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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.

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.