Diagenetic Products Research Articles

Mineral formation and element migration in fractures and pores of hydrogenetic ferromanganese nodules

Ferromanganese (FeMn) nodules composed of nano-sized FeMn (oxyhydr)oxides are significant mineral resources rich in multiple critical metals (e.g., Co, Ni, Cu, and REY). The ubiquitous high porosity, large pore size, and pore connectivity in hydrogenetic nodules enable the constant influx of seawater and/or porewater driving widespread diagenetic processes (i.e., mineral dissolution and recrystallization) in the abundant fractures and pores. These diagenetic processes are critical to understanding the migration, partitioning, and distribution of the critical metals, but remain unclear yet. In this study, high-resolution in-situ analyses were conducted to investigate the mineralogical and geochemical characteristics of diagenetic precipitates in fractures and pores of a hydrogenetic FeMn nodule (MP5D33) from the central Pacific. Both oxic and suboxic processes were found in the fracture. The oxidized diagenetic products exhibit geochemical and mineralogical characteristics similar to thoese of hydrogenetic precipitates (e.g., vernadite), while the suboxic diagenetic processes primarily produced large-size 10 Å phyllomanganate. The diagenetic conditions within this studied fracture gradually shifted from oxic to suboxic, causing changes in the contents of multiple critical metals. The minerals in pores are mainly composed of 10 Å phyllomanganate that crystallized forms through heterogeneous nucleation on small-size vernadite in a suboxic environment. Additionally, minor todorokite likely transformed from 10 Å phyllomanganate undergoing long-term diagenesis. These findings shed some insights into the diagenetic process in the interior fractures and/or pores of FeMn nodules and enhance the understanding of the migration of critical elements associated with FeMn (oxyhydr)oxides during diagenesis in FeMn deposits.

Widespread dolomite recrystallization and porosity modification of upper Permian Zechstein carbonates, Symra discovery, Utsira High, Norwegian North Sea

The Utsira High is a prominent intrabasinal basement structure in which the eroded remnants of a widespread, Late Permian, Zechstein carbonate shelf are preserved locally. The western margin of the central Utsira High is mainly characterized by weathered basement rocks with a thin Mesozoic cover. However, the recently discovered Symra Field forms an isolated sedimentary inlier basin of the area, where deeply eroded Zechstein shelf carbonates are preserved within a half-graben. The Zechstein carbonates consist of mainly ZS2 marginal marine carbonate facies and are similar to those described elsewhere from the Zechstein Basin. They are unconformably overlain by Paleogene chalks. Based on detailed facies analysis of two recently drilled cores combined with detailed petrographic and stable isotopic analysis and supplemented by age dating of selected carbonate phases using U-Pb geochronology, we show that the Zechstein carbonates have been subjected to several phases of near-surface diagenetic alterations. Volumetrically the most dominant diagenetic product comprises nonplanar dolomites interpreted to have formed by recrystallization of a precursor reflux-type dolomite phase. The recrystallized dolomites retain enhanced reservoir quality in comparison to stratigraphic equivalent ZS2 reflux-type dolomites found elsewhere on the Utsira High. Based on U-Pb-derived age constraints, the recrystallization took place during a long-lived Late Triassic exposure event of the Utsira High, and a near-surface origin for the recrystallized dolomites is proposed. The porosity enhancement occurred contemporaneously with the dolomite recrystallization process and was facies controlled. Reservoir modifications associated with the exposure led to the dissolution of CaSO4 cement, and metastable dolomite phases, in addition to the enlargement of existing pores which had created zones of weakness prone to further dissolution. Overall, the Zechstein carbonates preserved on the Utsira High illustrate the complexity of the diagenetic alterations resulting from the uplift of a carbonate shelf, and its importance for reservoir quality.

Source and U-Pb Chronology of Diagenetic Fluids in the Permian Maokou Formation Dolomite Reservoir, Eastern Sichuan Basin, China

The hydrothermal dolomitization, facilitated by basement fault activities, had an important impact on the Permian Maokou Formation dolomite in the Sichuan Basin, which experienced complex diagenesis and presented strong reservoir heterogeneity. The source and age of diagenetic fluids in this succession remain controversial. In this study, various analyses were implemented on samples collected from outcrops and wells near the No. 15 fault in the eastern Sichuan Basin to reconstruct the multi-stage fluid activity and analyze the impact on reservoir development, including petrology, micro-domain isotopes, rare earth elements, homogenization temperature of fluid inclusions, and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb dating. The homogenization temperature of primary brine inclusions in fine-grained matrix dolomite and saddle dolomite is concentrated between 100 and 150 °C, which indicates that the impacts of abnormally high temperatures of other geological bodies. The δ13C and δ18O value and low 87Sr/86Sr value indicate that the diagenetic fluid of fine-grained matrix dolomite is mainly Permian seawater. The U-Pb ages of fine-grained matrix dolomite are ~260 Ma, which coincides with the age of the main magmatism of Emeishan Large Igneous Province (ELIP), and hydrothermal fluid provided a favorable high-temperature environment in the penecontemporaneous stage. While highly radiogenic 87Sr/86Sr compositions suggests those of saddle dolomite, the high-temperature Sr-rich formation fluid. The U-Pb ages of saddle dolomite are 245–250 Ma, which coincides with the age of the 255~251 Ma magmatism of ELIP. This indicates that those should be the diagenetic products of the ELIP hydrothermal fluid in the shallow burial stage. The U-Pb age of coarse-grained calcite is 190–220 Ma, and it should be the diagenetic product of the deep burial stage. Brine inclusions associated with primary methane inclusions were developed in coarse-grained calcite, with a homogenization temperature range of 140.8–199.8 °C, which indicates that the formation fluid activities were related to hydrocarbon charging. The Permian Maokou Formation dolomite was firstly formed in the penecontemporaneous shallow burial stage, and then it was subjected to further hydrothermal dolomitization due to the basement faulting and the abnormally high heat flow during the active period of ELIP. Hydrothermal dolomitization contributed to the formation and maintenance of intercrystalline and dissolution pores, whereas it also formed saddle dolomite to fill the pores, and reduce the pore space. The influence of deep fluid activities on reservoir evolution is further distinguished.

Effects of basin tectonic evolution on multi-phase dolomitization: Insights from the Middle Permian Qixia Formation of the NW Sichuan Basin, SW China

The platform-marginal shoal carbonates of the Middle Permian Qixia Formation in the NW Sichuan Basin, as important ultra-deep (>5000 m) hydrocarbon reservoirs, experienced multi-phase dolomitization and recrystallization during the long-term and complex basin tectonic evolution; however, there is no consensus on the dolomitization mechanism. Four types of dolomites have been identified: medium- and coarsely-crystalline dolomites within patchy dolomitized limestone (LD) at the top of the carbonate strata, massive medium to coarsely-crystalline dolomites categorized as porous dolomites (MD1) and tight dolomites (MD2) at the bottom of the carbonate strata, and cement coarsely saddle dolomites (SD) in the vugs and fractures. All dolomites exhibit similar REESN patterns to the host limestone with an obviously positively Ce anomaly, and show Sr isotopes falling in the range of Permian seawater. However, the LD dolomites display a higher Sr but lower Mn concentration, and more positive δ13C and δ18O than other types of dolomites. The petrographic and geochemical results suggest the dolomites of the Qixia Formation replaced precursor grainstone by the seawater-derived fluids. A comprehensive dolomitization model is proposed, incorporating seawater-reflux, intermediate-burial and tectonic-squeegee, and hydrothermal dolomitization for forming different types of dolomites. LD dolomites are early diagenetic products after selective dissolution under a near-surface or shallow burial environment, in which the weak-evaporated Permian seawater mixed with minor meteoric water accounting for dolomitizing fluids. MD1 and MD2 dolomites are likely to form due to the residually buried seawater for widely dolomitization driven by the Late Triassic tectonic compression. Subsequently, the dolomitic strata underwent hydrothermal modification due to rejuvenation of the thrust belt to enable saddle dolomites filling pre-forming porosity. This study integrates outcrop, petrographic, geochemical and dating analysis with paleogeography and tectonic events, providing a new perspective to establish a conceptual model for multi-phase dolomitization as an analog in comparative tectonic settings worldwide.

Paleolimnological Investigations in Coastal Sarıkum Lagoon, Sinop, Turkey

Lagoons are dynamic systems, making ecosystem management difficult. Paleolimnological approach is the only way to track past long term environmental changes and background conditions that are essential to build environmental management plans. Here, we investigated a sediment record from Sarıkum Lagoon, on the Black Sea coastal plain in North Anatolia, through a multiproxy paleolimnological approach to reveal long term environmental change and background conditions in the lake. For which, dry weight, organic and carbonate carbon, chlorophyll a (including its main diagenetic products), magnetic susceptibility and ostracod analysis were undertaken. The core was dated via radionuclide technique. At around 30 cm of the sediment record, there are abrupt transitions in all proxies, indicating a massive sediment input, a transition from transitional water to lacustrine environment, and lake’s biota also changed. Biological proxies preserved in the sediments suggest that the latest zone, after this event, is represented by lacustrine and hydrologically more isolated conditions from the Black Sea. Whatever the reason of that event (e.g. earthquake, flood), Sarıkum Lagoon has had lacustrine conditions during the last ~ 2 and half centuries following that abrupt event. This long term environmental change information should be considered while making future ecosystem management and conservation plans for the lake. (e.g. keep the lake connected with the Black Sea).

Diagenetic products, settings and evolution of the pre-salt succession in the Northern Campos Basin, Brazil

Abstract Despite all the effort made towards an understanding of the sedimentary, tectonic and diagenetic evolution of the pre-salt sequence and the Pre-Salt reservoirs of the Northern Campos Basin (East Brazil), two knowledge gaps have yet to be filled: (1) a detailed study of diagenesis in the crystalline basement and rift phases; and (2) the timing of diagenetic events that affected the pre-salt succession. In this study, samples from these geologic units were analysed for mineral composition and paragenetic evolution, fluid temperature and salinity, stable isotope compositions and laser ablation inductively coupled plasma mass spectrometer derived U–Pb ages of carbonate phases. The U–Pb ages of replacive and vein-filling cements reveal three tectono-diagenetic events, named the Barremian–Aptian (BADE, 125–117 Ma), the Albo–Cenomanian (ACDE, 103–98 Ma) and the Campanian–Maastrichtian (CMDE, 83–70 Ma). Each phase is characterized by distinct minerals, precipitation temperatures and burial conditions. The hydrothermal qualifier, identified by the temperature contrast between fluid and host rock, was initially high during BADE, then diminished over time (through ACDE) until it achieved equilibrium with the host rocks during CMDE. Diagenetic events are not coeval with magmatism but do coincide with known regional tectonic events described in the literature and are interpreted to be the result of increasing intraplate stresses. Multidisciplinary studies that include diagenetic events constrained by geochronological data will certainly lead to more robust conceptual geologic models, and, therefore, to a more reliable management of resources and strategies such as enhanced oil recovery, carbon capture, utilization and storage, and drinking water.

Diagenetic development of rhizoliths in the Ironshore Formation (Pleistocene) of the Cayman Islands

Rhizoliths in the Ironshore Formation on the Cayman Islands, up to 12 cm in diameter and 1 m long, developed in skeletal grainstones that accumulated on sand flats above high tide level. Internally, these well-preserved rhizoliths are characterized by well-defined concentric zones, centered around the central root opening, that reflect radial variations in calcite–aragonite content, decreases in micrite content, increases in porosity, and radial variations in trace element (e.g., Ca, Ni, S, Si, and Sr) concentrations. The stable isotopes, δ13C and δ18O, which vary in accord with the internal zones are more variable than the δ13C and δ18O of the host sediments. For the rhizoliths there is a high correlation between the δ13C and δ18O values and the calcite content. The contrast between the rhizoliths and their host sediments is largely a reflection of the more intense diagenetic processes that were largely mediated by the microbial populations that were concentrated in the rhizosphere that developed around each plant root. Microbial activity, which mediated micritization of the skeletal sands and micrite precipitation in the intergranular pores, was largely responsible for development of the internal zonation in the rhizoliths. Use of the δ13C values to determine if the rhizoliths developed in association with C3 or C4 vegetation failed because the δ13C values correlate with the calcite content that developed largely as diagenetic products.

Diagenetic Products, Settings and Evolution of the Presalt Succession in the Northern Campos Basin, Brazil

Abstract Despite all the effort made towards an understanding of the sedimentary, tectonic, and diagenetic evolution of the presalt sequence and the Pre-Salt reservoirs of the North Campos Basin (East Brazil), two knowledge gaps have yet to be filled: 1) a detailed study of diagenesis in the crystalline basement and rift phases, and 2) the timing of diagenetic events that affected the presalt succession. In this study, samples from these geologic units were analysed for mineral composition and paragenetic evolution, fluid temperature and salinity, stable isotope compositions, and LA-ICP-MS derived U-Pb ages of carbonate phases. The U-Pb ages of replacive and vein-filling cements reveal three tectono-diagenetic events, named Barremian-Aptian (BADE, 125-117 Ma), Albo-Cenomanian (ACDE, 103-98 Ma), and Campanian-Maastrichtian (CMDE, 83-70 Ma). Each phase is characterised by distinct minerals, precipitation temperatures, and burial conditions. The hydrothermal qualifier, identified by the temperature contrast between fluid and host rock, was initially high during BADE, then diminished over time (through ACDE) until it achieved equilibrium with the host rocks during CMDE. Diagenetic events are not coeval with magmatism but do coincide with known regional tectonic events described in the literature and are interpreted to be the result of increasing intraplate stresses. Multidisciplinary studies that include diagenetic events constrained by geochronological data will certainly lead to more robust conceptual geologic models, and therefore, to a more reliable management of resources and strategies such as Enhance Oil Recovery (EOR), Carbon Capture, Utilisation and Storage (CCUS), and drinking water. Supplementary material at https://doi.org/10.6084/m9.figshare.c.7103900

Evolution processes and diagenetic response of paleo-pore fluids in continental rift basin: A case study of Bonan Sag

Evolution and transport processes of pore fluids are accompanying with the burial process of sedimentary basins. Those processes lead to redistribution of materials and energies, and control the distribution of favorable reservoirs in sedimentary basins too. For petroliferous basins, confirming the evolution processes of pore fluids is useful for favorable reservoirs prediction. In this study, formation waters chemistry characteristics, skeleton particle compositions, and diagenetic products of sandstones were all take into consideration. It is to invert the evolution processes of pore fluids in continental rift basin. This study shows the changes of formation waters, diagenetic products and skeleton particle compositions are regularly with depth. The evolution processes of pore fluids are mainly controlled by basin sinking and uplifting, fluids concentration, mudstones compaction, organic matter evolution, diagenesis. The evolution of pore fluids in Bonan Sag have experienced seven stage: firstly, primary lake water; and then alkaline fluids from mudstone, which is caused by compaction; and then acidic fluids with organic acids; and then the alkaline fluids, which is caused by basin uplifting and water-rock interactions; and then acidic fluids with CO2, which is from cracking of organic matters; and then acidic fluids with CO2/H2S which caused by TSR; at last, alkaline fluids caused by deep buried and water-rock interactions. Changes of pore fluids (physical or chemical) lead parts of minerals in sandstones be unstable, and then dissolved by pore fluids. At the same time, part of dissolved particles in pore fluids reacted with each other, and then precipitated as diagenetic products.

An investigation into the characteristics of reactive silicon pools of coastal marine sediments

Many studies use traditional alkaline leach digestions to quantify biogenic silica (bSiO2) in sediments. Studies from river-plume systems demonstrate this methodology precludes information on other operational Si reactive pools, missing potentially valuable information on early diagenetic products. We build on previous studies by broadly examining the reactive Si pools in Mississippi River plume-influenced coastal sediments. The magnitude of Si released among operational pools across the region revealed large spatial variations in the conservation of detrital materials. Contrary to other analyses using different approaches, we suggest the availability of detrital sourced components (i.e. Al, Fe and Mn) may be a factor limiting authigenesis and resulting Si storage capacity in coastal sediments by this chemical mechanism. Overall, these data suggest authigenic precipitates may be forming as a function of bSiO2 accumulation, thereby implying that bSiO2 acts as the primary substrate for formation of authigenic products. The amount of authigenic Si products, relative to bSiO2, appears to be larger in this region than other systems studied to date, and this may be due to regional eutrophication which has led to an increase in sediment bSiO2 compounding the substrate available for authigenic precipitates. Thus, regions which have all experienced increased sediment bSiO2 accumulation coincident with eutrophication, may be primed for substantial authigenic product formation and increased Si sequestration.

Response to Comment on "Dying in the Sun: Direct evidence for elevated UV-B radiation at the end-Permian mass extinction".

Seddon and Zimmermann have raised questions about the evidence for increased UV-B flux across the end-Permian mass extinction (EPME) that was presented in our recent study, specifically regarding the measurement of UV-B-absorbing compound (UAC) levels in fossil pollen. We respond to these points, arguing that the comparison of FTIR spectra of >250 million-year-old Permian fossil pollen with ~700-year-old subfossil pollen is not valid and that negligible nonrandom interference derived from water vapor fluctuations during data generation cannot coincidentally produce a substantial UAC peak during the EPME. Furthermore, we refute the suggestion that the measured aromatic peak at 1600 cm-1 could have been influenced by diagenetic products from other organic constituents of pollen. The most productive route forward will be to generate sporomorph geochemical data from additional Permian-Triassic boundary sections to test the results put forward in our study.

Oxidation rates and redox stabilization of ferrous iron in trioctahedral smectites

Iron(II)-bearing trioctahedral smectites (saponites) form during anoxic alteration of basaltic rock. They are predicted to have been widespread on the early Earth and are observed in the oceanic subsurface today. Smectite structures, including the occupancy of sites in the octahedral sheet, affect iron redox behavior but the rates and products of trioctahedral smectite oxidation have been largely unexplored to date. In this study we synthesized two Fe(II)-bearing trioctahedral smectites, one moderate (22 wt% Fe) and one high (27 wt% Fe) in iron content. We then examined the rate, extent, and products of their oxidation by dissolved oxygen, nitrite, and hydrogen peroxide. Dissolved oxygen caused partial oxidation of Fe(II) in the smectites with 14 to 43% of Fe(II) unoxidized after 20 to 30 days of exposure. The rate and extent of oxidation correlated with the dissolved oxygen concentration and the Fe(II) content of the clay. The incomplete oxidation in these experiments is consistent with the mixed-valent trioctahedral smectites observed in oxidized natural samples but contrasts with the complete reoxidation by oxygen shown by chemically- or microbially-reduced dioctahedral smectites. Oxidation of structural Fe(II) by 5 mmol L−1 nitrite was negligible for the moderate-iron smectite and yielded only ∼17% oxidation after 54 days of reaction for the high-iron smectite. Hydrogen peroxide caused rapid and near-complete oxidation of both clays. Powder X-ray diffraction, variable-temperature Mössbauer spectroscopy, and extended X-ray absorption fine structure spectroscopy together detected no crystalline or short-range-ordered secondary phases and show that oxidized iron remained in the trioctahedral smectite structure. The recalcitrant Fe(II) pool in oxidized trioctahedral smectites exists in less distorted sites than Fe(II) in the initial clays. Its unreactive nature at prolonged reaction times indicates an elevated redox potential generated by the local coordination environment. Slower oxidation rates create a larger recalcitrant Fe(II) pool, suggesting kinetic competition between oxidation and a process involved in redox stabilization, such as electron exchange between octahedral iron sites or deprotonation of hydroxyl groups in the structure. The resistance to complete oxidation of trioctahedral ferrous smectites and their full retention of iron demonstrates that transitions from anoxic to oxic conditions generate mixed-valence smectites rather than a mixture of new phases. Identifying the diagenetic products of mixed-valent trioctahedral smectites may provide an indicator in the rock record of past redox cycling. Substantial portions of structural Fe(II) in trioctahedral smectites display slow abiotic oxidation kinetics and represent potential electron donors for both microaerophilic iron oxidizing and nitrate-reducing, iron-oxidizing microorganisms in altered mafic rocks and related settings.

Coarsening of tufa textures through early diagenesis: Modern and fossil examples from Bonito region (Central-West, Brazil)

Diagenetic modifications in tufa are difficult to distinguish from primary processes as they occur soon after sedimentation. Active and inactive fossil, probably Pleistocene to mid-Holocene, tufas in the Bonito region (Brazilian Midwest) were investigated to characterize primary and early diagenetic changes. Textural observations, analyses of cathodoluminescence and point counting were performed to identify the main diagenetic processes in active and fossil tufa facies. Lithification, coarsening of the textures, and degradation of organic matter are the main diagenetic changes. Although in some cases microspar and some spar crystals are syn-sedimentary, most of them occur as early diagenetic products. Microbial influence, anisotropy of the porosity and dominant capillarity-controlled processes are all related to lithification. Micrite grading to microspar occurring in both modern and inactive tufa is interpreted as neomorphism. Inactive tufa presents cloudy spar crystals interpreted as neomorphic (pseudospar) and clear spar crystals formed as cements. High correlation coefficients and negative trends between contents of micrite-microspar (active tufa) and micrite-spar (inactive tufa) suggest neomorphism as the main diagenetic process, in agreement with textural observations in thin section. The lack of correlation between pores and micrite, microspar and spar in both tufas suggests that the main diagenetic processes are not pore-consuming, at the scale of the present work. Although the textural differences between active and inactive tufa from Bonito region are very subtle, all the diagenetic changes were limited to the time spanning the active depositional system.

Origin of breccia dolomites in the middle Devonian Guanwushan Formation, NW Sichuan Basin, China: Implications for ultra-deep carbonate reservoirs

Deep to ultra-deep carbonate rocks have become a target for hydrocarbon exploration. The formation mechanisms of these deep to ultra-deep reservoirs are very complicated, and no consensus has yet emerged. This paper reports an investigation into the Middle Devonian Guanwushan Formation in the NW Sichuan Basin, China, which is a typical ultra-deep dolomite reservoir. Drilling-core and thin-section observations identified breccia matrix dolomite (MD) and interbreccia-filling dolomite (FD), of which the FD is commonly associated with sphalerite and barite. Geochemical analyses, including C and O isotopes, trace and rare earth element (REE) contents, and fluid inclusions, were conducted to investigate the origins of the different types of dolomite. Results indicate that the MDs generally retain the signature of syn-depositional seawater (relatively positive δ18O, low Fe and Mn, left-leaning REE patterns, and negative Ce anomalies), with involvement of meteoric water (relatively high Al, Ti and ΣREE). In contrast, the FDs were the diagenetic product of deep hydrothermal fluids (depleted δ18O, high Fe and Mn, and enrichment in MREEs), and the homogenization temperature of fluid inclusions is much higher than the normal burial geothermal temperature. The breccia dolomite reservoirs of the Guanwushan Formation were deposited as reef-core micro-facies and formed during the depositional to early diagenetic stages, and were the result of subaerial seawater dolomitization and meteoric water karstification. However, hydrothermal fluids subsequently modified the reservoir by cementation and filling. The Guanwushan Formation is a typical case of an ultra-deep carbonate reservoir that formed during the depositional to early diagenetic stages but was filled at the burial stage. Therefore, the favorable reservoir zones associated with deposition and early diagenesis should be the targets of future exploration. The findings of this study are expected to be of general significance for deep and ultra-deep oil and gas exploration in other regions and countries.

Late Permian-Triassic sedimentary evolution of the Southern Adriatic area based on wells and cores analysis

Rock samples from two cores within the Triassic interval of the Puglia 1 well, were studied to reveal the sedimentary facies and diagenesis. The Core 1 (5.048–5.056 m deep) is characterized by laminated dolomudstones with thinly microbial laminae alternated to lenticular/tabular shaped anhydrite crystal levels, suggesting an intertidal/sabkha-type environment. The Core 2 (6.067–6.075 m deep), shows coarse-grained crystalline massive dolomite with remnants of oolites and shell fragments, suggesting a marginal shallow-water setting. Dolomitization is ubiquitous and occurred early after the deposition through the circulation of high saline dolomitizing fluids under general reducing conditions. The early-dolomitization preserved the dolomites from major burial diagenetic transformations, permitting only the ordering and the development of xenotopic textures in the dolomite crystals (aging). Primary anhydrite crystals were lately affected at least by one burial hydration-dehydration cycle, whereas fracture-filling anhydrite is a later-stage diagenetic product of circulating sulphate-rich fluids. Moreover, the integration of Puglia 1 well data with other six exploration wells and a seismic line allowed a possible reconstruction of the Permian-Triassic sedimentary evolution of the Southern Adriatic area. During Permian continental/coastal lagoon settings developed, while in the Ladinian, after a generalized subaerial exposure and an erosional phase, the extensional tectonic inputs linked to Tethys rifting brought to a relative sea-level rise and to the formation of carbonate shelves in NW-SE oriented tectonic depressions. During Carnian-early Norian a further extensional tectonic pulse coupled with a relative sea-level drop, led to the formation of NW-SE elongated intrashelf basin with a consequent enhancement of salinity and settlement of evaporative conditions. This triggered a massive evaporite deposition during the Norian, that filled the basinal areas. Lastly, in the late Norian-Rhaetian a marked relative sea-level rise, restored the connections with open-sea and shifted the evaporite deposition up to the shallowest parts of the basins that previously remained under subaerial conditions.

Pleistocene aragonite crust diagenesis mimics microbialite fabrics (Danakil Depression, Ethiopia)

Fibrous aragonite crusts occur in two consecutive Pleistocene successions in the Danakil Depression (Afar, Ethiopia). Lateral transitions between pristine and altered fibrous aragonite crusts document changes in texture associated with diagenesis. Crusts formed as essentially abiotic seafloor precipitates at the transition from marine to evaporitic conditions. Diagenesis started with the dissolution of aragonite fans at the interface between single fans in non-laminated crusts and along lamination planes in isopachous, irregular, or crudely laminated crusts. Incomplete dissolution resulted in the development of secondary porosity within a matrix of undissolved aragonite fibers. Subsequently, the porosity was filled with calcite that systematically encased remaining aragonite crystals. This was followed by the dissolution of remnant aragonite fibers, producing a network of elongated inter- and intracrystalline pores that were eventually filled with low-Mg calcite. The stepwise substitution of fibrous aragonite by low-Mg calcite resulted in sparry, sparry-cloudy, sparry-micritic (including clotted micrite), and peloidal textures, which obscure the fibrous nature of the original deposits. Stable C- and O-isotope compositions suggest that early diagenesis was driven by meteoric and evaporative fluids. These observations unequivocally demonstrate destructive diagenesis, resulting in secondary textures, which mimic micritic and grumous (peloidal and clotted) textures associated with sparry microfabrics.This suggests that these textures, classically interpreted as primary microbial precipitates and used as evidence of biogenicity in ancient microbialites, might be diagenetic products in some cases, even though at some stage, microbial processes and/or degradation of organic matter could have been involved in the diagenetic process.

Impacts of Depositional Facies and Diagenesis on Reservoir Quality: A Case Study from the Rudist‐bearing Sarvak Formation, Abadan Plain, SW Iran

AbstractAn integrated geological‐petrophysical analysis of the rudist‐bearing sequence of the Cretaceous Sarvak Formation is given one giant oilfield, and provides an improved understanding of this main reservoir in the Abadan Plain, in the Zagros Basin, SW Iran. The main objective of this study is to evaluate reservoir potential of the Sarvak Formation, and then to utilize the calibrated well log signature to correlate reservoir potential in un‐cored wells. Eight main facies are recognized and categorized in five facies groups: lagoon, shoal, rudist‐biostrome, slope, and shallow open marine, deposited on a shelf carbonate platform. Given the distribution of diagenetic products and their effects on pore systems, three diagenetic facies namely, (DF‐1) low dissolution and cementation; (DF‐2); high dissolution; and (DF‐3) high cementation are differentiated. The initial sedimentary characteristics in combination with distribution of diagenetic products play an important role in reservoir quality heterogeneity. The effect of diagenetic processes related to disconformities mainly depends on the facies nature below these surfaces. Grain‐dominated facies of shoal and rudist debris, observed below the Cenomanian–Turonian disconformity, are mostly characterized by high dissolution and interconnected pore systems. Finally, depositional and diagenetic facies in the studied wells are correlated by petrophysical well log data, leading to distribution of the reservoir zones. Data obtained can be utilized for efficient reservoir characterization of the Sarvak Formation and its equivalent units in the Arabian Plate.

Formation mechanism of thiadiamondoids: evidence from thermochemical sulfate reduction simulation experiments with model compounds

High concentrations of thiadiamondoids together with elevated H2S are indicative of the occurrence and extent of thermochemical sulfate reduction (TSR). However, the obscure formation mechanism and accumulation timing of thiadiamondoids restricts their application in TSR evaluation. In this study, TSR simulation experiments with MgSO4 and three model compounds, namely adamantane, 1-methyladamantane, and diamantane, were performed in gold tubes in high pressure reactors. The gaseous hydrocarbons and liquid products generated from the model compounds by TSR were identified and quantified. According to the H2S yields from the three model compounds, the susceptibility to TSR of three model compounds is 1-meththyladamantane > adamantane > diamantane. Four types of compound groups with diamondoid structure were detected in the liquid products: from high to low abundance these are diamondoidyl ketones, diamondoidyl alcohols, diamondoidthiols and thiadiamondoids, respectively. Thiadiamondoids can be generated from C0-alkylated diamondoid species by TSR. During the TSR process, the concentrations of both diamondoidyl alcohols and diamondoidthiols first increased within the maturity range 0.89–1.09 Easy%Ro and then decreased at maturity > 1.09 Easy%Ro. The rapid decrease in concentration of diamondoidthiols does not correspond to increasing thiadiamondoids, suggesting that diamondoidthiols may not be reaction intermediates for thiadiamondoids in laboratory experiment conditions. In the TSR simulation experiments, slowly increasing concentrations of diamondoidyl ketones correspond to slowly increasing concentrations of thiadiamondoids. For the synthesis mechanism of thiaadamantanes derived from thiaadaamantane-4,8-dione, we propose that the diamondoidyl ketones could be reaction intermediates in thiadiamondoid formation. The thiadiamondoids in petroleum could have two different formation mechanisms: (1) low-temperature cationic carbon ion rearrangement in diagenesis to early catagenesis stages, in which thiadiamondoids are derived from the diagenetic products of polycyclic thiols in the source rock; and (2) a free sulfur radical mechanism that occurs in the TSR process at high temperature in the middle catagenesis to metagenesis stages. During the TSR process, free sulfur radicals attack the diamondoidyl ketones generated in the first stage of TSR and open the cage structure. Then the carbonyl carbon atom is replaced by a sulfur atom, followed by cyclization to form thiadiamondoids. Based on the H2S yields in the TSR simulation experiments with diamondoid model compounds, most thiadiamondoids formed in the second stage of the autocatalyzed TSR process within the maturity range of 1.09–1.21 Easy%Ro.

Petrography and Clay Mineral Variations Across the Cretaceous/Paleogene (K/Pg) Boundary, Sulaimani, Kurdistan Region, Iraq

The Cretaceous/Paleogene (K/Pg) boundary marks the global extinction of many life forms, this boundary around Sulaimani city coincides with the boundary between Tanjero and Kolosh formations. These two units are flysch deposits of the Zagros Foreland Basin. This study focuses on petrography and clay mineralogy variations between these two units. Petrographic study, X-Ray Diffraction analysis, and Scanning Electron Microscope analysis were conducted on samples from different lithologies. The petrographic study of fine-grain lithologies shows that they are mainly wackestone without variation across the boundary. Sandstone petrography shows variation in framework components by increasing quartz, feldspar, and igneous fragments while argillaceous, carbonates, chert, and chalcedony fragments are decreased from Tanjero to Kolosh formations. Obtained results from XRD and SEM show that the clay mineral assemblage is dominated by corrensite (regularly interstratified chlorite/smectite) with discrete chlorite, smectite and traces of illite. The existence of these clay assemblage suggests that corrensite is the diagenetic product of smectite as it is an intermediate stage of smectite chloritization. Enrichment of precursor smectite and high content of unstable grains in sandstones across the K/Pg boundary is the result of low-intensity weathering in arid and seasonal climates of the source area. The smaller size of detrital smectite than other clay minerals is behind its enrichment in the deep marine basin for both Tanjero and Kolosh formations. Sandstone enrichment with sedimentary fragments in both formations related to uplifted sedimentary terrain in its hinterland, while variation across the boundary indicates gradual uplift of Zagros ophiolite zone in the early Paleogene.

Structural diagenesis and dolomitization of Cenozoic post-rift carbonates in the Red Sea rift basin: A multiproxy approach

Middle Miocene post-rift sediments are considered prolific subsurface reservoirs, representing one of the thickest sequences in the Red Sea rift basin. In the Umm Luj Basin of the eastern Red Sea, post-rift sediments are well exposed and represented by a carbonate-dominated system of the Raghama Formation. This formation was intensely fractured, following the rifting trend of the Red Sea. Such fractures and their associated diagenetic products could provide a significant archive of past tectonic fluid evolution. However, little is known about the origin and timing of the different fluid flows and diagenetic processes in the area. This study aims to resolve this issue by integrating fracture and multiproxy geochemical analyses of calcite precipitated in veins, fault breccia, and dolomitized host rock. The δ18O and δ13C isotopic compositions of calcite veins show tight clustering, varying between −10.6 and −9.5‰ and between −7.9 and −7.2‰, respectively. Meanwhile, the precipitated calcite along the fault breccia exhibited a closer to host rock isotopic composition (δ18O = −6.8‰; δ13C = −4.8‰). The δ18OVPDBof the dolomitized host rock shows a heavier average value, closer to the expected range of Miocene seawater. X-ray diffraction analysis shows that the veins have a high magnesium calcite content (up to 79.5%). In contrast, all the host rock samples, except samples 1-1BH, have dolomite contents of up to 94.3%, as well as breccia fragments. Thus, we argue that the structural diagenesis history of the study area comprises two distinct fluid members and tectonic events. The first member is the deposition of heavier isotopic composition related to dolomitization at slightly higher temperatures of up to 42.2°C. The second fluid flow member corresponded to a depleted isotopic calcite member with a temperature of 33°C. Compared with the Midyan Peninsula, the study area shares the same regional tectonic events, but the local tectonic and depositional settings could act as the determining factors of the dolomitization mechanism and meteoric alteration in each location. Hence, our results provide a new understanding of paleo-fluid circulation related to the evolution of tectonic events and highlights the value of integrating fracture and multiproxy geochemical analysis for structural diagenetic studies.