Diagenetic Alteration Research Articles

Using carbon and oxygen isotopes for global correlation in the Early Triassic Lower Mahil Formation, North Oman, despite potentially misleading diagenetic effect

The study probes into the carbon isotope curves of the Lower Triassic Mahil Formation in two sections situated in the Northern Oman Mountains. The primary objective of the research was to understand the relationship between facies and isotopes, involving the analysis of 174 micrite samples for oxygen and carbon isotopes. Strict measures were implemented during sample selection to mitigate potential influences such as cracks or weathering.The results unveiled diverse δ13C values within the same facies, indicating inconsistencies in carbon isotope trends. Intriguingly, despite notable diagenetic alterations, particularly in the Saiq Plateau section, the Lower Mahil Formation retained carbon isotope patterns with minimal diagenetic impact. However, a note of caution is warranted in the widespread application of these findings, as utilizing whole rock data for carbon isotope analysis may introduce uncertainties.Despite potential diagenetic effects, the carbon isotope records of the Lower Mahil Formation remain valuable for comprehending patterns associated with KS1, facilitating comparisons between local and regional carbon isotope signatures. This research contributes significantly to establishing correlations between outcrop and subsurface data, offering valuable insights into variations in Lower Triassic carbon isotopes.

Diagenetic patterns in Cretaceous belemnite rostra from Southern Tibet, China: Evaluating criteria to differentiate well preserved and altered fossils

Belemnites are valuable for reconstructing Jurassic and Cretaceous seawater temperatures, but their reliability hinges on the preservational quality of the fossil materials. Methods for assessing the preservational quality of fossils vary among studies. This research examined altered Cretaceous belemnites and seemingly well preserved Upper Jurassic belemnites from Tibet, aiming to evaluate established methods for differences between well preserved and altered fossil specimens. Results indicate that cathodoluminescence and the manganese content are particularly useful for detecting alterations, correlating strongly with the δ18O values. Elevated iron contents generally suggest alteration, though less reliable. Iron incorporation into the rostrum is not always connected to a change in δ18O value. Macroscopic features such as fractures, ventral invagination and apical line should be avoided in sampling. The colour of belemnite specimens can also serve as an indication of alteration, but is less reliable. Interestingly, the strontium content of belemnite rostra in our study negatively correlates with δ18O values, which contradicts most previous literature results. Finally, by disentangling patterns of diagenetic alteration and carefully correcting raw data for the effects of post-depositional alteration, it seems still possible to obtain useful carbon and oxygen isotopic data from fossil specimens that have suffered diagenetic alteration. Our conclusions are based on, and primarily apply to fossil materials from the Tibetan study area. However, fossils from neighbouring localities in India and Nepal show similar signatures, which shows that our observations are valuable for the broader Himalayan region and likely fossils that experienced similar orogenic processes elsewhere.

Diagenetic alterations induced by lamina-scale mass transfer and the impacts on shale oil reservoir formation in carbonate-rich shale of the Permian Lucaogou Formation, Jimusar Sag

Diagenesis plays a crucial role in shale oil reservoir formation. However, complex organic-inorganic interactions result in pore system formation and heterogeneity changes at the centimeter, or even micrometer scale, which controls the evolution of shale oil reservoirs. In this paper, an integrated approach including core observation, thin section observation, cathodoluminescence thin section observation, scanning electron microscope, μ-XRF analysis, electron microprobe analysis, AMICS analysis, LA-ICP-MS analysis, and isotope analysis was used to clarify the laminae combination carbonate-rich shale, diagenetic alterations in different types of shale laminae and the controls on shale oil reservoir formation. According to the lamina combination, the carbonate-rich shale in the Permian Lucaogou Formation can be divided into three types. They are shale consists of dolomitic lamina and terrigenous felsic lamina (Type A shale), shale consists of calcite-rich tufaceous lamina and terrigenous felsic lamina (Type B shale) and shale consists of calcareous lamina and tufaceous lamina (Type C shale). Dolomite cementation is the major diagenesis in terrigenous felsic lamina of Type A shale. The dolomite can be identified as the early stage forming from recrystallization of micrite dolomite, and the late stage precipitate from adjacent dolomitic lamina. In the terrigenous felsic lamina of Type B shale, however, the calcite cementation is the dominated diagenesis, which was sourced from adjacent calcite-rich tufaceous lamina. Also, some feldspar dissolutions occurred in terrigenous felsic lamina during organic matter evolution. For the Type C shale, the main diagenetic alteration is micrite calcite recrystallization in calcareous lamina. The dissolution and precipitation of carbonate minerals between different laminae of carbonate-rich shale reveal that the organic matter evolution significantly influence the diagenetic alterations of inorganic minerals. Particularly, the diagenetic mass transfer and redistribution at the micro-scale in a relatively diagenetic system were confirmed by organic-inorganic interactions in laminated shale. In this process, dissolution pores and recrystallization pores have provided considerable reservoir pores for shale oil.

Burial History and Hydrocarbon Potential of the Harudi and Fulra Limestone of Kachchh, Western India Constrained using Carbonate Clumped Isotope Thermometry

Abstract The clumped isotope paleothermometry is used to estimate minimum burial depth of the mid Eocene Harudi and Fulra Limestone Formations of Kachchh, western India. The Nummulites and other fossil foraminiferal genera were separated for conventional (δ18O, δ13C) and clumped isotopic (Δ47) measurements. The measurements were also made on whole rock (WR) to visualize the different phases of diagenetic alteration, expected depth of burial and capacity to generate hydrocarbon. The measured δ18O and δ13C values of WR samples vary between -3.09 and -2.21‰ and -0.78 and 0.96‰ respectively, with average values of -2.88‰ and 0.16‰. The δ18O and δ13C values of benthic foraminifera vary between -3.52 and -3.20‰ (Average: -3.35±0.013) and -1.04 and 0.81‰ (Average: 0.26±0.76) respectively. The measured Δ47 for the foraminifera translates to temperature of 35 and 49°C and between 26 and 48°C for WR indicating minor diagenetic alteration. The minimum estimates for the burial depth of the studied formations are up to 2 km which is based on the temperature difference between the ecological preference temperature of benthic foraminifera and the diagenetically altered foraminifera and δ13C of WR and foraminifera, indicating that rocks have potential to generate methane.

Microfacies and diagenetic alteration in a semi-deep to deep lacustrine shale: The Yanchang Formation in the Ordos Basin, China

The mineralogical development and diagenetic sequence of lacustrine shales in the Chang 7 Member of the Yanchang Formation in the Ordos Basin are detailed studied. A model of their depositional system and a diagenetic diagram are proposed in this study. Through detailed petrographic, mineralogical, and elemental analyses, four distinct shale types are identified: argillaceous shale, siliceous shale, calcareous shale, and carbonate, clay, and silt-bearing shale. The main diagenetic process in argillaceous shale is the transformation of illite to smectite, negatively impacting shale porosity. Siliceous shale undergoes carbonate cementation and quartz dissolution, contributing to increased porosity, particularly in mesopores. Calcareous shale experiences diagenesis characterised by carbonate formation and dissolution, with a prevalence of siderite. In carbonate, clay, and silt-bearing shale, the dissolution of K-feldspar contributes to illitization of kaolinite. Argillaceous shale, characterised by more clay minerals and lower mesopore volume, is identified as a potential hydrocarbon seal. Siliceous shale, with the highest pore volume and abundant inter-mineral pores, emerges as a promising shale oil reservoir. These findings contribute to a comprehensive understanding of shale properties, aiding in the prediction of shale oil exploration potential in the studied area.

Unravelling taphono-myths. First large-scale study of histotaphonomic changes and diagenesis in bone from modern surface depositions.

The use of diagenetic alterations in bone microstructure ('histotaphonomy') as indicators of funerary treatment in the past and for post-mortem interval calculations in forensic cases has received increasing attention in the last decade. Studies have used histological changes to conclude in-situ decomposition, mummification, infanticide and post-mortem interval. There has been very little attempt to experimentally validate the links between decomposition, depositional conditions, time-since-death and microscopic changes in human bone so that meaningful interpretations of archaeological and forensic observations can be made. Here, we address this problem experimentally using the largest sample of human remains from anatomical donors and the longest-term deposition framework to date. This study tests one key assumption of histotaphonomy; that putrefaction during the early stages of decay is reflected in bone microanatomy and composition. Seventeen human donors and six pigs were deposited on the surface in a known Australian environment and left to decompose between 463 and 1238 days. All remains underwent all stages of decomposition reaching skeletonisation. Rib and femur samples were analysed using conventional histological methods and scanning electron microscopy, by applying the Oxford Histological Index, and examining collagen birefringence, microcracking and re- and de mineralisation. Biomolecular changes of the femoral samples were analysed using Fourier-transform infrared (FTIR) spectroscopy. The results indicate that bioerosion in human bone does not occur due to putrefaction. There were no correlations between bone histology and the following variables: human vs pigs, season, primary vs secondary deposition, position, fresh vs frozen and time-since-deposition. Furthermore, no trends were observed between biomolecular changes and time-since-deposition. The study also shows that pigs cannot be used as substitutes for human remains for bone biodegradation research. This is the first, controlled, larger scale study of human remains providing a lack of support for a long-assumed relationship between putrefaction and bone histology bioerosion. Using bone degradation as an argument to prove putrefaction, in-situ decomposition and early taphonomic processes cannot be supported based on the experimental human data presented.

Calcareous nannofossil distribution in the upper Maastrichtian–lower Thanetian interval in the Izeh zone (Southwest Iran): biostratigraphic framework and stage boundary identification in the Eastern Tethys

The distribution of calcareous nannofossil species in the upper part ofGurpi Formation and lower part of Pabdeh Formation has been investigated in the Karta section (northwest of Izeh in Zagros Basin, Iran, Eastern Tethys) to provide a biostratigraphic framework from Maastrichtian to Thanetian interval. In this study, 31 genera and 52 species of calcareous nannofossils have been identified and based on the recognized bioevents and the resulting biozones, the studied interval spans from the top of the nannofossil zone UC19TP to the base of the NP7 zone and covers ~10.8Myr. In this study, the Gurpi Formation spans from the upper UC19TP to NP4 zones, while the lowest part of Pabdeh Formation covers NP5, NP6 and NP7 zones. The studied sequence is continuous and encompasses Cretaceous-Paleogene (K/Pg), Danian/Selandian and Selandian/Thanetian boundaries. The K/Pg boundary is characterized by nannofossil assemblage recognized worldwide, such as the occurrence of Biantholithus, Cruciplacolithus, Braarudosphaera, Praeprinsius, and calcareous blooms of Thoracosphaera spp. Adecrease in the abundance of nannofossils is observed in the upper part of the Maastrichtian to the lower part of the Danian; poor to moderate preservation of nannofossils and low diversified nannofossil assemblages cover most of the studied samples. Here, as a result of species poor to moderate preservation, we observe patterns of stepwise extinctions upward in the top of CC25 zone, that culminate with the last occurrence of eight species, just prior to the base of the UC20bTP. We interpret these patterns to be mainly the result of diagenetic alteration. Our dataset provides a record over a long-lasting interval of stepwise extinctions toward the K/Pg boundary that was likely caused by the diagenetically-enhanced Signor-Lipps effect.

New rock typing method for diagenetically modified carbonate reservoirs

The paper evaluates an efficiency of the existing rock typing methods for diagenetically modified carbonate reservoirs and proposes a new alternative rock typing index. Four existing rock typing techniques are applied to the target formation, subjected to considerable diagenetic alterations. Applied techniques do not provide sufficient results in terms of reliable correlation between porosity, permeability and irreducible water saturation, which is crucial for geological modelling. Therefore, a new rock typing index named KØS and calculated as a function of permeability (k), porosity (φ) and irreducible water saturation (Swir) is proposed for proper characterization of the carbonate formation. Contribution of depositional and diagenetic processes and associated microfeatures into parameters of the index is demonstrated by means of X-ray microCT and NMR experimental data. Comparative analysis of the proposed index with the existing ones shows that the KØS-derived rock types demonstrate the highest correlation coefficients between the key reservoir parameters. The defined rock types have distinguishable microstructures that confirm validity of the rock classification approach. All the entities of the KØS index are used for reserves calculations and commonly measured during routine core analysis: this enables its implementation at the most carbonate fields.

Magnesium isotopic compositions and origin of Neogene dolomites in Xisha Islands, South China Sea

A common consideration of dolomite-based records for paleoseawater geochemistry reconstruction is the process of dolomite precipitation, which is closely related to the source of relevant elements. “Island dolomite” has been proposed as having great potential for recording paleoseawater geochemical characteristics, however, dolomitization should be strictly constrained in advance. As a major element in dolomites, magnesium (Mg) and its isotopes are critically involved in dolomitization, which may serve as a tool for providing direct insights into this process. Here, we report the Mg isotopic compositions of reef dolomites from the well Xike−1 reef core in Xisha Islands, South China Sea. The geochemical and mineralogical characteristics suggest that these reef dolomites experienced limited diagenetic alteration, and retained primary geochemical characteristics after dolomitization was complete. These reef dolomites reached chemical equilibrium with contemporaneous seawater during dolomitization, which were buffered by coeval seawater. Our results suggest that the development of carbonate platform and the formation of reef dolomites in Xisha Islands were related to tectonic subsidence. These reef dolomites were deposited during periods of decreasing tectonic subsidence rate. Then, dolomitization model for these reef dolomites was established, which provides an effective basis for reconstructing the chemical characteristics of paleoseawater. The occurrence of near-invariant seawater δ26Mg value during the past ∼23 Myr was subsequently verified via cross-checks provided by different seawater δ26Mg archives in this and previous studies. Our results highlight the great potential of seawater-buffered dolomites with limited postdepositional diagenetic alteration for constructing paleoseawater geochemistry though the geological history.

Remobilization of HFSE, Y, and REE during Diagenetic Alteration of Heavy Minerals in Sandstones from the Chvalčov Site, Flysch Belt of the Outer Western Carpathians, Czech Republic

An in situ electron microprobe study of detrital minerals yielded important insights into the diagenetic history of the Cretaceous-to-Paleogene flysch sandstones from the Chvalčov site, Rača Unit, Flysch Belt of the Outer Western Carpathians. Detrital titanite and a Fe-Ti mineral (probably ilmenite) were almost completely altered to TiO2 minerals, which also newly crystallized in intergranular spaces of sandstone. Brookite, anatase, and, exceptionally, rutile were identified by Raman spectroscopy. Authigenic TiO2 phases show complex composition with occasionally elevated contents of Fe, Nb, Zr, V, Sc, Cr, Al, Y, and/or P, which were likely sourced from altered neighboring heavy minerals. In addition, rare authigenic LREE- and Y-enriched apatite rims were observed on detrital apatite. The remobilization of REE, Y, and HFSE was likely mediated by acidic early diagenetic fluids enriched in fluoride and sulfate anions. The superimposed formation of calcite cement was associated with the dissolution of detrital garnet, feldspars, and quartz. The compositions of detrital apatite and garnet (Alm60-82Prp4-30Sps0-24Grs0-19) are comparable with those from adjacent parts of the Flysch Belt. Detrital rutile is enriched in Nb, V, Cr, and Zr. Our study illustrates the intensity of diagenetic alteration of detrital minerals in flysch sandstones as well as the usefulness of in-situ electron-microprobe investigations for the recognition of processes influencing heavy minerals in diagenetically altered sediments.

Processes controlling rare earth element distribution in sedimentary apatite: Insights from spectroscopy, in situ geochemistry and O and Sr isotope composition

ABSTRACTIn phosphorites, the content and distribution of rare earth elements are linked to the environment of phosphogenesis. This paper focuses on the question of sources and processes controlling the rare earth element content of apatite from Belgian phosphorites formed during three major phosphogenic events in the Lower Palaeozoic, Late Cretaceous and Cenozoic. To constrain sources and processes, new data include petrological, mineralogical (including cathodoluminescence and Raman spectroscopy) and in situ trace element and Sr and O isotope analyses of apatite. Fluorapatite from Lower Palaeozoic P‐rich conglomerates has the greatest rare earth element enrichment. It is affected by metamorphism that led to deformation of apatite nodules and formation of garnet porphyroblast inclusions. The role of Fe‐oxyhydroxides in element scavenging is highlighted by some apatite nodules that maintain their primary middle rare earth element enrichment, while others are characterized by altered rare earth element patterns resulting from competition for these elements between co‐crystallizing minerals during deformation. A systematic shift towards lower δ18O and radiogenic Sr isotopic composition compared to contemporaneous seawater indicate interaction with 18O‐depleted meteoric fluids and a crustal component. By contrast, carbonate‐rich fluorapatite from the Late Cretaceous phosphatic chalk mostly keeps its primary trace element and isotopic signatures (close to seawater), although an external rare earth element addition is noted, as well as rare earth element redistribution induced by diagenetic alteration. Cenozoic carbonate fluorapatite nodules mostly present flat rare earth element patterns that are indicative of a detrital influence. Slight changes in rare earth element distribution are assigned to post‐depositional alteration, which also led to an increase in radiogenic Sr, with unchanged δ18O compared to seawater. The methodology followed here efficiently helps in deciphering the processes that modified the chemistry of apatite in the frame of major phosphogenic events.

Exploring uranium isotopes in shark teeth as a paleo-redox proxy

The uranium isotope composition (δ238U) of seawater is a powerful proxy for the extent of marine anoxia. For paleoredox reconstructions, carbonates are the most popular U isotope archive, but they have recently come under increased scrutiny as their δ238U values are subject to diagenetic alteration after deposition. Therefore, there is a need to investigate other archives that may record and preserve the original seawater δ238U signal. In this study, we explore whether shark teeth provide such an archive. Shark teeth enameloid consisting of crystalline fluorapatite is more resistant to post-depositional alteration and less sensitive to isotopic exchange than marine carbonates due to the lower solubility of the crystalline fluorapatite. Since U is readily incorporated into phosphate, shark teeth could incorporate and preserve the original δ238U signature of seawater.To assess whether U isotopes in shark teeth can record seawater signatures, we measured the U isotopes (both δ238U and δ234Usec) in 39 fossil shark teeth from various locations, including Banks Island (Arctic), the Gulf of Mexico (GOM), and Pisco Basin (Peru), and ranging in age from modern to Cretaceous. Our results show that U concentrations are negligible in modern shark teeth (<1 ppb) but elevated in fossil samples (up to several hundred ppm), indicating that U is incorporated into shark teeth postmortem during burial. The δ238U values range from −0.72 to +0.57 ‰, and the δ234U values from −162.1 to +969.7 ‰. The data indicate that (i) diagenetic overprinting of seawater U isotope ratios is common among shark teeth, and (ii) δ238U data are influenced by local depositional environments. Nonetheless, the U isotope variations observed in shark teeth are comparable to those seen in marine carbonates, indicating that the samples with less diagenetic alterations might offer useful insight into the past extent of ocean anoxia.

Enigmatic carbonate isotope values in shark teeth: Evidence for environmental and dietary controls

Shark teeth are abundant in the fossil record and integrate physiological information, ecological interactions, and paleo-oceanographic conditions in their chemistry. Fossil shark teeth are well suited for stable isotope analysis because their enameloid is resistant to diagenetic alteration due to its high chemical stability. Although often used in paleoecological studies of mammals, carbonate carbon isotope compositions (δ13CCO3) in shark enameloid have remained enigmatic. Here, we investigate multiple stable isotope systems (δ13Corg, δ13CCO3, δ18OCO3, δ18OPO4) within modern shark teeth to determine relationships between the different systems and build an interpretative framework for future studies of both modern and fossil sharks. There is a weaker than expected correlation between δ18OPO4 and δ18OCO3 values in modern shark teeth (r2 = 0.44), which contrasts with mammalian studies to date and suggests this metric is not an appropriate test for diagenetic alteration in fossil shark teeth. Organic carbon isotope composition (δ13Corg) measured from modern dental collagen ranges from −16.0‰ to −10.8‰. The enameloid δ13CCO3 values we measured are much higher than collagen, ranging from −6.0‰ to 10.3‰, and there is no direct relationship between δ13Corg and δ13CCO3 values in shark teeth. Instead, we found the fractionation (ε) between δ13Corg and δ13CCO3 values to correspond with δ18OCO3 values but not δ18OPO4 values. This could be due to the carbon source in shark enameloid being partitioned between dietary carbon and dissolved inorganic carbon (DIC) or physiological differences in the tooth formation process changing the fractionation of carbonate isotopes. We applied the fractionation factor from modern teeth to carbonate isotope compositions of fossil shark teeth to predict δ13Corg values. Although the carbon sources to shark enameloid carbonate needs further investigation, our results suggest that fossil shark teeth could provide insights into carbon cycling of ancient marine ecosystems.

Helium - Relationships to other reservoir gases and some implications for exploration: The New Mexico Example

Helium (He) is the second most abundant element in the universe after hydrogen but is relatively rare on earth. He occurs as two stable isotopes, 3He and 4He. 4He is the dominant isotope in crustal gases and is a radiogenic decay product of uranium and thorium mainly in granitic basement rocks. 3He is dominantly primordial and primarily originates from the earth’s mantle. 3He may also be formed by radiogenic decay of 6Li (Lithium) which may be found in argillaceous sediments deposited in evaporitic settings. Although He occurs in most natural gases, it almost always occurs in extremely low, subeconomic concentrations, less than 0.1%. It is rare in concentrations more than 1%. A very few small reservoirs have gases with more than 7% He. Other gases that constitute the dominant components of helium-bearing natural gases are nitrogen (N2), carbon dioxide (CO2), and methane (CH4). The highest He concentrations occur where the dominant gas is N2 but most He has historically been produced as a byproduct of gases that are hydrocarbons. Hydrocarbons are generated from petroleum source rocks. Their presence in a reservoir is dependent upon the presence of a mature source rock in the basin and a migration path between the source rock and the reservoir. Large accumulations of CO2 in the southwestern U.S. resulted from the degassing of rising Tertiary magmas and subsequent migration of the gases into crustal reservoirs. N2 appears to originate mostly from degassing of the mantle but may also be formed in some strata by the thermal maturation of kerogens or by diagenetic alteration of clays or organic compounds in red bed sequences. The presence of economic concentrations of He in reservoir gases is dependent not only on an adequate source of 4He generated from granitic basement rocks but also on accommodating flux rates of N2, CO2, and CH4. These gases differ in their origins, places of generation and rates of generation, migration and emplacement. While basement-derived 4He and N2 enter reservoirs at slow rates over long periods of geologic time, hydrocarbons and CO2 enter the reservoir over much shorter time periods and dilute the 4He and N2. Basement-derived gases may be characterized by differing N2:He ratios which may indicate greater rates of He production within the crust in some areas. Exploratory drilling for He on Chupadera Mesa in the late 1990’s and early 2000’s encountered He-rich gases in Lower Permian strata. Isotopic analyses suggest that 93% of Chupadera Mesa He originated from radiogenic decay in crustal rocks while 7% is derived from the mantle or with a possible contribution by evaporitic Permian shales. Marked differences in the CO2 concentrations in different strata indicate that some strata acted as carrier beds for magmatically-derived CO2 while strata with N2-rich and CO2-poor gases were isolated from CO2 sources.

A clumped isotope diagenetic framework for the Ediacaran dolomites: Insights to fabric‐specific geochemical variabilities

AbstractWhile marine dolomites formed under near surface conditions have been considered to be potentially reliable archives of past oceanic conditions, this interpretation comes with significant challenges because diagenetic alteration frequently produces diverse fabrics with large geochemical variability. It has been suggested that the Ediacaran dolomites in South China (Hamajing Member, Dengying Formation) recorded the oceanic conditions present at the time they formed, yet these dolomites are composed of five different fabrics (stromatolitic, micritic, oolitic, saddle dolomites and fibrous–radial dolomite cements) and show large variations in multiple geochemical isotope proxies (carbon, oxygen, clumped, magnesium and the sulphur of carbonate‐associated sulphate). This study establishes a paragenetic sequence for these dolomites by combining the clumped and the oxygen isotopic compositions, thereby assessing whether they are geochemically representative of the original seawater. Using this diagenetic framework, the micritic and stromatolitic dolomites show a closed‐system behaviour (low water–rock ratios; &lt;0.3) and are largely resistant to the hydrothermal alteration during late diagenesis. In contrast, the ooid and cement fabrics have been affected by the hydrothermal fluid precipitating saddle dolomite in the open‐system condition with the high stimulated water–rock ratios (&gt;1). Furthermore, in a closed‐system environment, the elevated δ24Mg and δ34S values in the stromatolitic dolomite reflect the isotopic Rayleigh fractionation that enriches the 26Mg and 34S through rock‐buffered recrystallization, coupled with microbial sulphate reduction. These results demonstrate that the complex signals in early marine dolomite should be carefully evaluated when used as a palaeoproxy.

An FTIR-based model for the diagenetic alteration of archaeological bones

Bones and teeth from archaeological records are direct evidence of past individuals and they represent valuable archives for palaeo-anthropological and palaeoenvironmental studies. However, pristine information may be obliterated by the diagenetic alteration of bone specimens. Thus, defining in detail their preservation state is fundamental to assess the potential of extracting information about the past life of the individual, as well as to investigate the palaeo-environment at the burial site. For this reason, we have selected a set of archaeological samples (petrous bones and tooth roots) of different origin and chronology that have experienced diverse environmental and burial conditions, from arid and semi-arid to temperate regions in a time span from prehistoric to fresh bones. Thus, the selected samples underwent a variety of diagenetic processes, resulting in different types and extent of alteration patterns. Here we have applied a minimally-invasive sampling strategy and an analytical protocol based on Fourier transform infrared spectroscopy (FTIR) in order to ensure the investigation of a statistically significant set of samples. We provide a method to describe and quantify the changes in the physico-chemical properties of both the organic and inorganic constituents of bones, by establishing a set of parameters calculated from the FTIR spectra and evaluating their sensitivity in describing the alteration types and extent in bone specimens. Through FTIR spectral analysis, we have defined the most suitable parameters to effectively describe the alteration degree of bone specimens. Notably, the results have shown that, despite the complexity of multifactorial diagenetic processes, their measurable effects on differently altered bones can be described by a common mechanism. This indicates a progressive loss of collagen which is paralleled to the recrystallization of bioapatite and the loss of structural carbonate, regardless of the origin and chronology of the specimens. Thus, here we propose an FTIR-based model for bone diagenesis of general validity, highlighting that distinct diagenetic processes affecting the archaeological bones have acted according to common alteration mechanisms.

Target-based sedimentary diagenesis simulation and three-dimensional diagenesis evolution modeling

Deposition and diagenesis are important factors affecting the quality of oil and gas reservoirs. Previously reported diagenesis studies mainly involve qualitative analysis, such as the determination of diagenetic events and diagenetic evolution sequences and modeling of two-dimensional diagenetic evolution, with less effort devoted to quantitative numerical simulation and three-dimensional diagenetic evolution models. This study proposes a new method for target-based diagenesis simulation by integrating the discrete element method, the quartet structure generation set method, and morphological algorithms. This new hybrid method can quantitatively simulate the effects of mineral grain deposition, compaction, cementation, dissolution, and replacement. Since specific minerals can undergo specific diagenetic alterations through mineral labeling, the sequence and path of diagenesis can be predefined in this method. Taking the sandstone reservoir of Lower Member 3 of the Shahejie Formation in Linnan Sag, Huimin Depression, Bohai Bay Basin, East China as an example, the new simulation method used the constructed diagenetic evolution sequence based on thin section identification and geochemical analysis to systematically simulate deposition and diagenesis in the study area and to construct a three-dimensional diagenetic evolution model. The predicted porosity, permeability, and pore size distribution of the simulated digital rocks are in good agreement with the experimental values, which validates the high accuracy of the simulation method. Based on these three-dimensional multi-mineral evolution models, the evolution trends of porosity and permeability during the sedimentation and diagenesis process were elucidated, revealing the variation in reservoir quality in the studied area and indicating reservoir sweet spots. The constructed 3D diagenesis evolution models are widely applicable and can be used not only for permeability simulation but also for the study of electrical, acoustic, and mechanical properties of reservoirs.

Depositional and sequence stratigraphic controls on diagenesis in the Upper Cambrian-Lower Ordovician Barik Formation, central Oman: Implications for prediction of reservoir porosity in a hybrid-energy delta system

This study aims to employ an integrated depositional and sequence stratigraphic approach to assess the control of diagenesis on reservoir porosity of a hybrid-energy delta system. The study focuses on examining the outcropped Upper Cambrian-Lower Ordovician Barik Formation representing a highstand system tract of a hybrid-energy delta in the Haushi-Huqf region of Central Oman, which is equivalent to the subsurface gas reservoirs within the Interior Oman Salt Basins. The petrographic, mineralogical, and geochemical assessments revealed that the Barik Formation primarily consists of moderately to very well-sorted coarse-grained siltstones to fine-grained sandstones, with a composition mainly comprisingfeldspathic- and subfeldspathic arenite and the reservoir porosity is controlled by various degrees of diagenetic processes. Eodiagenetic alterations comprise mechanically infiltrated clays and kaolinitization of silicate grains, including detrital feldspars, micas, and mud matrix. The mechanically infiltrated clays (e.g., smectite) were introduced into the sandstones by the percolation of muddy waters under the control of tidal pumps. Although these clays may affect reservoir porosity during eodiagenesis, they positively preserve reservoir porosity during deep burial (mesodiagenesis) by inhibiting cementation by feldspar and quartz overgrowths. Kaolitization of silicate detrital grains was facilitated by the influx of meteoric waters into the sandstones as the delta progrades during the highstand system tract. During progressive burial and lack of extensive eodiagenetic cement, compaction continued and reduced reservoir porosity. Illitization and chloritization of the mechanically infiltrated smectitic clays took place; however, they limited the cementation by quartz and feldspar overgrowths. The quartz and feldspar overgrowths on detrital grains with discontinued smectitic clay coats supported the detrital grains from further compaction and, thus, helped in reservoir porosity preservation. Moreover, extensive feldspar dissolution and kaolinitization has occured during mesodiagenesis and enhanced the reservoir porosity. Subordinate amounts of calcite, dolomite, and gypsum cement have a minor reduction in the reservoir porosity of the Barik Formation. Hence, it can be concluded that, on the one hand, lack of eodiagenetic cementation favored intense reservoir porosity reduction due to compaction. Nevertheless, on the other hand, the mechanically infiltrated clays during the eodiagenetic stage helped preserve the reservoir porosity by hindering the formation of quartz and feldspar overgrowths during the mesodiagenetic stage. This study may serve as an analog model for a similar hybrid-energy delta sandstone reservoirs to better understand the type, and spatial and temporal of diagenetic alterations and their role in controlling the reservoir quality in deeply buried sandstone reservoirs.

Lithological and diagenetic variation of mixed depositional units in the middle permian saline lacustrine deposition, Junggar Basin, NW China

Mixed depositional reservoirs are widely distributed in Junggar Basin, NW China. These reservoirs are featured by complex lithology, ultra-low permeability and extremely heterogeneous pore structures that markedly impact field development efficiency. This study illustrated the distribution of lithological combinations within a single sand bar in the Middle Permian Lucaogou Formation, Jimsar Sag, Junggar Basin and revealed the diagenetic control on the reservoir quality of a single sand bar based on thin sections, scanning electron microscopy (SEM), helium porosity and air permeability measurement, pressure-controlled mercury injection (PMI), rate-controlled mercury injection (RMI), X-ray computed tomography (CT), and collected and published data. The results show that the heterogeneity of microscopic pore structures within a single sandbar is controlled by the distribution of various lithological combinations and different levels of diagenetic alteration. The results show that there are mainly three types of lithology developed in a single sand bar. The reservoir quality of dolomitic siltstone reservoirs is the best, followed by silty dolomite reservoirs. The reservoirs quality of dolomicrite reservoirs is the worst. Three main lithological combinations can be identified within a single sand bar. Controlled by climate, lake level fluctuation and provenance, lithological combination A, characterized by blended mixing of dolomitic siltstone, silty dolomite and dolomicrite, is mainly developed in the middle of single sand bar. Lithological combination B, characterized by saltatory mixing of interbedded dolomitic siltstone and dolomicrite, and lithological combination A developed on the side of the sand bar near the shallow lake, while lithological combination C with blended mixing of interbedded dolomitic siltstone and silty dolomite developed on the side near provenance. Strong compaction is the main factor of the decrease of reservoir quality of sand bar reservoirs. Carbonate cementation promotes the densification of reservoirs. The irregular flaky clay minerals lead to the exponential decline of permeability, and dissolution is the main kind of diagenesis to improve reservoir quality. The dolomitic siltstone reservoirs in the middle of a single sand bar have the best reservoir quality because the overlying dolomicrite layers resist compaction, resulted in a certain amount of primary pores remained. Besides, the dolomitic siltstone reservoirs are far from the sand-mudstone interfaces, which leads to the low carbonate cement content. Furthermore, abundant dissolution pores in dolomitic siltstone reservoirs improve the reservoir quality. These research results are crucial to reservoir evaluation and development in similar mixed depositional tight reservoirs.

Carbon and oxygen isotope microanalysis of calcite in the Permian Kupferschiefer system, Saale subbasin, eastern Germany

The Kupferschiefer district in Central Europe contains some of the world's highest-grade sediment-hosted stratiform Cu (SSC) deposits. In the Saale subbasin (eastern Germany), high-grade sulfides formed via replacement of calcite cement in the continental sandstones of the uppermost Rotliegend (S1), the overlying organic-rich marine mudstones of the Kupferschiefer (T1) and the Zechstein Limestone (Ca1) units. The spatial distribution of the calcite cement, therefore, had a fundamental role to play in the Cu mineralizing system. In this study, we investigate the origin of the calcite cement (and crosscutting calcite veins) using detailed petrography (cathodoluminescence, CL; scanning electron microscopy, SEM), major element chemistry (electron probe microanalyzer, EPMA), and secondary ion mass spectrometry (SIMS) microanalyses of δ13C and δ18O values in drill core samples (n = 47) from the Saale subbasin. The calcite cement in the S1, T1 and Ca1 has a similar CL response and major element chemistry, suggestive of a common origin. Overlapping δ13C and δ18O values in calcite cement in samples from the S1 and T1 in the Sangerhausen and Wallendorf drill cores also suggest that the calcite cement was derived from fluids of similar composition. The low δ13C values of calcite cement in samples from the S1 (−13‰ to 4.3‰, VPDB) and T1 (−10‰ to 0.7‰) indicates carbonate alkalinity was sourced mainly from seawater-derived fluids and the oxidation of organic matter. The wide range of δ18O values in the calcite cement in the S1 (∼18‰ to 31‰, VSMOW) and T1 (∼ 22‰ to 31‰) samples suggest they are derived from pore fluids with a chemical composition influenced by early diagenetic alteration of detrital clasts, mainly dissolution of volcanic rock fragments, with minor contributions from the influx of meteoric waters and evaporated seawater. The negative δ13C values (down to −15‰) in calcite veins from the T1 and Ca1 indicate sources of carbonate alkalinity derived from organic matter degradation. Our data demonstrate that no isotopic hydrothermal alteration haloes can be inferred from the δ13C and δ18O values in calcite cement associated with the high-grade sulfide mineralization. The lack of systematic isotopic variability in the calcite cement likely indicates the mineralizing fluid flux or temperature was not sufficient to overprint the background sources of isotopic variability, which may help to explain the modest size of SSC deposits in this part of the Kupferschiefer district.