Abstract Examination of petrophysical properties (acoustic velocity, porosity, permeability, and density) and petrographical characteristics (texture, facies composition, and diagenesis) of more than 250 core plugs from the Middle Jurassic carbonates of the eastern Paris Basin provides insights into the parameters controlling acoustic velocities in relatively low-porosity carbonate rocks (Φ < 20%). The pore-type observations reveal distinct acoustic velocities in samples with intergranular macropores and samples with micropores in subhedral micrite, such that velocities in microporous mudstone–wackestone (lagoonal) deposits are greater than in macroporous grainstone (shoal) samples, at a given porosity range (15–20%). The standard Wyllie and Raymer transforms fit very well with the linear regression between acoustic velocity and porosity from mudstone or lagoonal facies. Marls and fine-grained deposits interpreted as lagoonal facies include statistically significant correlation (r = 0.9) between velocity and porosity. However, the data suggest that the wide scatter in velocity–porosity relationship from grainstones are not the result of different sorting, grain size, pore type, dolomite content, or clay content. Instead, early cementation greatly influences acoustic properties during diagenesis, and are interpreted to account for the high variability of velocities over a given porosity range. Specifically, at a given porosity, acoustic velocities in compacted grainstone that did not undergo early cementation are higher than in early-cemented grainstone. Petrographic observations suggest that early cementation limits mechanical compaction, creating a heterogeneous medium from the earliest stages of diagenesis (non-touching grains, preservation of intergranular macropores that are partially to totally filled by later blocky calcite cement). The abundant interfaces between micritized ooids, early cement fringes, and blocky calcites in grainstones may induce significant wave attenuation. As a result, the standard time-average equations fail to predict the effect of diagenetic features such as early cementation on sonic velocity. Conversely, an absence of early cementation favors mechanical compaction, grain-to-grain contact, and suturing. The result is a homogeneous micritized grain-supported network that may facilitate wave propagation. Through demonstration of the key role of early cementation in the explanation of variability in acoustic properties, the results of this study illustrate the complicated factors influencing velocity transforms in carbonates (Wyllie and Raymer), i.e., classical tools for predicting reservoir properties. These insights on the interpretation of Vp and the refinement of velocity–porosity transforms in grainstone units may be broadly applicable to enhancing seismic-based exploration in carbonate successions.
Combined Sr and Caisotopic tracers have beenappliedto stratabound fluorite deposits in the central part ofBurgundy (France). These deposits are spatially related to an unconformity between aPalaeozoic granitic and metamorphic basement and Late Triassic sediments. The aim of this work is to trace the origin of fluid(s)fromwhich stratabound fluorite deposits formed. We suggest thatthe variations of the δ44CaSWin fluorite (-0.1 to +0.2‰ relative to seawater sw) could be explained by1)precipitation from a fluid dominated by a Ca isotopic signature similar to that of seawater (calcite or dolomite dissolution from the host-rock) and a Sr isotopic ratio higher than seawater (granitic basement) and involves no fractionation, the preferred scenarioor 2)cristallization from a fluid with a lowδ44CaSW(carbonate and silicate sources) involving fractionationprocesses.
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Abstract The Calcaires du Barrois Formation is a succession of dominantly micritic limestone of Kimmeridgian to Tithonian age, outcropping in the eastern part of the Paris Basin. This is an active karstic aquifer of interest to Andra (French National Agency for Radioactive Waste Management), who are studying the feasibility of a deep geological repository of radioactive waste in an underground research laboratory located approximately 450 m below the surface. Surface installations of the Industrial Centre for Geological Disposal project are planned in the upstream recharge zone of the aquifer. It is of primary interest to characterize the Calcaires du Barrois Formation to provide guidelines for the planning and sizing of these facilities, to minimize the impact on the aquifer system. An integrated study was designed for this purpose, linking petrography (thin section and scanning electron microscopy), carbon and oxygen stable isotope geochemistry, X-ray diffraction, petrophysics and geomechanics. It is based on the analysis of three key cored wells penetrating the formation at different relative depths. The Calcaires du Barrois have undergone several stages of diagenesis that have defined the current properties. Unconformities associated with the Jurassic–Cretaceous transition led to prolonged early subaerial exposures during which freshwater flowed efficiently through the upper half of the formation. Through mineralogical stabilization, among other processes, microporosity was preserved in micrites in this interval consisting of clean limestone with thin marl layers. The lower half of the formation, more argillaceous, was not or was only slightly affected by this early meteoric diagenesis, and recrystallization and cementation of micrites occurred during burial diagenesis, involving chemical compaction. Later, during the return to the surface associated to the Cenozoic orogens, another phase of meteoric diagenesis affected the uppermost few metres below the outcropping portions of the formation, but without modifying significantly the previously acquired petrophysical properties. Consequently, an intra-formational boundary was progressively developed at around 75 m (from the top reference). This boundary separates (1) a lower half of the Calcaires du Barrois with dense and tight micrites, showing high Young's modulus values, and a moderate intensity of fractures, from (2) an upper half with microporous micrites showing low Young's modulus values, and almost devoid of fractures. A transitional zone of about 30 m thick, with intermediate properties, sitting above this boundary and below the only thin metre-scale macroporous grainstone level of the formation, accommodated most of the deformation linked to the Cenozoic Western European orogens and is intensively fractured. The current hydrogeological model considers a purely sedimentological boundary to delimit two sub-aquifers within the Calcaires du Barrois Formation. This will have to be reappraised since it is here demonstrated that the real boundary is located significantly higher in the formation and is inherited from a multi-stage diagenetic history. These findings will complement and influence planning for the Industrial Centre's project.
