The Control of Diagenesis and Mineral Assemblages on Brittleness of Mudstones

The variation in mineral composition will affect the rock brittleness, thus the change of mineral assemblages during diagenesis has a potential control on the brittleness of mudstones. In this study, thin section, X-ray diffraction (XRD), and Scanning Electron Microscope (SEM) analyses were used to investigate compositional and microscopic features of mudstones. With the enhancement of diagenesis, three mineral assemblages were divided due to the diagenetic evolution of minerals. Quartz, feldspar, dolomite, chlorite, and illite were regarded as brittle minerals and (quartz + feldspar + dolomite + illite + chlorite)/(detrital mineral + carbonate + clay mineral) was defined as the brittleness evaluation index The mudstone brittleness changed slightly during early diagenesis but increased gradually with enhancement of diagenesis in the late diagenesis stage. Quartz and feldspar were scattered above the clay matrix and the contact of grains was limited, therefore, the contribution of detrital minerals to the brittleness was affected by the properties of clay minerals. The diagenetic transformation of clay minerals resulted in the reduction of ductile components (smectite/I-Sm and kaolinite) and increase of brittle components (illite and chlorite), leading to the enhancement of integral rigidity of the mudstones. Meanwhile, the improved crystallization of carbonate in late diagenesis stage enlarged the carbonate grains which resulted in rigid contact between grains. These results highlighted the influence of diagenesis on mudstone brittleness. Therefore, for evaluation of mudstone brittleness, attention should be paid to the diagenesis process besides mineral composition.

Tracking the Origin and Evolution of Diagenetic Fluids of Upper Jurassic Carbonate Rocks in the Zagros Thrust Fold Belt, NE-Iraq

Utilizing sophisticated tools in carbonate rocks is crucial to interpretating the origin and evolution of diagenetic fluids from the Upper Jurassic carbonate rocks along the Zagros thrust-fold Belt. The origin and evolution of the paleofluids utilizing in-situ strontium isotope ratios by high resolution laser ablation ICP-MS, integrated with stable isotopes, petrography and fieldwork are constrained. Due to the lack of information on the origin of the chemistry of the fluids, the cements that filled the Jurassic carbonate rocks were analysed from the fractures and pores. This allowed us to trace the origin of fluids along a diagenetic sequence, which is defined at the beginning from the sediment deposition (pristine facies). Based on petrography and geochemistry (oxygen-, carbon- and strontium-isotope compositions) two major diagenetic stages involving the fluids were identified. The initial stage, characterized by negative δ13CVPDB values (reaching −10.67‰), involved evaporated seawater deposited with the sediments, mixed with the input of freshwater. The second stage involved a mixture of meteoric water and hot fluids that precipitated as late diagenetic cements. The late diagenetic cements have higher depleted O–C isotope compositions compared to seawater. The diagenetic cements display a positive covariance and were associated with extra- δ13CVPDB and δ18OVPDB values (−12.87‰ to −0.82‰ for δ18OVPDB and −11.66‰ to −1.40‰ for δ13CVPDB respectively). The distinction between seawater and the secondary fluids is also evident in the 87Sr/86Sr of the host limestone versus cements. The limestones have 87Sr/86Sr up to 0.72859, indicative of riverine input, while the cements have 87Sr/86Sr of (0.70772), indicative of hot fluid circulation interacting with meteoric water during late diagenesis.

Deformations of weakly lithifed rocks of the Bazhenov-Georgievsky complex

Research subject. Deposits of the Upper Jurassic Bazhenov-Georgievsky complex situated in central West Siberian regions. At a frst approximation, the deposits are represented by interlayering pure or carbonatized radiolarites and organic-clay mixites. Materials and methods. A collection of core samples taken from the deposits was analyzed to study the geological and geophysical features of borehole sections, photographs of core samples, and micrographs of thin section. Soft sediment deformations, which appeared within the Bazhenov-Georgievsky complex as a result of earthquakes, were investigated using data obtained by Chinese researchers. Results. It is shown that, at different stages of diagenesis, layers with sharply differing density and mechanical properties came into contact, which led to various deformations of weakly lithifed layers during earthquakes. In the early and middle diagenesis, mobile carbonated radiolarites deformed plastic clay interlayers with the formation of carbonate nodules. In the middle diagenesis, in radiolarites, areas of weak lithifcation and mobile radiolarites coexisted. Along with plastic deformations, the processes of autobrecciation and autofluid fracturing of radiolarites, abrasion erosion of the boundaries of clayey varieties took place. In the late diagenesis, lithifed varieties predominated in radiolarites and clayey silicites; as a result of deformations, radiolarites and the argillaceous bridges separating them were transformed into fragments, polished surfaces, and pellets. If, under the influence of differential loads, fluidization (softening) and movement of weakly lithifed radiolarites occurred, then the rates of such movements in adjacent layers inevitably differed. Each of the described cases is demonstrated by the core material of specifc boreholes and petrographic thin sections. The scale of the phenomenon is shown from the signs of loading of millimeter interlayers to pressure shafts of brecciated limestones with a thickness of 7–10 m. Conclusions. The West Siberian Plate is epicontinental; therefore, the most intense earthquakes occurred at consolidated basement block boundaries during isostatic subduction and compensations in the Neocomian clinoform ultrafast sedimentation zones. Soft sediment deformations occurred within the Bazhenov-Georgievsky layers recorded the rock history of intense seismic events of the past, their temporary and spatial distribution.

Lithological models of reservoir rocks for upper cretaceous deposits in East Caucasian Region

The article describes the results of lithological and petrophysical investigations that would be a base for characterization of reservoir rocks in Upper Cretaceous deposits. These investigations include thin sections description, SEM and NMR analysis. As found that three main factors have constrained final quality of reservoir rocks: 1) depositional settings favorable for coccoliths and chalk sedimentation; 2) late diagenesis changes – compaction and recrystallization degree; 3) fracture intensity.

Geochemistry of Framboids

The stoichiometry of pyrite in framboids is unknown. The trace element content of framboids has been reported since framboids usually constitute the earliest pyrite phase in a sediment and therefore are more likely to pick up trace element variations in contemporary seawater. The trace element ratios in sedimentary framboids are similar to those in the host shales. Analyses of hydrothermal framboids are fewer, and As, Sb, and Tl appear to be enriched in hydrothermal framboids, with As, Sb, Ni, and Co also being enriched in framboids formed during metamorphism. In contrast with trace element distributions, no spatial variations in sulfur isotopic compositions have been reported within individual framboids. Framboids pick up a more accurate measure of the sulfur isotopic composition of the prevailing dissolved sulfide and are likely to retain this over geologic time. Although it is probable that pyrite framboids collect the local environmental trace element variations, interpretations of the results in terms of paleoenvironmental reconstructions are currently complex. The original sequestration of trace elements is likely to be in part determined by the pyrite crystal chemistry, and there may be a limit to how much of any given trace element can be sequestered by pyrite. This is likely to be enhanced during late diagenesis and early metamorphism and it is not altogether clear how individual trace elements behave over geologic time.

Differentiating early from later diagenesis in a Cretaceous sandstone and petroleum reservoir of the Cedar Mountain Formation, Utah

ABSTRACT Previously published anomalous whole-rock stable isotopic values from the Poison Strip Sandstone Member of the Cretaceous Cedar Mountain Formation (CMF) of eastern Utah are of uncertain origins. This study investigated the diagenetic history and the processes responsible for these anomalous data. Accordingly, we integrated photomicroscopic techniques including polarized light microscopy, epifluorescence and cathodoluminescence (CL) imaging, micromilling of stable isotope samples, and fluid-inclusion heating and freezing measurements to this end. The key observations involved the microscopic mapping of calcite cement stratigraphy using CL imaging to permit the analysis of stable isotopes of calcite cements that crystallized during early and late diagenesis. The mapping of calcite cement zones of sufficient submillimeter size to mill out and isolate microgram-sized stable isotope samples enabled this discrimination. Early diagenetic calcite cements have the most positive δ18O values (-10 to -8.5‰ Vienna Pee Dee Belemnite [VPDB]) of all components. The pattern of δ13C and δ18O variation in this early diagenetic cement indicates affinities with early meteoric diagenesis previously documented in published literature on the CFM. The late diagenetic calcite cements yield the most negative δ18O values (-18 to -16‰ VPDB). We interpret the late diagenetic cements to be responsible for the anomalously low whole-rock δ18O values previously reported from the Poison Strip Sandstone Member. Our discoveries of bitumen in late-stage pore fillings and liquid petroleum in the fluid inclusions of late diagenetic calcite cements of the Poison Strip Sandstone Member explain the lower whole-rock organic matter δ13C values and anomalous Δ13C values reported from the unit. Comparatively lower carbonate δ18O and organic δ13C values originally derived from whole-rock analyses of samples from the Poison Strip Sandstone Member resulted from high-temperature basinal diagenesis (hydrothermal circulation and/or petroleum migration), rather than the alternative interpretation of early diagenesis related to a Cretaceous paleoclimatic perturbation. Our results are illustrative of methods to resolve the long-standing geologic problem of discriminating and characterizing products of early vs. late diagenesis in terrigenous clastic sedimentary strata.

First U-Pb dating of fossilized soft tissue using a new approach to paleontological chronometry

Previous U-Pb dating of fossils has had only limited success because of low uranium content and abundance of common Pb as well as element mobility during late diagenesis. We report the first accurate U-Pb dating of fossilized soft tissue from a Pliocene phosphatized bivalve mold using laser ablation–inductively coupled mass spectrometry (LA-ICPMS). The fossilized soft tissue yields a diagenetic U-Pb age of 3.16 ± 0.08 Ma, which is consistent with its late Pliocene stratigraphy and similar to the oldest U-Pb age measured on accompanying shark teeth. Phosphate extraclasts give a distinctly older age of 5.1 ± 1.7 Ma, indicating that they are likely detrital and may have furnished P, promoting phosphatization of the mold. The U-Pb ages reported here along with stratigraphic constraints suggest that diagenesis occurred shortly after the death of the bivalve and that the U-Pb system in the bivalve mold remained closed until the present. Shark teeth collected from the same horizon show variable resetting due to late diagenesis. Data were acquired as line scans in order to exploit the maximum Pb/U variation and were regressed as counts, rather than ratios, in three-dimensional space using a Bayesian statistical method.

Origin of carbonate cements in deep sandstone reservoirs and its significance for hydrocarbon indication: A case of Shahejie Formation in Dongying Sag

Carbonate cements are primary cement types formed in deep sandstone reservoirs of Dongying Sag. We have proposed three stages of carbonate cements with different origin and material sources: carbonate cements in early stage are rim-shaped high-Mg calcite, which is the product of quasi-contemporaneous period; and calcite filled with primary pores without obvious compaction and diagenetic transformation is mudstone compaction during the drainage process. Carbonate cements in middle stage are calcite and dolomite filled with feldspar secondary dissolved pores. The rich Ca2+, Mg2+, and CO 3 2 − {\text{CO}}_{3}^{2-} in overpressure fluid enter the reservoir and mix with Ca2+ in the original formation water. Carbonate cements in late stage are ferrocalcite and ankerite that filled the dissolution pores of early- and middle-stage carbonate cements. They were products of CO 3 2 − {\text{CO}}_{3}^{2-} formed by organic acid splitting decomposition in late diagenesis and CO 3 2 − {\text{CO}}_{3}^{2-} formed by dissolution of carbonate cements in early and middle stages, combined with Mg2+, Ca2+, Fe2+ plasma in pore fluid. Dissolution–reprecipitation of the lacustrine carbonate rocks is responsible for obvious positive drift in the δ 13CPDB‰ values of carbonate cements. Carbonate cements in middle stage and late stage, respectively, represent the early hydrocarbon charging of Dongying Formation and the end of Guantao Formation to the present.

Effects of Diagenesis on the Reservoir Quality in the Upper Sands of Lower Goru Formation, Badin Block, Lower Indus Basin, Pakistan

The core samples of B member of upper sand reservoir rocks of the lower Goru Formation from three wellsof the Badin block were studied using thin section, XRD, and SEM techniques to investigate the diagenetic trends andtheir effect on reservoir quality. Microscopic study indicates that the B sand unit is mature with quartz as thepredominant mineral constituent with variable amount of feldspar and lithic minerals. The QFL plot indicates that mostof the samples are plotted in the field of quartz arenite, sub-litharenite and sub-arkose respectively. A few samplesbelong to the category of litharenite and feldspathic litharenite. The feldspars were partially to completely altered tokaolinite and other clay minerals. Coarse-crystalline or micro-crystalline calcite is the predominant cementing material.Bulk rock XRD analysis also confirms that the main mineral constituents of there samples are quartz and calcite invariable proportions. The undulose extinction and fracturing of quartz grains indicate that the area remained understress. Moreover, such fracturing is post-depositional and therefore is the product of late diagenesis. Scanning ElectronMicroscopic (SEM) images at 50 micrometer (μm) size show irregular type of fracturing within the quartz grains. Thislate stage fracturing of quartz has also generated various types of channels which may serve as secondary porosity. Thequartz overgrowth was observed in some samples due to late stage diagenesis. The micro-crystalline cement in the formof calcite is mostly present within the pores in fractured quartz. The results indicate that the diagenesis played asignificant role in improving the reservoir characteristics of B sand by increasing the porosity due to quartz fracturing,feldspar dissolution, alteration and quartz corrosion. Hence, better understanding of reservoir heterogeneities inporosity through diagenetic studies can be helpful in evaluating potential reservoir horizons for hydrocarbonaccumulation on spatial and temporal scales.