Characterisation of the transmissivity field of a fractured and karstic aquifer, Southern France

International audienceGeological and hydrological data collected at the Terrieu experimental site north of Montpellier, in a confined carbonate aquifer indicates that both fracture clusters and a major bedding plane form the main flow paths of this highly heterogeneous karst aquifer. However, characterising the geometry and spatial location of the main flow channels and estimating their flow properties remain difficult. These challenges can be addressed by solving an inverse problem using the available hydraulic head data recorded during a set of interference pumping tests.We first constructed a 2D equivalent porous medium model to represent the test site domain and then employed regular zoning parameterisation, on which the inverse modelling was performed. Because we aim to resolve the fine-scale characteristics of the transmissivity field, the problem undertaken is essentially a large-scale inverse model, i.e. the dimension of the unknown parameters is high. In order to deal with the high computational demands in such a large-scale inverse problem, a gradient-based, non-linear algorithm (SNOPT) was used to estimate the transmissivity field on the experimental site scale through the inversion of steady-state, hydraulic head measurements recorded at 22 boreholes during 8 sequential cross-hole pumping tests. We used the data from outcrops, borehole fracture measurements and interpretations of inter-well connectivities from interference test responses as initial models to trigger the inversion. Constraints for hydraulic conductivities, based on analytical interpretations of pumping tests, were also added to the inversion models. In addition, the efficiency of the adopted inverse algorithm enables us to increase dramatically the number of unknown parameters to investigate the influence of elementary discretisation on the reconstruction of the transmissivity fields in both synthetic and field studies.By following the above approach, transmissivity fields that produce similar hydrodynamic behaviours to the real head measurements were obtained. The inverted transmissivity fields show complex, spatial heterogeneities with highly conductive channels embedded in a low transmissivity matrix region. The spatial trend of the main flow channels is in a good agreement with that of the main fracture sets mapped on outcrops in the vicinity of the Terrieu site suggesting that the hydraulic anisotropy is consistent with the structural anisotropy. These results from the inverse modelling enable the main flow paths to be located and their hydrodynamic properties to be estimated

Représentation de la réussite scolaire et professionnelle : homogénéité et différenciation en fonction du sexe et de l'origine sociale d'étudiants en lettres et sciences humaines

Jamet G., Massonnat Jean, Mathet M. Représentation de la réussite scolaire et professionnelle : homogénéité et différenciation en fonction du sexe et de l'origine sociale d'étudiants en lettres et sciences humaines. In: Bulletin de psychologie, tome 35 n°353, 1981. Psychologie de l'éducation. pp. 242-256

Influence of the observation scale on permeability estimation at local and regional scales through well tests in a fractured and karstic aquifer (Lez aquifer, Southern France)

International audienceDetermination of permeability in fractured and karstic carbonate reservoirs is of great importance as reservoirs of this type represent a significant proportion of the aquifers and petroleum reservoirs in the world. In fact at a given scale the permeability values determined from the hydrodynamic response to a well test can vary dramatically from one observation well to another. In this study we investigate such permeability variations and their origins at both local and regional scales.Permeability values for a fractured and karstic carbonate aquifer (Lez spring hydrogeological catchment, Southern France) have therefore been analyzed from well tests conducted at local and regional scales: (i) interference tests at the field site scale, (ii) pulse tests at the reservoir scale.Analyzing and comparing the hydrodynamic responses to the pumping tests at both scales show that (i) mean estimated permeability values can increase with observation scale in this particular carbonate reservoir, (ii) at a given scale these values depend dramatically on the location of the observation well used for permeability estimations as the connectivity between the well and the high permeability flow path network depends on this location, and (iii) the water table level or the drilling depth of the observation well appear to be key parameters when estimating permeability values as they also control the connectivity between the well and the high permeability flow path network

Analogue modelling of fault reactivation : tectonic inversion and oblique remobilisation of grabens

Journal of Structural Geology, v. 24, n. 11, p. 1741-1752, 2002. http://dx.doi.org/10.1016/S0191-8141(01)00129-8International audienc

RESERVOIR PROPERTIES OF BARREMIAN – APTIAN URGONIAN LIMESTONES, SE FRANCE, PART 1: INFLUENCE OF STRUCTURAL HISTORY ON POROSITY-PERMEABILITY VARIATIONS

Upper Barremian – Lower Aptian inner platform “Urgonian” limestones in the Mont de Vaucluse region, SE France, consist of alternating metre-scale microporous and tight intervals. This paper focuses on the influence of structural deformation on the reservoir properties of the Urgonian limestone succession in a study area near the town of Rustrel. Petrographic, petrophysical and structural data were recovered from five fully-cored boreholes, from the walls of a 100 m long underground tunnel, and from a 50 m long transect at a nearby outcrop. The data allowed reservoir property variations in the Urgonian limestones to be studied from core to reservoir scale. Eleven Reservoir Rock Types (RRTs) were identified based on petrographic features (texture, grain size), reservoir properties (porosity, permeability), and the frequency of structural discontinuities such as fractures, faults and stylolites. Tight and microporous reservoir rock types were distinguished. Tight reservoir rock types were characterised by early cementation of intergranular pore spaces and by the presence of frequent structural discontinuities. By contrast microporous reservoir rock types contained preserved intragranular microporosity and matrix permeability, but had very few structural discontinuities. Observed vertical alternations of microporous and tight rock types are interpreted to have been controlled by the early diagenesis of the Urgonian carbonates. Deformation associated with regional-scale tectonic phases, including Albian – Cenomanian “Durancian” uplift (∼105 to 96 Ma) and Pyrenean compression (∼55 to 25 Ma), resulted in the modification of the initial petrophysical properties of the Urgonian limestones. An early diagenetic imprint conditioned both the intensity of structural deformations and the associated circulations of diagenetic and meteoric fluids. Evolution of the Reservoir Rock Types is therefore linked both to the depositional conditions and to subsequent phases of structural deformation

RESERVOIR PROPERTIES OF BARREMIAN–APTIAN URGONIAN LIMESTONES, SE FRANCE, PART 2: INFLUENCE OF DIAGENESIS AND FRACTURING

International audienc

Fault zones in limestones : impact on karstogenesis and groundwater flow (Lez aquifer, southern France)

The Lez aquifer in southern France comprises low-porosity karstified limestones and provides drinking water for similar to 400,000 inhabitants. Population growth and climate change have increased the stress on the water resources. In order to provide long-term protection and to optimize the water supply, the hydrogeology of the Lez aquifer must be better characterized. This study focused particularly on the St-Clement major fault zone (12 km long with a 500-m normal throw) which was structurally characterized using accurate geological mapping of the area, outcrops analysis and geophysics tools. The research highlights and explains the close relationship between the fault and the karstic occurrences. Moreover, tracer tests and piezometric head variations in boreholes have shown (1) strong interconnection between the observed karstic formations and (2) the major role of St-Clement fault on mass and pressure transfers in the aquifer. At the reservoir scale, the other major faults of the Lez aquifer, such as Corconne-Matelles or Gourg Noir faults, have shown some common morphologic and dynamic characteristics, and suggest a similar hydrogeological functioning. This study then extends this model to a larger scale. It proposes that, in aquifers of low-porosity carbonates, fault zones control the development of the main karstic network which, in turn, controls the main groundwater flows. Thus, faults should be reconsidered in order to improve the vulnerability studies and the quality of karstic aquifer modelling. Therefore, this report can contribute to protecting the groundwater resource, improving yields and optimizing groundwater supply exploitation in this type of aquifer

Zonas de falha em calcários: impacto na carstogênese e no fluxo da água subterrânea (aquífero Lez, sul da França)

International audienceThe Lez aquifer in southern France comprises low-porosity karstified limestones and provides drinking water for ~400,000 inhabitants. Population growth and climate change have increased the stress on the water resources. In order to provide long-term protection and to optimize the water supply, the hydrogeology of the Lez aquifer must be better characterized. This study focused particularly on the St-Clément major fault zone (12 km long with a 500-m normal throw) which was structurally characterized using accurate geological mapping of the area, outcrops analysis and geophysics tools. The research highlights and explains the close relationship between the fault and the karstic occurrences. Moreover, tracer tests and piezometric head variations in boreholes have shown (1) strong interconnection between the observed karstic formations and (2) the major role of St-Clément fault on mass and pressure transfers in the aquifer. At the reservoir scale, the other major faults of the Lez aquifer, such as Corconne-Matelles or Gourg Noir faults, have shown some common morphologic and dynamic characteristics, and suggest a similar hydrogeological functioning. This study then extends this model to a larger scale. It proposes that, in aquifers of low-porosity carbonates, fault zones control the development of the main karstic network which, in turn, controls the main groundwater flows. Thus, faults should be reconsidered in order to improve the vulnerability studies and the quality of karstic aquifer modelling. Therefore, this report can contribute to protecting the groundwater resource, improving yields and optimizing groundwater supply exploitation in this type of aquifer

Late Barremian–early Aptian ammonite bioevents from the Urgonian-type series of Provence, southeast France: Regional stratigraphic correlations and implications for dating the peri-Vocontian carbonate platforms

This work provides a new ammonite age-calibration of the rudistid limestones of the Urgonian-type Provence carbonate platform (Southeast France) based on sampling along three ~200 km-long platform to-basin transects and re-examination of historical collections. Ammonite key findings indicate that the first rudistid platform stage (including the Agriopleura and requieniidemonopleurid beds) develops and spreads northward through the Toxancyloceras vandenheckii-Gerhardtia sartousiana zones interval (lower upper Barremian). This stage is interrupted by the tectonically-induced deepening of the southern Provence domain during the Imerites giraudi Zone while the northern regions records the massive deposition of PalorbitolinaeHeteraster beds. Recovery of the rudistid carbonate system is illustrated by the development of caprinid-bearing rudistid limestones in the North Provence domain through the Martelites sarasini Subzone (lower Martelites sarasini Zone, uppermost Barremian), which shows a bidirectional progradation toward the South Provence and Vocontian basins. The caprinid-bearing limestones terminate at a shortterm exposure and are overlain by chertyeoobioclastic deposits spanning the Pseudocrioceras waagenoides Subzone (upper M. sarasini Zone) to the lower Deshayesites forbesi Zone. A regional-wide flooding of the study area is illustrated by the abrupt change to a marl-dominated regime occurring in the upper D. forbesi Zone. Compared to the previous datings, the Barremian/Aptian boundary should be relocated in the lower part of the post-caprinid, chertyeoobioclastic deposits although its precise level cannot be fixed due to the lack of a continuous ammonite record. Ammonite age-calibration of the surrounding Urgonian rudistid platform series is discussed and gives evidence of a comparable twofold demise of the peri-Vocontian rudistid biota during the uppermost Barremian. Accordingly, the link between the final demise of the peri-Vocontian rudistid biota and the onset and/or culmination of the mid-early Aptian Oceanic Anoxic Event (OAE) 1a should be reconsidered