mechanical properties of shale gas reservoir rocks part 2 ductile creep brittle strength and their relation to the elastic modulus we studied the elastic moduli ductile creep behavior and brittle strength of shale gas reservoir rocks from barnett haynesville eagle ford and fortst john shale in a series of triaxial laboratory experiments we found a strong correlation between the shale compositions in particular the volume of clay plus kerogen and intact rock strength frictional strength and viscoplastic creep viscoplastic creep strain was approximately linear with the applied differential stress the reduction in sample volume during creep suggested that the creep was accommodated by slight pore compaction in a manner similar to instantaneous strain there was more viscoplastic creep in samples deformed perpendicular to the bedding than parallel to the bedding the tendency to creep also correlated well with the static young s modulus we explained this apparent correlation between creep behavior and elastic modulus by appealing to the stress partitioning that occurs between the soft components of the shales clay and kerogen and the stiff components quartz feldspar pyrite and carbonates through a simple 1d analysis we found that a unique relation between the creep compliance and elastic modulus independent of composition and orientation can be established by considering the individual creep behavior of the soft and stiff components that arises from the stress partitioning within the rock this appears to provide a mechanical explanation for why long term ductile deformational properties can appear to correlate with short term elastic properties in shale gas reservoir rocks

field determination of the three dimensional hydraulic conductivity tensor of anisotropic media 2 methodology and application to fractured rocks the analytical solutions developed in the first paper can be used to interpret the results of cross hole tests conducted in anisotropic porous or fractured media in the particular case where the injection and monitoring intervals are short relative to the distance between them the test results can be analyzed graphically from the transient variation of hydraulic head in a given monitoring interval one can determine the directional hydraulic diffusivity kd e ss and the quantity d ss by curve matching here kd e is directional hydraulic conductivity parallel to the unit vector e pointing from the injection to the monitoring interval ss is specific storage and d is the determinant of the hydraulic conductivity tensor k the principal values and directions of k together with ss can then be evaluated by fitting an ellipsoid to the square roots of the directional diffusivities ideally six directional measurements are required in practice a larger number of measurements is often necessary to enable fitting an ellipsoid to the data by least squares if the computed kd e ss values fluctuate so severely that a meaningful least squares fit is not possible one has a direct indication that the subsurface does not behave as a uniform anisotropic medium on the scale of the test test results from a granitic rock near oracle in southern arizona are presented to illustrate how the method works for fractured rocks at the site the oracle granite is shown to respond as a near uniform anisotropic medium the hydraulic conductivity of which is strongly controlled by the orientations of major fracture sets the cross hole test results are shown to be consistent with the results of more than 100 single hole packer tests conducted at the site

interlayer material transport during layer normal shortening part ii boudinage pinch and swell and migmatite at sondre stromfjord airport west greenland slow deformation of rock at metamorphic conditions involves an interplay of ductile flow fracturing and mass transfer boudinage and pinch and swell structures from a precambrian terrane in west greenland illustrate this point macroscopic fractures between separating boudins are expected to be 0 1 mm or less in width it is in such narrow openings simultaneously dilating and being filled that deposition from an oversaturated pore fluid leads to macroscopic differentiation the microscopic process thought to underly macroscopic mass transfer is as follows pore fluid occurring throughout the deforming rock has a pressure lower than the normal stresses existing between grains it will therefore be oversaturated compared to a hydrostatic equilibrium situation in which all solid solid contacts experience normal stresses equal to the pore pressure local solution and deposition cause local rearrangements in the pore network and contribute to local ductile flow only heterogeneous deposition in separate mechanically controlled and dynamically reestablished groupings and alignments of sites gives rise to macroscopic differentiation products the macroscopic theory developed in part i and the microscopic view presented here are combined in an interpretation of some common features of boudinage and pinch and swell structures at high metamorphic grade the rocks are melted to some small degree the principles governing differentiation are not affected by changing the pore fluid from a hydrous phase to melt veins in anatectic terranes grow as relatively competent deposits from a highly incompetent melt permeating the deforming rock

ultrasonic velocity and anisotropy of hydrocarbon source rocks an experimental study of the physical properties of black kerogen rich shales also including maturation analysis scanning electron microscope sem observations and physical modeling revealed fairly peculiar petrophysical parameters specifically these rocks have very low porosity and density but most importantly both p and s ultrasonic velocities normal to bedding are extremely low whereas they are much higher parallel to bedding giving rise to a strong anisotropy even at high confining pressures we found that these parameters primarily reflect kerogen content microstructure and maturation level of these rocks we found also that microcracks inferred from ultrasonic velocity measurements occur only in mature shales these microcracks are parallel to the bedding plane and further enhance strong intrinsic anisotropy notably at low effective pressure our results show that on a small scale kerogen rich shales are transversely isotropic rocks and can be effectively modeled using the thin laye

determination of shale stiffness tensor from standard logs a technique allowing inversion of the shale stiffness tensor from standard logging data sonic velocities density porosity and clay content is developed the inversion is based on the effective medium theory the testing of the technique on laboratory measurements of the elastic wave velocities in shale samples shows that the inversion makes it possible to predict the elastic wave velocities vp vs1 and vs2 in any direction within an error of a few per cent the technique has been applied for the stiffness tensor inversion along a well penetrating a shale formation of the mississippian age altered by thin layers of limestone it is demonstrated that the symmetry of a stiffness tensor inverted at the sonic frequency 2 khz is slightly orthorhombic and taking into account the experimental errors can be related to the vertical transverse isotropy symmetry for the productive interval of the shale formation the thomsen parameters and average respectively 0 32 0 25 and 0 21 which indicate anelliptic behaviour of the velocities in this shale the coefficients of anisotropy of this shale interval are around 24 and 20 for the compressional and shear waves respectively the values of the inverted velocities in the bedding plane for this interval are in good agreement with the laboratory measurements the technique also allows inversion of the water saturation of the formation sw and the inverted values are in agreement with the sw values available for this formation a backus like upscaling of the inverted stiffness tensors is carried out for the lower and upper bounds of the frequency band used in the crosswell tomography 100 hz and 500 hz these results can serve as an initial velocity model for the microearthquake location during hydrofracking of the shale formation

a continuum representation of fracture networks part i method and basic test cases abstract a method to represent fracture networks as grid cell conductivities in a continuum model is described and evaluated the method is developed for a sparsely fractured rock with a conductivity field that is dominated by major fractures and fracture zones for such a fracture system it is believed that an accurate description of the correlation and anisotropy structure is essential the proposed method will capture these features of the fracture system the basic approach of the method is to evaluate accurate cell conductivity values for a staggered grid arrangement i e velocities are stored at cell walls and scalars at cell centres the balance of forces i e the darcy law is evaluated for a control volume centred around the point where the velocity component is stored fractures are assumed to have a thickness and conductivity the conductivity of a velocity control volume is calculated from the intersecting volume between the fracture and the velocity control volume intersecting volumes with their respective conductivities are added up to form the control volume conductivity the general conclusion is that the method results in accurate simulations it is expected that the method can be utilised in many practical groundwater simulations this statement is based on the results from three test cases with analytical solution and one test case where results are compared with those from established theories

parameterization of elastic stress sensitivity in shales stress dependency and anisotropy of dynamic elastic properties of shales is important for a number of geophysical applications including seismic interpretation fluid identification and 4d seismic monitoring using sayerskachanov formalism we developed a new model for transversely isotropic ti media that describes stress sensitivity behavior of all five elastic coefficients using four physically meaningful parameters the model is used to parameterize elastic properties of about 20 shales obtained from laboratory measurements and the literature the four fitting parameters namely specific tangential compliance of a single crack ratio of normal to tangential compliances characteristic pressure and crack orientation anisotropy parameter show moderate to good correlations with the depth from which the shale was extracted with increasing depth the tangential compliance exponentially decreases the crack orientation anisotropy parameter broadly increases with depth for most of the shales indicating that cracks are getting more aligned in the bedding plane the ratio of normal to shear compliance and characteristic pressure decreases with depth to 2500 m and then increases below this to 3600 m the suggested model allows us to evaluate the stress dependency of all five elastic compliances of a ti medium even if only some of them are known this may allow the reconstruction of the stress dependency of all five elastic compliances of a shale from log data for example

estimating physical parameters of cracked porous oil reservoirs by inverting shear wave splitting summary we present the problem of determining crack density porosity and permeability of cracked porous carbonate reservoirs by examining the origin of anisotropy considered from a physical model of porous reservoir rock with a system of vertical fluid filled cracks this stabilizes the non linear optimization inversion procedure and determines a set of significant reservoir parameters on a basis of qp qs1 and qs2 velocities measured in a vsp experiment

anisotropy of experimentally compressed kaolinite illite quartz mixtures the anisotropy of physical properties is a well known characteristic of many clay bearing rocks this anisotropy has important implications for elastic properties of rocks and must be considered in seismic modeling preferred orientation of clay minerals is an important factor causing anisotropy in clay bearing rocks such as shales and mudstones that are the main cap rocksofoilreservoirs thepreferredorientationofclaysdepends mostly on the amount of clays and the degree of compaction to studytheeffectoftheseparameters wepreparedseveralsamples compressingattwoeffectiveverticalstressesamixtureofclays illite and kaolinite and quartz silt with different clay quartz ratios the preferred orientation of the phases was quantified with rietveld analysis on synchrotron hard x ray images pole figures for kaolinite and illite display a preferred orientation of clay platelets perpendicular to the compaction direction increasing in strength with clay content and compaction pressure quartz particles have a random orientation distribution aggregate elastic properties can be estimated by averaging the singlecrystal properties over the orientation distribution obtained from the diffraction data analysis calculated p wave velocity anisotropy ranges from 0 pure quartz sample to 44 pure clay sample highly compacted but calculated velocities are much higher than measured velocities this is attributed to uncertainties about single crystal elastic properties and oriented micropores and limited grain contacts that are not accounted for in themodel inthiswork wepresentaneffectivemethodtoobtain quantitative data helping to evaluate the role of clay percentage and compaction pressure on the anisotropy of elastic properties ofclay bearingrocks

interrelations of mesoscopic structures and strain across a small regional fold virginia appalachians abstract small regional folds such as the clover hollow anticline of the narrows thrust sheet in southwest virginia u s a are considered to be large buckle folds expressing lateral shortening above a subsurface decollement cleavage mesoscopic and regional folds and contraction faults have developed in these rocks under anchimetamorphic conditions in a single protracted deformation during thrust sheet emplacement the contraction faults dominate the structure at all scales three fault associations isolated contraction faults contraction faults in series and complex fault zones with intense folding determine the pattern and intensity of local structures regional displacement transfer of strain along and across faults has produced local variations in structural style duplex like systems of second order faults terminate laterally into zones of intense folding and third order faulting fold tightness cleavage intensity strain magnitude and total longitudinal strain e t are maximum in these regions contraction faults in this thrust sheet have propagated along zones of high strain rate associated with mesoscopic folding and intense cleavage regional hinge migration and greater structural complexity along the southeast limb of the clover hollow anticline are considered to be due to emplacement of the adjacent thrust sheet

effective elastic properties of solid clays clay minerals are perhaps the most abundant materials in the earth s upper crust as such their elastic properties are extremely important in seismic exploration seismic reservoir characterization and sonic log interpretation because little exists in the literature on elastic properties of clays we have designed a method of measuring effective elastic properties of solid clays clays without pores in this method clay minerals are mixed with a material with known elastic properties to make composite samples elastic properties of these clay minerals are then inverted from the measured elastic properties of the composite samples using the weighted hashin shtrikman average using this method we have measured 66 samples of 16 types of clays in this paper we present a comprehensive data set of elastic properties of solid clays that commonly occur in or are related to petroleum reservoirs although uncertainties up to 10 exist the data set reported here is by far the most comprehensive set of e

seismic anisotropy in sedimentary rocks part 2 laboratory data part one of this paper presents a method for measuring seismic velocities and transverse isotropy in rocks using a single core plug this method saves at least two thirds of the time for preparing core samples and measuring velocities in transversely isotropic ti rocks using this method we have measured velocity and anisotropy of many shale and reservoir rocks from oil and gas fields around the world we present some of the data in this paper which include seismic velocity and anisotropy in 17 brine saturated shale samples 1 gas and brine saturated coal sample 8 brine saturated sands 12 gas saturated sands 32 gas saturated carbonate samples and 25 brine saturated carbonate samples the results show that clays and fine layering in sedimentary rocks are the main causes of seismic anisotropy very little intrinsic anisotropy exists in unfractured reservoir rocks such as sands sandstones and carbonates under reservoir conditions in contrast all shales were found seismically anisotropic anisotr

the connectivity of two dimensional networks of spatially correlated fractures 1 continuum percolation thresholds for fracture systems are based on models with random fracture locations resulting in uniformly clustered systems in which all connections are fracture intersections fractures in natural systems can be either clustered or anticlustered and a significant portion of the connectivity is achieved through fracture abutments or splays analytical and numerical methods are combined to define critical properties of isotropic networks of constant length lines as a function of fracture clustering and of the ratio of abutments to intersections a second suite of numerical results indicates that the influence of these parameters varies as a function of the fracture length distribution a preliminary predictor of critical density of anisotropic nonrandom systems with lognormal length distributions is derived and applied to a suite of natural fracture systems predicted connectivities are closer to the observed connectivities when the predictor is used than when connectivity is estimated from the random threshold

anisotropic effective medium modeling of the elastic properties of shales shales are complex porous materials normally consisting of percolating and interpenetrating fluid and solid phases the solid phase is generally comprised of several mineral components and forms an intricate and anisotropic microstructure the shape orientation and connection of the two phases control the anisotropic elastic properties of the composite solid we develop a theoretical framework that allows us to predict the effective elastic properties of shales its usefulness is demonstrated with numerical modeling and by comparison with established ultrasonic laboratory experiments the theory is based on a combination of anisotropic formulations of the self consistent sca and differential effective medium dem approximations this combination guarantees that both the fluid and solid phases percolate at all porosities our modeling of the elastic properties of shales proceeds in four steps first we consider the case of an aligned biconnected clay fluid composite composed of ellipsoidal inclusion

elastic properties of anisotropic synthetic calcite muscovite aggregates 1 a set of synthetic aggregates containing mixtures of calcite and muscovite were compacted with varying uniaxial loads ranging from 20 mpa up to 400 mpa their elastic properties have been measured using compressional and shear waves in confining hydrostatic pressures up to 475 mpa measured seismic velocities are shown to depend on 1 the ratio of calcite to muscovite 2 the uniaxial load used during sample manufacturing and 3 the porosity amount prior to velocity measurements the matrix framework may also affect the seismic velocities but this effect is not easily quantifiable in general measured seismic velocities decrease with increasing muscovite content and porosity elastic properties have been calculated based on texture measurements which were obtained by neutron diffraction the calculated velocities are based on the calcite and muscovite single crystal elastic tensors their orientation distribution functions and their modal fractions a large discrepancy is apparent between calculated and measured velocities where the former always overestimate the actual velocities shear waves display less of a difference between calculated and measured values than do compressional waves indicating that waves propagating by shearing are less affected by sample porosity and matrix framework a better agreement between measured and calculated seismic velocities arises when the initial porosity is taken into account using a differential effective medium model seismic anisotropy is evident from both measured and calculated velocities and is low for samples containing pure calcite but becomes prominent as the muscovite concentration increases the intensity of anisotropy further depends on the uniaxial load used during sample compaction and the initial porosity

ultrasonic velocities in cretaceous shales from the williston basin compressional and shear wave velocities were measured in the laboratory from 1 bar to 4 kbar confining pressure for wet undrained samples of cretaceous shales from depths of 3200 and 5000 ft in the williston basin north dakota these shales behave as transversely isotropic elastic media the plane of circular symmetry coinciding with the bedding plane for compressional waves the velocity is higher for propagation in the bedding plane than at right angles to it and the anisotropy is greater for the 5000 ft shale for shear waves the sh wave perpendicular to bedding and the sv wave parallel to bedding propagate with the same speed which is about 25 percent lower than that for the sh wave parallel to bedding in general compressional and shear velocities are higher for the indurated 5000 ft shale than for the friable 3200 ft shale all velocities increase with in increasing confining pressure to 4 kbar the 3200 ft shale exhibits velocity hysteresis as a function of pressure whereas this effect is a

elastic moduli of anisotropic clay claymineralsareimportantcomponentsinshales controlling their elastic properties and anisotropy the elasticity of crystalline clay minerals differs significantly from that of clay in situ because of the ability of clay particles to bind water in the majority of published works only isotropic moduli for in situ clays are reported however anisotropy is inherent in the clay elasticity we develop an inversion technique for determination of thestiffnesstensorofinsituclayfromtheshale sstiffnesstensor as an example we obtain the stiffness tensor of a water clay composite from the data on the water saturated greenhorn shale sample whose clay composition consists of almost equal amounts of illite and smectite and comparable amounts of kaolinite and chlorite the stiffness tensor of the water clay composite is found for the greenhorn shale with step by step inversion based upon an effective medium theory the inversion uses a nonlinear optimization technique with bounds imposed on the estimated parameters in the inversion we apply different approaches of the effective medium theory using a published method referred to here as the generalized singular approximation gsa the gsa method makes it possible to take into account the microstructure of shales the resulting elasticity constants of the anisotropic transversely isotropic in situ clay composite are c11 23 7 c33 8 5 c44 0 8 c66 5 7 and c13 3 1 in gpa and the density equals 2 17 g cm 3 the thomsen parameters for the clay composite are 0 89 3 10 and 0 34 the elasticity constants found for this clay composite can be used in the theoretical analysis of shales that have a similar composition of clay but with different mineral compositions the inversion technique developed can be used for general shale water clay composites when the mineral composition and orientation of the clay platelets are known

fluid assisted particulate flow of turbidites at very low temperature a key to tight folding in a submarine variscan foreland basin of sw europe 1 the problem addressed in this article is how sedimentary formations like turbidites in a foreland basin which include layers with apparently great competence contrast can be tightly folded in a regular manner under very low temperature and pressure this raises two major issues the rheological behavior of the rocks at the time of folding and the role played by fluids in order to understand very low temperature folding and the structural evolution of a submarine foreland basin we carried out detailed structural work in turbidites with alternating sandstone and shale for which estimated peak temperature conditions were top diagenetic to very low grade metamorphism folds are tight to isoclinal with local collapsed hinges which implies that the incompetent shale was mobile enough to flow away from strongly flattened areas we did not find evidence for cataclastic flow or crystal plasticity at mesoscopic and microscopic scales other structures mostly boudins foliations conjugate brittle faults and quartz veins associated with folds denote anisotropic compaction by fluid extraction during regional shortening this is possible if the folded rocks were unconsolidated fluid saturated sediments the estimated low peak temperature is consistent with the shale being unlithified poorly cemented grains are free to slide past one another under shallow burial or high pore pressure conditions following this line of thought we consider independent particulate flow assisted by fluids under very low confining pressure bean bag analogy as the rock deformation mechanism active during the described intense folding similar deformation is likely occurring and has occurred in other submarine accretionary wedges

seismic reflectivity of the sediment covered seafloor effects of velocity gradients and fine scale layering summary knowledge of the reflectivity of the sediment covered seabed is of significant importance to marine seismic data acquisition and interpretation as it governs the generation of reverberations in the water layer in this context pertinent but largely unresolved questions concern the importance of the typically very prominent vertical seismic velocity gradients as well as the potential presence and magnitude of anisotropy in soft surficial seabed sediments to address these issues we explore the seismic properties of granulometric end member type clastic sedimentary seabed models consisting of sand silt and clay as well as scale invariant stochastic layer sequences of these components characterized by realistic vertical gradients of the p and s wave velocities using effective media theory we then assess the nature and magnitude of seismic anisotropy associated with these models our results indicate that anisotropy is rather benign for p waves and that the s wave velocities in the axial directions differ only slightly because of the very high p t os wave velocity ratios in the vicinity of the seabed our models nevertheless suggest thats wave triplications may occur at very small incidence angles to numerically evaluate the p wave reflection coefficient of our seabed models we apply a frequency slowness technique to the corresponding synthetic seismic wavefields comparison with analytical plane wave reflection coefficients calculated for corresponding isotropic elastic half space models shows that the differences tend to be most pronounced in the vicinity of the elastic equivalent of the critical angle as well as in the post critical range we also find that the presence of intrinsic anisotropy in the clay component of our layered models tends to dramatically reduce the overall magnitude of the p wave reflection coefficient as well as its variation with incidence angle

boudinage structure some new interpretations based on elastic plastic finite element simulations abstract an elastic plastic finite element method based on the prandtl reuss equations of plastic flow and involving equivalent stresses and strains is used to study boudinage structure our choice of data for the simulations was guided by published stress strain curves for marble matrix and quartzite boudin the essential parameters being yield stress and rock hardness defined by the slope of the stress strain curve all models assume an initial fracture and slight separation and therefore only simulate post fracture behaviour the simulations suggest that boudin shape is determined by boudin hardness maximum stresses are concentrated in the corners which therefore shows the most shape modification matrix hardness determines the amount of boudin separation direct comparison with natural examples is restricted to boudins suffering no significant pre fracture plastic deformation i e rectangular and barrel shaped boudins although other types are likely to have the characteristics of barrel and pinch and swell styles the simulations do not consider the nature and timing of boudin defining fractures but these are important in determining the style of boudinage which ultimately develops some mechanical problems associated with the infilling of inter boudin gaps by ductile rock matrix are discussed and two models proposed the first based on yielding fracture mechanics is used to explain boudins with wedge shaped or otherwise nonmatching ends the second a hydraulic model is proposed to account for gaps between rectangular boudins that are filled by ductile rock matrix

storm generated sedimentary structures in subtidal marine facies with examples from the middle and upper ordovician of southwestern virginia abstract the role of storms is of major importance in forming stratification in open shelf facies though storm processes on modern shelves are not fully understood enough is known to see how sediments are influenced storms transport sand away from coastal areas and suspend and transport sand and mud on the open shelf when wave base is lowered storms generate winnowed lag deposits of shells or gravel but coarser sediment and shells of megafauna are little transported except in very near shore environments the martinsburg formation middle and upper ordovician of southwestern virginia was deposited on a storm influenced open marine platform in contrast to the classic turbidite flysch basin of the martinsburg in the great valley of virginia to the northeast deposition of most martinsburg sediments was due to storms sediments deposited during single storm events occur in characteristic fining upward sequences 1 to 80 cm thick in both carbonate and terrigenous clastic dominated areas of the platform individual storm deposits consist of a sharp erosional base overlain by a couplet of whole fossil packstone and laminated calcarenite siltite or quartzose siltstone to very fine sandstone which is gradational to shale packstones are typically intraclastic have shells aligned parallel o bedding and convex up often with adhering or contained mud and have abundant infiltration textures such as cement filled shelter voids packstones grade abruptly to a laminated bed with horizontal plane lamination large and small scale in phase undulatory lamination and wave ripple cross lamination commonly occurring in ordered vertical sequences laminated beds contain escape burrows are weekly graded and have irregularly mounded upper bed surfaces laminated beds grade upward to shale containing horizontal branching burrows and marine fossils the faunal content of the fining upward sequences matches that of the interbedded shale storm generated beds vary greatly in thickness laterally and frequently pinch out completely fining upward sequences formed by scouring winnowing and suspension of the sand and mud during the height of the storm to form the erosional base and autochthonous lag of whole fossils storms preferentially preserve whole fossils rather than fragmenting them infiltration of finer sediment into the lag deposit and deposition of the laminated bed occurred as the storm waned laminated beds are molded into a distinctive suite of sedimentary structures by wave generated currents the shale represents deposition of finest storm suspended material and continued hemipelagic sedimentation similar sequences are widely distributed in the geologic record variability of storm generated sequences can be related to changing water depth thus storm deposits may be useful in basin analysis to estimate relative water depths and trace transgressive and regressive events

microlithon alteration associated with development of solution cleavage in argillaceous limestone textural trace elemental and stable isotopic observations abstract comparison of microlithons the rock between adjacent cleavage domains in strongly cleaved argillaceous limestone with samples of uncleaved limestone from the same stratigraphic horizon lower devonian kalkberg formation hudson valley of eastern new york indicates that the process of cleavage formation results in significant changes in microlithon fabric and in the chemistry of microlithon calcite petrographic observation indicates that at least 50 of the calcite in microlithons is recrystallized as calcite fibers whose long axes are subparallel to cleavage domains the fibrous calcite occurs as overgrowths on larger grains in thin bed parallel veins and in diffuse extension zones areas of the microlithons in which inequant crystals of microcrystalline calcite are aligned parallel to cleavage domains chemical analyses show that calcite in microlithons of strongly cleaved limestone is enriched in mn and sr relative to calcite in uncleaved limestone 18 o and possibly 13 c values for calcite in strongly cleaved rock are significantly smaller than values for calcite in uncleaved rock these results indicate that microlithon calcite is completely recrystallized i e much more calcite is recrystallized than is visible in thin section and that it underwent exchange with a fluid external to the local rock system calculation of the water rock ratio suggests that 0 1 2 6 volumes of water may have exchanged with the calcite during cleavage formation comparison of the minimum integrated and the inferred instantaneous water rock ratios suggests that bulk fluid flow occurred during microlithon recrystallization comparison of the amount of shortening across cleavage domains with the amount of extension in microlithons and of the volume of calcite dissolved in domains with the amount of fibrous presumably added calcite in microlithons suggests that microlithons are a major sink for the calcite dissolved in domains during cleavage formation and that at this locality development of disjunctive solution cleavage in the kalkberg formation resulted in volume constant plane strain

three dimensional morphology and connectivity of stylolites hyperactivated during veining abstract the limestone of the palaeozoic jack formation in north queensland australia contains bedding parallel stylolites that locally increase in amplitude adjacent to calcite veins the veins formed in response to inhomogeneous strain locally superposed on the tightly folded strata stylolites were locally hyperactivated during vein formation and provided calcite for vein filling three dimensional reconstructions of the morphology of the stylolites based on detailed serial profiles demonstrate that the side faces of columnar stylolites form an interconnected fluid pathway through the rock the connective morphology defines a drainage network comprising highly tortuous major trunks of high amplitude side faces linked locally by minor pathways of variable amplitude the permeability pathway anastomoses in three dimensions making it impossible to image in two dimensions alone

hydrocarbon generation induced microcracking of source rocks laboratory measurements of ultrasonic velocity and anisotropy in kerogen rich black shales of varying maturity suggest that extensive bedding parallel microcracks exist in situ in most mature source rocks undergoing the major stage of hydrocarbon generation and migration given the normal faulting regime with the vertical stress being the maximum principal stress typical of most sedimentary basins this microcrack alignment cannot be accounted for using simplified fracture mechanics concepts this subhorizontal microcrack alignment is consistent with 1 a model of local principal stress rotation and deviatoric stress reduction within an overpressured formation undergoing hydrocarbon generation and with 2 a strong mechanical strength anisotropy of kerogen rich shales caused by bedding parallel alignment of kerogen microlayers microcracks originate within kerogen or at kerogen illite interfaces when pore pressure exceeds the bedding normal total stress by only a few mpa due to the extremely low fractu

elastic anisotropy modeling of kimmeridge shale 1 anisotropy of elastic properties in clay rich sedimentary rocks has been of long standing interest these rocks are cap rocks of oil and gas reservoirs as well as seals for carbon sequestration elasticity of shales has been approached by direct velocity measurements and by models based on microstructures here we are revisiting the classical kimmeridge shale studied by hornby 1998 by first quantifying microstructural features such as phase volume fractions grain shapes and grain orientations and pore distributions with advanced analytical methods and then using this information in different models to explain bulk elastic properties it is shown that by application of a self consistent algorithm based on eshelby s 1957 model of inclusions in a homogeneous medium it is possible to explain most experimental elastic constants though some discrepancies remain which may be due to the interpretation of experimental data using a differential effective medium approach an almost perfect agreement with experimental stiffness coefficients can be obtained though the physical basis of this method may be questionable the influence of single crystal elastic properties grain shapes preferred orientation and volume and shapes of pores on elastic properties of shale is explored

upscaling of elastic properties of anisotropic sedimentary rocks summary in this paper the term upscaling means the theoretical prediction of rock s elastic properties at lower frequency seismic or cross well data using higher frequency logging data on sonic velocities vp vs1 and vs2 porosity and density the mineral composition and water saturation derived from other logs are used due to the special treatment of sonic logging data provided by the dipole sonic probe all the sonic velocities are obtained in the principal coordinate system of the rock s stiffness tensor the upscaling procedure includes two steps the first step involves the solution of an inverse problem on reconstruction of the parameters of the rock s microstructure from the logging data the inversion is based on the effective medium theory as a result of the inverse problem solution the effective stiffness tensor is found for depths at which the sonic wave velocities are measured at the second step the velocities of waves at given lower frequencies are calculated as propagating in a layered medium the number of layers in the medium depends on the given frequency and logging step each layer of the medium has the stiffness tensor found at the first step this upscaling procedure has been applied to a wellbore for which the dipole sonic data are available the rocks penetrated by the well are shales in general the resulting medium exhibits orthorhombic symmetry at sonic frequency this symmetry results from the preferential orientation of clay platelets and grain related cracks and vertical cracks the existence of the latter is indicated by the dipole sonic tool depending on the microstructure parameters orientation of clay platelets and cracks pore crack connectivity and shale mineralogical composition the shales at lower frequency have either transversely isotropic symmetry with the vertical axis of symmetry a k a vti or orthorhombic symmetry

linking preferred orientations to elastic anisotropy in muderong shale australia the significance of shales for unconventional hydrocarbon reservoirs nuclear waste repositories and geologic carbon storage has opened new research frontiers in geophysics among many of its unique physical properties elastic anisotropy had long been investigated by experimental and computational approaches here we calculated elastic properties of cretaceous muderong shale from australia with a self consistent averaging method based on microstructural information the volume fraction and crystallographic preferred orientation distributions of constituent minerals were based on synchrotron x ray diffraction experiments aspect ratios of minerals and pores determined from scanning electron microscopy were introduced in the self consistent averaging our analysis suggested that phyllosilicates i e illite mica illite smectite kaolinite and chlorite were dominant with 70 vol the shape of clay platelets displayed an average aspect ratio of 0 05 these platelets were aligned parallel to the bedding plane with a high degree of preferred orientation the estimated porosity at ambient pressure was 17 vol and was divided into equiaxial pores and flat pores with an average aspect ratio of 0 01 our model gave results that compared satisfactorily with values derived from ultrasonic velocity measurements confirming the validity and reliability of our approximations and averaging approach

effects of porosity and clay content on wave velocities in sandstones the ultrasonic compressional vp and shear vs velocities and first arrival peak amplitude ap were measured as functions of differential pressure to 50 mpa and to a state of saturation on 75 different sandstone samples with porosities ranging from 2 to 30 percent and volume clay content c ranging from 0 to 50 percent respectively both vp and vs were found to correlate linearly with porosity and clay content in shaly sandstones at confining pressure of 40 mpa and pore pressure of 1 0 mpa the best least squares fits to the velocity data are vp km s 5 59 6 93 2 18c and vs km s 3 52 4 91 1 89c deviations from these equations are less than 3 percent and 5 percent for vp and vs respectively the velocities of clean sandstones are significantly higher than those predicted by the above linear fits about 7 percent for vp and 11 percent for vs which indicates that a very small amount of clay 1 or a few percent of volume fraction significantly reduces the elastic moduli of sandstones for shaly

syntectonic vein development in a thrust sheet from the external french alps abstract the geometry and age relations of syntectonic veins within calcareous rocks of one imbricate sheet within a thrust belt in the external french alps are described the earliest veins developed during the main ductile deformation by cleavage parallel extension the majority of the syntectonic veins developed towards the end of the deformation and after the formation of second folds they include a conjugate set of normal shears an abundant set of upright extension veins and en echelon sets the dominantly simple shear strain making up the main ductile phase of deformation occurred by a mechanism of grain to grain pressure solution the stretching lineation records the overall direction of thrust sheet movement a change in the microchemical mechanism of pressure solution is thought to have caused the change from first to second phase deformation as recorded by slaty cleavage and crenulation folds in the field from the shear and vein geometries directions of principal stress have been inferred the directions rotated throughout the deformation the maximum principal stress being inclined to bedding during simple shear strain becoming normal to bedding during the phase of abundant vein growth and becoming vertical at the very end of the deformation

texture and anisotropy analysis of qusaiba shales scanning and transmission electron microscopy synchrotron x ray diffraction microtomography and ultrasonic velocity measurements were used to characterize microstructures and anisotropy of three deeply buried qusaiba shales from the rub alkhali basin saudi arabia kaolinite illite smectite illite mica and chlorite show strong preferred orientation with 001 pole figure maxima perpendicular to the bedding plane ranging from 2 4 6 8 multiples of a random distribution m r d quartz feldspars and pyrite crystals have a random orientation distribution elastic properties of the polyphase aggregate are calculated by averaging the single crystal elastic properties over the orientation distribution assuming a nonporous material the average calculated bulk p wave velocities are 6 2 km s maximum and 5 5 km s minimum resulting in a p wave anisotropy of 12 the calculated velocities are compared with those determined from ultrasonic velocity measurements on a similar sample in the ultrasonic experiment which measures the effects of the shale matrix as well as the effects of porosity velocities are smaller p wave maximum 5 3 km s and minimum 4 1 km s the difference between calculated and measured velocities is attributed to the effects of anisotropic pore structure and to microfractures present in the sample which have not been taken into account in the matrix averaging

an empirical method for estimation of anisotropic parameters in clastic rocks clastic sediments particularly shale exhibit transverse isotropic properties with the symmetric axis perpendicular to bedding vti these anisotropic rock properties are important in seismic imaging prestack seismic analysis and reservoir characterization laboratory tests are the main techniques used in measuring rock anisotropic properties vertical seismic profiling vsp and seismic refraction are the in situ techniques used to determine anisotropic parameters all of these methods provide significant insight to the anisotropic properties of the subsurface however limitations exist in the applications of these methods because of cost issues and restrictions on the number of measurements that can be conducted therefore new techniques that are cost effective easy to use and able to measure continuous anisotropic profiles are desirable this paper describes a methodology which involves the use of conventional rock properties to derive anisotropic rock properties

mathematical modelling of anisotropy of illite rich shale summary the estimation of illite rich shale anisotropy to account for the alignment of clays and gas or brine filled cracks is presented via mathematical modelling such estimation requires analysis to interpret the dominance of one effect over another this knowledge can help to evaluate the permeability in the unconventional reservoir stress orientation and the seal capacity for the conventional reservoir effective media modelling is used to predict the elastic properties of the illite rich shale and to identify the dominant contributions to the shale anisotropy we consider two principal reasons of the shale anisotropy orientation of clay platelets and orientation of fluid filled cracks in reality both of these two factors affect the shale anisotropy the goal of this study is first to separately analyse the effect of these two factors to reveal the specific features in p and s wave velocity behaviour typical of each of the factors and then consider a combined effect of the factors when the cracks are horizontally or vertically aligned to do this we construct four models of shale the behaviour of p and s wave velocities is analysed when gas and water filled cracks embedded in a host matrix are randomly oriented or horizontally or vertically aligned the host matrix can be either isotropic or anisotropic of vti symmetry in such a modelling we use published data on mineralogy and clay platelet alignment along with other micromechanical measurements in the model where the host matrix is isotropic the presence of a singularity point when the difference changes its sign in shear wave velocities is an indicator of brine filled aligned cracks in the model with the vti host matrix and horizontally aligned cracks filled with gas an increase in their volume concentration leads to that the azimuth at which the singularity is observed moves toward the symmetry axis in this case if the clay content is small around 20 per cent the singularity point may even vanish the thomsen parameters are helpful in fluid type indication in shale an indicator of gas filled aligned cracks is if aligned cracks in illite rich shale are brine filled negative value of indicates brine filled cracks in illite rich shale a shale with brine filled cracks exhibits higher vp vs ratio in the vertical direction as compared to the gas filled shale a disorientation of clay platelets and brine filled cracks may lead to that the singularity point is absent for brine saturated shale as well in this case one can also observe and decreased values of vp vs in the vertical direction as in the case of gas filled cracks in the presence of vertically aligned cracks shales exhibit distinctly revealed features of orthorhombic symmetry the results have important applications where seismic measurements are applied to predict the maturity state of the shale

interaction of bed parallel stylolites and extension veins in boudinage abstract a form of discontinuous boudinage styloboudinage involves extension along and shortening across layers due to the contemporaneous development of extension veins and localized bed parallel stylolites styloboudins develop under anchimetamorphic conditions and occur in the thinnest parts of hummocky bedded limestone layers enclosed by calcareous mudrock within the martinsburg formation of southwest virginia u s a they show complex subfabrics defined by sets of orthogonal veins and stylolites the boudins are late stage deformation structures associated with reactivation of early bed parallel stylolites and development of bed normal calcite veins pressure solution shortening across these stylolites is 10 15 whereas finite elongations recorded by veins ranges from 40 70 in thin parts to 10 25 in the widest parts of layers latest pull apart and veining in some cases utilized existing stylolites at high angles to bedding

estimating spatially variable representative elementary scales in fractured architecture using hydraulic head observations 1 water resources within large scale fractured aquifers are typically evaluated using continuum models while any arbitrary scale of continuum can be imposed to represent fracture architecture using the representative elementary volume eliminates errors that result from estimating network connectivity in traditional theory representative elementary scales res are equal in magnitude and geometry throughout a domain and are normally determined using a structural indicator such as porosity or effective hydraulic conductivity we present a fluid based methodology analyzing scale dependent energy variation to estimate res using relatively low cost readily available hydraulic head data hydraulic head predictions of res hyres reveal a spatial variation in elementary scale consistent with the flow field and fracture architecture porosity predictions of res porres incorporate the structural effect of disconnected fracture regions but do not account for fluid behavior res estimated from hydraulic conductivity kres are sensitive to fracture connectivity which controls fluid movement but are difficult to quantify because of scale and spatial variability hyres may be the superior approach for evaluating water resources because it is sensitive to fracture connectivity and avoids complications with scale and spatially dependent averaging our method is sensitive to fracture density clustering connectivity and flow direction but is insensitive to the magnitude of hydraulic gradient hyres is a novel approach to estimating spatially variable res that improves water resource evaluation and flow characterization in fractured aquifers

relationships between compressional wave and shear wave velocities in clastic silicate rocks new velocity data in addition to literature data derived from sonic log seismic and laboratory measurements are analyzed for clastic silicate rocks these data demonstrate simple systematic relationships between compressional and shear wave velocities for water saturated clastic silicate rocks shear wave veloci ty is approximately linearly related to compressional wave velocity and the compressional to shear velocity ratio decreases with increasing compressional velocity laboratory data for dry sandstones indicate a nearly constant compressional to shear velocity ratio with rigidity approximately equal to bulk modulus ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water saturated and dry trends for dry rigidity equal to dry bulk modulus gassmann s equations predict velocities in close agreement with data from the water saturated

the influence of crenulation cleavage development on the bulk elastic and seismic properties of phyllosilicate rich rocks abstract the anisotropy of seismic wave propagation is strongly influenced by the mineralogy and microstructure of rocks phyllosilicates are elastically highly anisotropic and are therefore thought to be important contributors to seismic anisotropy in the continental crust crenulation cleavage is one of the most common microstructural fabrics found in multiply deformed phyllosilicate rich crustal rocks we calculated the bulk elastic properties and resulting wave velocities for rock samples that preserved three different stages of crenulation cleavage development an initial planar foliation a moderately developed crenulation cleavage and a well developed crenulation cleavage mineral orientation maps were obtained using electron backscatter diffraction and calculations were made using asymptotic expansion homogenization combined with the finite element method the difficulties involved with sample preparation and data acquisition of phyllosilicate rich rock samples are also discussed we compare our results to more conventional methods for calculating an aggregate stiffness matrix from a mineral orientation map namely voigt and reuss averages these averages do not account for grain scale interactions and therefore deviate from the results calculated using asymptotic expansion homogenization our results show that the rocks characterized by a planar foliation and a moderately developed crenulation cleavage are highly anisotropic with p wave anisotropies up to 30 9 and s wave anisotropies up to 34 2 whereas the rock characterized by a well developed crenulation cleavage is only mildly anisotropic with a p wave anisotropy of 15 5 and s wave anisotropy of 10 7 progressive development of the fabric also causes the orientations of p and s wave velocity maxima and s wave polarization directions to change markedly despite the high anisotropy imparted by a planar schistosity the variety of folds and fabrics typically found in phyllosilicate rich rocks within larger scale crustal volumes will tend to mute the anisotropy possibly to the point of appearing nearly isotropic

compaction trend versus seismic anisotropy in shaly formations shales comprise more than 60 of sedimentary rocks and form natural seals above hydrocarbon reservoirs their sealing capacity is also used for storage of nuclear wastes the world s most important conventional oil and gas reservoirs have their corresponding source rocks in shale furthermore shale oil and shale gas are the most rapidly expanding trends in unconventional oil and gas shales are notorious for their strong elastic anisotropy i e so called vertical transverse isotropy this vertical transverse isotropy characterised by a vertical axis of invariance is of practical importance as it is required for correct surface seismic data interpretation seismic to well tie and amplitude versus offset analysis a rather classical paradigm makes a clear link between compaction in shales and the alignment of the clay platelets main constituent of shales this would imply increasing anisotropy strength with increasing compaction our main purpose is to check this prediction on two large databases in shaly formations more than 800 samples from depths of 0 6 km by extracting the major trends in the relation between seismic anisotropy and compaction the statistical analysis of the database shows that the simultaneous increase in density and velocity a classical compaction signature is quite weakly correlated with the anisotropy strength as a consequence compaction can be excluded as a major cause of seismic anisotropy at least in shaly formations also the alignment of the clay platelets can explain most of the anisotropy measurements of both databases finally a method for estimating the orientation distribution function of the clay platelets from the measurement of the anisotropy parameters is suggested

permeability tensor for discontinuous rock masses rock masses which commonly contain a large number of discontinuities are treated as homogeneous anisotropic porous media to formulate the corresponding permeability tensor this has been successfully achieved by introducing a symmetric tensor crack tensor which depends only on the geometry of the related cracks aperture size and orientation the principal directions associated with the symmetric crack tensor are coaxial with those of the permeability tensor the first invariant of the crack tensor is proportional to the mean permeability while the deviatoric part is related to the anisotropic permeability these results are well supported by numerical experiments on the permeability of cracked media by long remer wilson and witherspoon an actual rock mass moderately jointed granite was studied to see whether the crack tensor can be determined in situ stereology based on geometrical statistics provides a sound basis for determining the crack tensor in situ the crack tensor is obtained by

on thermohydrologic conditions near high level nuclear wastes emplaced in partially saturated fractured tuff 2 effective continuum approximation this paper presents an effective continuum approximation for modeling of fluid and heat flow in fractured porous media the approximation is based on the thermohydrologic behavior observed in detailed simulations with explicit consideration of fracture effects see the companion paper part 1 the crucial concept in the development of an effective continuum approximation is the notion of local thermodynamic equilibrium between rock matrix and fractures where applicable it provides a substantial simplification of the description of fluid and heat flow in fractured porous media we derive formulas for effective continuum characteristic curves relative permeabilities and capillary pressures in terms of the properties of fracture and matrix continua respectively numerical simulations demonstrate that under favorable conditions the effective continuum approximation closely matches predictions obtained from an explicit modeling of fracture effects it is also demonstrated that the approximation breaks down under unfavorable conditions very tight rock matrix a simple criterion for the applicability of an effective continuum approximation is derived from consideration of diffusive processes a quantitative evaluation shows the criterion to be consistent with results of our numerical simulations

velocity anisotropy in shales a petrophysical study using ultrasonic velocity and anisotropy measurements on a variety of shales with different clay and kerogen content clay mineralogy and porosity at a wide range of effective pressure we find that elastic anisotropy of shales increases substantially with compaction the effect is attributed to both porosity reduction and smectite to illite transformation with diagenesis a means of kerogen content mapping using velocity versus porosity crossplot for shales is shown matrix anisotropy of shales dramatically increases with kerogen reaching the maximum values of about 0 4 at total organic carbon toc 15 20 a strong chemical softening effect was found in shales containing even minor amounts of swelling smectite clay when saturated with aqueous solution this effect results in a significant p wave anisotropy reduction as compared to dry and oil saturated shales since mature black shales are normally oil wet this effect can only have a local significance restricted to the wellbore wall accurate measurements of phase velocities including velocities at a 45 direction to the bedding plane allow us to immediately calculate elastic stiffnesses and anisotropic parameters intrinsic high pressure properties of shales display an e 0 relation introduction of the bedding parallel microcracks in overpressured shales results in a decrease when fully fluid saturated and a increase when partially gas saturated with a characteristic effect on the shape of the p wave velocity surface at small angles of incidence filtering the contribution of the intrinsic anisotropy of shales it is possible to estimate the pore fluid phase microcrack density and aspect ratio parameters using seismic anisotropy measurements

prediction of sonic velocities in shale from porosity and clay fraction obtained from logs a north sea well case study abstractprediction of sonic velocities in shales from well logs is important for seismic to log ties if the sonic log is absent for a shaly section for pore pressure anomaly detection and for data quality control an anisotropic differential effective medium dem was used to simulate elastic properties of shales from elastic properties and volume fractions of silt and wet clay a hypothetical composite material that includes all clay minerals and water anisotropic elastic coefficients of the wet clay were assumed as a first order approximation to be linearly dependent on wet clay porosity wcp here by wcp we mean a ratio of a pore volume occupied by water to a total volume of the wet clay effects of silt inclusions on elastic coefficients of shales were taken into account by using the anisotropic differential effective medium model silt inclusions were modeled as spherical quartz particles simulated elastic coefficients of shales were used to calculate compressional and shear velocities and th

seismic anisotropy of shales shales are a major component of sedimentary basins and they play a decisive role in fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure shale anisotropy needs to be quantified to obtain reliable information on reservoir fluid lithology and pore pressure from seismic data and to understand time to depth conversion errors and non hyperbolic moveout a single anisotropy parameter thomsen s parameter is sufficient to explain the difference between the small offset normal moveout velocity and vertical velocity and to interpret the small offset avo response the sign of this parameter is poorly understood with both positive and negative values having been reported in the literature is sensitive to the compliance of the contact regions between clay particles and to the degree of disorder in the orientation of clay particles if the ratio of the normal to shear compliance of the contact regions exceeds a critical value the presence of these regions acts to increase and a change in the sign of from the negative values characteristic of clay minerals to the positive values commonly reported for shales may occur misalignment of the clay particles can also lead to a positive value of for transverse isotropy the elastic anisotropy parameters can be written in terms of the coefficients w 200 and w 400 in an expansion of the clay particle orientation distribution function in generalized legendre functions for a given value of w 200 decreasing w 400 leads to an increase in while for fixed w 400 increases with increasing w 200 perfect alignment of clay particles with normals along the symmetry axis corresponds to the maximum values of w 200 and w 400 given by w max 200 10 8 2 and w max 400 3 2 8 2 a comparison of the predictions of the theory with laboratory measurements shows that most shales lie in a region of the w 200 w 400 plane defined by w 400 w 200 w max 400 w max 200

deformation of helderberg limestones above the blind thrust system of the central appalachians helderberg limestones have been subjected to late burial solution and layer parallel shortening by cleavage formation with twinning they also record deformation during passive macrofolding due to emplacement of the underlying blind thrust system including passive cleavage rotation during flexural flow perhaps by grain boundary sliding cleavage intensification in gentle fold limbs and cleavage dilation in steep limbs by cleavage normal untwinned calcite fibers cleavage dilation was the second dilation event as bedding solution structures were previously opened with bedding normal fiber growth during layer parallel shortening thus pre existing solution structures were commonly exploited as fluid conduits and precipitation sites

an equivalent continuum model for coupled stress and fluid flow analysis in jointed rock masses to provide a set of governing equations for solving the coupled stress and fluid flow a rock mass which commonly contains a large number of geological discontinuities is treated as an anisotropic elastic porous medium with the corresponding elastic compliance and permeability tensors hydro mechanical equivalents to the discontinuous mass the hydromechanical equivalents with the special emphasis on the permeability tensor are formulated on the assumption that any crack can be replaced by a set of parallel planar plates connected by two springs two dimensional numerical analyses on seepage flow networks support the validity of the permeability tensor some field evidence is analyzed to examine the applicability of the present model for the practical purpose of rock hydraulics