osmotic generation of anomalous fluid pressures in geological environments osmotic pressures are generated by differences in chemical potential of a solution across a membrane but whether osmosis can have a significant effect on the pressure of fluids in geological environments has been controversial because the membrane properties of geological media are poorly understood anomalous pressures large departures from hydrostatic pressure that are not explicable in terms of topographic or fluid density effects are widely found in geological settings and are commonly considered to result from processes that alter the pore or fluid volume which in turn implies crustal changes happening at a rate too slow to observe directly yet if osmosis can explain some anomalies there is no need to invoke such dynamic geological processes in those cases here i report results of a nine year in situ measurement of fluid pressures and solute concentrations in shale that are consistent with the generation of large up to 20 mpa osmotic pressure anomalies which could persist for tens of millions of years osmotic pressures of this magnitude and duration can explain many of the pressure anomalies observed in geological settings they require however small shale porosity and large contrasts in the amount of dissolved solids in the pore waters criteria that may help to distinguish between osmotic and crustal dynamic origins of anomalous pressures

fluid overpressure estimates from the aspect ratios of mineral veins abstract several hundred calcite veins and mostly normal faults were studied in limestone and shale layers of a mesozoic sedimentary basin next to the village of kilve at the bristol channel sw england the veins strike mostly e w 239 measurements that is parallel with the associated normal faults the mean vein dip is 73 n 44 measurements field observations indicate that these faults transported the fluids up into the limestone layers the vein outcrop trace length 0 025 10 3 m and thickness 0 1 28 mm size distributions are log normal taking the thickness as the dependent variable and the outcrop length as the independent variable linear regression gives a coefficient of determination goodness of fit of r2 0 74 significant with 99 confidence but natural logarithmic transformation of the thickness length data increases the coefficient of determination to r2 0 98 indicating that nearly all the variation in thickness can be explained in terms of variation in trace length the geometric mean of the aspect length thickness ratio 451 gives the best representation of the data set with 95 confidence the true geometric mean of the aspect ratios of the veins lies in the interval 409 497 using elastic crack theory appropriate elastic properties of the host rock and the mean aspect ratio the fluid overpressure that is the total fluid pressure minus the normal stress on the fracture plane at the time of vein formation is estimated at around 18 mpa from these results and using the average host rock and water densities the depth to the sources of the fluids below the present exposures forming the veins is estimated at between around 300 m and 1200 m these results are in agreement to those obtained by independent isotopic studies and indicate that the fluids were of rather local origin probably injected from sill like sources water sills inside the sedimentary basin

osmotic model to explain anomalous hydraulic heads laboratory experiments have shown that compacted clays act as osmotic membranes when they separate aqueous solutions of unequal ionic concentration theoretically osmotically induced differential hydraulic pressure in groundwater systems can be relatively high the magnitude depends primarily upon concentration differences across the membrane type of ions type of clay and pore size in experiments thin compacted clay membranes commonly exhibit varying degrees of osmotic efficiency due to ion leak age through the clay in natural systems the membrane and the solution containers are not as distinct and well defined as they are in the laboratory moreover the membrane is commonly thick inhomogeneous and composite in a buried triassic basin at the savannah river plant near aiken south carolina it is suspected that osmosis causes the saline water in the basin center to be slightly geopressurized in relation to freshwater in the overlying coastal plain aquifer two wells have heads of 7 88 and 12 98 bars 114 3 and 188 3 psi above the head in the coastal plain aquifer the head in each of these wells approximates the osmotic equilibrium head calculated from solution concentration of water produced by each well 12 000 and 18 500 mg l respectively other wells penetrating the top and edge of the triassic basin probably penetrate a zone where ion leakage gives rise to less saline water thus these wells are not geopressurized

extension tectonics in the wessex basin southern england the permian to cretaceous tectonic evolution of the wessex basin was controlled by horizontal tensional and vertical isostatic forces within the lithosphere the gross morphologies of its constituent structures were governed by the location of variscan thrust and wrench faults in the upper and middle crust which suffered extensional reactivation in tensional stress fields oriented approximately nw se several episodes of crustal extension can be resolved in early permian early triassic early jurassic and late jurassic early cretaceous times these were characterized by the rapid subsidence of fault bounded basins and commonly by erosion of adjacent upfaulted blocks superimposed upon the fault controlled subsidence dominant during periods of fault quiescence and becoming increasingly important with time a component of regional subsidence is considered to have a thermal origin this suggests that crustal extension was accompanied by some form of not necessarily uniform lithospheric thinning subsidence analyses assuming local airey isostasy give cumulative crustal extension factors of 20 28 beneath the grabens a more reasonable assumption of regional airey compensation indicates basinwide crustal extension of 13 17 which is consistent with birps offshore deep seismic reflection data

use of vertical profiles of δ18o to constrain estimates of hydraulic conductivity in a thick unfractured aquitard vertical distributions of 18o were used to constrain hydraulic conductivity k in a thick unfractured aquitard previous investigations utilizing permeability tests at a variety of scales keller et al 1989 indicated that bulk k at the site ranges from 3 5 0 11 m s to 1 2 10 10 m s advective displacements calculated on the basis of these k values lead to a wide range of predicted distributions of natural tracers after 11 000 years comparison of 18o distributions and simulated advection diffusion profiles provide a more precise constraint on advective displacement and hence on k the simulations cast in terms of a peclet number p v2t d ranging in value from 0 to 12 indicate that transport is diffusion controlled and that k is actually less than 1 2 10 11 m s p greater than 0 1 k 1 2 10 11 m s overestimates transport this result indicates that conventional permeability tests in very tight media tend to overestimate k in practical terms advective transport at the site is negligible and transport can be simulated with diffusion only p o

fault controlled sequential vein dilation competition between slip and precipitation rates in the austin chalk texas abstract multi layered calcite veins in a dilatant jog of a left stepping left slipping shallowly buried fault segment are composed of alternating millimeter to submillimeter thick calcite veinlets and host lithons forming a coarse crack seal texture the grain fabrics in calcite veinlets are mostly equant or irregular suggesting face controlled grain growth in a fluid filled cavity the relatively thick veinlets can be developed by progressive fault slip and veinlet opening simultaneously with calcite precipitation under low effective stress continuous changes in the oxygen isotopic compositions of the calcite veinlets along the length of veins suggest that the individual calcite veinlets were sequentially developed from the footwall to the hanging wall there is no particular evidence that these veins represent excursions in fluid pressure or instantaneous fracture opening related to episodic fault slip the fracture formation and filling cycle could have taken place along a continuously slipping fault contained within a porous rock with normal fluid pressure

hydraulic and chemical pulse tests in a shut in chamber imbedded in an argillaceous formation numerical and experimental approaches hydraulic and chemical pulse tests were performed in four shut in chambers to obtain hydraulic and chemical parameters of the callovo oxfordian shale the osmotic tests i e chemical pulses were carried out by exchange of the formation water for a different solution either more or less concentrated two sets of two osmotic tests were done in march and october 2006 these osmotic tests are superimposed on a purely hydraulic response because of the drilling of the well causing a pressure drop the pressure records hydraulic and osmotic responses were interpreted using a 2 d model to obtain the hydraulic and chemical parameters the osmotic efficiency inferred for the callovo oxfordian shale is on the order of 0 012 for a concentration of 0 43 mol l 1 and 0 12 for a concentration of 0 086 mol l 1 these results suggest that the callovo oxfordian shale behaves as a weakly semipermeable membrane and only 0 1 0 15 mpa of the 0 2 0 5 mpa can be explained by these osmotic efficiency values

selective reverse reactivation of normal faults and deformation around reverse reactivated faults in the mesozoic of the somerset coast abstract normal faults exposed in the triassic jurassic limestones and shales of the somerset coast were formed during the mesozoic development of the bristol channel basin reverse reactivation of some of these normal faults occurred during late cretaceous to early tertiary north south contraction the contraction is also evident from thrusts and conjugate strike slip faults preferential reactivation of the normal faults is attributed to 1 decreased fault plane friction 2 domino block rotation 3 displacement magnitude and 4 fault connectivity the geometries of overlapping and underlapping zones in reactivated fault zones are dependent on the existing structural geometry two distinctive styles of displacement accommodation occur between reverse reactivated normal faults 1 formation of a network of strike slip faults conjugate about nne ssw and 2 oblique steeply dipping reverse faults interaction between strike slip and an existing fault is dependent on whether the normal fault was reactivated the range of structures related to the north south contraction has been incorporated into a single deformation model controlled by the northwards movement of the hanging wall of the quantock s head fault pure dip slip movement occurred in the centre of its curved fault trace with a sinistral component at the western tip and a dextral component of displacement and strike slip block rotations occurred at the eastern tip shortening of these blocks was achieved through development of a strike slip fault network and nw striking thrusts in an underlap zone loading of the footwall by the hanging wall block modified the local stress system to allow formation of oblique steeply dipping reverse faults

experimental support for a predictive osmotic model of clay membranes osmosis has been cited as a mechanism for explaining anomalously high fluid pressures in the subsurface clays and shales act as membranes and osmotic flux across these units may result in pressures sufficiently high to explain these anomalies the theoretical osmotic pressures as calculated solely from solution properties can be quite large however it is not yet resolved whether these geologic membranes are sufficiently ideal to generate such pressures

a slug test to assess the osmotic and hydraulic properties of argillaceous formations pulse tests in pressurized chambers represent a very efficient method to estimate the hydrodynamic properties of low permeability formations useful analytical solutions for purely hydraulic pulse test interpretation have been proposed in the present note an extension of the classical model by bredehoeft and papadopulos 1980 is developed for a single hydraulic and or chemical osmotic pulse test involving laplace transforms this solution is used to identify graphical variables in an osmotic pulse test record which can be used to estimate the relevant hydrochemical parameters

analytical and numerical solutions for alternative overpressuring processes application to the callovo oxfordian sedimentary sequence in the paris basin france 1 previous studies that made use of basin models have shown that the normal geological evolution of the paris basin does not generate the observed albeit weak excess pressures in some shale layers of the basin other processes that may have created the overpressures currently neglected in such models are investigated here terms accounting for osmotic effects and tectonic stress changes are successively added to the diffusivity equation the effect of changes in outcrop boundary conditions is also calculated with a pseudo two dimensional analytical solution these solutions are applied to the callovo oxfordian shale formation in the eastern part of the paris basin france it is shown that a long term transient osmotic effect starting in the tertiary could explain in part the observed excess pressures in the callovo oxfordian shale assuming effective diffusion coefficients of 1 5 10 12 m2 s 1 in line with the measurements and a pore radius b around 20 a for the shales however because of the uncertainty on the value of the shale pore radius additional head measurements and osmotic experiments on samples should be made to fully establish the possibility of an osmotic process our study also shows that recent changes in hydrodynamic boundary conditions could also explain excess pressure distribution in this shale layer it is plausible that a combination of the two processes could best explain the distribution and intensity of the overpressures tectonic stress changes do not appear to be important it is shown that for such processes to maintain high pressures strong and recent increase in tectonic compressive stress would be required

joints in the mesozoic sediments around the bristol channel basin abstract analysis has been carried out at four locations on the edges of the bristol channel basin to illustrate the later phases of deformation of a sedimentary basin and to illustrate the control on joint patterns of subtle changes in the stress system the characteristics of the joints are described and influences on joints are determined including the roles of faults folds and beds there is a low coefficient of correlation between joint spacing and bed thickness except in very thin limestone beds which have a high density of joints the lengths and spacings of earlier joint phases are usually greater than those of later phases later joints normally abut against earlier joints the joints abut the latest faults but are not displaced by them so the joints post date the main alpine contraction the joints formed in five main phases during reduction of the alpine stresses phase 1 joints are sub parallel to the regional compression direction 160 180 phase 2 joints are perturbed by faults often curving towards points of stress concentrations along the faults phase 3 joints are sub parallel to the earlier e w striking fold axes phase 4 joints are cross joints and phase 5 joints form polygonal patterns within joint bound blocks phases 2 and 3 do not occur in the absence of faults and folds and correspond with a reduction in horizontal compression and an increase in the importance of local factors phases 4 and 5 occur at all locations

stress field determinations in france by hydraulic tests in boreholes the stress field has been determined at eight different sites in france four in crystalline or metamorphic rocks four in sedimentary formations by hydraulic tests in boreholes for seven of these sites the complete stress field has been determined using the hydraulic tests on pre existing fractures htpf inversion method validity of the results is demonstrated by the good fit between a priori and a posteriori values for the data and by the low values of the a posteriori standard deviation on the unknowns for four sites results have been compared with those derived according to the hydraulic fracturing theory the two methods yield comparable results for the orientation of the maximum horizontal principal stress h a generally satisfactory fit for the magnitude of the minimum horizontal principal stress h but a very poor agreement for the magnitude of h this latter misfit has been attributed to the effect of fluid percolation prior to the actual opening of the fractures below a depth of 500 m a homogeneous h direction n150 e has been determined at the crystalline sites auriat echassieres le mayet de montagne chassoles all located in the northern massif central for three of these sites the vertical stress is significantly lower than the weight of overburden as computed from the rock density and the depth of the corresponding measurement at auriat and echassieres the stress field is consistent with a mostly strike slip faulting regime at le mayet de montagne and chassoles the maximum stress is vertical but nearly equal to h the stress field has been found to be much more heterogeneous at two of the four sites in sedimentary rocks because of the large variability in mechanical rock properties in such heterogeneous formations inversion with the htpf method must be limited to those data which pertain to the same rock horizon however because of its structure this heterogeneity does not restrict the possibility of defining large scale uniform principal stress directions all the results are consistent with the local seismotectonics

fluid overpressure and stress drop in fault zones using the average aspect length maximum thickness ratios of 379 mineral filled extension mode i veins from an active fault zone the fluid overpressure during their development with reference to the minimum compressive principal stress 3 is estimated at 20 mpa emplacement of such veins increases 3 and can generate a temporary stress barrier to the propagation of subsequent hydrofractures on meeting a subhorizontal stress barrier vertically propagating hydrofractures may change into water sills where the fluid pressure is at or above lithostatic in this model stress barriers and thus water sills can form at any depth in and in any type of fault zones for such a high fluid pressure the product of the coefficient of sliding friction and the normal stress in the modified griffith criterion becomes essentially zero and the driving stress associated with faulting equal to twice the in situ tensile strength of the host rock for typical in situ tensile strengths of 2 3 mpa the driving stresses for slip on overpressured fault planes is 4 6 mpa these results are in good agreement with the commonly measured average static stress drops of 3 6 mpa during earthquakes

time to reach near steady state in large aquifers 1 a new analytical solution of the flow equation has been developed to estimate the time to reach a near equilibrium state in mixed aquifers i e having unconfined and confined portions following a large hydraulic perturbation near equilibrium is defined as the time for an initial aquifer perturbation to dissipate by an average 95 across the aquifer the new solution has been obtained by solving the flow system of a simplified conceptual model of a mixed aquifer using laplace transforms the conceptual model is based on two assumptions 1 the groundwater flow can be reduced to a horizontal 1 d problem and 2 the transmissivity a function of the saturated thickness is assumed constant on the unconfined portion this new solution depends on the storativity of the unconfined portion the lengths of the unconfined and confined portions and the transmissivity assumed to be constant and equal in both portions of the mixed aquifer this solution was then tested and validated against a numerical flow model where the variations of the saturated thickness and therefore variations of the transmissivity were either ignored or properly modeled the agreement between the results from the new solution and those from the numerical model is good validating the use of this new solution to estimate the time to reach near equilibrium in mixed aquifers this solution for mixed aquifers as well as the solutions for a fully confined or fully unconfined aquifer has been used to estimate the time to reach near equilibrium in 13 large aquifers in the world for those different aquifers the time to reach near equilibrium ranges between 0 7 kyr to 2 4 107 kyr these results suggest that the present hydraulic heads in these aquifers are typically a mixture of responses induced from current and past hydrologic conditions and thus climate conditions for some aquifers the modern hydraulic heads may in fact depend upon hydrologic conditions resulting from several past climate cycles

flow of variable density formation water in deep sloping aquifers review of methods of representation with case studies abstract distributions of freshwater hydraulic heads have been and continue to be used in the representation and analysis of groundwater flow including the flow of variable properties formation water in deep sloping aquifers because of variations in pressure temperature and salinity the flow of formation waters in sedimentary basins is driven by potential forces resulting from pressure and elevation topography differences by buoyancy resulting from density differences and by osmosis caused by temperature and concentration gradients because of variations in both fluid and rock properties it is not possible to define a flow potential to be used in representing and analysing the flow neither is there a force potential from which a flow driving force can be derived and which could be used in flow representation and analysis even if osmotic effects are neglected buoyancy still plays a role which in certain cases may be important in horizontal or near horizontal aquifers the lateral flow of variable density water is correctly represented by freshwater potentiometric surfaces and the vertical flow may be analysed using environmental hydraulic head distributions however the flow of formation waters in sedimentary basins is mostly along bedding in confined sloping aquifers in this case short of calculating and representing either the velocity or the impelling force distribution freshwater hydraulic heads remain the only tool available for representing and analysing the flow by neglecting buoyancy errors in flow strength and direction are introduced which become significant if a dimensionless driving force ratio exceeds a threshold value estimated to be around 0 5 the review of representation and analysis methods for the flow of variable properties formation water in deep sloping aquifers is illustrated with two case studies from the alberta and llanos sedimentary basins

bayesian inversion of a chloride profile obtained in the hydraulically undisturbed opalinus clay mass transport and paleo hydrological implications abstract the bdb 1 deep inclined borehole was drilled at the mont terri rock laboratory switzerland and enabled to acquire relevant data on porewater composition through the opalinus clay opa and its bounding formations petrophysical measurements were carried out and included water content water accessible porosity and grain density determination conservative anion profiles were obtained by aqueous leaching and out diffusion experiments performed on drillcore samples and revealed to be consistent with previous studies carried out at the rock laboratory level diffusive properties were also investigated using three experimental setups cubic out diffusion radial diffusion and through diffusion these transport parameters were used as a priori values in a bayesian inversion using a markov chain monte carlo method to interpret the chloride profile in the opalinus clay based on a peclet number analysis using the transport parameters formerly acquired a purely diffusive scenario enabled specifying the paleohydrogeological evolution of the mont terri site from the folding and thrusting of the jura mountains to present time and transport parameters

investigation of double tropopause spatial and temporal global variability utilizing high resolution dynamics limb sounder temperature observations 1 this study examines the seasonal variation of the double tropopause dt using data from the high resolution dynamics limb sounder the combination of high data density and high vertical resolution available with this satellite instrument allows for the detailed analysis of fine scale structures such as the dt the spatial distribution of dt frequency is examined for all seasons from 2005 to 2007 global analysis of this distribution reveals that dts have a strong tendency to follow the zonal wind pattern and are present all year over the andes moreover during northern hemisphere winter spring there is a 15 decrease in the dt frequency over the eastern pacific and western atlantic two regions of wave breaking produced from a weakening of the zonal flow significant dt thickness values are also present over these regions but are found to extend further poleward than the corresponding frequency pattern a time series of dt frequencies is analyzed and shows an increase in daily frequencies during 2006 that highlights the interannual variability of this thermal structure for the first time dt duration is investigated in the extratropics utilizing a hovmoller diagram of dt frequency this representation highlights two preferred regions of formation the pacific and atlantic the slope of the feature gives it a speed of 18 m s in the northern hemisphere and a slightly faster speed for the southern hemisphere these speeds and the corresponding structures highlight a potential connection between upper tropospheric waves and dts

evolution of strength recovery and permeability during fluid rock reaction in experimental fault zones abstract physical and chemical fluid rock interactions are implicated in controlling earthquake nucleation and recurrence in particular interseismic compaction sealing and healing of fractured fault rocks can lead to strength recovery and stabilisation of fault zones in contrast these same processes can also assist increases in pore fluid pressures and consequent destabilisation of faults here we present high temperature hydrothermal experiments designed to assess the evolution of strength of fault zones in previously intact rock and also characterise the associated changes to porosity and permeability cores of fontainebleau sandstone were initially loaded to failure in a high pressure gas medium apparatus the failed specimens were then hydrothermally reacted at 927 c for variable duration under isostatic conditions and subsequently re fractured to determine the interseismic strength recovery in the most extreme case hydrothermally induced gouge compaction cementation and crack healing resulted in 75 strength recovery after reaction for 6 h isostatic hydrothermal treatment also resulted in dramatic reduction in porosity and permeability strength of the fault zone following hydrothermal reaction appears to be closely correlated to porosity consistent with previous studies on brittle failure of porous aggregates the experimental results show how hydrothermal reactions in fault zones may lead to two competing time dependent effects fault strengthening due to increased cohesion in the fault zone and fault weakening arising from elevated pore pressures within a well cemented low permeability gouge layer

importance of thermo osmosis for fluid flow and transport in clay formations hosting a nuclear waste repository abstract three osmotic processes have been identified in clay rich media electro and chemo osmosis flows of water caused by salinity and electrical potential gradients have so far attracted almost exclusive attention but despite the recurring concern about the importance of thermo osmosis a flow of water driven by a temperature gradient in argillaceous media it remains largely neglected here we propose a new formalism for estimating the thermo osmotic permeability based on a theoretical analysis at the molecular and pore scale then upscaled we show that the thermo osmotic permeability can be estimated only from surface charge density temperature pore size and salinity the possible prominent role of thermo osmosis in compacted shale layers with a temperature gradient is then exemplified this first general estimate of the thermo osmotic permeability can be used to improve our understanding of the influence on fluid flow and pressure fields of the natural geothermal gradient in sedimentary basins which is required e g for safety assessments of nuclear waste repositories in shale layers

chlorine transfer out of a very low permeability clay sequence paris basin france 35cl and 37cl evidence abstract the marine callovo oxfordian clay formation is found at a depth around 410m in the eastern part of the paris basin france it is a very low permeability formation investigated by the french agency for nuclear waste management andra to study the feasibility of a radioactive waste disposal examining hydrogeological and geochemical characteristics of the clay sequence may test confinement properties of this formation this study uses chlorine isotopes to investigate long term transport processes which may carry chemical elements out of the clay layer to the surrounding rocks detailed chlorine concentration and 37cl depth profiles are examined using pore waters and aquifer waters sampled in the clay formation and its surrounding aquifers the dogger at the bottom and the oxfordian kimmeridgian tithonian unit at the top they are discussed in terms of chlorine budget and hydrogeological processes clay pore waters and aquifer waters show strong chlorine concentration depletion a steep cl concentration gradient from the dogger at 500m in depth 2500 mg l to the oxfordian kimmeridgian tithonian aquifer near the surface 10 mg l is associated to a v shaped profile of the 37cl values modelling cl transport shows that a hydrodynamic dispersion process explains cl concentration and 37cl profiles in oxfordian limestone this process implies a mean upward flux of chloride in the 2 6 10 8 8 2 10 8 mole m2 yr range from the clay formation towards upper limestones where a westward advective flow disperses the chloride the modelling and knowledge of underground water transfer suggest a maximum effective cl hydrodynamic vertical dispersion coefficient vertical cl transport coefficient of 7 6 10 10 m2 s chlorine transfer through the callovo oxfordian clay since deposition 160my ago can be mainly described by the interplay of an early dilution and a later hydrodynamic dispersion event which has apparently erased most of the isotopic effects of diagenetic events such as early diffusion ion filtration etc

chemical and isotopic characterization of hydrocarbon gas traces in porewater of very low permeability rocks the example of the callovo oxfordian argillites of the eastern part of the paris basin abstract a methodology has been developed to determine chemical and carbon isotopic compositions of trace amounts of hydrocarbon gas compounds methane ethane propane iso and normal butane present as dissolved compounds in the porewater of the low permeability callovo oxfordian argillites in eastern paris basin france results indicate that the studied hydrocarbons contain significant amounts of ethane butane and propane in addition to methane carbon isotopic compositions reflect primarily thermogenic origin thermal cracking of organic matter and lack of any significant biodegradation because temperature did not exceed 50 c in the studied argillites investigated hydrocarbons must have originated in hotter deeper organic bearing formations possibly stephanian coals data supports the predominance of high maturity thermogenic gas in the upper part of the callovo oxfordian and low maturity thermogenic gas mixed with minor bacterially produced methane in the lower part of the formation a mixing between three end member gases models quite well the data one thermogenic gas with a low maturity 42 methane with a 13 c of 53 a gas with higher maturity 55 methane with a 13 c of 47 and a bacterial gas 99 45 methane with a 13 c of 80 this study illustrates that migration of hydrocarbon gases can take place in rocks with very low permeability and porosity such as compacted mudrocks given enough time it further suggests that the studied fluid migration and transfer in aquitards would help characterization and understanding of fluid movements in sedimentary basins as a complement to studies focused on water aquifers and hydrocarbon reservoirs chemical and isotopic composition of dissolved hydrocarbons in porewater can be used as natural tracers of fluid circulation in sedimentary basins in addition to more conventional tracers

geochemical evidence for efficient aquifer isolation over geological timeframes aquitards layers of rock having low permeability have been suggested as potential long term reservoirs for toxic materials such as nuclear or chemical waste but information about the isolation properties of aquitard layers is essential to evaluate whether they can indeed be used safely as reservoirs here we investigate the long term mobility of groundwaters between two aquifers surrounding an aquitard layer in the eastern recharge area of the paris basin france using helium isotopes as a geochemical tracer the deeper trias sandstone aquifer which lies above the crystalline basement accumulates radiogenic 4he and primordial 3he from large regions of the crust and mantle at rates comparable to the degassing of the whole crust1 and of mid ocean ridges2 we show that the overlying carbonate dogger aquifer which is separated from the trias aquifer by an aquitard layer consisting of a 600 m succession of shales and clays is stagnant and has been extremely well isolated from the trias over the past several million years this finding together with previous studies at the centre of the paris basin3 4 shows that diffusive mass transfer across aquitards is negligible and that cross formational flow in basins takes place preferentially in faulted areas

possible effects of erosional changes of the topographic relief on pore pressures at depth formation fluid pressures are said to be anomalous or abnormal if they differ from hydrostatic for the depth considered explanations of the origin and maintenance of anomalies seldom include the possibility of delayed adjustment of subsurface pore pressures to erosional modifications of the topographic relief yet the present paper shows that it is entirely possible for subaerial erosion to generate pore pressure changes at great depths by cross formational energy transfer in realistic field conditions conversely pressures may survive changes in the topography and be in a transient state for hundreds to millions of years thus appearing anomalous at the time of observation according to calculations for the red earth region in alberta canada present pore pressures in the devonian immediately above the precambrian basement at a depth of approximately 1400 meters may have been induced originally by erosionally shaped topography during pliocene times 12 to 1 m y b p but be adjusting now to new boundary conditions created by the erosional exposure of the sub cretaceous unconformity a 12 adjustment may already have taken place during the last 300 thousand years leaving nearly 90 of pliocene age pressures to survive to date as slowly decaying transient pressure relicts of the geologic past in view of the possible erosional effects on pore pressures demonstrated in this paper the question of anachronous formation pressures may deserve consideration in problems related to deep basin hydrology including regional groundwater resource development migration and accumulation of hydrocarbons radioactive waste isolation dating of various geologic events and so on

characterization of transport properties of argillaceous sediments application to the callovo oxfordian argillite 1 we consider the case of a cylindrical cored specimen of a saturated argillaceous sediment located between two reservoirs filled with an electrolyte one of the boundaries of the sample is submitted to a sharp pressure or salinity change the migration of the resulting pressure or saline front through the core specimen is responsible for the apparition of an electrical field which is called the streaming potential the diffusion potential for a salinity gradient respectively the temporal variation of this electrical field is recorded with ag agcl nonpolarizable electrodes and used to monitor the evolution of the pressure or saline front between the two reservoirs there is a finite interval of time before which the steady state condition holds through the core in both cases the electrical potentials exhibit a relaxation with a relaxation time constant depending on the square of the length of the cylindrical core specimen and on its hydraulic or ionic diffusivity the experiments described here provide new methodologies to determine the hydraulic and ionic diffusivities of clay rich materials these experiments confirm also the predictions of a transport model developed recently for such materials and illustrate a powerful geophysical method to obtain transport properties not only in the laboratory but also in the field using a network of nonpolarizable electrodes to remotely monitor dissipation processes occurring inside the investigated system

generation of high pore pressures in accretionary prisms inferences from the barbados subduction complex recent geological and geophysical exploration of several accretionary complexes reveals that pore pressures inside them are much higher than the hydrostatic values indirect inferences further suggest that such high pore pressures may be present inside most accretionary complexes in this study the mechanisms for generating excess pore pressures is investigated by focusing on the barbados ridge complex which is an end member of sediment rich accretionary prisms modeling is carried out using a two dimensional finite element procedure with constraints provided by the following data deep sea drilling project results on stratigraphy and lithology seismic results on sediment structure and thickness regional geology on tectonic history and laboratory measurements on mechanical properties of sediments coupling among the equations for fluid flow heat transfer and sediment deformation is made throughout the computation the results of the modeling predict that the principal decollement beneath the barbados ridge complex is a zone of high pore pressure high porosity and reversal in seismic velocity these characteristics may be responsible for the mechanical weakness of the decollement which allows continuous subduction of sediments below it and for its seismic signatures results of the modeling further suggest that tectonic overburden may be the most important mechanism for the generation of high pore pressures inside accretionary prisms the direct effect of tectonic compression is of secondary importance and the effect due to temperature increase with subduction is of little significance thrust faulting near the toe of accretionary prisms can disturb pore pressure patterns and produce localized highs rich sediment supply low permeability high subduction angle and high subduction velocity are all favorable factors in creating high pore pressures inside accretionary prisms

a nontectonic origin for the present day stress field in the paris basin france the large scale stress patterns observed in intra plate areas are often considered to result from far field boundary forces that drive plate tectonics however no present day deformation has been detected in the french paris basin yet significant deviatoric stresses are measured in limestone formations observed above soft argillite layers encountered in this region further the pore pressure measured in the argillite is larger than that measured in the surrounding permeable zones these observations suggest a presently active source of stress in this sedimentary system we propose that this stress field is not related to tectonics but only to gravity acting on the series of visco elastic orthotropic geomaterials that fill up the basin this visco elastic response is linked to pressure solution effects activated by pore pressure transients related to climatic variations these pressure transients develop in the fracture system that affects some of the geomaterials and imply a time dependant deformation field with time constants related to those of climatic variations this model outlines the influence of time dependent material properties on the present day stress and pore pressure fields that prevail at various depths it may be considered as a possible loading mechanism for the analysis of intra plate seismicity

effects of layering and anisotropy on fault geometry the geometry and orientation of faults are examined using several field examples of small scale 1 with dihedral angles usually of about 50 as predicted by the coulomb theory of failure in layered rocks the geometry of faults varies with the orientation of layering with respect to the stress field where 1 is approximately normal to layering or anisotropy conjugate faults also develop symmetrically about 1 where rocks have interbedded layers with different mechanical behaviours however faults tend to initiate orthogonal to the more brittle layers i e originate as extension fractures sub parallel to 1 but oblique to the less brittle layers as the fault steps through the layering pull aparts are developed which may reduce the dihedral angle where 1 is oblique c 25 75 to anisotropy one set of faults is developed at a high angle to layering with another at a low angle usually showing a ramp flat geometry large dihedral angles up to 90 may result and 1 does not bisect this angle where 1 is approximately parallel to layering two cases can be recognized where 3 is approximately normal to layering faults with layer parallel flats and contractional ramps develop where 2 is approximately normal to layering conjugate faults develop which are symmetrical about 1 the geometry resembling that in isotropic rocks these observations are in agreement with rock deformation experiments which show the strong effect of anisotropy on fault orientation but the observations incorporate the effects of layering of materials with different deformation characteristics

sealing efficiency of shales the sealing efficiency of shale layers is studied through the hydraulic parameters which are required for sustaining overpressure during geologically significant periods assuming a 1d sedimentary complex composed of a sealing layer overlying a permeable one we give approximate solutions for the dissipation of a given initial overpressure the time constant for relaxation involves the thickness hydraulic conductivity and specific storage of the seal and also those of the permeable layer the values of the various hydraulic parameters are discussed it is argued first that the specific storage corresponding to plastic deformation during burial compaction is larger than the one which would correspond to elastic deformation when taking into account i plausible values for specific storage of the upper shale layer and ii the storage effect of the lower permeable layer it is found that a shale layer of several tens of m with an hydraulic conductivity of the order of 10 15 ms 1 maintains overpressure for 1 myr for such hydraulic parameter values in the absence of on going pressuring forces initial overpressures would decay with a time constant corresponding to a decrease by a factor e 2 7 of 1 myr this is interpreted as supporting a dynamic origin for observed abnormal pressures

determination of principal stress magnitudes using calcite twins and rock mechanics data abstract this paper proposes a new method for determining principal stress magnitudes based on the use of the deviatoric stress tensor determined from computer inversion of calcite twin data additional evaluation of the vertical stress weight of overburden or of the mean stress is enough to calculate the complete stress tensor we emphasize that the best reasoning consists of combining calcite twin analysis with rock mechanics data failure friction criteria because the determination of the six parameters of the absolute stress tensor is overconstrained