservoir-quality, and oil-bearing heterogeneity of the Eocene deep-lacustrine mudstone in the Qibei Sub-sag, Bohai Bay Basin, China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2655, https://doi.org/10.5194/egusphere-egu25-2655, 2025.
EGU25-2783 | ECS | Posters virtual | VPS26
Sedimentary Characteristics and Sedimentary Model of Glutenite Fans in Shahejie Formation, Luojia areaWed, 30 Apr, 14:00–15:45 (CEST) | vP2.6
Glutenite Fans is one of the most favorable reservoirs for exploration and development in recent years and is widely distributed in the world. In recent years, major breakthroughs have been made in oil and gas exploration of glutenite fans in Luojia area in Luoxie 180 and Luo25 Wells. The Jiyang exploration area is a high mature exploration area in the east, which has entered the exploration stage mainly to search for subtle oil and gas reservoirs, and Glutenite Fans, as an important part of subtle oil and gas reservoirs, has become the most realistic and valuable exploration target at present.
The Luojia area has a complex structural background, with the development of fault structures in the area, and the development of two sets of glutenite fans bodies of different origin, and the lithology difference is great. The diagenesis is complex and the calcareous intercalation is widely developed, which is of great significance for reservoir reconstruction.
This paper takes the sand conglomerate of Es3 and Es4 members in Luojia area of Zhanhua Depression as the research object, synthesizes seismic, logging, core, analysis and test data, and carries out the research on the genetic types, sedimentary characteristics and diagenesis of the sand conglomerate controlled by different tectonic activities and provenance.
How to cite: yao, Y. and yang, Y.: Sedimentary Characteristics and Sedimentary Model of Glutenite Fans in Shahejie Formation, Luojia area, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2783, https://doi.org/10.5194/egusphere-egu25-2783, 2025.
EGU25-3009 | ECS | Posters virtual | VPS26
Characteristics of Shale Reservoirs in the Permian Fengcheng Formation, Hashan RegionWed, 30 Apr, 14:00–15:45 (CEST) | vP2.7
The Permian Fengcheng Formation is an important hydrocarbon source rock development sequence and exploration sequence in the Junggar Basin. The Hashan tectonic belt, located on the northwestern margin of the Junggar Basin, is a large-scale thrust nappe superposed structure. Having undergone multiple tectonic movements and tectonic uplift and denudation, it has lost the stratigraphic distribution characteristics of a foreland basin. The Fengcheng Formation developed on multiple thrust tectonic steps, resulting in difficulties in stratigraphic correlation and unclear understanding of the distribution characteristics of the original sedimentary system and the development characteristics of favorable reservoirs. Therefore, clarifying the distribution laws and genesis of diagenesis and establishing a reservoir-forming model for high-quality reservoirs are of great significance for the effective sedimentary reservoir mechanism and the prediction of favorable gas-bearing areas in the study area.
How to cite: Li, Y.: Characteristics of Shale Reservoirs in the Permian Fengcheng Formation, Hashan Region , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3009, https://doi.org/10.5194/egusphere-egu25-3009, 2025.
EGU25-4748 | ECS | Posters virtual | VPS26
Reservoir characteristics and diagenetic evolution of Lower Jurassic Sangonghe Formation in the hinterland of Junggar BasinWed, 30 Apr, 14:00–15:45 (CEST) | vP2.8
On the basis of core observation and description, multi-scale microscopic analysis and related reservoir physical property analysis, the petrological characteristics, reservoir characteristics and diagenetic characteristics of the Lower Jurassic Sangonghe Formation in the central area of Junggar Basin are systematically studied, and the diagenetic evolution sequence of the reservoir is further established. The results show that the reservoir in the studied interval has undergone three diagenetic processes: compaction, cementation and dissolution during its development and evolution after burial. The reservoir mainly goes through two stages: early burial compaction and late tectonic compression. There are various types of cementation, including carbonate, siliceous, clay mineral, gypsum and anhydrite. The overall intensity of dissolution in the reservoir is low, and it mainly develops in the interior or edge of easily soluble components such as feldspar and rock cuttings, and also develops in the edge of clay mineral bonding. Diagenetic evolution sequence of the reservoir in the study area is as follows: early calcite cementation - early chlorite cementation - acid dissolution/quartz enlargement/kaolinite cementation - illite cementation - gypsum/anhydrite cementation - late calcite cementation - iron calcite/iron dolomite cementation, mechanical compaction has developed in the whole burial evolution process.
How to cite: Guo, T. and Zhang, L.: Reservoir characteristics and diagenetic evolution of Lower Jurassic Sangonghe Formation in the hinterland of Junggar Basin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4748, https://doi.org/10.5194/egusphere-egu25-4748, 2025.
EGU25-7580 | Posters virtual | VPS26
Orderly variations in the spatial and geological characteristics of carbonate reservoirs in the northern Tarim Basin, ChinaWed, 30 Apr, 14:00–15:45 (CEST) | vP2.9
Deep marine carbonate rocks in the Tarim Basin, Northwest China, have significant burial depths, ancient ages, and complex diagenetic evolution. Multi-stage tectonic activities and periodic sea-level changes create unconformities that expose carbonate rocks, resulting in interlayer, syn-sedimentary, and epigenetic karst systems. These processes, along with host rock composition and faulting, shape carbonate reservoir distribution and properties. Dissolution is most intense in shallow water grainstones and packstones, where fracturing enhances fluid flow, serving as both reservoirs and migration pathways. Consequently, carbonate reservoir characteristics in the northern Tarim Basin vary systematically from north to south, shaped by variations in unconformity size, diagenetic patterns and fault activity intensity, reflecting the basin’s evolution from deposition to deep burial. In the Yakela area, the northernmost region, significant uplift and erosion have exposed Cambrian, sometimes even Sinian bedrocks beneath Cretaceous layers, forming buried hill dolomite reservoirs. Moving south to the Tahe area, a paleokarstic erosion zone has developed large-scale dissolved fracture-cavity reservoirs due to the combined effects of faulting, surface karstification, and river system development near the base Carboniferous erosion surface. Further south, in the Tahe slope zone, reservoirs are shaped by a combination of dissolution and faulting, with bedding-parallel dissolution pores and enlarged fractures becoming more prominent as proximity to the paleoerosion surface decreases. This reflects a decrease in karstification intensity and an increase in fault-induced fluid pathways. In the Shunbei area where marine carbonates are deeply buried, structural features such as fault slip surfaces and open fractures dominate reservoir formation, with tectonic activity and fluid flow through fractures driving diagenetic alterations. The spatial variations in diagenetic pathways—from initial deposition and uplift in the north to deep burial in the south—highlight the interplay of dissolution, tectonics, and fluid migration across varying depths and time scales, providing insights into the mechanisms that control carbonate reservoir formation and evolution globally.
How to cite: Fan, T.: Orderly variations in the spatial and geological characteristics of carbonate reservoirs in the northern Tarim Basin, China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7580, https://doi.org/10.5194/egusphere-egu25-7580, 2025.
