Guy Vasseur

Effects of mineralogical reactions on trace element redistributions in mantle rocks during percolation processes: A chromatographic approach

Mantle rock studies provide evidence of interaction with upwelling magmas. In erogenic lherzolites, one of the most conspicuous effects of these interactions is the development of harzburgite and dunite bands. Recent studies have suggested that these bands were formed at the expense of the host lherzolites by melt-rock reactions associated with magma percolation. In order to better understand the geochemical effects associated with percolation-reaction processes, we propose a numerical model of melt infiltration that takes into account modal variations in time and space resulting from melt-rock reactions. Melt volume variations are considered by means of porosity variations, and a local equilibrium approach is used for trace element exchange between melt and minerals. The transport of trace elements by the interstitial melt is described by a mass balance equation while the modal variations are constrained by the mineralogical trends observed in refractory peridotites massifs. The model is applied to RBE, Cr and Ni in percolated peridotites affected by an olivine-forming reaction, with the aim of reproducing the evolution of these elements in refractory peridotites from the Ronda massif. Our modelling can explain the negative correlation between the LREE/HREE ratio and the HREE content and between Cr and Ni in the Ronda refractory peridotites. Our results validate the hypothesis that, in the Ronda, the bands of refractory peridotites represent porous-flow channels formed by olivine-forming melt-rock reaction, at increasing melt volume. Because similar geochemical features are observed in ophiolitic peridotites and in mantle xenoliths, it is likely that melt-rock reactions associated with magmatic infiltration are widespread and represent important mantle processes.

Evolution of structural and physical parameters of clays during experimental compaction

Abstract To provide a better understanding of sedimentary basin geological history, it is important to describe correctly the evolution of the various physical, mechanical and hydraulic properties of clayey rocks as a function of burial depth. As a contribution to this field, a programme of experimental studies on reworked clay samples compacted under various load pressures in oedometric conditions has been set up. The evolution of samples under compaction was followed with microscopic and macroscopic measurements. In a specially designed oedometric cell, samples are compacted under different total stresses from 0.1 to 50 MPa. In this cell, cylindrical cores are submitted to progressive loading from both ends under controlled pore pressure conditions. The symmetrical loading allows more even deformation about the midplane of the sample. This device allows the evolution of hydraulic pressure, radial stress, displacement and expelled pore fluid to be followed as a function of time. In a first step, kaolinite was chosen because it retains a high permeability (compared with other clays such as illite or smectite), which allows compaction tests to be performed within a few days. A complete set of measurements was performed after the tests. These measurements are: (1) micro-structures investigated by means of transmission electron microscopy (TEM), mercury porosimetry, water removal under low water vapour pressure, granulometry and specific area measured by ethylene glycol adsorption; and (2) various physical parameters measured including hydraulic conductivity and thermal conductivity. TEM gives an understanding of the arrangement of particles. It was found that each particle is composed of several crystal units, each unit formed by ≈25 individual kaolinite layers. During compaction, these particles remain undeformed, but are rotated. The angular distribution of grain orientation is a function of the applied effective stress. This reorientation is in agreement with the observed decrease in porosity and pore size. It also explains the occurrence of a strong anisotropy in the thermal conductivity and hydraulic permeability. The combination of these experimental results allows a qualitative and quantitative understanding of the behaviour of kaolinite with respect to parameters such as permeability, porosity, mechanical and thermal properties, the knowledge of which are necessary for basin modelling.

Evolution of microstructures and of macroscopic properties of some clays during experimental compaction

Abstract In order to understand the evolution of clay sediments during their burial to three or four kilometers depth, remoulded and non-remoulded clay samples were oedometrically consolidated under loads ranging from 0.1–50 MPa and their microstructural, mechanical and transport properties were studied. For this study, four clays were selected on mineralogical grounds: a kaolinite (St Austell clay) two illitic clays (La Bouzule clay and Salins 14 clay) and an illite-smectite mixed clay (Marais Poitevin clay). Among those materials, two are almost pure clays (St Austell clay and Salins 14 clay) and were compacted in a remoulded state, the other two have a clay fraction about 60% and were studied in non remoulded state for Marais Poitevin clay and both remoulded and non remoulded for La Bouzule clay. The oedometric tests have been performed in classical oedometers and in a new oedometric cell specially designed to measure lateral stresses and to perform hydraulic conductivity measurements under compaction load. The two pure clays show an evolution of their void ratio differing from what is expected considering their liquid limits. The behaviour seems to be governed by the size of the clay particles (around 1 μm for St Austell clay and around 0.1 μm for Salins 14 clay). Marais Poitevin clay, very slightly compacted in its natural state, and remoulded La Bouzule clay have consolidation coefficients (Cc) and void ratios consistent with their liquid limits, a relationship reported in soil mechanics literature (Skempton 1944; Burland 1990; Nagaraj et al., 1994). The natural La Bouzule clay shows very low void ratios and Cc, due to overconsolidation. The microstructure observed by TEM showed that clay particles are platy and made of aggregates of individual crystals, except for Salins 14 clay where individual crystals do not aggregate. The evolution of the microstructure with applied stress shows no change in the particles structure but a progressive reorientation of the particles perpendicular to the load axis. These microstructural observations explain the transport properties measured and their evolution. The thermal conductivity of kaolinite measured in the axial and radial direction reflects the evolution of anisotropy with compaction observed on particles orientation. The different hydraulic conductivities observed on the various samples are compared to those obtained with the Kozeny-Carman formula using two types of specific surface determination. This comparison emphasizes the importance of inter and intraparticule pore size and of their intimate organisation.

1-D Modelling rock compaction in sedimentary basins using a visco-elastic rheology

Abstract Recent experimental studies on unlithified sediments suggest that the compaction is in part viscous. At the basin scale and on a geological time scale, processes such as pressure solution can be approached by creep deformation, subjected to a viscous rheology. In the present work, we have studied porosity reduction and fluid pressure development resulting from the visco-elastic compaction of a sedimentary basin during its formation. Model equations include continuity equations, Darcy’s law and a visco-elastic rheology law which relates the strain rate to the effective stress and to the rate of change of this effective stress. Under the assumption that permeability is a power law function of porosity, the equations become essentially non-linear. Model results illustrate how the decrease of porosity starts at the base of the basin and spreads upward with increasing time. In a wide range of input parameter values calculations indicate a zone of almost linear increasing of pore pressure just below the basin surface, a transition zone of rapidly increasing fluid pressure with a large pressure gradient and a zone of lithostatic fluid pressure below. This is consistent with the general features of zoning of fluid pressure distribution in overpressured areas but zones with high pressure also correspond to low porosity at depth. The relative thickness of the zones depends on time, subsidence velocity and the physical parameters of sediments which can be combined in order to define a characteristic compaction length and a characteristic compaction time. In the upper zone, the decrease of porosity results in a boundary layer; within this layer, the porosity decreases from its initial value down to its minimum value. The deeper zone appears when the time of formation of the given depth basin exceeds the characteristic compaction time and the thickness of the basin is in order of compaction length. Zones of fluid overpressure may also develop due to the spatial variations of the physical properties of the sediments.

Elastoplastic deformation of porous media applied to the modelling of compaction at basin scale

Abstract For simulation and modelling of coupled phenomena occurring during basin evolution, the mechanical aspects of rock deformation are generally restricted to vertical compaction characterized by a simple relation between the effective vertical stress and the rock porosity. Elasto-plasticity leads to a more general formulation which, in principle, allows for the calculation of horizontal deformation and stress field. Various aspects of this application of continuum mechanics to the compaction of sedimentary rocks at basin scale are presented. Firstly, the problems of mechanical deformation and of fluid flow—or pressure evolution—are shown to be intimately coupled through the effective stress concept. The elasto-plastic Cam-Clay rheology is recalled as a satisfactory approach of the stress-strain relationship for fine-grained sediments. This gives the complete bases for numerical modelling of the hydro-mechanical problems related to sedimentary basin evolution. Secondly, two numerical codes which are of standard use in civil engineering problems are tentatively applied to basin modelling. The first code (CESAR) is a finite element one which fully takes into account the hydro-mechanical couplings. The slow sedimentation process, whereby the geological structure is progressively built, can be accounted for by incremental deposition of layers. In practice the computation is so time-consuming that only restricted simulation on existing sedimentary structure can be seriously considered. A second computer code (FLAC) based on finite difference method is then applied. Some special development makes it possible to account for the geometrical evolution (build-up) of a basin and some cases studies are presented to show the importance of lateral deformation during the development of a margin-type basin. However these possibilities were obtained at the expense of a fixed fluid pressure field and we did not succeed in coupling the hydraulical and mechanical computations. Thirdly, a simple incremental mechanical model is proposed for completely solving the coupled hydro-mechanical problem in the case of progressive sedimentation. A numerical solution is obtained in the 1-D case and gives results which are consistent with some published ones. Since it is 1-D, this solution offers only a few advantageous features at present. However generalization to several dimensions can be imagined.

Natural hydraulic cracking: numerical model and sensitivity study

Abstract Natural hydrofracturing caused by overpressure plays an important role in geopressure evolution and hydrocarbon migration in petroliferous basins. Its mechanism is quite well understood in the case of artificial hydraulic fracturing triggered by high-pressure fluid injection in a well. This is not so for natural hydraulic fracturing which is assumed to initiate as micro-cracks with large influence on the permeability of the medium. The mechanism of natural hydraulic cracking, triggered by increasing pore pressure during geological periods, is studied using a fracturing model coupled to the physical processes occurring during basin evolution. In this model, the hydraulic cracking threshold is assumed to lie between the classical failure limit and the beginning of dilatancy. Fluid pressure evolution is calculated iteratively in order to allow dynamic adjustment of permeability so that the fracturing limit is always preserved. The increase of permeability is interpreted on the basis of equivalent fractures. It is found that fracturing is very efficient to keep a stress level at the rock’s hydraulic cracking limit: a fracture permeability one order of magnitude larger than the intrinsic permeability of the rock would be enough. Observations reported from actual basins and model results strongly suggest that natural hydraulic cracking occurs continuously to keep the pressure at the fracturing limit under relaxed stress conditions.

Quantitative evaluation of synsedimentary fault opening and sealing properties using hydrocarbon connection probability assessment

Hydraulic behaviors of faults in sedimentary basins have been paid close attention in studies of hydrocarbon migration and accumulation because of their important functions in basin hydraulic circulations. In previous studies, however, the function of faults in hydrocarbon migration is characterized by the sealing capacity of faults. In fact, sealing is only an impressive and time-dependent aspect of the hydraulic behavior of faults, which may act as seals during some periods and as pathways some time later. Therefore, in hydrocarbon migration studies, sealing indices may successfully be used in some cases but not in others. In this article, we introduce an empirical method (termed the fault-connectivity probability method) for assessing the hydraulic connecting capacity of a fault for hydrocarbon migration over geological time scales. The method is based on the recognition that observable hydrocarbon in reservoirs should result from the opening and closing behavior of the fault during the entire process of hydrocarbon migration. In practice, the cumulative petroleum migration through a segment of the fault zone is identified by the presence (or not) of hydrocarbon-bearing layers on both sides of the segment. Data from the Chengbei step-fault zone (CSFZ) in the Qikou depression, Bohai Bay Basin, northeast China, were used to develop this method. Fluid pressure in mudstones, normal stress perpendicular to fault plane, and shale gouge ratio are identified as the key factors representing fault-seal capacity. They are combined to define a nondimensional fault opening index (FOI). The values of FOI are calculated from the measured values of the key factors, and the relationship between FOI and fault-connectivity probability on any fault segment is established through statistical analysis. Based on the data from the CSFZ, when the FOI is less than 0.75, the fault-connectivity probability is 0; when FOI ranges from 0.75 to 3.25, the corresponding fault-connectivity probability increases from 0 to 1 following a quadratic polynomial relationship; when FOI is greater than 3.25, the fault-connectivity probability is 1. The values of fault-connectivity probability can be contoured on a fault plane to characterize the variations of hydraulic connective capacity on the fault plane. The applicability of this concept for other oil fields (in particular, the quantitative relationship between FOI and fault-connectivity probability) has still to be ascertained.

Some new developments for modelling the geological compaction of fine-grained sediments: introduction

Abstract From 1992–1995 the European Union supported a programme of research entitled “Interdisciplinary Basin Studies”. One objective of the programme was to develop a better understanding of the compaction of fine-grained sediments. Some of the results of the research are presented in the subsequent four papers, which are introduced and summarised in this contribution.

An experimental study of secondary oil migration in a three-dimensional tilted porous medium

A three-dimensional physical experiment was conducted to study secondary oil migration under an impermeable inclined cap. Light-colored oil was released continuously at a slow rate of about 0.1 mL/min from a point at the base of an initially water-saturated porous model. With buoyancy as a primary driving force, a vertical cylindrical shape of an oil migration pathway was observed first, and then a layer-shaped lateral migration pathway was observed beneath the top inclined sealing plate once the oil cluster had reached the top cap. Magnetic resonance imaging was used to observe the migration processes—for example, morphology of the migration pathway, intermittency of oil bubbles, and variation of oil saturation within the migration paths. Results show that the snap-off phenomenon (related to fast local imbibition processes) occurred more commonly during vertical migration than it did during lateral migration. The lateral migration pathway that parallels to the top inclined cap has a typical vertical thickness of 2 to 4 cm (0.8–1.6 in.) (i.e., roughly 40–80 pores). This thickness is consistent with the prediction derived from scaling laws related to pore size and Bond number. Along the lateral migration direction, the sectional area and the horizontal width of the migration pathway fluctuate significantly, although the average oil saturation along the pathway remains almost the same. After stopping the initial oil injection, the sectional area of the migration pathway shrinks significantly. Therefore, we believe that this significant shrinking of the migration pathway is the main reason why only a relatively small volume of oil and gas has been lost during secondary migration.

Time constant of hydraulic-head response in aquifers subjected to sudden recharge change: application to large basins

Analytical formulae are proposed to describe the first-order temporal evolution of the head in large groundwater systems (such as those found in North Africa or eastern Australia) that are subjected to drastic modifications of their recharge conditions (such as those in Pleistocene and Holocene times). The mathematical model is based on the hydrodynamics of a mixed-aquifer system composed of a confined aquifer connected to an unconfined one with a large storage capacity. The transient behaviour of the head following a sudden change of recharge conditions is computed with Laplace transforms for linear one-dimensional and cylindrical geometries. This transient evolution closely follows an exponential trend exp(−t/τ). The time constant τ is expressed analytically as a function of the various parameters characterizing the system. In many commonly occurring situations, τ depends on only four parameters: the width ac of the main confined aquifer, its transmissivity Tc, the integrated storage situated upstream in the unconfined aquifer M = Suau, and a curvature parameter accounting for convergence/divergence effects. This model is applied to the natural decay of large aquifer basins of the Sahara and Australia following the end of the mid-Holocene humid period. The observed persistence of the resource is discussed on the basis of the time constant estimated with the system parameters. This comparison confirms the role of the upstream water reserve, which is modelled as an unconfined aquifer, and highlights the significant increase of the time constant in case of converging flow.RésuméDes formules analytiques sont proposées pour décrire le premier ordre de l’évolution temporelle de la charge hydraulique dans les grands systèmes d’eau souterraine (tels que ceux trouvés en Afrique du Nord ou dans l’Est de l’Australie) qui sont soumis à des modifications drastiques de leurs conditions de recharge (comme ceux du Pléistocène et Holocène). Le modèle mathématique est basé sur l’hydrodynamique d’un système aquifère mixte composé d’un aquifère captif relié à un aquifère libre caractérisé par une grande capacité de stockage. Le comportement transitoire de la charge hydraulique suite à un changement soudain des conditions de recharge est calculé avec des transformées de Laplace pour des géométries unidimensionnels et cylindriques linéaires. Cette évolution transitoire suit de près une tendance exponentielle exp(−t/τ). La constante de temps τ est exprimée analytiquement en fonction des différents paramètres qui caractérisent le système. Dans de nombreuses situations qui se produisent souvent, τ dépend de quatre paramètres: la largeur ac de l’aquifère principal captif, sa transmissivité Tc, le stockage intégré situé en amont de l’aquifère libre M = Suau, et un paramètre de courbure prenant en compte les effets de convergence/divergence. Ce modèle est appliqué à la désintégration naturelle des grands bassins aquifères du Sahara et de l’Australie après la fin de la période humide de l’Holocène moyen. La persistance observée de la ressource est examinée sur la base de la constante de temps estimée à l’aide des paramètres du système. Cette comparaison confirme le rôle de la réserve d’eau en amont, qui est modélisée en tant qu’aquifère libre, et met en évidence l’augmentation significative de la constante de temps dans le case d’un écoulement convergent.ResumenSe proponen formulas analíticas para describir la evolución temporal de primer orden de la carga hidráulica en grandes sistemas de agua subterránea (como las que se encuentran en el norte de África o este de Australia) que están sujetas a modificaciones bruscas de sus condiciones de recarga (como los del Pleistoceno u Holoceno). El modelo matemático se basa en la hidrodinámica de un sistema acuífero mixto compuesto de un acuífero confinado conectado a uno no confinado con una gran capacidad de almacenamiento. El comportamiento transitorio de la carga hidráulica luego de un cambio brusco en la condiciones de recarga se calcula con las transformadas de Laplace para geometrías unidimensionales y cilíndricas lineares. Esta evolución transitoria sigue estrechamente la tendencia exponencial exp(−t/τ). La constante de tiempo τ está analíticamente expresada como una función de varios parámetros que caracterizan el sistema. En muchos casos que ocurren comúnmente, τ depende solo de cuatro parámetros: el ancho ac del principal acuífero confinado, su transmisividad Tc, el almacenamiento integrado situado agua arriba en el acuífero no confinado M = Suau, y un parámetro de curvatura que tiene en cuenta los efectos de convergencia / divergencia. Este modelo se aplica al decaimiento en grandes cuencas acuíferas del Sahara y Australia a partir del final del período húmedo del Holoceno medio. Se discute la persistencia observada en el recurso sobre la base de la constante de tiempo estimada con los parámetros del sistema. Esta comparación confirma el papel de las reservas de agua aguas arriba, la cual se modela como un acuífero no confinado, y pone de manifiesto el significativo incremento de la constante de tiempo en el caso de la convergencia de flujo.摘要提出了描述经历过补给条件剧变(诸如更新世和全新世时期的剧变)大型地下水系统(如北非或澳大利亚东部发现的大型地下水系统)中水头的一级时间演化的解析公式。数学模型基于一个混合含水层系统的水动力学,这个混合含水层系统为一个承压含水层连接着另一个储量巨大的非承压含水层。通过线性一维和圆柱几何Laplace变换计算了补给条件突然变化的水头瞬时特性。这个瞬时演变紧紧遵循着指数趋势经验值(-t/τ)。时间常数τ 解析表达为描述系统的各种参数的函数。在许多经常出现的情况下,τ 只取决于四个参数:主要承压含水层的宽度ac 、承压含水层的导水系数Tc 、非承压含水层上游的综合储量M = Suau 以及表示聚合/散发效应的曲率参数。这个模型应用在了中全新世潮湿阶段结束后撒哈拉和澳大利亚大型含水层盆地的自然衰退中。根据利用系统参数估算的时间常数论述了观测到的资源持久性。这个对比确认了模拟为非承压含水层的上游水储量的作用,强调了汇聚水流情况下时间常数的显著增大。ResumoFórmulas analíticas são propostas para descrever a evolução temporal de primeira ordem da carga em grandes sistemas de águas subterrâneas (como aqueles encontrados no Norte de África ou no leste da Austrália) que são sujeitos a modificações drásticas das suas condições de recarga (como as dos tempos Pleistoceno e Holoceno). O modelo matemático é baseado na hidrodinâmica de um sistema aquífero misto, composto por um aquífero confinado conectado a um aquífero livre com grande capacidade de armazenamento. O comportamento transiente da carga hidráulica, seguido de uma súbita alteração das condições de recarga, é computado com a transformada de Laplace para geometrias lineares unidimensionais e cilíndricas. Essa evolução transiente segue de perto uma tendência exponencial exp(−t/τ). A constante temporal τ é analiticamente expressa como uma função dos vários parâmetros que caracterizam o sistema. Em muitas ocasiões que ocorrem comumente, τ depende apenas de quarto parâmetros: da extensão ac do aquífero confinado principal, da sua transmissividade Tc, do armazenamento integrado situado a montante no aquífero livre M = Suau, e de um parâmetro de curvatura considerando efeitos de convergência/divergência. Este modelo é aplicado ao decaimento natural das grandes bacias aquíferas do Saara e da Austrália após o final do período húmido do Holocénico médio. A persistência observada do recurso é discutida com base na constante temporal estimada com os parâmetros do sistema. Esta comparação confirma o papel da reserva a montante, que é modelada como um aquífero livre, e salienta o incremento significativo da constante temporal no caso de fluxo convergente.