Consistent experimental investigation of the applicability of Biot-Gassmann’s equation in carbonates

International audienceCarbonate formations are characterized by multiscale heterogeneities which control theirflow and acoustic responses. At the laboratory scale, carbonate rocks already do not show astrong correlation between P- and S-wave velocities and porosity. The velocity disparitiesbetween carbonates of similar mineralogy and porosity result from different microstructuresderived from their sedimentary facies and subsequent diagenetic transformations. The stilldiscussed applicability of Biot-Gassmann’s equation for fluid substitution in carbonate rocksremains another key issue. We propose an integrated experimental workflow that allows aconsistent checking of the applicability of Biot-Gassmann’s equation and provide key geologicaland microstructural information to understand the petroacoustic signature of carbonate rocks.This approach is implemented on samples representative of two different carbonate formations.The obtained results demonstrate the applicability of Biot-Gassmann's equation for the twostudied carbonate families and show the interrelation of mineralogy and porosity distribution intheir acoustic response

Combined controls of sedimentology and diagenesis on seismic properties in lacustrine and palustrine carbonates (Upper Miocene, Samos Island, Greece)

International audienceFor the subsurface characterization of carbonates, linking physical properties (e.g. porosity and seismic reflectors) with their geological significance (e.g. sedimentary facies and diage-nesis) is of primary importance. To address this issue, we study the lacustrine and palustrine carbonates on Samos Island through a geological and geophysical characterization of a sed-imentary succession. The microstructures of the samples are described, and the samples' physical properties are measured (porosity, P-wave velocity and density). The results show that the identification of only the primary (i.e. sedimentary) microstructure is not sufficient for explaining the huge variations in porosity and P-wave velocity. Hence, we highlight two early diagenetic processes that strongly impact the microstructures and control the physical properties: (i) neomorphism occludes porosity and increases the P-wave velocity of mud-and grain-supported microstructures, which implies a mineralogical stabilization of the grains; (ii) conversely, the dissolution process creates porosity and decreases the P-wave velocity of grain-supported microstructures if the mineralogical composition of the grains is not previously stabilized. These two diagenetic processes thus depend on the primary microstructures and mineralogy of the sediments. This work aims to explain the variations in porosity and P-wave velocity for each defined primary microstructure. A 1-D seismogram is then built to highlight seismic reflectors with a metre-scale resolution. These reflectors are associated with several geological contrasts. Hard kicks (positive amplitude reflectors) match well with exposure surfaces related to palaeosols. They correspond to contrasts between non-modified primary microstructures and highly neomorphosed microstructures. Conversely, soft kicks (negative amplitude reflectors) are linked with diagenetic contrasts (e.g. neomorphosed mi-crostructures versus non-modified primary microstructures) and sedimentary contrasts that can be overprinted by diagenesis (e.g. neomorphosed mud-supported microstructures versus dissolved grain-supported microstructures). This study highlights that high-resolution seismic reflectors of lacustrine and palustrine carbonates are strongly related to the spatial contrasts of primary microstructures overprinted by early diagenesis

Effective medium modeling of diagenesis impact on the petroacoustic properties of carbonate rocks

International audienceCarbonate formations are highly heterogeneous, and the velocity-porosity relationships are controlled by various microstructural parameters, such as the types of pores and their distribution. Because diagenesis is responsible for important changes in the microstructure of carbonate rocks, we have extended the standard effective medium approach to model the impact of diagenesis on the carbonate elastic properties through a step-by-step effective medium modeling. Two different carbonate rocks deposited, respectively, in lacustrine and marine environments are considered in this study. The first key step is the characterization of the diagenesis, which affected the two studied carbonate sample sets. Effective medium models integrating all of the geologic information accessible from petrographic analysis are then built. The evolution of the microstructural parameters during diagenesis is thoroughly constrained based on an extensive experimental data set, including X-ray diffraction analysis, different porosimetry methods, and ultrasonic velocity measurements. A new theoretical approach including two sources of compliance is developed to model the specific behavior of carbonates. A compliant interface is introduced around the main carbonate grains to represent grain contacts and the different pore scales are taken into account through multiscale modeling. Finally, direct calculations with the model provide elastic wave velocities representative of the different diagenetic stages. An extrapolation to permeability evolution is also introduced. This approach allows the identification of the acoustic signature of specific diagenetic events, such as dolomitization, dissolution, or cementation, and the assessment of their impact on the elastic properties of carbonates

Simulation stochastique couplée fades et diagenèse. L'exemple de la diagenèse précoce dans la formation Madison (Wyoming, USA)

Cette etude propose une approche integree visant areproduire ala fois la repartition des facies sedimentaires et des phases diagenetiques associees, au sein d'un modele statique de reservoir. Dans le Wyoming (USA), la Formation Madison (d'age Mississippien) est une formation carbonatee, epaisse de 200 a340 m, affleurant dans plusieurs zones du bassin d'avant-pays des Bighorn. Au sein de cette serie, nous avons identifie neuf facies sedimentaires, groupes en trois sequences de facies (basees sur l'empilement vertical des facies): I) une sequence inter- a supratidale; 2) une sequence subtidale peu profonde a intertidale; 3) une sequence subtidale profonde et ouverte. Ces facies ont ensuite ete integres au sein d'un modele de depots synthetique, correspondant a une rampe carbonatee evoluant graduellement vers une geometrie de plate-forme dont seule la partie la plus interne est reconnue. Ce travail a permis de proposer un cadre chronostratigraphique pour la serie, qui couvre au moins six sequences de depots de troisieme ordre (certaines d'entre elles etant localement absentes car erodees). L'etude de la diagenese s'est concentree sur I'identification et la succession des phases diagenetiques precoces (micritisation, cimentation calcitique, dolomitisation ...). Pour les besoins de la modelisation, sept "empreintes diagenetiques" ont ete definies, chacune d'entre elles correspondant a une succession de phases diagenetiques coexistant au sein d'un meme facies sedimentaire. De plus, nous avons quantifie la proportion relative de chaque empreinte diagenetique affectant un facies sedimentaire. Un modele maille 3D a alors ete bati afin de reproduire I'organisation des facies des trois premieres sequences de depots (qui sont les mieux documentees). Le maillage est donc base sur les quatre limites de sequences reconnues sur toutes les coupes. Les relations entre facies sedimentaires et diagenese ont ete utilisees pour definir les parametres de simulation. Ces dernieres sont basees sur des algorithmes plurigaussien et emboites. Finalement, nous proposons une discussion sur la distribution des heterogeneites reservoirs potentielles, prenant en compte a la fois les caracteristiques sedimentaires (facies, architecture, continuite laterale... ) mais egalement I'impact de la diagenese

Morphologie, évolution latérale et signification géodynamique des discontinuités sédimentaires. Exemple de la marge Ouest du Bassin du Sud-Est (France)

M. Hervé PHILIP PrésidentM. Gilles MERZERAUD Directeur de ThèseM. Michel SERANNE Co-Directeur de ThèseM. François GUILLOCHEAU RapporteurM. Bernard PEYBERNES RapporteurM. Marc FLOQUET ExaminateurM. Christophe RIGOLLET ExaminateurIn the Lodève region (« Caussenard High », southwestern margin of the South-East Basin), liasic deposits are composed of peritidal dolomitic facies of Hettangian age, overlained by Sinemurian shallow water carbonate ramp deposits, capped by a major hardground. A major change in sedimentation then occurred, with the development of black marls typical of deep water conditions. Detailed descriptions of eleven field sections, including a precise analysis of the sedimentary facies and of their organisation has enabled us to show that this sinemurian series can be divided in two sedimentary groups, corresponding to two types of shallow platforms.The first group is mainly composed of low-energy facies (tidal flat and lagoonal facies). The second one is characterized by the absence of dolomitic tidal flat facies and by the increase of opened and external facies. The facies partitioning has shown that lateral facies changes occur rapidly over very short distances (100s of meter). Moreover, no consistent landward or seaward direction seems to be indicated by the pattern of facies changes. It is thus impossible to rationalise the studied facies within a traditional shoreline model as previously proposed for this region. We suggest a new carbonate platform model, based on the observations presented in this paper (Fig.11). The Lodève region platform is hence envisaged to have been a “mosaic” of shallow, intertidal to supratidal “islands” between which restricted subtidal or open conditions could coexist over very short distances (hundreds of meter). The local tectonic setting, characterized by numerous synsedimentary faults (SW-NE and NW-SE), seems to locally control the sedimentation, by defining parts of the basement on which shallow, peritidal facies are able to develop, and subsiding parts where subtidal facies occur.Numerous sedimentary discontinuities developed in this particular setting. On the basis of several universally applicable criteria (geometry, morphology biological activity, mineralization, early diagenesis...), four groups of surfaces have been identified : 1) emersion surfaces (karstic surfaces, palaeosoils, desiccation cracks) ; 2) condensation surfaces (softgrounds, firmgrounds, different types of hardgrounds) ; 3) Submarine erosion surface ; 4) “polyphase surfaces” which exhibit emersion, erosion and condensation criteria. Detailed sedimentological study has enabled us to show the lateral continuity of these surfaces, and their morphological variations. The vertical distribution of surface types in the studied sections has also been discussed, and compared with the sequential framework. Correlations between “Caussenard high”, “ardèche” paleomargin of the subalpine basin” and Sologne paleomargin of the Paris basin, have enabled us to distinguish major discontinuities. Lotharingian hardgound of the “Caussenard high” series seems to record a regional tectonic event, known as Lotharingian crisis and observed in the Causses Basin, the Quercy or the Paris Basin. The Hettangian-Sinemurian transition, well-known in the Paris Basin, could be more enigmatic in the Caussenard High series, consisting of a repetition of karstic surfaces in the dolomitic series.La série liasique du Seuil Caussenard, située sur la marge Sud-Ouest du Bassin du Sud-Est Français est constituée de dépôts dolomitiques hettangiens épais, suivis de faciès à dominante calcaire, d'âge Hettangien supérieur à Lotharingien. Cette série carbonatée est coiffée par un niveau condensé durci, associé à de l'érosion, qui marque la transition Lias calcaire – Lias marneux (Toarcien). Le levé banc par banc de onze coupes géologiques balayant l'intervalle Sinémurien s.l., a permis de montrer que cette série s'organise en deux ensembles sédimentaires correspondant à deux types de plates-formes carbonatées qui se relaient au cours du temps. Le premier ensemble voit surtout se développer des faciès de faible énergie (de replat de marée, de lagune). Le second ensemble voit la disparition des faciès dolomitiques et la multiplication des faciès ouverts, profonds. L'étude régionale de la répartition latérale de ces faciès a montré des variations rapides et sur de très faibles distances (de la centaine de mètres au kilomètre). De plus, il s'avère impossible de définir une réelle polarité « amont-aval » entre une zone continentale et un domaine marin franc à l'échelle régionale et il est rapidement apparu comme impossible de hiérarchiser les faciès sédimentaires le long d'un profil de dépôts de plate-forme barrée classique. Nous proposons donc un nouveau modèle de dépôts basé sur ces observations. La région de Lodève est envisagée comme une mosaïque de zones hautes, très peu profondes, à caractère inter- à supratidal, entre lesquelles peuvent se développer des zones plus profondes, infralittorales à circalittorales. Le contexte tectonique local, caractérisé par la présence de nombreux réseaux de failles SW-NE et NW-SE, dont certaines ont joué de façon synsédimentaire, contrôle la formation de ces zones hautes et basses, et affecte la sédimentologie et l'épaisseur de la série du Lias calcaire.Cette première phase de l'étude a permis de définir le contexte géodynamique régional, dans lequel se développent les nombreuses discontinuités sédimentaires. En se basant sur différents critères, d'ordre morphologique, biologique ou diagénétique, les discontinuités ont été classées en quatre grandes catégories : surfaces d'émersion (surfaces karstiques, paléosols, coupelles de dessiccation) ; niveaux de condensation de différents types (softgrounds, firmgrounds, différents types de hardgrounds) ; surfaces d'érosions sous-marines ; surfaces composites présentant en association des critères d'érosion, d'émersion et de condensation. L'étude sédimentologique et le découpage en unités génétiques ont permis de proposer des corrélations régionales dans ces dépôts du Lias et d'examiner le « comportement latéral » des différentes discontinuités limitant ces unités (continuité, changement de morphologie ou de nature, maturité, etc...). Il a également été possible de discuter de leur répartition verticale dans la série en fonction du cadre séquentiel et suivant le contexte paléotopographique et tectonique local.Les corrélations réalisées entre la série du Seuil Caussenard, la marge Ardéchoise et la marge de Sologne (Bassin de Paris) ont enfin permis de déterminer l'existence de discontinuités majeures. Le hardground au sommet du Lotharingien marque un évènement tectonique pluri-régional (connu sous le nom de crise Lotharingienne) enregistré dans les Causses, le Quercy ou encore le Bassin de Paris. La transition Hettangien-Sinémurien bien visible dans le Bassin de Paris ne se marque, sur le Seuil Caussenard, que par un « niveau diagnostique », au sein duquel s'observent plusieurs surfaces karstiques

Morphologie, évolution latérale et signification géodynamique des discontinuités sédimentaires (exemple du Lias de la marge ouest du bassin du sud-est (France))

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Nature and distribution of diagenetic phases and petrophysical properties of carbonates: The Mississippian Madison Formation (Bighorn Basin, Wyoming, USA)

International audienceThe purpose of this study is to characterize factors controlling the nature and distribution of the diagenetic phases that gave rise to carbonate reservoirs. To do so, a pluridisciplinary approach was carried out, integrating sedimentologic and diagenetic studies on the Madison Formation (Lower Carboniferous, 357-340 My) which is a carbonate reservoir in subsurface.The Madison Formation, outcropping in the Bighorn Basin (Wyoming, USA), is a 340 m thick carbonate series composed of four to seven 3rd-order depositional sequences (S1 to S7) depending of the palaeogeographic location. The first three sequences (S1 to S3) were deposited under arid and warm conditions during Tournaisian times which favoured high accumulation of carbonates leading to a morphological change from a quite flat ramp (S1 to S2) to a wide platform (S3). It also probably favoured the early calcite cementation (isopachous and syntaxial calcite cements) of the subtidal deposits and the early dolomitization (D1) of the supratidal to intertidal sabkha ones. In addition, the very flat profile occurring during S3 was also responsible for the postponed dolomitization (D2) of S1-S2 due to reflux of brines, at various degrees depending on the palaeogeographic location. The deposition of S4 to S7 under humid conditions during Visean times were associated with 1) a decrease of the carbonate accumulation and of the dolomitization; 2) dissolution processes at micro- (pores network) and macro-scales (karst, collapse breccia...) and calcite cementation (C1). The spatial distribution of all these first diagenetic phases acts as a controlling factor on the distribution of the later burial diagenetic phases. Thus, the mesogenetic calcite cements are mostly observed in the secondary porosity created by dissolution (karsts, collapse breccias) or dolomitization. The burial diagenesis of the Madison Formation were characterized by 1) a dolomitization phase (D3) in proximal parts of the platform leading to an increase of porosity and permeability and 2) calcite cementations (C2-C3) in the distal parts of the platform leading to a decrease of these properties.The Madison Formation provides a good spatial representation of sedimentary and diagenetic heterogeneities that may occur in the carbonate reservoirs due to various palaeogeographic locations, climates, burial history and structures

New insight of sedimentological and geochemical characterization of siliciclastic-carbonate deposits

This article is a first attempt of combining sedimentological analysis and geochemical systematics of the Alveolina Limestone Formation as a tool to identify the major stratigraphic surfaces, and to improve the sequence stratigraphy interpretation. This formation is Early Eocene in age and crops out in several well-exposed cliffs in the Serraduy – Roda de Isabena area (Graus-Tremp basin, NE Spain). Within this succession, nineteen carbonate and siliciclastic facies have been identified and grouped in environmental facies associations (based on their vertical stacking and lateral relationships) : 1) coastal plain; 2) clastic deltaic complex; 3) shallow carbonate inner-ramp; 4) midramp; 5) outer-ramp; 6) reefal facies. The depositional architectures studied in the Serraduy area can be directly assessed on the field, and a 3D reconstruction is proposed. This enables us to build a synthetic depositional model and to identify five small-scale T/R cycles, bounded by different kinds of sedimentary discontinuities : angular unconformity, firmground, erosional surface… In parallel, geochemical analyses (C and O isotopes, major, minor and trace elements) were carried out to help at hierarchizing the cycles and the boundaries previously identified. Four of them may be considered as major stratigraphic surfaces, corresponding either to regional-scale angular unconformities, or to exposure surfaces. The latter are characterized by a selective dissolution, a slight but sharp decrease in δ13CV-PDB and in Mg, Fe and Sr contents below the surface. The absence of typical sedimentary criteria of exposure (with the exception of these geochemical signatures) may be explained by short-term exposure, an arid to semi-arid climate, and a dominant low-magnesian calcite original mineralogy, precluding the development and the preservation of widespread vadose diagenetic products. A new sequence stratigraphy model for the Alveolina Limestone Fm is finally proposed and discussed