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The Middle Jurassic carbonates of the eastern part of the Paris Basin display surprisingly low values of porosity and permeability (Φ

http://www.sciencedirect.com/science/article/B6X1W-4TNCG9T-1/2/0237cb95f51234546c22d2610c694281; International audience; The aim of this paper is to outline a methodology to transform depth intervals into geological time intervals and thus to estimate duration; for example the duration of biostratigraphical units or of hiatuses. The latter point is particularly important because a continuous time recording is often assumed in geological modelling. Andra (National Radioactive Waste Management Agency) has conducted studies in its Meuse/Haute-Marne Underground Research Laboratory located at a depth of about 490 m in a 155-million-year-old argillaceous rock: the Callovo-Oxfordian argillite. The study of the variations of that clay layer can be conducted by the analysis of logs and particularly high-resolution logs. Working in a geochronological system helps to understand the physical properties of rocks and to make very precise well-to-well correlations. To transform the present coordinates, a high-resolution tool is necessary: the FMI® (Fullbore Formation MicroImager, Schlumberger). First, well-to-well correlations are established between seven wells at different scales. Relative variations of the sedimentation rate are observed locally. Second, FMI® data are studied in detail to extract as much information as possible. For example, the analysis of FMI® images reveals a clear carbonate-clay interbedding which displays cycles. Third, geostatistical tools are used to study these cycles. The variographic analysis of conventional log data shows one-metre cycles. With FMI® data, smaller periods can be detected. Variogram modelling and factorial kriging analysis suggest that three spatial periods exist. They vary vertically in the boreholes but cycle ratios are stable and similar to orbital cycle ratios (Milankovitch cycles). The three periods correspond to eccentricity, obliquity and precession. Since the duration of these orbital cycles is known, depth intervals can be converted into time intervals (duration) and thus give real sedimentation rates and a quantification of hiatus duration

The Callovo-Oxfordian claystones located at 500 m depth at Bure (Meuse, France) are currently being investigated by Andra (the French National Radioactive Waste Management Agency) for testing the feasibility of long-term and deep geological nuclear waste disposal. In order to evaluate its potential as a geological barrier, it is very important to study, assess and describe its physico-chemical variability. The molecular biomarker composition of 150 samples of these claystones and their surrounding limestones carry diverse information on the sources of the sedimentary organic matter, the chemistry of the depositional environment, the preservation and diagenesis conditions. It also allows assessing the degree of lateral and vertical variability of the organic matter within these sedimentary series. The abundance of unsaturated biomarkers, the distribution of steroids and hopanoids and CPI values >2 prove the thermal immaturity of the organic matter. The co-occurrence of plankton, bacteria and land plant biomarkers indicate that the organic matter is a mixture of marine and continental contributions. The data also reveal that the organic matter was deposited under oxic and open-sea conditions except for a brief event of photic zone anoxia at the beginning of the Middle Callovian. In the claystones, the geosynthesis of diasterenes is favored to the detriment of the formation of steranes, especially in smectite-rich levels, and the organic matter is rapidly isolated from oxidizing then reducing conditions after the deposition due to the protective effect of clays. On the scale investigated, the claystones are characterized by a unique molecular facies and are thus homogenous from their organic content point of view. Yet, detailed investigation of specific molecular families indicates changes related to major claystone–limestone transitions. The homogeneity of these claystones can be explained by the paleogeographic position of their depositional setting and the plane and sub-horizontal paleotopography on which they were deposited. This study demonstrates the efficiency of organic geochemistry in the assessment of the variability of geological barriers.

International audience; An integrated stratigraphic study was conducted on a Kimmeridgian succession of 3rd-order cycles including marls and limestones in a shelf context at Bure (Paris Basin). The study was possible due to the exceptional opportunity provided by well-boring activities related to the construction of an underground laboratory of the French National Agency for Radioactive Waste Management (ANDRA). Studies of macrofossils, microfossils, sedimentology, clay mineralogy, isotopic composition (C, O) of shells and organic molecular geochemistry lead to a detailed description of the stratigraphic column, which allows us to address the history of sea-level and climate changes. Six transgressive-regressive cycles are recognised in the studied Kimmeridgian succession. In these 3rd-order cycles, the deepest environments are systematically represented by marls and organic-rich sediments whilst the shallowest are represented by limestones. These cycles do not correspond to changes in temperatures or carbonate production rates at a regional or global scale. On the contrary, long-term palaeontological, sedimentological, and geochemical changes during the Early Kimmeridgian are interpreted as climatically induced. These climatic changes are considered as responsible for bringing significant granular carbonate production to an end, in contrast to carbonate mud that was deposited in alternation with marls throughout the Late Kimmeridgian.

Porous micritic facies, either primary chalks or resulting from secondary destructive micritization, can constitute important hydrocarbon or water reservoirs. Characterization of reservoir properties and the understanding of factors which controlled the distribution of porosity are of primary interest to evaluate the prospective reserves. Middle and late Oxfordian limestones of the eastern Paris Basin show several horizons with porosities higher than 20%. The porosity is mainly microporous and located either within secondary micritized grains or in the micritic matrix. Using SEM, cathodoluminescence, as well as confocal microscopy, stable oxygen and carbon isotope ratios, and petrophysical measurements, a scenario for the evolution of the micropores is proposed. Lime mudstones to packstones constitute the majority of the high-porosity facies (HPFs). Inner lagoonal deposits are more micritized and thus more porous than grainstones, and facies rich in leiolitic oncoids and echinoid clasts are less impacted by micritization. Micritization was responsible for an increase of the intragranular porosity in most grain types. During both eogenesis and shallow burial, mineralogical stabilization dissolved aragonitic particles and allowed precipitation of calcite rhombs. This process was probably enhanced below surfaces of subaerial exposure. During burial, Ostwald ripening allowed the growth of larger micrite crystals at the expense of smaller ones during early Berriasian and late Aptian recharges of deep aquifers when the northern margin of the basin was exposed. Overgrowths on micrite crystals were more important in intervals strongly affected by chemical compaction, which favored oversaturation of waters with respect to calcite. In low-porosity horizons (LPFs), the dense micritic texture of oncoids and the monocrystalline architecture of echinoid clasts prevented an intense micritization, while the strong chemical compaction enhanced poronecrosis. Telogenetic fracturing created new fluid pathways that favored inputs of meteoric fluid in porous micrite and allowed the continuation of Ostwald ripening during Cenozoic times. As a whole, mesogenetic inputs of waters undersaturated with respect to calcite in deep aquifers during exposure of basin margins are a more efficient process than early subaerial exposure for enhancing aggrading neomorphism and appearance of microporous micrites. Initial mineralogical heterogeneities also impact the intensity of chemical compaction and thus the stratigraphical distribution of microporous limestones.

In order to aid the interpretation of plant biomarker assemblages found in the geological record in terms of palaeoflora composition, a new experimental approach, based on artificial maturation of extant plants using confined pyrolysis was developed. A representative Pinaceae species ( Abies pinsapo ) was selected for study because these plants synthesize large amounts of abietanoic acids for which the diagenetic pathway is well known. Pyrolysis parameters were defined so as to allow the formation of all the abietanoic acid-derived diagenetic products encountered in ancient sediments. With pyrolysis parameters of 700 bar, 24 h and 280 °C, partial degradation of dehydroabietic acid leads to the formation of all the typical aromatic abietanes (e.g. simonellite, retene, etc.) found in the geosphere. Synthesis of saturated abietanes (norabietanes, abietanes) was activated with the pyrolysis of a mixture comprising a reducing agent (LiAlH 4 ) and the plant material. We also show that the distribution of plant biomarkers formed under these two pyrolysis conditions is consistent with those of the fossil Pinaceae in published palaeochemotaxonomic studies. Consequently, the procedure could be extended using a broad range of plant taxa in order to predict their fossil molecular signatures. New palaeochemotaxonomic trends for other taxa could therefore be found. In the future, the data thus gathered could not only be useful for the reconstruction of palaeoflora populations and palaeoclimates but also for archaeological and other research areas.

Resume L'Andra mene, depuis 1994, un ensemble de recherches pour determiner les proprietes hydrogeologiques, dont la permeabilite, de la formation d'argilites du Callovo-Oxfordien dans l'Est du bassin de Paris et, plus particulierement, sur le site du laboratoire souterrain situe sur la commune de Bure, dans le Sud du departement de la Meuse. La determination d'un tel parametre dans une argile raide de tres faible permeabilite pose de nombreuses questions, a la fois theoriques (la loi de Darcy est-elle applicable ?), methodologiques et techniques (comment obtenir des valeurs representatives de la formation ?). Cet article presente la demarche et les moyens qui ont ete adoptes pour permettre d'atteindre cet objectif, c'est-a-dire approcher de la facon la plus fiable et precise possible la valeur de ce parametre, son incertitude, son anisotropie et sa variabilite eventuelle en fonction de la lithologie. Les resultats obtenus a partir des mesures sur echantillons et a l'aide d'une serie de tests, aussi bien en forages profonds realises depuis la surface qu'en forages courts depuis la galerie d'experimentation a 445 m de profondeur, sont coherents, bien que les methodes et les echelles d'investigation soient differentes. A l'echelle du site du laboratoire, la permeabilite est inferieure a 10 −12 m s −1 sur toute l'epaisseur de la formation d'argilite, avec une valeur minimum estimee a 10 −14 m s −1 . Les premiers resultats des tests dans la galerie d'experimentation du laboratoire souterrain, a 445 m de profondeur montrent, en grand, la tres faible permeabilite du Callovo-Oxfordien. Pour citer cet article : J. Delay et al., C. R. Geoscience 338 (2006).

Mesozoic limestones from the eastern part of the Paris Basin display, mostly in strata rich in bioherms, abundant macro-cavities (vugs) and some connected micro-fractures almost completely filled with euhedral calcite crystals (late calcite spar). The interparticle porosity of limestones is also filled with euhedral calcite spar cements similar to those observed in vugs and fractures, and rarely displays the preservation of the structures typical of early marine cements. These calcite spars (both from vugs/fractures infillings and from cemented porosity) were studied in drilled cores cross-cutting the Kimmeridgian to Bathonian series with the main objective of determining the nature and flow regime of the fluids responsible for the strong porosity reduction observed in that part of the Paris Basin Mesozoic limestones. The approach combined mineralogical and geochemical studies (O and C stable isotopic compositions measured by conventional and SIMS methods) with temperature estimates from fluid inclusion analyses. The δ 13 C values of late calcite are close to those of the host limestones and correlate with the δ 13 C changes in host rocks, indicating a local source of C. By contrast, late calcites from vugs, fractures and also in the cemented interparticle porosity of limestones have δ 18 O values around +21‰/SMOW, values which are distinct from those of bulk host limestones (+25‰ to +27‰). The δ 18 O value of the fluid is estimated to be −6.8 to −2.5‰ for a crystallisation temperature between 32 and 42 °C determined from fluid inclusion studies. Thus, the parent fluid of cemented interparticle porosity and vug- and fracture-infillings likely contained a component of meteoric water, in contrast to the previous belief that the reduction of porosity in those series was related to early marine diagenesis. Possible situations in which such meteoric fluid could have occurred are: (i) a stage when the limestone aquifers were directly connected to surface water, and/or (ii) channelling of water, from deeper zones in the basin, through regional fault systems. The O isotopic compositions of the calcite cements in the underlying 140 m – thick Callovo-Oxfordian argillites and those of the less recrystallised Bathonian to Kimmeridgian limestones are those expected for unmodified marine calcite: δ 18 O=ca. +28‰ SMOW. Therefore, from the isotopic point of view, the Callovo-Oxfordian argillites remained essentially unaffected by the paleo-fluid circulation documented in the adjacent limestones series. This indicates the argillites acted as an efficient permeability barrier in the past.

Explosive volcanic activity is recorded in the Upper Jurassic of the Paris Basin and the Subalpine Basin of France by the identification of five bentonite horizons. These layers occur in Lower Oxfordian (cordatum ammonite zone) to Middle Oxfordian (plicatilis zone) clays and silty clays deposited in outer platform environments. In the Paris Basin, a thick bentonite (10–15 cm), identified in boreholes and in outcrop, is dominated by dioctahedral smectite (95%) with trace amounts of kaolinite, illite and chlorite. In contrast, five bentonites identified in the Subalpine Basin, where burial diagenesis and fluid circulation were more important, are composed of a mixture of kaolinite and regular or random illite/smectite mixed-layer clays in variable proportions, indicating a K-bentonite. In the Subalpine Basin, a 2–15 cm thick bentonite underlain by a layer affected by sulphate–carbonate mineralization can be correlated over 2000 km 2 . Euhedral zircon, apatite and biotite crystals have been identified in all the bentonites. The geochemical composition of the bentonites in both basins is characterized by high concentrations of Hf, Nb, Pb, Ta, Th, Ti, U, Y, Zr and low concentrations of Cr, Cs and Rb. Biostratigraphical and geochemical data suggest that the thick bentonite in the Paris Basin correlates with the thickest bentonite in the Subalpine Basin, located 400 km to the south. These horizons indicate that significant explosive volcanic events occurred during the Middle Oxfordian and provide potential long-distance isochronous marker beds. Immobile element discrimination diagrams and rareearth element characteristics indicate that the original ash compositions of the thickest bentonites correspond to a trachyandesitic source from a within-plate alkaline series that was probably related to North Atlantic rifting.

The hydrothermal reactivity of a smectite saturated with K was studied experimentally at 300°C and 100 bar in (Na,K) chloride solutions (Na/K = 0, 50 and 100, liquid/solid ratio = 10/1). X-ray diffraction, TEM and microprobe results show: (1) a partial to total dissolution of the initial smectite layers; and (2) the crystallization of newly-formed euhedral I-S. Random I-S is formed after 7 days, but an ordered mixed-layer I-S containing

26 pages; International audience; A detailed sedimentological, geochemical and mineralogical study is carried out on the Early Bajocian to Early Callovian (Middle Jurassic) limestones of the Paris Basin. Objectives are to document and explain the facies changes in the context of the climate/environmental evolution at the NW European scale. Deposits include 18 lithofacies which are stacked into 10 third-order depositional sequences. At a greater time scale, 4 biosedimentary packages are distinguished by their allochems associations. (1) An intracratonic carbonate environment with coral reefs and crinoid-rich facies is typical of the Early Bajocian. (2) A major facies change occurred at the Early/Late Bajocian transition with a shift from crinoid- and coral-rich facies to ooid-rich facies. During the Late Bajocian, a southward-dipping ooid ramp with successive progradational trends was emplaced. (3) A large lagoon protected by ooid shoals developed during the Bathonian in a muddy ramp setting. (4) At the Bathonian/Callovian boundary, a second major change occurred with the lagoonal facies being superseded by an ooid-bioclastic (crinoids, corals) ramp associated with a waning of carbonate productivity (retrogradational trend). Our geochemical study including a compilation of bibliographic data allows us to refine the existing sea surface palaeotemperature pattern. A cooling from the latest Early Bajocian to the Late Bajocian and a subsequent warming from the earliest Callovian to the Early/Middle Callovian transition are described using a proxy of sea surface temperatures. Together with δ13C and mineralogical data, our new δ18O values suggest a palaeo-climatic/palaeo-environmental control of facies in this shallow carbonate ramp environment.

International audience; The aim of this paper is to outline a methodology to transform depth intervals into geological time intervals. Working in a geochronological system aids in the understanding of the physical properties of rocks and to do very precise correlations between wells. To transform the present coordinates a high resolution tool is necessary : the FMI® (Fullbore Formation MicroImager, Schlumberger). First, well log correlations are established between seven wells at different scales. Relative variations of sedimentation rate are observed locally. Second, FMI® images analysis are used to reveal clear carbonate - clay interbedding which display cycles. Third, to study these cycles, geostatistical tools are used. The variographic analysis of conventional log data shows cycles of one meter. With FMI® data, smaller cyles can be detected. Variogram modelling and factorial kriging analysis suggest that three spatial periods exist. They vary vertically and laterally in wells but cycle ratios are stable and similar to orbital cycle (Milankovitch cycle) ratios. The three periods correspond to eccentricity, obliquity and precession. If orbital cycles are known, depth intervals can be converted into geological time intervals and thus give real sedimentation rates and quantification of hiatus duration.

Shaly formations are the focus of many research programs and consortia sponsored by petroleum companies and/or waste management organizations; given that they act as seals for oil- or gas-bearing reservoirs or as host rock for underground waste disposals, their integrity (e.g., the possible presence of water-bearing fractures) is a critical factor in risk assessment. To model their rheological properties through time, the observed clastic injectites are used as markers of their mechanical evolution. Aptian–Albian marly formations of the Vocontian Basin (southeast France) are the basis of this study; massive turbidite systems associated with large-scale clastic injectite networks have been described in exceptional outcrops. Field data have permitted the identification of early fracturing in the host formation; the injection of sand is an early event, contemporaneous with the deposition of massive sand bodies. The paleocompaction curve has been calculated, and the porosity evolution of the sediments has been restored from sea floor to about 500 m (1640 ft) burial. Then, the original configuration of dikes can be reconstituted. Boundary conditions of various numerical modeling have been derived from this extensive reliable data set. Numerical static simulations of the behavior of marly formations are presented, testing the possible function of heterogeneous lithology, bedrock geometry, or loading by sudden massive sand deposition; they indicate that early fracturing is physically possible in the presented scenarios. The next step will be to simulate in dynamic conditions the opening and filling of some of these cracks by hydrofracturing. We dedicate this chapter to the memory of our colleague and friend, Stephen T. Horseman.

A set of regularly spaced specimens, made up of dark-coloured calcareous argillites, were collected from borehole-cores from the Early Callovian up to the Middle Oxfordian, in the eastern Paris Basin. These specimens have been studied for their magnetic susceptibility, natural and artificial remanence, and for their anisotropy of magnetic susceptibility. Compared with sulfate, carbonate and iron contents, these magnetic properties provide information on both the nature and the relative amounts of the constitutive minerals. To a first approximation, these data outline the main end-members of these argillites, namely (i) carbonate-enriched and less susceptible, (ii) clay-enriched and more susceptible, and (iii) more remanent and partly due to the silt fraction. The middle of the Callovo-Oxfordian formation, called the maximum clay zone, is marked by the highest recorded susceptibility, mainly due to the paramagnetic iron-bearing clay minerals, and by a rather low remanence. The anisotropy of susceptibility provides information on the petrofabric of mostly the clay fraction. The fabric is highly planar, in agreement with the subhorizontal preferred orientation of the clay platelets, but the always present linear anisotropy of the argillites, although modest in strength, points to ∼north–south trending directions throughout most of the formation. These directions agree with the detrital fractions coming from the northern Rhineland, Bohemian and London–Brabant massifs. To cite this article: L. Esteban et al., C. R. Geoscience 338 (2006).

Vascular plant biomarkers are used here as a chemostratigraphic tool for tracing both palaeoflora and palaeoclimatic changes during the Callovian and Oxfordian in the Paris Basin. The retene/cadalene ratio shows low values in claystones from the Middle Callovian to the end of the Lower Oxfordian, then increases drastically from the end of the Lower Oxfordian (cordatum Zone) synchronously with the progressive development of the Oxfordian carbonate platform. The relative proportion of retene, a conifer biomarker, is strongly correlated with the abundance of some of its diagenetic precursors and with saturated diterpanes. Some of these are very common and abundant in the Pinaceae family while specific markers for non-Pinaceae conifers are not detected. Thus, the increase in the retene/cadalene ratio around the cordatum Zone reflects the rising proportion of Pinaceae on the London-Brabant Massif, which was the nearest land from the locations studied. As Pinaceae show many morphological adaptations to dry climate compared to the other plants reported in the Jurassic taphofloras of the Paris basin, their increasing proportion indicates an increasing aridity. This climatic change is also supported by published geochemical, palaeobiogeographical, palynological, palaeobotanical and mineralogical data. A comparable increase in the relative proportion of retene was also described for the Oxfordian of Western Australia, which could be an argument for a worldwide increase in aridity at the end of the Lower Oxfordian.

A long core (1110 m) drilled at Montcornet (northeastern Paris Basin) provides early Jurassic magnetostratigraphic data coupled with biochronological control. About 600 paleomagnetic samples were obtained from a 148-m-thick series of Hettangian and Sinemurian rocks. A composite demagnetization using thermal (up to 300°C) followed by alternating field technique (up to 100 mT) is used to separate the magnetic components. A low unblocking temperature component (