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Introduction: Mantle-derived CO2, as an important component of hydrothermal fluids, is widely distributed in petroliferous basins. While previous experimental studies have suggested that CO2 can improve sandstone reservoir quality through mineral dissolution in open fluid setting, they have overlooked its nagetive effects to sandstone reservoir quality by carbonate cementation. Additionally, the roles of various fluid environments in CO2-reservoir interactions have not been studied in detail. Methods: To systematically investigate the influences of CO2 on sandstone reservoirs, we examine a typical mantle-derived CO2 gas reservoir, Bohai Bay Basin, China. This study employs integrated methods, including electron microscopy, scanning electron microscopy, X-ray diffraction, stable C- and O-isotope analysis, and physical property data. The aim is to investigate the evidence and mechanisms by which mantle-derived CO2 impacts sandstone reservoirs, particularly focusing on its effects in open and closed fluid environments. Results and Discussion: Our findings reveal that dawsonite and ankerite are prevalent within the mantle-derived CO2 gas reservoir, while isotopic analysis of carbonate cements indicates values (δ13C: −9.0‰ to −1.6‰; δ18O: −21.7‰ to −12.7‰) consistent with mantle-derived CO2 and hydrothermal fluids. These pieces of evidence indicate that CO2-rich hydrothermal fluids participate in water-rock interactions, thereby significantly influencing the diagenesis of reservoirs. Further, we notice that CO2 reservoirs adjacent to faults exhibit an open fluid environment, characterized by superior porosity and permeability, more quartz, but fewer feldspar, carbonate, and clay minerals compared to those in closed fluid environments. Notably, kaolinite predominates in open fluid environments, while illite/smectite (I/S) is more common in closed settings. The dual roles of mantle-derived CO2 are highlighted in our analysis: while it enhances reservoir storage and permeability through mineral dissolution, the carbonate cement generated by CO2-water-rock interaction can also adversely affect reservoir quality. In open fluid environments, CO2 facilitates the dissolution of feldspar and carbonate minerals, promoting the timely removal of dissolution by-products (clay mineral) and inhibiting carbonate cementation, thereby improving reservoir properties. Conversely, in closed fluid environments, decreasing CO2 concentrations with depth leads to diminishing dissolution effects and increased carbonate cementation, resulting in reduced reservoir porosity and permeability. Overall, the significance of this study is to correct the deviation in the impacts of CO2 on sandstone reservoirs at laboratory setting through case study of typical mantle-source CO2 gas reservoir. This work can be applied to the studies of reservoir homogeneity and sweet spots in regions with hydrothermal and mantle-derived CO2 activities. However, due to the limitation of CO2 content range (about 15%–70%) in the study case, we are unable to investigate the effects of low-concentration CO2 on sandstone reservoirs, which may affect the generalizability of this work. Besides, the formation temperature and pressure, and salinity of formation water, should be considered when dealing with other cases. [ABSTRACT FROM AUTHOR]

Understanding the petrographic and geochemical characteristics of different tectonic settings is essential to improving traditional methods of identifying the provenance of palaeo-sedimentary environments. This research focuses on the K2b unit, which consists of mixed carbonate and volcanic lithic sandstones located in Iran with the age of the Turonian-Early Campanian period. Five sedimentary lithotypes with distinct petrographic and geochemical signatures were identified in the Turonian/Lower Campanian K2b unit, which consists of coarse-grained fan-delta strata. Carbonate and volcanic lithic fragments are the most characteristic components of mostly quartzo-lithic to felspatho-litho-quartzose sandstone layers, documenting a unidirectional paleocurrent pattern with the north-eastward direction. The original composition has undergone diagenesis, resulting in the precipitation of authigenic carbonates and depleting Rare Earth Elements (REE) except for Eu, which shows a positive anomaly. The K2b fan delta was situated on the margin of the proto-south Caspian back-arc basin, and the sediments were sourced from the undissected magmatic arc. The gravity flow deposits in the K2b unit were likely triggered by tectonic activity in the catchment area of the fan delta, leading to uplift and formation of relief associated with the regional Mosha Fault and, on a larger palaeogeographic scale, with subduction of the Arabian plate beneath the Iranian plate during Turonian-Early Campanian. [ABSTRACT FROM AUTHOR]

Paleoenvironmental archives in East Antarctica have revealed significant changes during the Holocene, marked by ice sheet retreat leading to the isolation of submarine basins. These basins offer valuable insights into past climate, glaciology, and oceanography shifts that impact sedimentary processes. In this study, environmental magnetism and biogeochemical proxies to investigate the Mid-to-Late-Holocene transitions in Pup Lagoon, a coastal isolation basin is presented. Our findings reveal distinct stratigraphic zones reflecting shifts from marine to lacustrine environments. The results reveal predominant mechanical weathering in the Stornes region, producing coarse-grained "soft" ferrimagnetic minerals. Notably, a period of warm oceanographic conditions between 6000 and 4722 cal. yr BP was characterized by mixed magnetic grain sizes and ultrafine superparamagnetic grains, indicating relatively oxic open waters in the basin. Subsequent shifts to reducing conditions coincide with persistent marine sea ice cover from 4722 to 2634 cal. yr BP, favoring the retention of coarse-grained ferrimagnets. Finer magnetic grain sizes between 2634 and 2185 cal. yr BP was attributed to the increased freshwater inputs associated with the Mid-Holocene Hypsithermal. Further, diagenetic changes under persistent sea ice cover between 2185 and 1970 cal. yr BP led to the selective dissolution of fine-grained ferrimagnets. Transitioning to freshwater isolated basin conditions between 1970 and 588 cal. yr BP, fine ferrimagnet precipitation indicate oxic to suboxic conditions alongside drier conditions. Biogenic productivity increased post-isolation, which was reflected in increased (Total Organic Carbon) TOC and (Total Nitrogen) TN percentages. Additionally, the presence of greigite in the isolated phase sediment indicates reducing conditions owing to organic matter decomposition. Notably, χfd% exhibits an inverse trend to sea ice concentration, potentially indicating anoxic-dysoxic conditions due to the presence of sea ice. These observations align with broader regional sea ice concentration changes, emphasizing the interconnected behavior of local and regional factors shaping Antarctic coastal environments. [ABSTRACT FROM AUTHOR]

This study marks the first detailed depositional, diagenetic and organic geochemical investigation of Jurassic Isha Formation in North Waziristan, Khyber Pukhtunkhuwa, Pakistan. Detailed systematic sampling was carried out from exposed outcrop along the Mirali road in North Waziristan for microfacies analysis and diagenetic studies to unravel the depositional environment and diagenetic alterations. The oraganic maturity and hydrocarbon generation potential were assessed by total organic content and Rock-Eval pyrolysis. Outcrop analogue revealed that the formation predominantly comprises of thick to thin bedded oolitic fractured limestone, bioclastic and dolomitic limestone. In the petrography, a total of eight microfacies (MF1–MF8) were constructed and were directed different depositional enviroments. Based on the integrated field data and microfacies analyses, the Isha Formation is interpreted to be deposited on a homoclinal ramp particularly in peritidal, lagoon, and carbonate shoals. Sequences stratigraphic setup shows intervallic Transgressive and Regressive Systems Tracts. The TSTs are mostly grainstone and packstone facies whereas the RSTs are wackestone and mudstone microfacies which demonstrates a single 2nd order cycle and then multiple 3rd order transgressive and regressive cycles for the deposition of Isha Formation. The total organic content and Rock-Eval pyrolysis results were ploted on VanKerevlen diagram which indicate that the source rock potential fall within the type III kerogen. The HI versus Tmax and PI versus Tmax cross plots indicate that the formation lies within the zone of maturation, generation potentiality indicate that the formation has fair to poor potential and hydrocarbons are indigenous. [ABSTRACT FROM AUTHOR]

Rhenium (Re) and uranium (U) are essential proxies in reconstructing past oceanic oxygenation evolution. However, their removal in continental shelf sediments, hotspots of early diagenesis, were previously treated as quantitatively unimportant sinks in the ocean. Here we examine the sedimentary reductive removal of Re and U and their coupling with organic carbon decomposition, utilizing the 224Ra/228Th disequilibria within the East China Sea shelf. We identified positive correlations between their removal fluxes and the rates of sediment oxygen consumption or organic carbon decomposition. These correlations enable an evaluation of global shelf reductive sinks that are comparable to (for Re) or higher than (~4-fold for U) previously established suboxic/anoxic sinks. These findings suggest potential imbalances in the modern budgets of Re and U, or perhaps a substantial underestimation of their sources. Our study thus highlights shelf sedimentary reductive removal as critical yet overlooked sinks for Re and U in the modern ocean. The sources and sinks of Re and U in the modern ocean may be imbalanced, according to sedimentary porewater analysis and assessment of reductive Re and U removal on the continental shelf [ABSTRACT FROM AUTHOR]

Seamounts are ubiquitous topographic units in the global oceans, and the Caiwei Guyot in the Magellan Seamounts of the western Pacific is a prime example. In this study, we analyzed a well-dated sediment core using magnetic properties, sediment grain size, and metal enrichment to uncover regional ventilation history during the middle Pleistocene and explore potential linkages to global climate changes. Our principal findings are as follows: (1) The median grain size is 3.3 ± 0.2 mm, and clay and silt particles exhibit minimal variation, with average values of 52.8 ± 1.8% and 38.2 ± 1.6%, respectively, indicating a low-dynamic process; (2) Three grain-size components are identified, characterized by modal patterns of ~3 mm (major one), ~40 mm, and 400-500 mm, respectively; (3) Magnetic coercivity of the deep-sea sediments can be classified into three subgroups, and their coercivity values are 6.1 ± 0.5 mT, 25.7 ± 1.0 mT, and 65.2 ± 2.1 mT. Based on these results, we propose a close linkage between magnetic coercivity and metal enrichment, correlating with changes in deep-sea circulation intensity. Conversely, sediment grain-size changes seem to be more strongly influenced by eolian inputs. Consequently, we suggest that regional ventilation has weakened since ~430 ka, likely linked to a reduction in Antarctic bottom water formation. [ABSTRACT FROM AUTHOR]

ABSTRACT This is a comment on the paper of Singh et al. (Geological Journal, 2024:1–20) on Strontium isotope stratigraphy of marine Oligocene–Miocene sedimentary successions of Kutch Basin, western India. Kutch hosts stratotypes of some lithostratigraphic and chronostratigraphic units of India. In this study, the Sr‐isotope estimated ages of the studied formations deviate significantly from their known ages based on biostratigraphy. The authors have not validated the interpreted ages with biostratigraphy. We believe a scrupulous screening of samples and validation of Sr‐isotope data with biostratigraphy are two essential requirements of Sr‐isotope stratigraphy. Unfortunately, Singh et al. made a new contribution to Kutch stratigraphy that falls short of meeting both conditions, leading to incorrect ages of the regional chronostratigraphic units of India. [ABSTRACT FROM AUTHOR]

We present a suite of major element stable isotope (δ13C, δ18O, δ44/40Ca, δ26Mg), and selected trace element (Sr/Ca and Mg/Ca) data from Pleistocene sediments from the Great Barrier Reef (IODP Expedition 325), as well as Holocene surface sediments from the Bahamas (Triple Goose Creek, Andros Island) to identify geochemical fingerprints associated with early marine and meteoric diagenesis. Sediments from both sites exhibit co-variation in δ13C and δ18O values, depletion in trace elements, and distinct geochemical trends in δ26Mg and δ44/40Ca values that reflect differences between diagenetic alteration in marine and meteoric fluids. While marine diagenesis results in lower Sr/Ca ratios, higher δ44/40Ca values, and little effect on bulk sediment δ26Mg values, meteoric diagenesis leads to lower Sr/Ca ratios, lower δ44/40Ca values, and lower δ26Mg values. Using a numerical model of diagenesis, we show how diagenetic alteration by meteoric fluids must occur after an initial period of diagenetic alteration by marine fluids, a two-stepped diagenetic history that complicates the interpretation of geochemical data in meteorically altered marine carbonate sediments. Finally, we discuss how paired metal isotopes may serve as a robust indicator of meteoric alteration in ancient shallow-water marine carbonate sediments. [ABSTRACT FROM AUTHOR]

Microbialite reservoirs are of great importance in oil and gas exploration. However, there is still a lack of comprehensive studies on the formation mechanisms of thrombolitic reservoirs, a specific type of microbialite. This research focuses on the oldest thrombolitic dolomite reservoir located within the Precambrian Dengying Formation in the central Sichuan Basin, southwestern China. A multi-disciplinary approach was employed to characterize different thrombolite facies and elucidate the formation mechanism of thrombolitic dolomite reservoirs and their controlling factors, involving core observation, thin-section analysis, cathodoluminescence, scanning electron microscope (SEM) microscopy, elemental analysis using LA-ICP-MS, and carbon and oxygen stable isotope analysis. Based on variations in texture, four types of thrombolite were identified: 1) distinct clotted thrombolite, 2) diffuse and regular clotted thrombolite, 3) diffuse and irregular clotted thrombolite, and 4) composite clotted thrombolite. Notably, the diffuse clotted thrombolitic dolomite is the prevalent lithology in the reservoir. Through modification by meteoric water, organic acid fluids, and hydrothermal fluids, a reservoir with dominant porosity in the form of primary growth-framework pores, dissolution pores, and vugs was formed. This resulted in the development of two high-quality reservoir intervals within the Second Member and at the top of the Fourth Member of the Dengying Formation. The growth-framework porosity of the thrombolites, epigenetic karstification, and tectonic fracturing were mainly conducive to reservoir development. However, various types of cementation have reduced porosity and connectivity within the reservoir. Overall, this study is a valuable example of the methodology required to understand meso- and microstructures of deep-buried thrombotic dolomite reservoirs, including their heterogeneities and diagenesis, as the original structures influence diagenesis. [ABSTRACT FROM AUTHOR]

This manuscript documents the diagenetic progression from deposition to early burial of freshwater low‐Mg calcite mud within a Holocene sequence of the Everglades and Florida Bay (Florida, USA). During initial, in situ burial, the texturally‐complex, organic‐rich mud, exhibits high susceptibility to alteration, within a primarily dissolution‐driven realm. Seaward, a brackish‐water transitional burial environment currently displays evidence of mixed precipitation and dissolution. This brackish stage burial alteration may be due to fluid exchange (a relatively high fluid‐to‐rock ratio) in a submerged estuary setting. Lastly, the burial environment in Florida Bay records a transgression from freshwater to near‐normal marine conditions. This island setting currently exhibits a diagenetic trend towards low‐Mg calcite precipitation in intervals of both the marine and freshwater mud. Burial of freshwater mud beneath nearly 3 m of marine mud during rising sea level appears to have preserved a considerable amount of original textures, as compared with younger freshwater deposits now within the transitional brackish‐water setting. Geochemical and textural data within the freshwater mud sections suggest that complex early diagenesis begins shortly after burial at the site of formation, and perhaps even during deposition. The findings herein document three major implications for coastal zone paludal carbonates: (i) that incipient textural diagenesis in low‐Mg calcite crystals can be extremely early and the stability and petrographic fidelity of these deposits are vulnerable due to the open, trellis‐like structure of the crystals; (ii) relatedly, the stable isotope signature of the original low‐Mg calcite crystals is also vulnerable to alteration by secondary precipitation within the open crystal structure; and (iii) during marine transgression, the paludal calcite crystals likely experience a shift from interstitial fluids that are dissolution‐dominated (freshwater) to more precipitation‐dominated as marine fluids overtake the pore fluid regime. Because palustrine deposits are used as palaeoenvironmental archives, it is important to understand their diagenetic and preservation potential. This study has implications for those concerned with the deposition, reactivity and preservation potential of continental and microbial carbonates. [ABSTRACT FROM AUTHOR]

Planktonic foraminiferal shells are widely used to investigate past oceanographic and climatic variations via their trace elements and stable isotopes. However, these geochemical methods may be compromised by the presence of diagenetic high‐Mg calcite. In this study, dolomite crystals are observed in planktonic foraminifera from International Ocean Discovery Program Site U1464 sediments on the Australian Northwest shelf, a shallow marine environment (<300 m). This study investigates the formation of the dolomite using new scanning electron microscopy, X‐ray diffraction, stable isotope (carbon and oxygen), Mg/Ca and rare earth element data, combined with shipboard geochemistry and available literature. This study finds that foraminiferal dolomite formation was probably associated with early diagenesis, microbial and ageing processes. Two potential diagenetic processes (seafloor diagenesis and post‐depositional diagenesis) are proposed, and both indicate that the favourable redox condition of dolomite formation is low oxygen, which may activate methanogenic catalysis to facilitate dolomite formation. It is postulated that high‐Mg calcite may be the precursor of foraminiferal dolomite in this case, and that dissolution–precipitation may be the ageing process of dolomite formation. In contrast to deeper, more normal pelagic environments, this study shows that the marine environment can facilitate dolomite precipitation by higher fluxes of organic matter to the seafloor. [ABSTRACT FROM AUTHOR]

This study deals with unraveling the diagenesis-induced porosity evolution in a mixed clastic-carbonate sequence of the Middle Permian Indus Basin, Pakistan. Multiple data sets including outcrop, petrography, cathodoluminescence, scanning electron microscopy (SEM), mineralogy, and geochemical isotopic compositions were integrated to establish a link between porosity evolution and diagenesis. The spatial thickness and facies variations of the strata at outcrop scale are inherently controlled by the underlying bathymetry of the basin with deepening westward trend. The low values of δ18O of the target strata, relative to average values of the Permian carbonate, hints to diagenetic alteration in the strata. The data sets used in this study reveal modification of the strata in four environments, that is, i) early marine diagenesis indicated by micritization, pervasive dolomitization and isopachous fibrous cements, followed by ii) meteoric dissolution, and iii) shallow burial diagenetic processes including the precipitation of blocky cement, compaction of skeletal and non-skeletal allochems, and stylolites, and iv) a deep burial environment, characterized by pressure solution, and micro-fractures. The clastic intervals host subangular to subrounded quartz grains, floating textures, and almost complete absence of deleterious clay minerals, consequently resulting in the preservation of primary porosity. The primary porosity of carbonate intervals is preserved in the form of intercrystalline and intracrystalline porosity. The secondary porosity evolved through various diagenetic phases in the form of fractures and dissolution. The diagenetic solution mediated by organic matter in carbonates may have experienced both bacterial decomposition and thermochemical sulfate reduction, precipitating sulfides within the pores. The plug porosity/permeability analyses generally suggest high porosity in the siliciclastic unit, and carbonates with wackestone fabric while lower values were observed for the inner shelf pure carbonate facies. However, both intervals show very low permeability values probably due to isolated moldic pores and intense micritization. Therefore, clastic intervals may provide an opportunity to serve as a moderate reservoir; however, the carbonate intervals possess very low permeability values and could generally be considered as low-moderate reservoir potential. [ABSTRACT FROM AUTHOR]

Aragonite is a metastable mineral, which is easily transformed into calcite, and generally difficult to preserve in the stratum. However, large amounts of aragonites were found in the Paleogene shale of the Jiyang Depression. The characteristics and preservation mechanisms of these aragonites were analyzed through a series of analytical methods, including cathodoluminescence, field-emission scanning electron microscopy (FESEM), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), microarea carbon and oxygen isotopes, Sr isotopes, and dissolution simulation experiments under high temperature and high pressure. The research results show that: ①Aragonite in the Paleogene shale of the Jiyang Depression is related to algal microbial fossils, primarily composed of coccoliths and characterized by two emission peaks at 420 nm and 480 nm in cathodoluminescence; ②The primary factor allowing biological aragonite to be preserved is the immaturity of the organic matter and the deficiency of abundant organic acids necessary for its dissolution or transformation, which is confirmed by the evidence of organic matter maturity and simulation experiments of organic acid dissolution on aragonite under high-temperature and high-pressure conditions. Additional factors that may aid in the preservation of aragonite are the ideal sedimentation conditions, the defense of organic coating, and the enclosed environment with tiny pores, low porosity, and low permeability; ③These aragonite-rich shales, characterized by coccolithophores, provide a solid evidence for seawater intrusion into terrestrial lake basin, and have a significant implication for the source and storage of shale oil. [ABSTRACT FROM AUTHOR]

Methane seeps, prevalent in ocean basins globally, indicate upward methane migration from the subsurface, often evident as gas chimneys in seismic reflection data. The footprint of this methane migration is often indicated by methane-derived authigenic carbonate (MDAC), a product of anaerobic oxidation of methane (AOM). Despite extensive research on MDAC from present-day seafloors and outcrops, understanding methane migration footprints from subsurface rock samples remains limited. Therefore, this study aims to investigate methane migration footprints from subsurface rock samples taken from a proven area of gas migration. This study utilized cutting samples from well XS-01 located in the Makassar Strait, offshore Indonesia. The well was drilled through a gas chimney into Oligocene carbonate reservoirs hosting a substantial methane column (102 m). Analysis of 44 cutting samples involved petrographic examination, Scanning Electron Microscope (SEM) imaging, and X-ray Diffraction (XRD) analysis to discern mineralogical content and diagenetic features signaling the presence of gas. A diagenetic texture called clotted peloidal micrite (CPM) was discovered within foraminifera fossils and around fractures based on petrographic analysis. CPM is a type of MDAC and predominantly occurs in fine-grained, siliciclastic rocks, indicating gas migration. This migration is interpreted to occur: (i) during the early burial stages originating from microbial activity since the Oligocene (biogenic gas); (ii) following matured source rock that reached its peak maturity from the Middle Eocene to the Pliocene (thermogenic gas); or (iii) interference of these two processes. The migration route persists until present day as evidenced by gas chimney and seabed pockmarks identified in seismic reflection data. This study emphasizes the importance of subsurface rock samples, such as cuttings, in uncovering gas migration footprints. Especially, where seismic data is unavailable or could not image fluid flow features. In addition, this study also provides a new perspective on diagenesis along methane migration route, complementing most of the research that is primarily focused on reservoir diagenesis. [ABSTRACT FROM AUTHOR]

Archaeological bone chemical composition is modified post-mortem by diagenesis processes, and over decades, several authors have proposed different protocols to avoid post-depositional contamination that can carry to misleading interpretations about the lifestyle and origin of ancient populations. In this work, a methodological approach based on rare earth elements analysis was developed to determine diagenetic alterations on femurs, humeri, and skull surfaces, and internal layers from thirteen individuals exhumed during fieldwork in the Fatih Sultan Mehmet Mosque at Rozafa Castle (Shkodër, Albania). Major, minor, and trace elements, including rare earth elements, were measured employing spectrometric techniques, and the obtained data were statistically processed by principal component analysis and one-way ANOVA to select the best preserved bones. The results show that in general, the internal parts of bones, especially skulls, suffered post-depositional chemical contamination. Finally, to show the effectiveness of the proposed approach, a diet reconstruction employing log(Sr/Ca) and Zn/Ca was tested, obtaining results that are in line with the literature describing a diet based on a mixed economy, mostly agricultural products with low protein intakes. [ABSTRACT FROM AUTHOR]

Tight oil reservoirs are considered important exploration targets in lacustrine basins. High-quality reservoir prediction is difficult as the reservoirs have complex distributions of depositional facies and diagenesis processes. Previous research has found that the diagenesis process of tight oil sandstones varies greatly in different depositional facies. However, diagenesis variation in different depositional facies is still poorly studied, especially in distributary channels of shallow water delta deposits in lacustrine basins. Based on the description of core samples, the observation of rock slices, the interpretation of well logging data, and the analysis of porosity and permeability data, the differences in the lithofacies types, diagenesis processes, and pore structures of different distributary channels have been clarified. Ultimately, a model of diagenesis and reservoir heterogeneity distribution in the shallow-water delta of Chang 8 Member of the Yanchang Formation in the Caijiamiao area of the Ordos Basin has been established. This research indicates that the main distributary channels in the study area are dominated by massive bedding sandstone lithofacies, while the secondary distributary channels are primarily characterized by cross-bedding sandstone lithofacies. There are significant differences in the compaction, dissolution, and cementation of authigenic chlorite and carbonate among different parts of the distributary channels. Plastic mineral components, such as clay and mica, are abundant in sheet sands, and are more influenced by mechanical and chemical compaction. Influenced by the infiltration of meteoric water and hydrocarbon generation, dissolution pores are relatively well-developed in the underwater distributary channel reservoirs. A large amount of carbonate cementation, such as calcite and siderite, is found within the sandstone at the interface between sand and mud. The occurrence of authigenic chlorite exhibits a clear sedimentary microfacies zonation, but there is little difference in the kaolinite and siliceous cementation among different microfacies reservoirs. Finally, a model of diagenetic differences and reservoir quality distribution within dense sand bodies has been established. This model suggests that high-quality reservoirs are primarily developed in the middle of distributary channels, providing a theoretical basis for the further fine exploration and development of oil and gas in the study area. [ABSTRACT FROM AUTHOR]

Diagenetic spheroids are rounded mineralised structures in sedimentary rocks that span several orders of size dimensions. These objects include rosettes spanning tens of micrometres; millimetre-sized granules; and centimetre- to metre-scale botryoids, nodules, and concretions. Connections between diagenetic spheroids and chemically oscillating reactions (COR) have been established based on chemical and pattern similarities. Nevertheless, it is unknown if there are variations in their occurrences throughout geological time, especially during periods of global environmental change. This is because COR are pattern-forming reactions where carboxylic acids are decarboxylated by strong oxidants like sulphuric acid, bromate, or iodate. The hypothesis is that there should be a greater number of diagenetic spheroids at periods of increased atmospheric and oceanic oxygenation levels. Hence, this work presents the first compilation of occurrences of diagenetic spheroids between 0.541 and 1.8 billion years ago (Ga). The compiled patterned objects are those with concentric laminations and radially aligned crystals, which are patterns akin to equidistant circularly concentric waves that radially diffuse in COR. Another characteristic of diagenetic spheroids, similar to that found in botryoidal minerals, is the destructive interference that forms circular twins; this is identical to the COR pattern formed when circular waves meet. The timeline of Proterozoic diagenetic spheroids produced in this work reveals a significant increase in occurrences across the Ediacaran, which is attributed to an increased environmental oxidation state. During this interval, seawater is known to have become richer in sulphate and iodate, which create ideal conditions for the abiotic and diagenetic oxidation of biomass. Therefore, increased occurrences of diagenetic spheroids in the Neoproterozoic possibly represent sedimentological evidence for abiotic decarboxylation reactions and widespread COR in productive environments. The distribution of diagenetic spheroids observed during the Proterozoic may also be more broadly applicable to other epochs of Earth's history. [ABSTRACT FROM AUTHOR]

Illitization of smectite in sedimentary systems, a process akin to "reverse weathering," is a diagenetic process that has significant implications for sediment paragenesis and hydrocarbon exploration. However, the potential influence of chemical weathering on the illitization of smectite and its possible control of the neogenesis of titanium (Ti)-oxides remain unclear. Altered volcanic tephra layers (i.e., K-bentonites) characterized by an interstratified illite-smectite (I-S) clay mineralogy serve as an excellent medium to investigate the illitization of smectite. In this study, we first investigated the fine structure of clay minerals and in situ nano-mineralogy of Ti-bearing minerals from altered volcanic ashes and then undertook a meta-analysis of geochemical compositional data for bentonites spanning a wide range of ages and depositional environments globally. We found that Ti mainly occurs as discrete micrometer-scale magmatic srilankite and nanoscale authigenic anatase crystals. During the weathering process, the magmatic srilankite partly dissolved under acidic conditions, resulting in a local buildup of Ti in porewaters. The I-S displays a platy habit and curved edges and is found closely associated with anatase crystals under TEM. Our compilation results combined with microscopic evidence suggest that chemical weathering may potentially promote the illitization reaction by changing the chemical composition of the fluids through increased terrestrial inputs and by creating larger pore spaces through the decomposition of weatherable components. Positive correlations between K and Ti are especially common in (K-)bentonites that are dominated by I-S, suggesting that I-S can adsorb Ti during the weathering process and provide a suitable site for the nucleation of anatase. Our study highlights the role of chemical weathering in the illitization of smectite and their combined effect on the formation of Ti-oxides. [ABSTRACT FROM AUTHOR]

We review published studies characterizing the Thamama‐B reservoir zone in the upper Kharaib Formation (late Barremian) in Abu Dhabi oilfields and at outcrops in Oman. Available data for oxygen and carbon isotope compositions, fluid inclusion measurements, cement abundance and formation water composition are interpreted in terms of a paragenetic model for the Thamama‐B in field F in Abu Dhabi where the interval is deeply buried. The present synthesis provides a useful basis for understanding and predicting reservoir quality in static models and undrilled prospects, as well as for planning promising directions for further research. The goals of this study were to summarize the geologic setting and petrology of the Thamama‐B reservoir and its surrounding dense zones, and to examine how sedimentology, stratigraphy and diagenesis have interacted to control porosity and permeability. Results that may have useful applications for similar microporous limestone reservoirs in general include: the depositional environments and stratigraphy of the subject strata;a model for how porosity variations result mainly from calcite cementation sourced from stylolites, with little dependence on lithofacies other than the localization of chemical compaction by depositional clay linked to sequence stratigraphy;the use of solidity (rock thickness with porosity removed) as a check on porosity creation by burial dissolution;observations linking high‐permeability streaks with storm lag beds and fractures;the concept of strata being gradually buried through a relatively static salinity‐stratified water column;integration of conventional and clumped stable‐isotope data with petrologic observations to constrain the timing of porosity evolution. [ABSTRACT FROM AUTHOR]

Reservoir quality prediction in deeply buried reservoirs represents a complex challenge to geoscientists. In sandstones, reservoir quality is determined by the extent of compaction and cementation during burial. During compaction, porosity is lost through the rearrangement and fracture of rigid grains and the deformation of ductile grains. During cementation, porosity is predominantly lost through the growth of quartz cement, although carbonate and clay mineral growth can be locally important. The degree of quartz cementation is influenced by the surface area of quartz available for overgrowth nucleation and thermal history. Clay grain coats can significantly reduce the surface area of quartz available for overgrowth nucleation, preventing extensive cementation. Using a coupled-effect compaction and cementation model, we have forward-modelled porosity evolution of surface sediments from the modern Ravenglass Estuary under different maximum burial conditions, between 2000 and 5000 m depth, to aid the understanding of reservoir quality distribution in a marginal marine setting. Seven sand-dominated sub-depositional environments were subject to five burial models to assess porosity-preservation in sedimentary facies. Under relatively shallow burial conditions (<3000 m), modelled porosity is highest (34 to 36%) in medium to coarse-grained outer-estuary sediments due to moderate sorting and minimal fine-grained matrix material. Fine-grained tidal flat sediments (mixed flats) experience a higher degree of porosity loss due to elevated matrix volumes (20 to 31%). Sediments subjected to deep burial (>4000 m) experience a significant reduction in porosity due to extensive quartz cementation. Porosity is reduced to 1% in outer estuary sediments that lack grain-coating clays. However, in tidal flat sediments with continuous clay grain coats, porosity values of up to 30% are maintained due to quartz cement inhibition. The modelling approach powerfully emphasises the value of collecting quantitative data from modern analogue sedimentary environments to reveal how optimum reservoir quality is not always in the coarsest or cleanest clastic sediments. [ABSTRACT FROM AUTHOR]

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Copyright of Special Oil & Gas Reservoirs is the property of Special Oil & Gas Reservoirs Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In order to examine further the characteristics of micropore-throat structures of the tight oil reservoir in the Jiufotang Formation in the Houhe region, this study used whole rock X-ray diffraction, routine physical property analysis, and routine thin section observations to analyze the material composition and physical properties of the tight oil reservoir. CT scanning, high-pressure mercury infiltration, and other test methods were employed to analyze the characteristics of the pore-throat structures in the tight oil reservoir. In addition, the Pearson correlation coefficients quantified the relationships between nine parameters and pore-throat structures. The parameters with high correlations were optimized for analysis, and a comprehensive classification scheme for micropore-throat structures in the tight oil reservoir in the study area was established. The results show that the reservoir in the Jiufotang Formation in the Houhe region is composed of feldspathic and lithic arkosic sandstone, with feldspar and clast pore dissolution pores as the main type of reservoir pore space. The tight oil reservoir has small pore-throat radius, complex structures, poor connectivity, and high heterogeneity. It generally contains micron-sized pores with submicron to nanometer throat widths and small- and medium-sized pores to fine micropore-throat structures. Porosity, permeability, coefficient of variation, skewness coefficient, and average pore-throat radius, were selected for k-means cluster analysis. The micropore-throat structures of the tight oil reservoir were divided into three categories: classes I, II, and III. The study area is dominated by class II pore throats, accounting for 58%. Diagenesis mainly controls the pore-throat structure. These results provide an effective reference for the identification and evaluation of favorable sweet spots in tight oil reservoirs in similar blocks in China. [ABSTRACT FROM AUTHOR]

The linkage between the variation of the seawater Mg budget and the long-term carbon cycle can be elucidated by seawater Mg isotope composition (δ26Mg). However, obtaining primary seawater δ26Mg signatures from marine archives is challenging due to the widespread alteration of diagenesis. Aragonite, a common primary marine carbonate, can effectively record seawater δ26Mg but is prone to alteration and transformation into calcite during early diagenesis. Therefore, a comprehensive understanding of Mg isotope behavior during the aragonite–calcite transition is essential to enhance the applicability of aragonite δ26Mg. In this study, we investigate the variation of aragonite δ26Mg during diagenesis with a limited supply of Mg by conducting a series of well-controlled aragonite–calcite transition experiments in a closed system. The experimental conditions encompass temperatures of 60 and 90 °C, the presence of Ca and Na in the solution, varying Na concentrations, as well as the presence of calcite seed. Results demonstrate that the significant decrease of bulk carbonate δ26Mg is accompanied by a substantial amount of Mg released into the solution during the aragonite–calcite transition, and the amount of released Mg is controlled by fluid chemistry via altering Mg partitioning in calcite. Furthermore, Mg isotope fractionation during calcite precipitation is influenced by temperature, ionic strength, and the presence of calcite seed, while kinetics played a negligible role in our experiments. Combined with previous experiments, the temperature-dependent Mg isotope fractionation during calcite precipitation in unseeded experiments is Δ26Mg cal-sol = (−0.13 ± 0.06) × 106/T2 – (0.47 ± 0.68). This fractionation is systematically higher than that of seeded experiments by 0.3–0.6 ‰ from 15 to 90 °C and can mainly be attributed to differences in surface free energy between homogeneous and heterogeneous calcite nucleation. These findings offer fundamental understandings of the Mg isotope behavior during the aragonite–calcite transformation, providing useful insights for interpreting the variation of δ26Mg in experimental and natural carbonates and facilitating ancient seawater δ26Mg reconstruction. [ABSTRACT FROM AUTHOR]

Temperature affects the timing of the transformation of amorphous silica (opal-A) into crystalline (opal-CT) exponentially. Thus, in hot subseafloor environments opal-A is expected to convert into opal-CT at relatively shallow burial depths, where in situ temperatures do not exceed ∼56 °C, as it has been previously observed at various deep-sea sites and in onshore rock outcrops as well as assessed during lab experiments. The response of biosilica (biogenic opal-A) diagenesis to steep geothermal gradients (∼224–529 °C/km) at extremely high sedimentation rates (∼1 m/kyr) was examined in cores from off-axis boreholes drilled by the International Ocean Discovery Program (IODP) Expedition 385 in the actively spreading, intrusive sill-riddled Guaymas Basin at the Gulf of California (Mexico) rifted margin. At three sites drilled by IODP Expedition 385 (U1545, U1546, and U1547), the conversion from amorphous opal (−A) to crystalline opal (−CT) occurs in relatively deep (up to ∼330 mbsf) and unexpectedly hot (in situ temperatures of ∼74–79 °C) subseafloor conditions. This observation indicates a significantly slower reaction kinetics of biosilica transformation than previously reported. A compilation of empirical data that include biosiliceous basins with a similarly hot subseafloor (Sea of Japan and Bering Sea) yield new kinetic parameters that account for the slower rates of silica transformation. Thus, current kinetic models for the prediction of opal-A to −CT conversion face limitations when burial rates exceed those typical of biogenic sedimentation in open-ocean conditions. At Guaymas Basin Site U1545, where there is no evidence of sill-related metamorphic overprint, the d-spacing of the opal-CT (1 0 1) peak correlates linearly with in situ temperature between ∼75 and 110 °C throughout the opal-CT zone, thus, providing a local silica paleothermometry proxy that can be used to calculate the maximum temperature to which opal-CT sediment has been subjected. [ABSTRACT FROM AUTHOR]

Tight carbonate reservoirs are complicated by their vast heterogeneity, which is attributed to diagenetic and depositional processes. The new discovery of natural gas in the tight carbonate reservoir of the Upper Cretaceous Kawagarh Formation (KF− UC) in Pakistan makes it crucial to study the factors controlling reservoir properties. This study investigated the sedimentology and reservoir characteristics of KF− UC using integrated petrography, scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS), and petrophysical techniques. The microfacies analysis of KF− UC indicates mudstone and wackestone, identified in the proximal and distal outer ramp settings of the homoclinal carbonate ramp. Diagenetic processes such as compaction, micritization, dolomitization, cementation, neomorphism, and dissolution, which represent meteoric and marine phreatic mixing zones and burial diagenetic settings, have modified the reservoir characteristics of KF− UC. The net porosity of KF− UC could be enhanced by dolomitization, dissolution, and fracturing, making it a better petroleum reservoir. With an average porosity of 2.3%, microporosities such as intercrystalline/interparticle, intracrystalline/intraparticle, vuggy, and fracture were identified in KF− UC, designating it as a tight carbonate reservoir. The outcrop and well data exhibit excellent correlation; nevertheless, the well’s net pay is inadequate due to the influence of complex tectonic structures along significant faults within the study area. Furthermore, the results of this study were compared with the Gurpi Formation of Zagros Basin, Iran, and the Aruma Formation of Central Saudi Arabia, highlighting similar sedimentological characteristics, thus emphasizing the need to initiate such projects in other regions with similar settings to further strengthen petroleum exploration. [ABSTRACT FROM AUTHOR]

The lower Triassic Kangan Formation is one of the most important carbonate reservoir rocks in southern Iran and the Persian Gulf. This formation is part of the Dehram Group and consists of a carbonate-evaporite sequence, including limestone, dolomite and anhydrite. This carbonate sequence has been deposited on a gently-sloping homoclinal carbonate ramp in a warm and dry climate conditions. In this study, by carefully examining geological reservoir zones (GRZs) in three wells in the central (wells A and B) and northern (well C) Persian Gulf, in terms of facies changes, sedimentary environment, diagenesis and sequence stratigraphy, it is possible to determine the reservoir quality by considering the reservoir heterogeneity. Petrographic observations show that the Kangan Formation consists of fifteen microfacies related to four facies belts, including sabkha, lagoon, tidal flat and shoal environments. Facies and environmental changes in the Kangan Formation indicate three third—order and seven fourth-order sequences in the central and northern Persian Gulf. Each sequence includes TST (Transgressive System Tract) and HST (Highstand System Tract) related to sabkha, intertidal, lagoon and shoal environments. The main diagenetic processes in the reservoir are dolomitization, dissolution and cementation. The connection between the depositional facies, sedimentary environment and diagenetic processes (dolomitization, anhydrite and calcite cementation and dissolution), allowed for the identification of seven geological reservoir zones (GRZs) related to the fourth-order sequences. These sequences and GRZs demonstrate vertical and lateral heterogeneity of the reservoir, observed as variation in GR log changes, lithology, facies frequency, diagenetic features and reservoir properties among the studied wells. GRZ-1 to GRZ-3 in the northern Persian Gulf and GRZ-4 to GRZ-7 in the central Persian Gulf show better reservoir quality. Facies analysis of the Kangan Formation indicates higher energy conditions for the central parts than the northern parts. [ABSTRACT FROM AUTHOR]

Carbonate rocks hold considerable volumes of hydrocarbons in various basins around the globe. Understanding the complex pore systems is a key factor in unlocking the potential of carbonate rocks. Matrix properties are also a challenge in carbonate reservoirs. Exploration geoscientists include the regional portfolio targeting the sweet spots for every basin. This study defines the reservoir quality and character of carbonate rocks with scope to the complete carbonate succession of north Western Desert and Abu Gharadig basin (AGB) as one of the most prolific basins in Egypt. More than 301 core samples are used in this study to represent all carbonate rock types. The workflow includes core data classification (grouping), petrographical investigation, reservoir mineralogy, and matrix parameters integrated with well logs and elastic properties. The provided work investigates the effect of pore structure and mineralogy on the petrophysical properties of the rock and its impact on elastic properties as well. Carbonate reservoirs in AGB are divided into three main categories: type-I, type-II, and type-III. Reservoirs enhancement by dolomitization and dissolution are controlling the distribution of pores, which are the key factors to delineate their potential. This study defines the characteristics of each group and the attractive formations for future exploration and development. The main differences in rock type, composition, diagenetic processes, and elastic properties were described. The rock brittleness index was calculated from dynamic properties for each carbonate type to get more insights about the rock brittleness and elasticity and predict variations of each group for future consideration in the basin. [ABSTRACT FROM AUTHOR]

This study investigates the diagenetic history and timing of hydrocarbon migration in the lower Permian Ingleside Formation as revealed in the Ingleside roadcut at Owl Canyon area, Colorado. Fourteen beds were identified within the studied exposure and are composed of quartz arenite sandstone, limestone, dolomite, or siltstone. Observations from outcrop and thin sections, including calcite veins, carbonate-hosted vugs, and carbonate cement, indicate carbonate mobility throughout the outcrop. Stylolites parallel to bedding were probably formed by pressure solution related to compaction, and pressure solution of carbonates is one possible source of carbonate that could have precipitated in veins or as a cement. The Ingleside Formation was affected by other diagenetic processes, including feldspar dissolution and alteration and several stages of cementation. Hematite, calcite, dolomite, kaolinite, and quartz overgrowth cements are the major types of cements identified within the studied exposure. Hematite cement was determined to have precipitated very early, followed immediately by the precipitation of poikilotopic carbonate cement. Blocky calcite and blocky dolomite cements, the most common cements within the formation, formed after the hematite and poikilotopic cements. Kaolinite cement may have precipitated in association with feldspar dissolution and alteration or occurred with feldspar dissolution during modern weathering. Hydrocarbon migration may have taken place before the of the blocky carbonate cement. The CaO in sandstone samples is due to the occurrence of calcite and dolomite cement. The MgO in sandstone samples is related to dolomite content. Furthermore, the low concentration of K2O in sandstone samples could be attributed to the low proportion of K-feldspar, and other K-rich minerals and possibly reflects loss of K during diagenesis. The relationship fact that S and MgO are both more abundant in samples from the middle and upper parts of the outcrop than in deeper samples suggests that they were added by diagenetic fluids. This study of the Ingleside outcrop provides insights on the diagenesis, possible timing of possible hydrocarbon migration through the Ingleside Formation, and geochemical and mineralogical composition of the exposure, which was used to interpret the diagenetic history throughout the outcrop. Therefore, it adds to the understanding of hydrocarbon migration and hydrocarbon pathways in this part of the Denver Basin. [ABSTRACT FROM AUTHOR]

Sedimentological and rock magnetic analysis was performed on 67 turbidite samples recovered from Lithostratigraphic units I–III at IODP Site U1520 with the aim to characterise the sedimentary processes and post-depositional diagenesis within a Quaternary sequence in the Hikurangi Trough, New Zealand. Lithostratigraphic Unit I was rapidly emplaced with frequent turbidite deposition with high sedimentation rate (∼1.9 m/kyr). In contrast, in units II–III, the turbidites were finer grained and more thinly bedded, and were deposited at less than half the deposition rate of Unit I. The turbidites were classified into three clusters based on the magnetic properties. Clusters 1 and 2 are characterised by a high coercivity and SIRM/χ, but low to moderate Bcr/Bc. The recognition of gyroremanent magnetisation acquisition in their clusters indicates the diagenetic sulfidization. Cluster 3 was represented by low coercivity and SIRM/χ, but high Bcr/Bc, indicating lack of sulfidization. Restricted occurrence of Cluster 3 in Unit I may be ascribed to the high sedimentation rate, which prevents sulfidization at the sulfate–methane transition zone. Degree of sulfidization might be related with sedimentary features of turbidites in the trough, such as grain size, bed thickness and depositional rate. [ABSTRACT FROM AUTHOR]

Surajkund Formation of Central Narmada Basin exhibits fining upward sequences of pebbly conglomerate, coarse-fine grained sandstone, siltstone and association of seven lithofacies, namely massive pebbly conglomerate, coarse-medium grained sandstone with large scale tabular cross bedding, massive coarse grained sandstone, coarse to medium grained sandstone with horizontal parallel bedding, fine grained sandstone with parallel lamination, fine grained sandstone with ripple lamination and siltstone, indicates their deposition in mixed load meandering river. Granulometric studies of Surajkund sediments also support the fluvial depositional environment. Soft sediment deformation structures documented in the siltstones suggest sediment liquification due to earthquake shocks. Abundant development of nodular, bedded calcretes and rhizoliths within these sediments are indicative of semi-arid climate and related subaerial exposure. These sediments are prominently lithic arenites, and clay mineralogy as well as geochemistry indicate deposition in the proximity of source, short distance of transport and mixed provenance of a variety of pre-Quaternary rocks such as Precambrian metamorphic rocks and granites, Vindhyan and Gondwana Supergroups, Deccan Trap basalt and laterite. Evidences of fresh water phreatic as well as vadose zone diagenesis linked to the semi-arid climatic conditions, together with subaerial exposure of these sediments, are seen in thin sections, which are supported by δ13C (av. −5.67%) and δ18O (av. −3.88%) values of calcretes. These values also suggest calcretes formed due to pedogenic and shallow groundwater processes in warm climate with C4-dominated vegetation. OSL date of one sample from Surajkund Formation gave an Ionian Age of Pleistocene Epoch. [ABSTRACT FROM AUTHOR]

Strontium (Sr) isotopes measured in fossil remains have been a useful tool to assess the geographical origin and even migrations of humans and other animals. In particular, dental enamel generally represents the ideal material, as it is dense and less prone to diagenetic replacement of Sr post-burial. However, fossil teeth can often be precious artefacts and difficult to access for destructive analysis. Here, we assess whether measuring Sr isotopes in fossil dental calculus could be used at least as a rangefinder to determine the geographical origin of an individual. We measured trace element concentrations in modern calculus (from a local dental practice), and trace element concentrations and 87Sr/86Sr ratios in human fossil calculus, dentine, and enamel from specimens collected in York, UK. Comparing trace element concentrations between modern and fossil calculus show that metals present in fossil calculus are mostly acquired post-burial, including Sr. The relationship between 87Sr/86Sr and Rb/Sr ratios in fossil calculus, dentine, and enamel suggests that the diagenetic end member would have a 87Sr/86Sr ratio consistent with the one modelled for the York region, but a low Rb/Sr. Without calculus data, dentine and enamel data would have probably suggested a lower 87Sr/86Sr ratio for a diagenetic end member, expecting high Rb/Sr values. Thus, while Sr isotopes in fossil calculus may not be useful to identify the geographical origin of an individual, they may be useful in constraining the composition of the diagenetic end member. Combining Sr isotopes in fossil dental calculus and enamel could be a more robust approach to identify geographical origin than using enamel alone. [ABSTRACT FROM AUTHOR]

Chert is abundant in Archean and Paleoproterozoic rocks and is commonly densely packed with authigenic Fe(II)-silicate nanoparticles such as greenalite, indicating a close relationship between iron and silica deposition. We investigate the relationship between Fe(II)-silicate minerals and dissolved silica during precipitation, settling, and diagenesis using anoxic synthesis, sorption, and heating experiments. Excess silica is associated with the solid during precipitation, resulting in high molar Si/Fe ratios (<1.52) that exceed that of stoichiometric greenalite (0.67). At pH 8–8.5, silica sorbs to the surface, reaching sorption densities of 0.68 mmol Si per mmol Fe(II)-silicate. Furthermore, excess Si is released upon heating as the Fe(II)-silicate gel crystallizes. We suggest that Fe(II)-silicate minerals acted as an effective Si shuttle between the water column and the sediments in Archean and Paleoproterozoic marine environments, providing sites for the growth of early diagenetic chert, consistent with observations from the sedimentary record. Our results explain the exceptional preservation of greenalite in early chert and indicate that these minerals could provide a robust archive of marine geochemical data. [ABSTRACT FROM AUTHOR]

Shallow-water platform carbonate δ13C may provide a record of changes in ocean chemistry through time, but early marine diagenesis and local processes can decouple these records from the global carbon cycle. Recent studies of calcium isotopes (δ44/40Ca) in shallow-water carbonates indicate that δ44/40Ca can be altered during early marine diagenesis, implying that δ13C may also potentially be altered. Here, we tested the hypothesis that the platform carbonate δ13C record of the Kinderhookian–Osagean boundary excursion (KOBE), ∼353 m.y. ago, reflects a period of global diagenesis using paired isotopic (δ44/40Ca and clumped isotopes) and trace-element geochemistry from three sections in the United States. There is little evidence for covariation between δ44/40Ca and δ13C during the KOBE. Clumped isotopes from our shallowest section support primarily sediment-buffered diagenesis at relatively low temperatures. We conclude that the δ13C record of the KOBE as recorded in shallow-water carbonate is consistent with a shift in the dissolved inorganic carbon reservoir and that, more generally, ancient shallow-water carbonates can retain records of primary seawater chemistry. [ABSTRACT FROM AUTHOR]

Copyright of Petroleum Geology & Experiment is the property of Petroleum Geology & Experiment Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Magnetic iron oxides are commonly enriched in soils and sediments on Earth's surface. Magnetic properties are widely employed in soil taxonomy, sediment tracing and paleoclimate reconstruction as indicators of associated pedogenic and depositional processes. However, in coastal regions, frequent shifts in pedogenesis and diagenesis accompanied by changes in hydrodynamic and geochemical conditions from land to sea can influence the formation and preservation of magnetic particles in sediment sequences. To discern the origin and nature of magnetic particles driven by these processes, we systematically examined soils and sediments around a tropical lagoon that has been closing for two hundred years. We found that the finest superparamagnetic particles are enriched along with hematite from inland soils with high‐Fe and high‐Al backgrounds, which was driven by pedogenesis. Single‐domain ferrimagnetic particles are enriched along with amorphous iron oxides in the lagoon with high‐Mg and high‐Mn backgrounds, which was driven by early diagenesis in quiet and reducing depositional environments. Coarse titanomagnetite particles are strongly enriched in inshore sediments with high‐Si and high‐Ti backgrounds, which was driven by seawater elutriation. However, magnetic particles in barrier bar soils have mixed features depending on the neighboring environment. Identifying magnetic particles derived from different soils and sediments in tropical coastal regions can assist in tropical coastal paleoclimate and paleoenvironment reconstruction on the basis of magnetic properties along coastal sediment sequences. Plain Language Summary: Magnetic particles and iron oxides in soils and sediments can be used for sediment tracing and paleoclimate reconstructions. Interpretations are more complex in coastal environments and are accompanied by paleoenvironmental changes. In this study, we collected samples from around a tropical lagoon to systematically analyze the effects of different coastal locations on magnetic particles and iron oxides in soils and sediments. We find that the finest magnetic particles are enriched along with hematite in inland soils. Medium‐sized magnetic particles are enriched along with amorphous iron oxides in the lagoon in quiet and reducing environments. The coarsest magnetic particles are enriched in inshore sediments as a result of seawater waves. However, magnetic particles in barrier bar soils have mixed features depending on the surrounding environment. Our work assists in discerning magnetic iron oxides from different soils and sediments for paleoclimate and paleoenvironmental reconstruction research in tropical coastal areas. Key Points: Soil and sediment samples around a tropical lagoon were analyzed via geochemical and magnetic methodsDifferent origins of magnetic particles are associated with hydrodynamic and geochemical backgroundsDifferent types of magnetic particles can be used to reconstruct coastal paleoenvironments [ABSTRACT FROM AUTHOR]

To reveal the diagenetic sequence of reservoir rocks in the central part of the deep depression basin, the Wuerhe Formation in Junggar Basin was taken as an example to conduct the detailed studies on its sedimentary facies, diagenetic sequence, and the micropore structure evolution rules based on the comprehensive data from a super deep exploration well C-6 (approximately 7,000 m in depth). First, an arid environment fan delta sedimentary model of the Wuerhe Formation was established, and its sedimentary evolution law was clarified as a gradual transition from a fan delta front to a fan delta plain during the water-regression process until the lake dried up. Then, the diagenesis types and evolution sequence of the Wuerhe Formation, and the influence degrees of the compaction, cementation, and dissolution on the rock formation process were clarified. Finally, the diagenesis and pore evolution model was established, and the greatest impact factors of the late reservoir densification were clarified. Based on this research, the diagenesis and pore evolution processes of the deep rocks in the studied deep central sag were ultimately revealed to provide useful guidance for the deeply buried oil and gas reservoir exploration.

Archaeological bone chemical composition is modified post-mortem by diagenesis processes, and over decades, several authors have proposed different protocols to avoid post-depositional contamination that can carry to misleading interpretations about the lifestyle and origin of ancient populations. In this work, a methodological approach based on rare earth elements analysis was developed to determine diagenetic alterations on femurs, humeri, and skull surfaces, and internal layers from thirteen individuals exhumed during fieldwork in the Fatih Sultan Mehmet Mosque at Rozafa Castle (Shkodër, Albania). Major, minor, and trace elements, including rare earth elements, were measured employing spectrometric techniques, and the obtained data were statistically processed by principal component analysis and one-way ANOVA to select the best preserved bones. The results show that in general, the internal parts of bones, especially skulls, suffered post-depositional chemical contamination. Finally, to show the effectiveness of the proposed approach, a diet reconstruction employing log(Sr/Ca) and Zn/Ca was tested, obtaining results that are in line with the literature describing a diet based on a mixed economy, mostly agricultural products with low protein intakes.

Methane seeps, prevalent in ocean basins globally, indicate upward methane migration from the subsurface, often evident as gas chimneys in seismic reflection data. The footprint of this methane migration is often indicated by methane-derived authigenic carbonate (MDAC), a product of anaerobic oxidation of methane (AOM). Despite extensive research on MDAC from present-day seafloors and outcrops, understanding methane migration footprints from subsurface rock samples remains limited. Therefore, this study aims to investigate methane migration footprints from subsurface rock samples taken from a proven area of gas migration. This study utilized cutting samples from well XS-01 located in the Makassar Strait, offshore Indonesia. The well was drilled through a gas chimney into Oligocene carbonate reservoirs hosting a substantial methane column (102 m). Analysis of 44 cutting samples involved petrographic examination, Scanning Electron Microscope (SEM) imaging, and X-ray Diffraction (XRD) analysis to discern mineralogical content and diagenetic features signaling the presence of gas. A diagenetic texture called clotted peloidal micrite (CPM) was discovered within foraminifera fossils and around fractures based on petrographic analysis. CPM is a type of MDAC and predominantly occurs in fine-grained, siliciclastic rocks, indicating gas migration. This migration is interpreted to occur: (i) during the early burial stages originating from microbial activity since the Oligocene (biogenic gas); (ii) following matured source rock that reached its peak maturity from the Middle Eocene to the Pliocene (thermogenic gas); or (iii) interference of these two processes. The migration route persists until present day as evidenced by gas chimney and seabed pockmarks identified in seismic reflection data. This study emphasizes the importance of subsurface rock samples, such as cuttings, in uncovering gas migration footprints. Especially, where seismic data is unavailable or could not image fluid flow features. In addition, this study also provides a new perspective on diagenesis along methane migration route, complementing most of the research that is primarily focused on reservoir diagenesis.

Upper Permian Longtan Formation transitional shale has become an important exploration layer, but the Longtan Formation shale (LFS) has a complex mineralogical composition, which affects the subsequent diagenesis and diagenetic evolution, and restricts the subsequent geologic exploration of shale gas. In this article, the observation of drilling cores, argon ion polishing-scanning electron microscope, Vitrinite reflectance, and X-ray diffractometer were used to analyze the type and characteristics of diagenesis of the LFS and clarify the stage of diagenetic evolution. The results show that the main diagenesis in the LFS is compaction, cementation, thermal maturation of organic matter (OM), dissolution, and transform of clay minerals. Among them, OM hydrocarbon generation, clay mineral transformation, and dissolution are pore-enhancing diagenetic events. Compaction and cementation are pore-reducing diagenetic events. The transitional and marine shales have similar characteristics of diagenesis, but there are big variations in the diagenesis of OM hydrocarbon, authigenic quartz, and siderite. The complex depositional environments of the marine-continental transition environment have resulted in a variety of rock types, which in turn influenced the diagenesis types and diagenetic evolution process. Compared with the transitional shales of the Shanxi and Taiyuan formations in the South China North Basin, the LFS are characterized by high clay content, low quartz content, complex mineral compositions, and a higher degree of thermal evolution.

The shale in the 7th member (Chang 7) of Triassic Yanchang Formation on the southeastern margin of the Ordos Basin is characterized by shallow burial depth, light oil quality, large variability in oil well production capacity, and significant resource potential. Identifying the main controlling factors of reservoir enrichment is key to efficient exploration. Based on analysis results of cast thin sections, physical properties, and scanning electron microscope (SEM) tests of the Chang 7 shale member in the Fuxian area, combined with core, well logging, and seismic data, its layer characteristics are described and the main controlling factors are discussed. The results show that the Chang 7 shale in the research area extensively develops gray, dark gray, and gray black source rocks. The lithology is mainly mudstone and mud shale, with organic matter types mainly classified as type Ⅰ-Ⅱ2. The vitrinite reflectance (Ro) values of source rocks range from 0.81% to 1.10%, indicating strong hydrocarbon generation potential. The sandstone reservoirs are mainly fine-grained feldspathic sandstone, with pore types primarily consisting of intragranular pores, residual intergranular pores, dissolution pores, and primary intergranular pores. Porosity ranges from 2.0% to 16.0%, and permeability ranges from 0.01×10-3 to 1.20×10-3 μm2, indicating tight reservoirs. The oil-bearing properties of the reservoir are impacted by the physical properties of the sandstone interlayers and proximity to faults: coarser-grained reservoirs with better physical properties exhibit better oil and gas bearing potential. Reservoir properties are controlled by two factors, sedimentary microfacies and diagenesis. Subaqueous distributary channel microfacies have the best properties, followed by mouth bar microfacies. Strong early diagenetic chlorite cementation and weak calcite cementation contribute to the formation of sweet spots in the physical properties. The development and nature of faults play a crucial role in the Chang 7 shale oil enrichment. Statistical analysis shows that it is difficult to obtain industrial oil flow when the fault displacement exceeds 10 m and the wellbore is within 1 km of the fault. However, when the fault displacement is less than 7 m or when the wellbore is more than 1 km away from a large fault, industrial oil flow is more easily obtained.

The Triassic-Jurassic successions of the Sulaiman Fold-Thrust Belt, Pakistan, are predominantly composed of limestone with minor proportions of shale, marl, and sandstone, accumulated at the northwestern passive margin of the Indian Plate within the Palaeo-Tethys. The current work is an outcome of field observations, thin-section petrographic studies, and scanning electron microscopy (SEM), aiming to reconstruct the palaeoenvironments and diagenetic pathways. Facies analysis and microscopic descriptions revealed sixteen microfacies, including mud-supported microfacies (mudstone and wackestone), grain-supported microfacies (packstone and grainstone) and calcisiltite. The Triassic succession includes calcisiltite and radiolarite, deposited below the CCD. Carbonates of the study area reflect both early-stage diagenesis (micrite envelopes, syntaxial rim cement, drusy mosaic calcite cement, and microbial micritization) - and late-stage diagenesis (chemical compaction, partial mechanical compaction, filling of fractures, and stylolites). Specific types of cement including dog tooth, syntaxial overgrowths, circumgranular, granular, radiaxial acicular and drusy mosaic calcite cements, indicate an early diagenetic stage of the marine phreatic zone. The Triassic-Jurassic carbonates have been affected by various diagenetic processes, which include cementation, compaction, dissolution, replacement and de-dolomitization, indicating primarily alteration by marine phreatic diagenesis, later burial diagenetic processes and mechanical breakdown of the rocks subsequently controlled by tectonics. Sea level changes, punctuated by tectonics and the paleogeographic evolution of the Indian Plate, controlled these depositional and diagenetic changes. [ABSTRACT FROM AUTHOR]

The geochemical characteristics and origin of Lower Jurassic dolomites from the Sarki and Sehkaniyan formations in the Imbricated Zone of Northern Iraq were investigated through analysis of major, trace, and rare earth elements, as well as stable isotopes (carbon and oxygen). The consistent CaO and MgO contents, their positive correlation, and elevated Mg/Ca values suggest an early penecontemporaneous sedimentary origin for dolomitization. These dolomites exhibit low Fe, Mn, and Sr contents, along with moderately low Na levels. Furthermore, they display low total rare earth element (ΣREE) contents, indicating low formation temperatures and a lack of light rare earth element (LREE) enrichment, while relatively negative δ18O values dismiss the influence of hydrothermal fluids on dolomitization. The patterns of rare earth elements normalized to the Post-Archean Australian Shale (PAAS) standard reveal a depletion of LREE compared to heavy rare earth elements (HREE), along with distinct negative Ce and positive Eu anomalies in most dolomites. The δ13C and δ18O values of the dolomites exhibit lower values than the contemporaneous seawater of the Lower Jurassic, which suggest the role of organic matter degradation and influence of meteoric water, respectively. Based on the petrographic and geochemical characteristics, the dolomite of the Lower Jurassic formations was mainly formed in the near-surface penecontemporaneous stage with late diagenetic recrystallization especially for Sehkaniyan dolomite. [ABSTRACT FROM AUTHOR]

Carbonate rocks are the most important sedimentary rocks since they hold almost 60% of the world’s oil reserves. Therefore, it is very important to evaluate the factors that control the formation and deposition of these rocks. The current study aimed to better understand the depositional facies and sequence stratigraphy of Eocene Chorgali Formation from Barian and Nathia Gali Sections of Hazara Basin, NW Himalayas, Pakistan. The field observation shows that the formation comprises of grey to pale grey compacted limestone, argillaceous limestone, and greenish grey shale. From microscopic observations, eight and nine microfacies were constructed form both Barian and Nathia Gali section respectively. The formation is also rich from benthic foraminifera. The constructed microfacies and foraminiferal assemblage shows that the carbonates of Chorgali Formation were most likely deposited in the inner ramp to proximal middle ramp setting. Frequent variations in the microfacies reveals that the Eocene Chorgali Formation was deposited in a 3rd order cycle representing several second order system tracts. Transgressive Systems Tracts (TSTs) and High Stand System Tracts (HSTs) based on sea level variations derived from microfacies in the vertical section. The sea-level curve of the present study deviates from the established global scheme, indicating the influence of intense local tectonics. The carbonate unit of the Chorgali Formation also represents various diagenetic alteration like micritization, bioturbation, dissolution, cementation, compaction and dolomitization. Overall, processes like fracturing, dissolution, and dolomitization enhanced the reservoir quality while cementation infills the available pore spaces and significantly reducing the reservoir quality. [ABSTRACT FROM AUTHOR]

In the western boundary region of Central Iran with Sanandaj-Sirjan, considering the widespread carbonate units in the northeastern Isfahan area, the barite deposits have considerable potential. The Komsheche barite, hosted in the Middle Triassic Shotori dolomite Formation, is the most active mine in the region. Barite mineralization has occurred in two styles, banded-layered and veined-brecciated. Fluid inclusions in the bedded barite homogenize at temperatures from 78 to 122 ºC, with salinity between 10.9 to 17.0 wt.% eq. NaCl, while the homogenization temperature for two-phase inclusions of the second type barite is between 130 to 187 ºC and associated halite-bearing tri-phase inclusions range from 192 to 210 ºC, with an average salinity of 14.3 and 36.6 wt.% eq. NaCl, respectively. The δ34S values of the samples range from 18.40 to 26.34 per mil CDT, and their δ18O values range from 8.9 to 14.7 per mil SMOW. Based on conducted studies, ore-forming fluids were formed in an open and near-bottom seawater system during the early diagenetic stage. The heavy isotope values related to pore water are locked in a closed system during the final stages of diagenetic. Sedimentary type, associated minerals, REE composition, finding of fluid inclusion and isotope reveal that Komsheche barite has the most similarity to diagenetic/cold-seep types of marine barites. Introduction In the western border of Central Iran zone with Sanandaj-Sirjan, according to the extent of carbonate units in the northeastern area of Isfahan, barite deposits have considerable potential. Part of the barytization is formed in the Middle Triassic host rock (such as Komsheche and Lamar) and part in the Lower Cretaceous host rock (such as Pinavand, Bagharabad, Maste-Kouh, Khase-Tarash and east of Mourcheh-Khort), which indicates that these sequences are prone to more discoveries in the future. These barites are unique in terms of sedimentary structure, and the Komsheche deposit is one of the representative examples of strata bound barite deposits in the Triassic sedimentary sequences of this area. Considering that the development of barite mineralization in the Triassic carbonate sequence is still in a halo of ambiguity, in this research, in addition to stratigraphic studies and investigation of the characteristics of mineralization, the mechanism of barite deposition has been tried using the mineralogical studies, the stable isotopes composition, and the fluid inclusions of barite are described. Materials and methods Field surveys were conducted to study ore layers by observing sedimentary characteristics and collecting samples from both host rock and ore. These samples were then analyzed in the laboratory through petrographic and mineralogical studies in the laboratory. Five samples were analysed by ICP-MS at the Zarazma laboratory to quantify major, minor, and rare earth elements. The study of fluid inclusions was conducted using the Linkham THM600 at University of Isfahan. The sulfur and oxygen isotopic composition in the Komsheche deposit was measured using a mass spectrometer. Values of δ34S and δ18O are expressed in parts per thousand or per mil (‰) using the Standard Canon Diablo Troilite (CDT) and Standard Sea Water (SMOW), respectively. Results and discussion The oldest rocks in the Komsheche mining area are assigned to the weathered orange-brown Shotori deposits of the Middle Triassic. These deposits are widespread and have a rough appearance. The lower layer, related to the Sorkh-Shale Formation (Lower Triassic), is not visible in Komsheche. The Nayband Formation, from the Upper Triassic age, consists of coaly shales and dark siltstone with interlayers of quartzitic sandstone. The Jurassic sequence is not visible in the Komsheche area, and no rocks containing Jurassic fossils have been observed in the red conglomerate of the Cretaceous base. The lack of sedimentation from the end of the Late Triassic to the Early Cretaceous may be due to the Cimmerian orogeny. The thin layers of sandstone and red conglomerate at the base of the Lower Cretaceous, with a thickness of up to 2 meters thick, consist of Shotori dolomite and siliceous rocks. This sequence begins without metamorphism but has a sudden change in lithology from clastic to carbonate. In the eastern parts, there is a thick layer of coarse-grained limestone, while the western parts are a sequence of olive-green marls, limestones, and sandy limestones of the Late Cretaceous age with orbitulina and ammonite fossils. Field studies, facies analysis, and laboratory research focusing on shape, mineralogy, texture, and grade indicate that the barite mineralization at the Komsheche mine can be categorized into two main types: banded and veined/brecciated. a) Type 1: The banded facies, which is widespread, contains lower-grade mineral material. It shows a stratification consistent with the host rock, with barite and host rock alternating over a thickness of less than 10 meters and a length of 700 meters. These facies, part of the Shotori dolomite sequence, is the primary mineralized horizon. The presence of interfinger structures of barite and dolomite crystals suggests simultaneous formation in unconsolidated sediments. b) Type II: The veined/brecciated facies, located near the center of the ore deposit, occurs as vein and shear masses formed along reverse faults. This facies represents the high-grade portion of the mine, and it is mined by through open-pit mining. These veins formations are commonly found at the boundary between the Shotori carbonates and the Nayband Formation, with dolomitization and limited silicification in the host rock. Barite veins are typically less than 1.5 meters thick, are surrounded by shear zones less than 2 meters wide. A homogenization temperature (Th) was conducted on 17 primary fluid inclusions (F.I) trapped in barite minerals from both the stratabound and veined/brecciated facies of the Komsheche deposit. The results of the analyzed fluid inclusions are presented in Table 3. In the first type barite, most fluid inclusions are single-phase liquid (L) or two-phase liquid-rich (L>V). The homogenization temperature ranges from 78 to 122 ºC, and the salinity levels range from 10.9 to 17.0 wt.% eq. NaCl. For the second type of barite, the fluid inclusions are predominantly two-phase liquid-rich (L>V), single-phase liquid (L), or three-phase with a solid phase of halite (LVH). The homogenization temperature for the two-phase inclusions of the second type is higher, ranging from 130 to 187ºC, while the three-phase inclusions with halite range from 192 to 210 ºC. The salinity levels for the second type of two-phase inclusions range from 9.5 to 16.0 wt.% eq. NaCl, and for the three-phase inclusions, it ranges from 36.2 to 37.0 wt.% eq. NaCl. The sulfur isotope values (δ34S) of the barite samples range from 18.40 to 26.34 ‰ CDT, while their oxygen isotope values (δ18O) range from 9.8 to 14.7 ‰ SMOW. The sulfur isotope values indicate isotopic diversity. A comparison with previous data (22.6 to 26.7 ‰) reported by (Forghani Tehrani, 2003; Rajabzadeh, 2007) shows differences in the distribution of measured sulfur isotope values, as shown in Table 4. According to the latest diagenetic models, sedimentary barite layers are formed at the sulfate-methane transition zone (SMTZ) during hydrocarbon migration. Based on this study, it was found that barite mineralizing fluids in the initial diagenesis stage were found to have temperatures below 120 ºC and a salinity of about 13 wt.% eq. NaCl. These fluids form near the seafloor in a shallow sea open system where sulfate supply is associated with anaerobic oxidation of methane as a mechanism for the reduction of sulfate to H2S. Barite formation occurs in shallow marine environments through by downward sulfate diffusion and the transfer of hydrocarbons and barium from depth upwards. The periodic formation of barite layers is related to the stability of the SMTZ. In an open system, barite retains the oxygen and sulfur isotope values of the coeval seawater. Barite precipitation occurs by fluid cooling, fluid mixing, and/or water-rock interaction. In the final diagenesis stages, heavy isotope amounts are linked to formation water in closed systems with high water- rock ratios. Veined barite, which is younger, may forms at deeper levels along faults over an extended period, completely replacing the host rock entirely. It is unlikely that veined barite forms near the sea floor. Deep fluids are heating by the geothermal gradient. This research suggests that the barite in question is similar to marine barites found off the coast of Southern California in diagenetic/cold seep environments.

Abstract Identification of the sex of modern, fossil and archaeological animal remains offers many insights into their demography, mortality profiles and domestication pathways. However, due to many-factors, sex determination of osteological remains is often problematic. To overcome this, we have developed an innovative protocol to determine an animal’s sex from tooth enamel, by applying label-free quantification (LFQ) of two unique AmelY peptides ‘LRYPYP’ (AmelY;[M+2] $$^{2+}$$ 2 + 404.7212 m/z) and ‘LRYPYPSY’ (AmelY;[M+2] $$^{2+}$$ 2 + 529.7689 m/z) that are only present in the enamel of males. We applied this method to eight modern cattle (Bos taurus) of known sex, and correctly assigned them to sex. We then applied the same protocol to twelve archaeological Bos teeth from the Neolithic site of Beisamoun, Israel (8-th–7-th millennium BC) and determined the sex of the archaeological samples. Since teeth are usually better preserved than bones, this innovative protocol has potential to facilitate sex determination in ancient and modern bovine remains that currently cannot be sexed.