Daniel Beaufort

Illitization of Diagenetic Kaolinite-to-dickite Conversion Series: Late-stage Diagenesis of the Lower Permian Rotliegend Sandstone Reservoir, Offshore of the Netherlands

ABSTRACT This paper describes the diagenetic evolution of clay minerals in the Rotliegend sandstone reservoir under contrasting burial histories in the Broad Fourteens basin (Dutch sector, Southern North Sea). The diagenetic modifications affecting the crystal structure of clay minerals (both kaolin-group and illitic minerals) were studied using X-ray diffraction (XRD). The XRD study includes oriented and random mounts of various size fractions, numerical processing (decomposition) of XRD profiles, and simulation of one-dimensional and three-dimensional XRD patterns. Petrographic observations, differential thermal analysis, K/Ar geochronology, and geochemical considerations complement the XRD study and allow determination of the sequence of mineral crystallization and the morphological evoluti n of clay minerals and place further constraints on the absolute timing of diagenetic events and on the nature of the fluids responsible for clay-mineral crystallization. From deposition time ( 275 Ma) to the Kimmerian orogeny ( 155 Ma), crystallization of kaolinite at the expense of K-feldspars was favored by acid fluids from the underlying Carboniferous Coal Measures source rocks; kaolinite crystallization is followed by a steady kaolinite-to-dickite transformation affecting both the structure and the morphology of kaolin-group minerals. The structural characteristics of kaolin-group minerals are related to the burial history of the sediments prior to the Kimmerian orogeny. During the Kimmerian orogeny, rapid illitization of kaolin-group minerals was favored both by increased heat flow in the sedimentary pile and by widespread presence of faults, which permitted significant fluid flow probably from the Zechstein Formation. The morphological and structural characteristics of illitic minerals, i.e., illite content of illite/smectite mixed layer (I/S), ratio of illite to I/S, and three-dimensional structure of illitic minerals, do not represent the progress of a smectite-to-illite transformation, but these characteristics clearly reflect the temperature during illitization of kaolin.

Kinetic constraints on illitization reactions and the effects of organic diagenesis in sandstone/shale sequences

Abstract Based on water-rock interaction modelling and kinetic considerations, the present study is aimed at testing the impact of organic matter maturation on two kinds of diagenetic reactions leading to the alteration of the petrophysical properties of sandstones: (1) Clay mineral conversion to illite with K-feldspar being a local potential source of K; and (2) Dissolution-precipitation processes produced in sandstones by the action of water-soluble organic species derived from adjacent shales. Assuming firstly that the chemical reactions take place in closed systems, the nature and timing of diagenetic reactions in marine sandstone/shale formations were modelled for a 50–120°C temperature range in order to improve understanding of the factors that control the illitization reaction with K-feldspar coexisting with aluminous clay. Illite is modelled here as a muscovite type mineral. We tested the effects of an energy barrier on illite growth by allowing or preventing the muscovite/illite precipitation reaction to occur, while using several illitization reaction rates. We also compared the stable mineral parageneses predicted for organic material-free systems with those predicted in the case of organic diagenesis (release of CO 2 , CH 4 , acetic and oxalic acids). Similarities and discrepancies between numerical results and natural mineral assemblages suggest that the illitization reaction depends on the nature of the reacting clays. Kaolinite conversion to end-member illite involves high-energy conditions (> 2 kcal mol −1 ), which are not met when the pore water equilibrates with the mineral matrix from undersaturated conditions in a closed system. To overcome this barrier, the fluid should be oversaturated with respect to K-feldspar. An external source of K or a pH increase in an open system is necessary for this reaction. No particular effect of organic diagenesis on this reaction was found in the present study. On the other hand, smectite-to-illite conversion involves a lower energy barrier and can operate in closed systems where K-feldspars are the source of K. The maturation of organic matter may speed up the smectite-to-illite conversion rate by increasing the Gibbs Free Energy of illite growth. Interactions between sandstone and shale were examined by modelling the transfer of aqueous species of organic origin from shale to sandstone. Fluid expulsion from shale had little or no effect on diagenesis in adjacent sandstones during each flushing cycle, mainly because organic protons and ligands were neutralized in the source rock. However, the diffusion of shale-derived cations through sandstone as organometallic complexes (Mg > Ca > > Fe > > Al) appears to be an efficient process during carbonate cementation in sandstones, where oxidizing conditions enhance the decomposition of such complexes.

A new method for quantitative petrography based on image processing of chemical element maps: Part I. Mineral mapping applied to compacted bentonites

Abstract Most natural rocks or engineered materials display a multi-scale heterogeneity ranging from the nanometer to the centimeter. Their spatial textural heterogeneity can be approached from chemical element maps acquired using various techniques (SEM, EPMA, SXAM, synchrotron μ-XRF, TEM), depending on the chosen magnification. Chemical map processing that yields quantitative petrographic information is improved here according to newly developed mineral thresholding methods that accommodate mixtures and solid solutions. The complex case of an MX80 compacted bentonite is used as a test case. The 14 major chemical elements of this sample were mapped using an electron probe microanalyzer, and chemical map processing yielded a quantitative map of the 18 mineral species of bentonite with a spatial resolution of a few micrometers. The textural heterogeneity of the solid part of the sample is thus visualized and quantified on an area ranging between 0.1-1 cm2. The method also provides a complete modal analysis of the sample. The methodology is expected to have broad applications in Earth and materials sciences.

Experimental investigation of aqueous corrosion of R7T7 nuclear glass at 90°C in the presence of organic species

Abstract The initial dissolution kinetics of the French “R7T7” inactive LWR reference glass was investigated in organic media at 90°C by static experiments at imposed pH values. Tests were conducted with various organic acids or acid salts (formic acid, citric acid, humic acids, sodium citrate and sodium formate) at different concentrations and at pH values ranging from 3 to 9. The leachates were submitted to ICP-AES analysis, and the alteration films on the glass specimens were examined by SEM (equipped with an energy-dispersive chemical analysis systm) and XRD. Results are discussed for the following areas. Effect of pH: different mechanisms are involved at pH 2.5 and pH 9, but in both cases, the R7T7 glass dissolution kinetics are always controlled by a surface reaction mechanism. The most significant effects of the organic acids on the glass dissolution kinetics were observed at pH 3, which was not unexpected. Nature of the acid (or acid salt): this is an essential parameter. Formic acid (monoacid) and sodium formate had no effect on the initial glass dissolution rate nor on the nature of the alteration film; conversely, citric acid (triacid) and sodium citrate significantly affected glass dissolution. The role of citric acid in the surface reaction mechanism has been demonstrated experimentally at pH 2.5; it is also known to favor that extraction of Si (directly or indirectly) and Zr and probably of Fe. The humic acids are strong catalysts of the R7T7 glass dissolution reaction. The conclusions drawn from these observations corroborate other published findings, suggesting that the catalytic effect is appreciable only for species with at least two carboxylic acid functions. Effect of concentration: the effect of the concentration was observed at basic pH values in the presence of sodium citrate and humic acids. This parameter may thus be quantified and incorporated in the glass dissolution kinetic law.

Biotite chloritization process in hydrothermally altered granites

Abstract Chloritization of biotite in a hydrothermal environment was studied in two granitic rocks of the Massif Central (France). Four types of transformation were observed: (1) Mg-chlorite growth upon (001) biotite planes in contact with quartz; (2) biotite pseudomorphosed by a corrensite—residual biotite assemblage; (3) corrensite—residual biotite pseudomorphosed by a ferromagnesian chlorite; and (4) biotite pseudomorphosed by chlorite. These transformations induce an expansion of the site of the ferromagnesian minerals which can be observed by the corrosion of neighbouring minerals and fracturation of the rock around the new chlorites (open fractures or chloritic veinlets). If Al is considered as a strictly inert component, the reaction biotite → chlorite produces a decrease of the ferromagnesian mineral volume of ∼ 13% (Ferry, 1979). This is not in agreement with petrographic observations of the rocks studied here, therefore writing of a chloritization reaction must take into account a volume increase. In this case Al does not behave as an inert component but as a mobile one at the scale of the microsystem biotite—neighbouring crystals. Albitization of plagioclase which occurs simultaneously with the chloritization process is the Al furnisher. In addition, Fe and Mg must be brought into the microsystem during the transformation of biotite into chlorite, this explains why the Mg ( Mg+Fe ) ratio is not the same between biotite, chlorite growths and chlorite pseudomorphs. Because the bulk-rock composition is enriched in these components, the hydrothermal fluids must contain Fe and Mg before they flow into the rock. Chloritization in propylitized granites is not an isochemical process.

Crystal size of epidotes: A potentially exploitable geothermometer in geothermal fields?

Crystal size of epidotes crystallized in quartz + epidote veins is used as the basis for a new geothermometer from the fossil geothermal field of Saint Martin (Lesser Antilles). The epidote-bearing alteration paragenesis is developed as far as 3 km from a quartz diorite pluton at temperatures of 220-350 °C. The length/width ratio of the epidote grains is constant for all the analyzed samples and suggests isotropic growth environments. However, the length and width of the grains vary exponentially with temperature. The obtained results offer new perspectives for simple grain-size geothermomeery but must be extended to other geologic environments to clarify the influence of different rock types.

Pulsated flow regime in fractures: a possible explanation of local temperature gradients in hydrothermal systems

Abstract Several petrological studies of hydrothermal systems suggest the existence of temperature gradients in wall-rock adjacent to fractures. On the basis of a vein alteration system, in which a thermal gradient has been identified at a pluricentimetric scale, we propose a theoretical model of the thermal conditions of the wall-rock alteration: these results are then used to constrain the nature of the fracture flow regime. Knowing that phenomena of dissolution-recrystallization cannot occur if the temperature gradient appears for only a short time (for instance, less than one minute), we show that only a pulsated flow regime can explain such a phenomenon. Moreover, the conditions on the values of minimum crystallization temperature of the observed minerals imply that this pulsated regime cannot be described by a sinusoidal function but rather by a Fouriers series.