Diagenetic Imprint Research Articles

Chemical compaction features as potential barriers in the Permian-Triassic reservoirs of Southern Iran

Chemical compaction features are important diagenetic imprints that generally interrupt reservoir quality in most carbonate reservoirs. In the Permian-Triassic reservoirs of South Pars Field, they are recorded in various types and intensities. High- to low amplitude stylolites and solution seams are concentrated within the K1–K4 units of the upper Dalan and Kangan formations, depending on their facies characteristics, diagenetic alterations and dominant lithology. In these units, there are some meaningful trends between the chemical compaction zones and sequence stratigraphic positions including the systems tracts of third-order sequences. Control of chemical compaction on reservoir characteristics is evaluated by using the results of petrographic studies, petrophysical logs and core poroperm measurements. Internal reservoir architecture of the studied reservoirs is determined using the both hydraulic flow units and stratigraphic modified Lorenz plot approaches. Identified baffle and barrier units show close correspondence with chemically-compacted zones. This indicates that zones with high concentration of stylolites and solution seams can be considered as small-scale vertical flow barriers in the Permian-Triassic reservoirs of South Pars Field.

Integrated workflow for characterizing and modeling a mixed sedimentary system: The Ilerdian Alveolina Limestone Formation (Graus–Tremp Basin, Spain)

This paper proposes an advanced stochastic workflow to jointly model sedimentary facies and diagenesis. The formation of interest is the Early Eocene Alveolina Limestone Formation, which outcrops in the Serraduy area (Graus–Tremp Basin, NE Spain). Ten sedimentary lithotypes representing facies or facies associations of a mixed siliciclastic–carbonate ramp system were identified within the succession. A 3D model describing the depositional architecture is also proposed. The results from the diagenetic study evidenced the occurrence of several successive calcite cements, which were grouped into five diagenetic imprints for modeling. These imprints were then quantified to ease their integration into numerical models. The following step consisted in building a 3D gridded model with seven different modeling units. They were populated using a bi-plurigaussian simulation approach that reproduced both the sedimentary organization and the observed diagenetic imprint distributions. Last, the simulation results were validated referring to paleogeographic and diagenetic conceptual maps.

Assessing the reliability of foraminiferal Mg/Ca thermometry by comparing field-samples and culture experiments: a review

Trace elements incorporated into foraminiferal test carbonate are commonly used as palaeoproxies. For instance, benthic and planktonic Mg/Ca ratios are frequently used for reconstructing bottom and sea surface temperature (SST) changes, respectively. However, over the past few decades it has been shown that the incorporation of Mg 2+ into foraminiferal calcite is controlled by more than one environmental parameter, with significant variations of their sensitivities between culture and field-based studies. Opinions differ as to whether the laboratory conditions during culturing experiments or the natural conditions during field sampling (core-tops, sediment traps and plankton tows) better trace the wealth of information with improved accuracy. Laboratory culture experiments that isolate the effects of individual environmental parameters have been used to identify secondary controls on Mg uptake into planktonic foraminifer tests. However, field-based data (core-top sediments and plankton tows) from high salinity super-saturated settings have shown the additional presence of high-Mg inorganic precipitates leading to significant salinity (S) biases on the Mg/Ca palaeothermometer. Testing such synergistic effects between temperature, salinity and calcite saturation state would require an experimental design where all these parameters are varied systematically, but such experiments have yet to be conducted. Since the synergistic effects cannot presently be ruled out through culturing experiments, it is imperative to initially confirm the amplitude and geographic distribution of the detectable diagenetic precipitations (Scanning Electron Microscopy analysis) through field-based work, further quantify their importance (discrimination of distinct diagenetic stages and quantification of the diagenetic imprint) and finally estimate its potential effect on Mg/Ca-T calibration (e.g., overgrowth-corrected species-specific calibration equations). The example of the marginal high-salinity settings, among others, clearly highlights that the optimal use of Mg/Ca as a palaeotemperature proxy urgently requires the complementarity of both culture- and field-based data. To this end, we here present advantages and disadvantages to each approach. These insights reinforce the potential of the combined use of culture- and field-based foraminiferal studies, where possible, in order to minimize the observed inconsistencies, and to advance Mg/Ca thermometry by both providing a framework for better understanding the nature of Mg/Ca dependence on seawater temperature, and the effects of complicating factors.

Mid-latitude terrestrial climate of East Asia linked to global climate in the Late Cretaceous: COMMENT

Mid-latitude terrestrial climate of East Asia linked to global climate in the Late Cretaceous: COMMENT

Fish bone diagenesis in southeastern Brazilian shell mounds and its importance for paleoenvironmental studies

Brazilian shell mounds are relicts of prehistoric settlements characterized as distinct features on the sandy coastal plain, sandy bars of coastal lagoons and rocky island. They are composed of shells of edible marine mollusks, fish remains and wildlife bones intermixed with sand. In the coastal lagoon region of the Rio de Janeiro state these shell mounds range in age between 5100 and 1300 cal BP. In this study we analyzed the geochemistry of the bones of the whitemouth croaker that primarily offers insight on the nature of bone diagenesis and its use for radiochronology and as isotope paleo-thermometer. The x-ray diffractograms of bones from the Tarioba shell mound exhibit elevated calcite profiles in contrast to a reduced hydroxyapatite concentration. Two peaks of quartz and the presence of feldspar provide evidence of exogenous materials, e.g., sand or silt particles, infilling hollow spaces within the fish bones. The Beirada and the Ponte do Girau shell mounds are characterized by the highest peaks of apatite relative to calcite, which is clearly different from the result obtained for Tarioba. The Manitiba profile is similar to the Tarioba profile except for calcite peaks that are approximately three times lower than those for Tarioba. At a depth of 25 cm in the Tarioba shell mound, the redox sensitive index of diagenetic alteration reaches its maximum value, suggesting a zone of accumulation in the soil horizon redox condition. The δ18O derived temperatures profile based on mollusks showed a consistent range from 18.1 to 24.1 °C, indicating seasonal events of intense upwelling of the South Atlantic Central Water breaking the mean warm surface conditions of the Brazil Current. Diagenetic imprints of Tarioba shell mound suggest humid burial conditions with the loss of hydroxyapatite and the enrichment of exogenous calcite. Drier depositional environmental conditions are indicated in the Ponte do Girau and the Beirada shell mounds, while intermediary depositional environmental conditions characterize the Manitiba shell mound. In the period represented by the shell mound samples, the sea level exhibited significant oscillations and portions of the inner shelf were exposed, which may have resulted in an anomalous derived temperature records. These results are compatible with events of anomalous temperatures occurring in the sea level during the time span of occupation of these shell mounds.

Magnetic fingerprinting of hydrodynamic variations and channel erosion across the turbidity maximum zone of the Yangtze Estuary, China

Magnetic measurements were conducted on surface sediments and suspended particles collected across the turbidity maximum zone of the Yangtze Estuary, in order to examine the spatial distribution of magnetic mineral assemblages and the factors responsible for this distribution. The results indicate that magnetic properties are dominated by ferrimagnetic grains. Bulk magnetic susceptibility (χ) and saturation isothermal remanent magnetization (SIRM) values show positive correlations with the proportion of the >63μm fraction in the North Channel, while anhysteretic remanent magnetization (χARM) is significantly correlated with the proportion of the <16μm fraction in both surface sediments and suspended particles. Such a bimodal distribution of ferrimagnetic minerals in the sand and finer fractions is confirmed by particle size-specific measurements. Sediments in the North Channel have the highest χ and SIRM values but lowest χARM/SIRM ratios, which is consistent with the coarsest particle size due to strong hydrodynamics, i.e. currents. Within each channel, χ and SIRM values are higher in sediments from shallower water depth due to energetic conditions resulting from waves. Compared with surface sediments, suspended particles have lower χ and SIRM values but higher χARM/SIRM ratios due to lower sand fractions. The increasing trend of χARM/SIRM of suspended particles along the pathway of sediment transport indicates weakening hydrodynamics from the inner estuary to the outer estuary. Diagenesis is another factor influencing magnetic properties in addition to particle size. Channel erosion leads to local exposure of buried sediments on the channel bed. As a result of a stronger diagenetic imprint, they show magnetic properties different from the recently deposited sediments. Our results indicate that magnetic properties cannot only indicate spatial variations in hydrodynamics, but also provide insight into sediment erosion/deposition processes. Combined granulometric and magnetic methods can therefore be used to interpret hydrodynamics and track changes in estuary morphology.

Limits to the sensitivity of living benthic foraminifera to pore water carbon isotope anomalies in methane vent environments

Episodic 13C depletions in the carbon isotopic composition of benthic foraminiferal tests preserved in the stratigraphic record have been interpreted as an active incorporation of methane-derived carbon. Understanding the extent to which these isotope excursions reflect basin-wide fluxes of methane carbon to bottom waters versus a local supply of methane carbon within the sediments in which benthic foraminifera live, or a postmortem diagenetic imprint is critical to the interpretation of δ13C paleoceanographic proxies. Here we evaluate the impact of chemical gradients measured in pore waters adjacent to active methane vents on carbon assimilation by living benthic foraminifera and show that those living near methane vents do not assimilate the distinctly 13C depleted methane-derived dissolved inorganic carbon into their tests from the pore water in which they were found. Our observations can be explained by the recently articulated physiological limits imposed on deep-sea fauna by low-oxygen and high-pCO2 environments. Understanding the importance of the different processes involved in the observed disequilibrium between the carbon isotopic composition of the benthic forams and the pore waters where they were found has important implications on the reliability of carbon isotopic composition of benthic foraminifera for paleoceanographic reconstructions. In particular, the observation on the inhospitability of these environments for benthic foraminifera at least for reproduction and growth raises the issue on the overprint either in the late adult stages of foraminifera that grew in a different neighboring environment or during early diagenesis in these geochemically active environments.

Dinosaur eggshell isotope geochemistry as tools of palaeoenvironmental reconstruction for the upper Cretaceous from the Tremp Formation (Southern Pyrenees)

The isotopic compositions (δ13C and δ18O) of dinosaur eggshells have been widely used in palaeoenvironmental studies, although the geochemical signatures of eggshells are not usually contrasted with other proxies. In this work, the isotopic signatures of eggshells from a large Maastrichtian succession from the Tremp Formation (Southern Pyrenees, Spain) are compared to those of carbonate pedogenic nodules occurring in the same levels. The isotopic signatures of eggshells vary according to the stratigraphic unit and geographical location. A group of samples from several localities corresponding to eggshells without significant diagenetic imprints has isotopic values differing from the associated nodules; The Late Cretaceous isotopic composition record from the Tremp Fm. is consistent that is, the eggshells have distinct primary signatures preserved.However, the eggshells from another locality, which exhibit neomorphed textures, display isotopic signatures similar to the associated pedogenic carbonate, which suggests a diagenetic isotopic signature and confirms alteration in the eggshells. Both microscopic and geochemical data suggest that an early meteoric diagenesis (pedogenesis) is responsible for the secondary signatures. The δ13C values in the carbonate pedogenic nodules indicate a carbon isotopic composition typical of C3 plants, although the slight difference in δ13C between the palaeosol carbonate of coeval successions may be due to slightly different palaeoenvironmental conditions. The small discrepancy in the δ13C calculated for C3 plants, from carbonate nodules and from eggshells may be because the palaeosol carbonate gives the isotopic composition of the vegetation grown at a local site whereas the δ13C from eggshells is a proxy for the ingested food in the area in which the dinosaurs lived. The oxygen isotopic compositions from palaeosol carbonate nodules have been used for calculation of the air temperature, and we may conclude that the mean air temperature in the studied area during the Late Cretaceous was approximately 21°C. The use of this temperature gives rational results in the calculation of the oxygen isotopic composition of palaeosol carbonate by means of the isotopic composition of the eggshell carbonate. Additionally, a relatively continuous isotopic record of nodules shows an overall vertical trend towards negative δ13C values and a high relative and steady δ18O content throughout the Maastrichtian. The K/Pg boundary was recorded by a negative excursion in δ13C in the carbonate nodules.

Geochemical evidence for sediment provenance in mudstones and fossil‐poor wackestones (Upper Jurassic, Majorca Island)

Terra Nova, 24, 437–445, 2012AbstractThe notion that diagenetic imprint on ancient carbonates significantly compromises palaeoenvironmental interpretations is revealed as an oversimplification. The enigmatic case at the core of this research is an extended, homogeneous, macrofossil‐poor deposit showing distinctive dark brown colour and very fine grain size (Aumedrá Fm., Majorca Island). This markedly contrasts with standard ammonitico rosso (AR) facies known from Jurassic epioceanic swells. Geochemical analysis of matrix micrite, carbonate cements and belemnite rostra are used to interpret the Aumedrá carbonates. Stable (Carbon and Oxygen) isotopes and elemental abundance are compared with data from under‐ and overlying deposits, and coeval sections. Conspicuously higher than expected C and O isotope signature, together with strontium enrichment and a typically low iron and manganese concentrations favour a distant, shallower, probably aragonitic sedimentary supply. Modern and ancient examples of carbonate deposition are integrated with possible hydrodynamic conditions justifying transport over the studied area. A potential NE source is envisagedTerra Nova, 00, 1–9, 2012

Anatomy, internal heterogeneities, and early fracture network of a Pleistocene carbonate coastal dune (Rejiche Formation, southeastern Tunisia)

Although eolian deposits are known to record the dominant winds, secondary conditions such as wind reversals during wintertime can also be observed in the petrographic composition and facies succession. Thus, eolian deposits are used here as a local paleoclimatic proxy. The spatial distribution of the depositional facies, early diagenetic imprints, and early fracture network of a coastal Pleistocene eolian ridge in southeastern Tunisia is described using a small-scale GIS model. Facies analysis indicates that coastal dune systems record seasonal cycles. The fracture density and directions are strongly influenced by the depositional facies type. Laminated facies present a higher fracture density compared to more homogeneous facies and show only one major fracture direction, while the more homogeneous facies display a bimodal distribution. Such a difference between these two groups is explained by the heterogeneous distribution of the early calcite cement within the laminated facies. No tectonic activity or overlying strata have affected the Pleistocene dunes under study. Therefore, the mechanism responsible for the fractures could only be related to the own weight of the eolianite and to its internal or underlying lithologic heterogeneity.

Contrast comparison of differential diagenetic pathways of Lower Tithonian carbonate materials from the Betic Cordillera (S. Spain): Evidence for physico-chemical paleo-seawater properties

The reconstruction of paleoenvironmental conditions is based on the study of ancient rock materials. For this purpose, skeletal materials are often assumed to have higher preservation potential, sometimes favored over the use of matrix micrite. However, factors such as ecology, depositional setting, mineralogy and diagenetic processes may obscure original geochemical signals, and care must be taken when reading such an intricate record. This work provides a thorough comparison between geochemical signatures of different carbonate materials, interpreted in the light of their paleoenvironmental significance. Burial and diagenetic processes are taken into account, contextualized by ammonite and belemnite ecology.Two sections deposited in epioceanic Tethyan areas and corresponding to the same Lower Tithonian ammonite biozone are under scope: a condensed pelagic and cephalopod-rich limestone (the Alamedilla sector) intercalated in thick mafic rocks piles, and a typical Ammonitico Rosso facies (the Cardador section). In contrast to depositional and taphonomic histories typical for a Tethyan top-swell site (i.e., rather calcareous Ammonitico Rosso), events of carbonate-mud deposition, rapid burial and early lithification, synsedimentary sliding and over imposition of firmground horizons characterize pelagic cephalopod-rich limestones. Differential taphonomy of carbonate skeletals agrees with differential preservation, favoring inner-cast preservation in Ammonitico Rosso and shell preservation (neomorphic calcite) in pelagic cephalopod-rich limestones.Stable carbon and oxygen isotope data, together with trace element data (Sr, Mg, Fe, Mn) are presented for a wide range of carbonate materials: matrix micrite, neomorphic ammonite shells, belemnite rostra and carbonate cements. Optical inspection (including cathodoluminescence imaging) and geochemical data suggest a fair preservation of the above mentioned materials, except for diagenetic marine burial carbonate cements. Carbon isotope values fall within the typical Tethyan Late Jurassic range (ca. 2‰), except data from belemnite rostra which are depleted around −1‰, which is interpreted as resulting from metabolic fractionation. Oxygen isotope values are more variable, but the clustering of different carbonate materials within a narrow range of values between −1 and 1‰ is consistent with conservative diagenesis. Warmer burial fluids explain lower values in later diagenetic carbonate cements. No evidence of pervasive diagenetic imprint is revealed by elemental composition. The increased abundance of Mg, Fe, and especially Mn (up to 1500ppm), is recognized as hydrothermal contamination. Proximity to hydrothermal sources (the Mid-Subbetic Volcanic Ridge) accords with higher Mn concentration at the Alamedilla site. Interestingly, the timing of the mineralogical stabilization of each carbonate material reveals a potential record of a post-sedimentary geochemical event.The obtained information demonstrates the complexity of ancient carbonate records, and encourages future research focused on the preservation potential of geochemical proxies throughout time. The studied case hopes to be a representative example to approach the interpretation of potential past and present rare analogs.

Stochastic Joint Simulation of Facies and Diagenesis: A Case Study on Early Diagenesis of the Madison Formation (Wyoming, USA)

The aim of this paper is to propose an integrated approach to reproduce both facies and diagenetic trends in a static reservoir model based on an outcrop case study. In Wyoming (USA), the Madison Formation (Mississippian) is a thick (up to 350 m) carbonate series, outcropping in several locations of the Bighorn foreland basin.Within these series, nine sedimentary facies have been identified. Based on their vertical stacking pattern, they are organized in small-scale facies sequences: 1) intertidal to supratidal facies sequence; 2) shallow subtidal to intertidal facies sequence; 3) deep subtidal facies sequence. These facies associations have been integrated in a synthetic depositional model, which corresponds to a carbonate ramp progressively evolving towards the most inner part of a platform. This enables to propose a sequence stratigraphy framework for the studied series, that represents at least six third-order sequences (some of them being locally eroded).The diagenetic study has been focused on the identification of the early diagenetic phases. Results from these analyses show the occurrence of several successive early diagenetic phases (micritization, marine calcite cementation, dolomitization, etc.). For modeling purposes, seven “diagenetic imprints” have been defined, each of them corresponding to a succession of diagenetic phases that can coexist in the same sedimentary facies. Moreover, as each sedimentary facies may be affected by several diagenetic imprints, a quantification of these imprints has been realized.A 3D gridded model designed for geostatistical modeling has been constructed in order to reproduce the facies organization of the three first third-order sequences (that are the best documented). The gridding is then based on the four sequence boundaries which have been recognized on every section. The relationships between sedimentary facies and diagenesis have been used to define lithofacies simulation rules. The simulations are based on the plurigaussian and nested algorithms. Finally, a discussion on the distribution of the potential reservoir heterogeneities is proposed, taking into account the sedimentary characteristics (facies, architectures, etc.) and the diagenetic impact.

Paleo-redox fronts and their formation in carbonate mound sediments from the Rockall Trough

Piston cores from the summits of coral topped carbonate mounds at the south west Rockall Trough margin reveal that the sediments have undergone significant post-depositional modifications affecting the original geochemical signature and mineralogical composition of the sediments. This diagenetic imprint provides information about the geochemical processes within cold-water coral mounds. The most prominent result of diagenetic alteration of the primary sediment composition is the absence or poor preservation of aragonitic coral skeletons in certain depth intervals associated with lithification. This study focuses on the enrichment of redox-sensitive elements and the dissolution of primary magnetic ferric iron minerals in the depth interval below lithification levels. By combining the magnetic susceptibility with the XRF signal of Fe and Ti specifically, intervals with susceptibility variations related to the conversion of strongly magnetic into weakly magnetic iron species can be defined. In all three studied cores a succession is recognised with a lithified interval with aragonite dissolution and low-Mg calcite precipitates that is underlain by an interval of magnetite dissolution and of iron and manganese enrichment. For the most recent lithified interval it is demonstrated that initial lithification occurred before an erosional regime was in place, most likely near the end of interglacial or at the start of glacial periods.

Multiscale characterization of pyritized plant tissues in blueschist facies metamorphic rocks

Pyritized plant tissues with well-preserved morphology were studied in rocks from Vanoise (western Alps, France) that experienced high-pressure, low-temperature metamorphic conditions in the blueschist facies during the Alpine orogeny. Organic and inorganic phases composing these fossils were characterized down to the nanometer scale by Raman microspectroscopy, scanning transmission X-ray microscopy and transmission electron microscopy. The graphitic but disordered organic matter composing these fossils is chemically and structurally homogeneous and mostly contains aromatic functional groups. Its original chemistry remains undefined likely because it was significantly transformed by diagenetic processes and/or thermal degradation during metamorphism. Various mineral phases are closely associated with this organic matter, including sulphides such as pyrite and pyrrhotite, carbonates such as ankerite and calcite, and iron oxides. A tentative time sequence of formation of these diverse mineral phases relative to organic matter decay is proposed. The absence of traces of organic matter sulphurization, the pervasive pyritization of the vascular tissues and the presence of ankerite suggest that the depositional/diagenetic environment of these metasediments was likely rich in reactive iron. Fe-sulphides and ankerite likely precipitated early and might have promoted the preservation of the fossilized biological soft tissues by providing mechanical resistance to compaction during diagenesis and subsequent metamorphism. In contrast, iron oxides which form rims of 100-nm in thickness at the interface between organic matter and Fe-sulphides may result from metamorphic processes. This study illustrates that it may be possible in some instances to deconvolve metamorphic from diagenetic imprints and opens new avenues to better constrain processes that may allow the preservation of organic fossils during diagenesis and metamorphism.

Diagenetic versus detrital origin of the magnetic susceptibility variations in some carbonate Frasnian–Famennian boundary sections from Northern Africa and Western Europe: implications for paleoenvironmental reconstructions

To provide a new insight into the diagenetic versus detrital origin of the magnetic susceptibility variations in ancient carbonate sequences, a study was conducted within four Frasnian–Famennian platform carbonate sections from Germany, France and Morocco. The study includes along-section magnetic susceptibility and carbonate content measurements complemented by analyses of magnetic hysteresis parameters, inorganic geochemistry and clay mineralogy. Our results show that the magnetic susceptibility evolution is dominantly controlled by the variations in the concentration of low-coercivity ferromagnetic magnetite grains and, to a lesser extent, of paramagnetic clays. In more detail, hysteresis ratios suggest the coexistence of two magnetite populations with significantly different grain size: (1) a dominantly coarse-grained detrital fraction including a mixture of multi-domain and single-domain particles (2) an authigenic fine-grained fraction composed of a mixture of single-domain and superparamagnetic particles. Despite a diagenetic imprint on the clay assemblages, no relationship is established between magnetic susceptibility and illite crystallinity, therefore discarding a noticeable distortion of primary within-section magnetic susceptibility evolution. The overall inherited character of the magnetic susceptibility fluctuations is corroborated by a significant correlation of magnetic susceptibility with terrigenous proxies (Zr, Th). The poorer correlation of magnetic susceptibility with the Fe content is consistent with the existence of a very fine-grained authigenic magnetite component that possibly induces a global magnetic susceptibility increase at the section scale, but no distortion of the within-section evolution. The magnetic susceptibility curves presented here provide a general record of climate-driven detrital influx and carbonate productivity through Frasnian–Lower Famennian times.

Effects of the Permian–Triassic boundary on reservoir characteristics of the South Pars gas field, Persian Gulf

AbstractThe Permian–Triassic boundary (PTB) is a world‐wide event characterized by the most extensive mass extinction in the history of life. In the Persian Gulf, the rock record of this time interval host one of the most important hydrocarbon reserves in the world: the South Pars Gas Field and its southern extension, the North Dome (or North Field). These carbonate and evaporite successions were sampled in eight wells for petrographic, geochemical and porosity–permeability studies. An important characteristic of the Dalan and Kangan formations is the centimetre‐scale lithological heterogeneities caused by facies changes and diagenetic imprints that led to the compartmentalization of these reservoirs. These Permian–Triassic (P‐T) sediments were deposited in a shallow marine homoclinal ramp. The PTB in this hydrocarbon field is represented by a reworked coarse‐grained intraclastic/bioclastic grainstone facies deposited during a marine transgression. Prolonged subaerial exposure in the P‐T transition caused hypersaline and meteoric diagenesis, including extensive cementation, dolomitization and some dissolution, influencing reservoir characteristics of bordering units. Both δ18O and δ13C values in this succession mirror worldwide excursions typical of other P‐T sections, with some variations due to diagenetic alterations. A pronounced decline in 87Sr/86Sr values, reflective of global seawater geochemistry for most of the Permian is evident in our data. Reservoir quality declines through the late Permian, as a result of facies change and diagenesis. The Late Permian is succeeded by a Triassic transgressive facies and decline in reservoir quality. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Effects of diagenesis on the Nd-isotopic composition of black shales from the 420 Ma Utica Shale Magnafacies

The Utica black shales were deposited in the Taconic Foreland basin ∼ 420 Ma ago. The organic matter in these shales is of marine origin and the timing of deposition of these shales has been constrained by graptolite biostratigraphy and bentonite chronostratigraphy. Rare earth element (REE) concentrations and Nd-isotopic ratios were measured in whole rock black shales with different grades of thermal maturity from the Utica Shale Magnafacies of Quebec (immature), Ontario (mature), and New York (post mature). These measurements were also made in the organic, carbonate, and sulfide fractions of the shales, that were isolated by sequential leaching. The purpose of this study is to understand how diagenesis (thermal maturation) may affect the Nd-isotopic compositions of different components as well as the whole rock black shales. Bulk rock REE concentrations of the Utica black shales are similar to average shales. However, these rocks show large range in 147Sm/ 144Nd (0.08–0.19) indicating diagenetic mobilization of the light-REE (LREE). Although thermal maturation of black shales may mobilize all LREE, our data indicate that mobility of La relative to Sm is greater in the mature shales compared to the post mature shales. In post mature shales, Nd is more mobile relative to Sm. Immature shales, however, show wide ranges in Sm/Nd and La/Sm that cannot be explained solely by diagenesis, implying source heterogeneity. Whole rock black shales and their isolated components show overlapping but characteristically different Nd-isotopic compositions. If the present day Nd-isotopic compositions of black shales are mostly an artifact of diagenesis (thermal maturation), which alters the Sm/Nd ratio of the rock, it can be argued that the different components of black shales are affected to different degrees for a given diagenetic process or different diagenetic processes affect the different components. Alternatively, the varying Nd-isotopic compositions of the shale components may reflect differences in provenance of the different shale components. Nd-isotopic composition of mature and post mature black shales of the Utica Magnafacies are controlled by thermal maturation of these rocks, as indicated by the correlation of ε Nd(0) with various LREE ratios such as, La/Sm and Sm/Nd. However, the observed variations in the Nd isotopic composition of immature shales dominantly reflect source heterogeneity. This is also supported by the correlation of ε Nd(0) and immobile heavy REE ratios, such as Dy/Yb, which reflect source compositions. The average ε Nd(0) compositions of the whole rocks and separated components vary with increasing maturity, the post mature samples showing the least radiogenic Nd and the mature samples with the most radiogenic Nd isotopic compositions. The initial Nd-isotopic compositions at 420 Ma of the Utica black shales overlap with the global seawater Nd-isotopic composition at that time. This similarity is observed for the samples with Sm/Nd ratios similar to average shales, suggesting no diagenetic imprint, and that the Taconic Foreland basin hosting the Utica Shale Magnafacies was probably connected to the global ocean during the time of deposition. Depleted mantle Nd model ages of the Utica black shales and their different components increase with increasing Sm/Nd. However, these model ages range from 1.6 to 2.3 Ga (average 1.9 Ga) for the samples with Sm/Nd similar to average shales. These ages indicate that the Taconic basin sediments of the Utica Shale Magnafacies were sourced from pre-Grenville-age rocks.

Aqueous and petroleum fluid flow associated with sand injectites

AbstractField, petrographic and fluid inclusion characteristics of sand injectites from five outcrop localities and from the subsurface of the Tertiary of the south Viking Graben are described. Although the case studies are from a wide variety of sedimentological, stratigraphic and tectonic settings, and hence their diagenetic evolutions differ significantly, it is possible and useful to assign diagenetic events to three distinct phases of fluid flow associated with sand injectites in sedimentary basins. Firstly, there is fluid flow associated with the injection of the fluid–sediment mix during shallow burial. Early diagenetic imprints in sand injectites reveal that basinal fluids, which may be released during movement along deeper‐seated faults, can be associated with this process and thus the injection process may reveal information on the timing of basin‐scale movement of fluids. Secondly, following the injection process, basinal fluids continue to migrate through uncemented injectites and mix with the ambient meteoric and/or marine pore fluids that invade injectites from the overlying and surrounding host sediments. Early, often pervasive, carbonate cementation is common within sand injectites and rapidly turns sand injectites into flow barriers during shallow (&lt;1 km) burial. If early carbonate cementation is not pervasive, fluid inclusions in late quartz cement (∼&gt;2 km of burial) reveal additional information on fluid flow associated with sand injectites during deeper burial. The latest phase of fluid flow occurs when sand injectites are reactivated as preferential fluid conduits during phases of deformation, when well‐cemented subvertical sand injectites become sites of focussed brittle deformation (fracturing). This study shows that sand injectites are a common and volumetrically important type of structural heterogeneity in sedimentary basins and that long‐lived fluid flow associated with sand injectites in very different settings can be assessed and compared systematically using a combination of petrography and fluid inclusion studies.

Origin and timing of sand injection, petroleum migration, and diagenesis in Tertiary reservoirs, south Viking Graben, North Sea

Petrographic, fluid-inclusion, and carbon and oxygen stable isotope studies of Tertiary injectite reservoirs in the south Viking Graben of the North Sea allow an understanding of the origin and timing of sand injection, petroleum migration, and diagenesis. Injection from shallowly (400 m; 1300 ft) buried Paleocene and Eocene unconsolidated sandstones occurred at the end of the Eocene, probably in response to earthquake activity. Liquid oil was already present in the parent sands prior to injection and leaked from the injectites to the seabed. Upward-migrating oil and basinal brines mixed with downward-invading mixed meteoric-marine pore fluids in the injectites, causing extensive biodegradation of the oil. Biodegradation of oil provided the driver for early carbonate cementation in injectites, causing diminished reservoir quality. However, early carbonate cementation also sealed off the injectites as potential escape routes for petroleum from the underlying parent sands. Oil (and gas) continued to migrate into the reservoir (parent) sands upon increased burial, causing a mixing of high-API oil with the early charged, extensively biodegraded low-API oil. The study of early diagenetic imprints reveals the evolution of injectite reservoirs, which forms the basis for understanding how to explore and develop them.

The structural and diagenetic evolution of injected sandstones: examples from the Kimmeridgian of NE Scotland

Injected sandstones occurring in the Kimmeridgian of NE Scotland along the bounding Great Glen and Helmsdale faults formed when basinal fluids moved upward along the fault zones, fluidizing Oxfordian sands encountered at shallow depth and injecting them into overlying Kimmeridgian strata. The orientation of dykes, in addition to coeval faults and fractures, was controlled by a stress state related to dextral strike-slip along the bounding fault zones. Diagenetic studies of cements allow the reconstruction of the fluid flow history. The origin of deformation bands in sandstone dykes and sills was related to the contraction of the host-rocks against dyke and sill walls following the initial stage of fluidized flow, and these deformation bands are the earliest diagenetic imprint. Early non-ferroan calcite precipitated in injection structures at temperatures between 70 and 100 °C, indicating that it precipitated from relatively hot basinal fluids that drove injection. Coeval calcite-filled fractures show similar temperatures, suggesting that relatively hot fluids were responsible for calcite precipitation in any permeable pathway created by dextral simple shear along the faults. During progressive burial, percolating sea water was responsible for completely cementing the still relatively porous injected sandstones with a second generation of ferroan calcite, which contains fluid inclusions with homogenization temperatures below 50 °C. During this phase, depositional host sandstones were also cemented.