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

specific surface area and pore size distribution in clays and shales one of the biggest challenges in estimating the elastic transport and storage properties of shales has been a lack of understanding of their complete pore structure the shale matrix is predominantly composed of micropores pores less than 2 nm diameter and mesopores pores with 2 50 nm diameter these small pores in the shale matrix are mainly associated with clay minerals and organic matter and comprehending the controls of these clays and organic matter on the pore size distribution is critical to understand the shale pore network historically mercury intrusion techniques are used for pore size analysis of conventional reservoirs however for unconventional shale reservoirs very high pressures 414 mpa 60 000 psi would be required for mercury to access the full pore structure which has potential pitfalls current instrumental limitations do not allow reliable measurement of significant portions of the total pore volume in shales nitrogen gas adsorption techniques can be used to characterize materials dominated by micro and mesopores 2 50 nm a limitation of this technique is that it fails to measure large pores diameter 200 nm we use a nitrogen gas adsorption technique to study the micro and mesopores in shales and clays and compare the results from conventional mercury porosimetry techniques our results on pure clay minerals and natural shales show that i they have a multiscale pore structure at different dimensions ii fine mesopores with a characteristic 3 nm pore size obtained with n2 gas adsorption are associated with an illite smectite group of clays but not with kaolinite iii compaction results in a decrease of pore volume and a reduction of pore size in the inter aggregate macropores of the illitesmectite clays while the fine intra tachoid mesopores are shielded from compaction iv for natural shales mineralogy controls the pore size distributions for shales and the presence of micropores and fine mesopores in natural shales can be correlated with the dominance of the illite smectite type of clays in the rock our assessment of incompressible 3 nm sized pores associated with illite smectite clays provides an important building block for their mineral modulus

intrinsic elasticity of a textured transversely isotropic muscovite aggregate comparisons to the seismic anisotropy of schists and shales 1 the anisotropic properties of phyllosilicate rocks are investigated through the modeling of the pore free intrinsic elasticity of a textured muscovite aggregate the elastic constants were calculated for textures as defined using orientation distribution functions odfs ranging from perfectly aligned to completely random the odfs were described using a gaussian distribution the width of which is controlled by the standard deviation of the orientations of the crystal c axis with respect to the sample foliation plane under the assumption that the sample has hexagonal transversely isotropic symmetry the values were obtained using the widely used voigt reuss hill averaging techniques as well as the geometric mean average gma the gma is more physically meaningful in that it requires that the elastic constants determined in either the stiffness or the compliance domains be invertible comparisons between these different averages indicate that strictly the voigt and reuss values do not necessarily provide the limiting bounds as is expected for the isotropic case despite this some characteristics of the elastic wave p wave anisotropy are only weakly dependent on the averaging procedure employed the calculated elastic constants are converted to the anisotropic and parameters in order to allow for comparison to existing measurements in the literature however these intrinsic results are not intended to provide a measure of the properties of phyllites per se but to provide a reference against which more sophisticated models that incorporate porosity layering and composition may be constructed

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

fabric anisotropy induced by primary depositional variations in the silt clay ratio in two fine grained slope fan complexes texas gulf coast and northern north sea article i nfo high resolution x ray texture goniometry hrxtg has been used to quantify the alignment anisotropy of phyllosilicate minerals in a series of diagenetically altered mudstone samples from the deep subsurface of the texas gulf coast and the northern north sea the sampled formations have undergone a simple burial history with no overprint of tectonic deformation that may have influenced the phyllosilicate orientation observations have been made in two constrained temperature windows 148 150 c north sea data and 169 210 c texas gulf coast wherein less than 20 smectite remains in the mixed layer phase illite smectite quantitative x ray powder diffraction xrpd shows quartz which is dominantly detrital and phyllosilicate content having an inverse relationship quantified illite smectite fabric intensity phyllosi licate alignments ranges from low alignment values of 2 42 m r d multiples of a random distribution to high alignment values 6 75 m r d and chlorite kaolinite ranges from 1 80 to 6 46 m r d we propose that the broad range of phyllosilicate fabric intensity observed reflects the range of quartz phyllosilicate ratios that are the imprint of depositional setting bedform truncation and initial grain size detrital quartz acts to disrupt fabric intensity by being non platy producing areas sheltered from effective stress pressure shadows and acting as a matrix support there is no evidence that bioturbation is a significant control on phyllosilicate alignment in these quartz phyllosilicate mixtures these findings have important implications for the role of grain scale alignment on seismic anisotropy in mudstone systems fabric intensity in post diagenetic samples can be predicted from 1 quartz content and 2 total clay content where

measurements of seismic anisotropy and fracture compliances in synthetic fractured media ultrasonic velocities were measured on a stack of synthetic material plexiglass at low 90 120 khz long wavelength range and high 480 khz short wavelength range frequencies plates were pressed together with uniaxial normal stress these measurements were repeated under different uniaxial pressures for two cases 1without circular rubber inclusions between synthetic fractures and 2with circular rubber inclusions between fractures calculation of anisotropy parameters and also normal and shear interface fracture compliances using linear calculus show reasonable variations anisotropy and compliances decreases as overburden pressure increases suggesting that anisotropy is caused by compliances at higher frequencies synthetic fractures seem to be stiffer than at lower frequencies for the long and short wavelength ranges similar variations were observed for anisotropy parameters and fracture compliances vs measurements in different directions show that for the case without rubber inclusions there is a good approximation of vti medium whereas for the case with inclusions there is a significant difference between vs travelling in directions parallel and perpendicular to the plates

a multiscale methodology for the analysis of velocity anisotropy in organic rich shale abstractchanges in the sources of velocity anisotropy and their relative magnitude as maturation progresses in organic rich shale are still incompletely characterized in the rock physics literature as a result of the increasing importance of organic rich shale as unconventional reservoirs a more thorough understanding of the elastic behavior of shale is needed we have formulated a comprehensive multiphysics multiscale experimental methodology for the characterization of the intrinsic syn lithification and extrinsic postlithification factors contributing to velocity anisotropy application of this methodology to unsaturated samples also enabled the characterization of the shale frame for fluid substitution modeling the methodological framework was then tested on a set of five naturally matured organic rich shale samples in this experimental methodology we combined classical rock physics measurements e g ultrasonic velocity and emergent high resolution imaging techniques such as x ray diffra

velocity anisotropy of shales and depth estimation in the north sea basin it is known that in the north sea basin the depths to major reflectors as determined from surface seismic data are often larger than the well log depths from a study of data sets which tie 21 wells i found a strong correlation between the occurrence of the depth error and the presence of shales in the subsurface assuming that the error is caused by elliptical velocity anisotropy in shales i measured the anisotropy from a comparison of the well log sonic data and the interval velocity profile obtained from the surface seismic data and also from a comparison of the seismic depth and the well log depth it was found that the two methods of measurements agree with each other and also agree qualitatively with the previous laboratory measurements of anisotropy in shale samples the results strongly suggest that the depth anomaly in the north sea basin is caused by the velocity anisotropy of shales a simple method to correct the seismic depth is given

applications of deterministic and stochastic rock physics modelling to anisotropic velocity model building modern anisotropic depth imaging requires transverse isotropy velocity models because the surface seismic experiment alone cannot uniquely determine all transverse isotropy parameters additional information is required traditional methods that use checkshots can only constrain anisotropy locally near the well and thus model building requires 3d extrapolation of well data we propose to use rock physics rock physics compaction modelling of shales and sandy shales can be used to constrain and predict anisotropic parameters of sediments whose depositional environment is governed by compaction in this study we present several applications of rock physics modelling to anisotropic velocity model building we show how both deterministic and stochastic rock physics modelling can be used for initial anisotropic velocity model building and as a constraint for anisotropic parameter estimation in all cases all predicted anisotropy models are physical and realizable in terms of effective medium theory and do not violate elasticity theory

microcrack induced elastic wave anisotropy of brittle rocks the failure of brittle rocks during compression is preceded by the formation growth and coalescence of microcracks elastic wave velocities are reduced in the presence of open microcracks and fractures and may therefore be used to monitor the progressive damage of the rock in general these microcracks are not randomly oriented and the rock displays an elastic anisotropy the elastic anisotropy due to cracks can be expressed in terms of a second rank and fourth rank crack density tensor for open cracks the contribution of the fourth rank crack density tensor to the elastic wave velocities is small these results are compared with recent measurements of the ultrasonic compressional and shear wave velocities for propagation parallel and perpendicular to an increasing axial stress applied at constant confining stress to berea sandstone inversion of the velocity measurements indicates that the microcracks propagate parallel to the maximum compressive stress in agreement with current rock mechanics theory a reasonable fit to the data is obtained using only the second rank crack density tensor even though at high confining stress the cracks are expected to be in partial contact along their length this is consistent with the model of elastic wave propagation in a medium containing partially contacting fractures published by white however measurements of off axis wave velocities are required to fully quantify the contribution of the fourth rank crack density tensor

microfracturing and microporosity in shales shales are ubiquitous rocks in sedimentary basins where their low permeability makes them efficient seals for conventional oil and gas reservoirs and underground waste storage repositories waste waters co2 nuclear fuels moreover when they contain organic matter they form source rocks for hydrocarbons that may escape towards a more porous reservoir during burial a process referred to as primary migration and when the hydrocarbons cannot escape these rocks can be exploited as oil or shale gas reservoirs while the presence of fractures at the outcrop scale has been described the existence of fractures at smaller scales their link with microporosity the mechanisms that created them their persistence over geological times and their effect on the petrophysical properties of shales represent scientific challenges for which drillings in various sedimentary basins over the past decades may hold timely key data here we review and synthetize the current knowledge on how microfractures and micropores in shales can be imaged and characterized and how they control their anisotropic mechanical properties and permeability one question is whether such microfractures when observed in outcrops or in drilled core samples extracted from boreholes are related to decompaction and do not exist at depth another question is whether veins observed in shales represent microfractures that were open long enough to have acted as flow paths across the formation the mechanisms of microfracture development are described some have an internal origin fracturing by maturation of organic matter dehydration of clays while others are caused by external factors tectonic loading importantly the amount of microfracturing in shales is shown to depend strongly on the content in 1 organic matter and 2 strong minerals the nucleation of microfractures depends on the existence of mechanical heterogeneities down to the nanometer scale their propagation and linkage to create a percolating network will depend on the presence of heterogeneities at the meso to macro scales such percolating microfracture networks could control both the long term sealing capabilities of cap rocks and the further propagation of hydraulic fracturing cracks finally possible areas of research for describing the mechanism of microfracture formation in greater detail and how this impacts the transport and mechanical properties of shales are also discussed

creation of synthetic samples for physical modelling of natural shale natural shale samples particularly well preserved drilled core samples are extremely difficult to obtain for laboratory research multiple tests must be carried out on one sample and some samples are disposed after destructive tests therefore rarity and non reusability of samples strongly restrict shale studies in this study based on statistical data from the world s major shale block a new type of synthetic shale was physically constructed via a process of interfusion stuffing and compaction using quartz clay carbonate and kerogen as the primary materials according to statistical data from the world s major shale blocks further evaluation of the synthetic shale involved the use of scanning electron microscopy imagery and analysis of its anisotropic characteristics in comparison with natural shale the synthetic shale had a laminated microstructure similar to natural shale and its velocity anisotropy corresponded to thomsen s anisotropy of a transversely isotropic medium the results of tests for homogeneity and repeatability indicated that the construction process was stable and that several identical synthetic samples which were satisfactorily similar to natural shale could be produced for both iterative and destructive tests the composition of each mineral as well as the density porosity permeability and anisotropy of the samples were all variable therefore a series of synthetic samples could be obtained with properties set to meet the requirements of petrophysical experimentation moreover gas or oil saturation was also considered in the construction of the synthetic shale meaning that the characteristics of gas or oil saturation or the complete range of data from dry to saturated samples could be tested using the synthetic shale

effective medium modeling of gas hydrate filled fractures using the sonic log in the krishna godavari basin offshore eastern india 1 indian national gas hydrate program expedition 01 has established that clay rich marine sediment from the krishna godavari kg basin in the eastern indian margin hosts one of the richest gas hydrate deposits in the world resistivity at bit images and pressure cores reveal that the gas hydrate morphology in clay rich sediment varies from complex vein structures grain displacing to invisible pore filling existing rock physics models which relate acoustic data to in situ gas hydrate concentrations generally assume isotropic pore filling gas hydrate which yields misleading concentration estimates for fractured fine grained sediments the anisotropic kg basin sediment presents additional complications here we apply differential effective medium theory to incorporate grain displacing morphologies by which gas hydrate is included as vertical ellipsoids with aspect ratios ranging from those of thin veins up to those of nodules in an elastic anisotropic background we have estimated gas hydrate concentrations from sonic velocities at hole 10d in the kg basin considering three basic gas hydrate morphologies i pore filling ii grain displacing and iii a combination of grain displacing and pore filling average gas hydrate saturations for these three cases are 35 42 27 30 and 33 41 of the total porosity respectively in the depth range 60 140 m below seafloor mbsf saturation is highest at 67 mbsf for any morphology but the values differ between morphologies for the pore filling morphology the maximum gas hydrate saturation of 56 is 18 22 higher than the grain displacing morphology and 2 9 higher than the combined morphology estimates differ by 6 of the sediment volume with rotations of gas hydrate veins from vertical to horizontal

preferred orientation of phyllosilicates effects of composition and stress on resedimented mudstone microfabrics abstract development of a preferred orientation of clay minerals is investigated in response to changes in vertical effective stress and composition in resedimented material boston blue clay boston blue clay bbc an illitic glacio marine clay composed of 57 clay sized particles

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

preferred orientation and elastic anisotropy of illite rich shale shales display significant seismic anisotropy that is attributed in part to preferred orientation of constituent minerals this orientation pattern has been difficult to quantify because of the poor crystallinity and small grain size of clay minerals a new method is introduced that uses high energy synchrotron x rays to obtain diffraction images in transmission geometry and applies it to an illite rich shale the images are analyzed with the crystallographic rietveld method to obtain quantitative information about phase proportions crystal structure grain size and preferred orientationtexture that is the focus of the study textures for illite are extremely strong with a maximum of 10 multiples of a random distribution for 001 pole figures from the three dimensional orientation distribution of crystallites and single crystal elastic properties the intrinsic anisotropic elastic constants of the illite aggregate excluding contribution from aligned micropores can be calculated by appropriate medium averaging the illitic shale displays roughly transverse isotropy with c11 close to c22 and more than twice as strong as c33 this method will lend itself to investigate complex polymineralic shales and quantify the contribution of preferred orientation to macroscopic anisotropy

a simplified model of effective elasticity for anisotropic shales a simple method to deal with cracklike pores in anisotropic matrix rock such as shales enhances analytical models and their applications actually clayrocks shales in particular are the dominant clastic component in sedimentary basins representing about two thirds of all sedimentary rocks shales are usually assumed to be transversely isotropic ti media they are known to be highly anisotropic because of 1 intrinsic elastic anisotropy of the solid phase matrix forming the rock more or less ordered clay layers and 2 anisotropy induced by the presence of cracklike pores we focus on this second component of anisotropy current analytical models deal with it but they are complex and are restricted in the case of matrix ti symmetry to cracks lying in the symmetry plane we simplify such models within a reasonably good approximation and develop an analysis scheme in which cracklike pore effects are calculated in an equivalent isotropic matrix this simplifies the theoretical approach and potentially broadens its application to any crack and or pore orientation e g damaged shale with horizontal and vertical perpendicular to the bedding plane cracks a high pressure confinement test provides experimental data for checking the proposed tool against a reference model in the case of cracklike pores lying in the bedding plane the results in terms of thomsen parameters are consistent with results from large scale field data

the elastic anisotrophy of shales shales constitute about 75 of the clastic fill sedimentary basins and have a decisive effect on fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure the elastic stiffnesses of a shale with partially oriented clay particles is expressed in terms of the coefficients w sub lmn in an expansion of the clay particle orientation distribution function in generalized legendre functions application is made to the determination of the anellipticity of shales for transverse isotrophy the anellipticity quantifies the deviation of the p wave slowness curve from an ellipse and is shown to depend on a single coefficient w sub 400 in the expansion of the clay particle orientation distribution function if w sub 400 is small the anellipticity may be neglected as is apparently the case for a near surface late tertiary shale studied by winterstein and paulson strongly aligned clay particles result in a positive value of w sub 400 and a positive anellipticity in agreement with the majority of the field measurements however less well ordered shales could have a significantly positive second moment w sub 200 but only a small positive or even negative value of w sub 400 for such shales the anellipticity would be small or negative despite amore preferred alignment of clay particles in the bedding plane numerical examples of clay particle orientation distribution functions leading to zero or negative anellipticity are given less

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

analysis of seismic anisotropy parameters for sedimentary strata abstractbased on a large quantity of laboratory ultrasonic measurement data of sedimentary rocks and using monte carlo simulation and backus averaging we have analyzed the layering effects on seismic anisotropy more realistically than in previous studies the layering effects are studied for different types of rocks under different saturation conditions if the sedimentary strata consist of only isotropic sedimentary layers and are brine saturated the value for the effective transversely isotropic ti medium is usually negative the value will increase noticeably and can be mostly positive if the sedimentary strata are gas bearing based on simulation results c13 can be determined by other ti elastic constants for a layered medium consisting of isotropic layers therefore can be predicted from the other thomsen parameters with confidence the theoretical expression of for an effective ti medium consisting of isotropic sedimentary rocks can be simplified with excellent accuracy into a neat for

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

seismic anisotropy of fractured rock a simple method for including the effects of geologically realistic fractures on the seismic propagation through fractured rocks can be obtained by writing the effective compliance tensor of the fractured rock as the sum of the compliance tensor of the unfractured background rock and the compliance tensors for each set of parallel fractures or aligned fractures the compliance tensor of each fracture set is derivable from a second rank fracture compliance tensor for a rotationally symmetric set of fractures the fracture compliance tensor depends on only two fracture compliances one controlling fracture compliance normal the other tangential to the plane of the fractures the stiffness tensor which is more useful in the consideration of elastic wave propagation through rocks can then be obtained by inversion the components of the excess fracture compliance tensor represent the maximum amount of information that can be obtained from seismic data if the background rock is isotropic and the normal a

determining formation s wave transverse isotropy from borehole flexural wave dispersion data abstractmany earth formations are characterized as transversely isotropic ti media in acoustic logging through a vertical borehole the s wave ti property has traditionally been determined from borehole monopole stoneley wave measurements but the feasibility of shear ti estimation from dipole flexural waves has not been fully investigated we have developed a methodology to determine the ti parameters from borehole dipole flexural wave data our analysis shows that the stoneley wave is sensitive to the ti property mainly in an acoustically slow formation and the sensitivity diminishes when the formation becomes faster the advantage of the flexural wave over the stoneley wave is that the former wave is sensitive to the ti property in the slow and fast formations provided the wave measurement is made in a broad frequency range in which the flexural wave dispersion characteristics from low to high frequencies can be used by calculating the theoretical flexural wave dispersion curve for the ti formati

the effect of anisotropy on the young s moduli and poisson s ratios of shales young s modulus and poisson s ratio are required for geomechanics applications such as hydraulic fracture design analysis of wellbore stability and rock failure determination of in situ stress and assessment of the response of reservoirs and surrounding rocks to changes in pore pressure and stress shales are usually anisotropic and models that neglect shale anisotropy may fail to describe geomechanical behaviour correctly anisotropy in shales results from a partial alignment of anisotropic clay particles kerogen inclusions microcracks low aspect ratio pores and layering for shales the young s modulus measured parallel to bedding e1 is usually greater than the young s modulus measured perpendicular to bedding e3 however the poisson s ratio 31 corresponding to stress applied perpendicular to bedding and strain measured parallel to bedding can be greater than equal to or less than the poisson s ratio 12 for stress applied parallel to bedding and strain measured parallel to bedding for transverse isotropy the elastic anisotropy resulting from a partial alignment of clay particles can be written in terms of the coefficients w200 and w400 which describe the impact of clay particle orientation on the anisotropy of shales disorder in the orientation of clay particles acts to reduce w400 faster than w200 since w400 is a higher order moment of the clay particle orientation distribution function than w200 this is confirmed by analysis of measured anisotropy parameters for shales a partial alignment of clay particles is consistent with the measured young s moduli for shales and with values of poisson s ratio 31 12 but not with values 31 12 these values can be explained if there exist kerogen inclusions microcracks or low aspect ratio pores aligned parallel to the bedding plane

compaction of quartz kaolinite mixtures the influence of the pore fluid composition on the development of their microstructure and elastic anisotropy abstract shales play important roles in sedimentary basins acting as both seals and reservoir rocks and knowledge of their anisotropic velocity trends is of practical importance for correct seismic image processing and inversion and for seismic to well tie whereas velocity depth trends are extensively studied for conventional reservoirs and critical factors that control their compaction are well understood little work has been done on shales compaction trends of shaly formations are controlled by a number of parameters such as clay mineralogy silt fraction and depositional environment this large number of parameters which are difficult if not impossible to control in naturally deposited shales make the study of shale compaction a statistically complex multivariate and non unique problem in such a situation laboratory experiments that allow precise planning of mixture mineralogy and chemical compositions of pore fluids as well as an accurate control of microstructural changes seems to be an attractive alternative in this work we have conducted two sets of compaction experiments on quartz kaolinite mixtures with 100 75 and 60 of kaolinite powders dry weight in the first set the use of a kcl brine solution to saturate the mixtures has led to the aggregation of the existing clay particles with each other and with silt particles in the other set the mixtures have been treated with a dispersant to separate existing clay aggregates into individual platelets the mixtures have been further compacted in an oedometric cell at uniaxial stresses up to 30 mpa compressional in vertical and horizontal directions and shear in vertical direction ultrasonic wave velocities have been measured during these compaction experiments and the p wave anisotropy has been estimated the effect of the chemical composition of the pore fluid on shale microstructure has been studied using the micro ct and sem image analysis neutron diffraction experiments have been conducted to understand the orientation of clay platelets at the final stage of the compaction our research shows that the chemical composition of pore fluid significantly affects the microstructure of the clay component and via this the elastic properties of the artificial shales the samples with the dispersed clay microstructure dcm have shown larger elastic anisotropies at the same porosity and significantly larger anisotropies at the same stress the dispersed clay platelets have orientated preferably parallel to bedding plane while the clay platelets in the samples with the aggregated clay microstructure acm show a wider angle distribution in the absence of quartz particles 100 kaolinite samples in the case of 40 of quartz fraction though the clay particles are more aligned in the acm sample than in the dcm one

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

products of the 1907 eruption of shtyubel volcano ksudach caldera kamchatka russia on march 28 1907 a sequence of explosive eruptions within the ksudach caldera complex located in the southern part of the kamchatka peninsula russia partially destroyed shtyubel9 volcano leaving a tuff cone with a 1 5 km in diameter horseshoe shaped crater filled with a deep lake the total estimated volume of erupted products is 2 4 km 3 of which 1 9 km 3 consists of juvenile andesitic to rhyolitic material the eruption can be subdivided into four phases the first phase opened the vent and dispersed about 0 02 km 3 of pyroclastic fallout within ksudach caldera the second phase produced a plinian column that distributed about 1 5 km 3 juvenile pyroclastic fallout to north northeast and southeast the third phase was characterized by a powerful blast explosion toward the north northeast in a 60 sector the blast cloud traveled at least 100 110 ms 1 to surmount topographic barriers 500 600 m high other explosions produced pyroclastic flows surges and breccias that cover about 115 km 2 with a volume of 0 15 km 3 the fourth phase consisted of a series of shallow hydromagmatic eruptions that emplaced five pyroclastic surge deposits with ash cloud surge beds at their tops to the west northwest two or three of these surge clouds must have exceeded velocities of 100 ms 1 in order to surmount ksudach caldera walls these deposits cover an area of 160 km 2 with a volume of 0 2 km 3 analyses of deposit structures particle textures and fragment components indicate that the first two phases were magmatically driven however the last two phases were increasingly hydromagmatic lack of erosional contacts between the deposits abundant impact sags at horizon tops and absence of reworked materials within the sequence suggest that the eruption lasted only a few hours to a few days juvenile products range from basaltic andesite to rhyolite in composition based on the variation of chemistry and texture of juvenile components we hypothesize that there was a zoned magma chamber below shtyubel9 volcano just prior to the 1907 event

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

the role of texture cracks and fractures in highly anisotropic shales organic shales generally have low permeability unless fractures are present however how gas oil and water flows into these fractures remains enigmatic the alignment of clay minerals and the alignment of fractures and cracks are effective means to produce seismic anisotropy thus the detection and characterization of this anisotropy can be used to infer details about lithology rock fabric and fracture and crack properties within the subsurface we present a study characterizing anisotropy using s wave splitting from microseismic sources in a highly anisotropic shale we observe very strong anisotropy up to 30 with predominantly vti vertical transverse isotropy symmetry but with evidence of an hti horizontal transverse isotropy overprint due to a ne striking vertical fracture set parallel to the maximum horizontal compressive stress we observe clear evidence of a shear wave triplication due to anisotropy which to our knowledge is one of only a very few observations of such triplications in field scale data we use modal proportions of minerals derived from x ray fluorescence data combined with realistic textures to estimate the contribution of intrinsic anisotropy as well as possible contributions of horizontally aligned cracks we find that aligned clays can explain much of the observed anisotropy and that any cracks contributing to the vertical transverse isotropy vti must have a low ratio of normal to tangential compliance zn zt typical of isolated cracks with low hydraulic connectivity subhorizontal cracks have also been observed in the reservoir and we propose that their reactivation during hydraulic fracturing may be an important mechanism to facilitate gas flow

comparison between analytical and 3d finite element solutions for borehole stresses in anisotropic elastic rock we present a rigorous validation of the analytical amadei solution for the stress concentration around an arbitrarily orientated borehole in general anisotropic elastic media first we revisit the theoretical framework of the amadei solution and present analytical insights that show that the solution does indeed contain all special cases of symmetry contrary to previous understanding provided that the reduced strain coefficients b11 and b55 are not equal it is shown from theoretical considerations and published experimental data that the b11 and b55 are not equal for realistic rocks second we develop a 3d finite element elastic model within a hybrid analytical numerical workflow that circumvents the need to rebuild and remesh the model for every borehole and material orientation third we show that the borehole stresses computed from the numerical model and the analytical solution match almost perfectly for different borehole orientations vertical deviated and horizontal and for several cases involving isotropic transverse isotropic and orthorhombic symmetries it is concluded that the analytical amadei solution is valid with no restriction on the borehole orientation or the symmetry of the elastic anisotropy

approximate traveltime inversion in downhole microseismic monitoring abstract in downhole microseismic monitoring accurate event location relies on the accuracy of the velocity model the model can be estimated along with event locations anisotropic models are important to get accurate event locations taking anisotropy into account makes it possible to use additional data two s wave arrivals generated due to shear wave splitting however anisotropic ray tracing requires iterative procedures for computing group velocities which may become unstable around caustics as a result anisotropic kinematic inversion may become time consuming in this paper we explore the idea of using simplified ray tracing to locate events and estimate medium parameters in the simplified ray tracing algorithm the group velocity is assumed to be equal to phase velocity in both magnitude and direction this assumption makes the ray tracing algorithm five times faster compared to ray tracing based on exact equations we present a set of tests showing that given perforation shot data one can use inversion based on simplified ray tracing even for moderate to strong anisotropic models when there are no perforation shots event location errors may become too large for moderately anisotropic media

the nanogranular acoustic signature of shale a multiscale micromechanics model has been developed for the prediction of anisotropic acoustic properties of shale the model is based on the recently identified nanogranular mechanical response of shale through indentation experiments it recognizes the dominant role of the anisotropic elastic properties of compacted clay in the anisotropic elasticity of shale at different length scales compared to contributions of shape and orientation of particles following a thorough validation at multiple length scales using mineral elasticity data nanoindentation experiment results and ultrasonic pulse velocity tests the model predictions compare adequately with measurements on kerogen free and kerogen rich shales and shaley sandstones the acoustic signature of shale thus is found to be controlled by two volumetric parameters that synthesize the porosity and mineralogy information the clay packing density and the silt inclusion volume fraction through a series of dimensionless isoparametric plots the micromechanics model predicts trends of increasing elastic anisotropy with increasing clay packing density or decreasing porosity which correspond to the intrinsic mechanical response of unfractured shale and quantifies the stiffness reduction induced by the presence of kerogen

hydrogen and oxygen isotope variations in the high himalaya peraluminous manaslu leucogranite evidence for heterogeneous sedimentary source abstract the manaslu granite belongs to the high himalaya leucogranitic belt which was produced by melting of the crust during postcollisional thrusting d and 18o values have been determined for whole rock and coexisting minerals from the 8 km thick manaslu massif and its 50 km long dyke sheet its country rocks and the formation 1 f1 paragneisses which are the source of the granite for the granite dmusc range from 70 to 85 and 18o w r from 10 9 to 12 8 h and o isotope fractionations among minerals are consistent with high temperature equilibrium and for oxygen closed system evolution a few samples coming mainly from the dyke swarm have very low d values down to 188 and biotite muscovite h isotope fractionations indicative of disequilibrium the d h ratios of associated magmatic tourmaline are essentially unmodified from the distribution of d values in the granite and its country rocks circulation of very low deuterium meteoric hydrothermal waters was extremely localized because these waters are depleted in deuterium by up to 50 relative to modern meteoric waters the manaslu area was either at an altitude substantially higher than that of today 2500 6000 m for the analyzed samples or a mountain chain once existed to the south the f1 gneisses have 18oquartz between 12 and 14 3 which confirms that the granite was generated from f1 but d values are 20 higher than in the granite such a difference can be a result of degassing of the magma and or introduction of fluid in the melting zone infiltration of low d fluid 90 into the hot but dry f1 probably triggered partial melting these fluids could have come from the dehydration of the midlands sediments which are separated from the overlying f1 by the main central thrust the correlations among 18o 87 sr 86 sr 20 ma and nd values in both f1 and the granite indicate that the variations of these isotopic ratios in the manaslu are inherited from those in f1 at the time of melting in turn these ratios in f1 are related to the proportion of quartz and phyllosilicates for the isotopic ratios of nd and o and to the quantity of radiogenic sr generated within the sediment which is a function of age and rb content amount of phyllosilicate some other himalayan leucogranites require either other crustal source rocks or the 18o and 87 sr 86 sr ratios of f1 vary along the himalaya

stress associated intrinsic and scattering attenuation from laboratory ultrasonic measurements on shales seismic attenuation is sensitive to stress induced subtle changes in the physical state of rocks in this study the stress and frequency associated attenuation is quantified through ultrasonic measurements on three differently oriented cylindrical shale samples under various axial stresses as an improvement to the single scattering model the elastic monte carlo method is employed to investigate multiple scattering attenuations by incorporating the boundary reflections and wave conversions our results show that as the axial stress increases the intrinsic attenuation decreases in all directions while the scattering attenuation decreases slightly in the direction perpendicular to the bedding but increases largely and nonlinearly in other directions these discrepancies result from different attenuation mechanisms both the intrinsic and scattering attenuation are found to be largest in the direction 45 to the bedding but least in the perpendicular direction the s wave attenuation is larger and more sensitive to stress changes than p wave attenuation due to its shorter wavelength as expected from sandstone examples the scattering attenuation in shales is significantly larger and more sensitive to stress changes than the intrinsic attenuation the frequency dependence of scattering attenuation suggests that the peak frequency with the maximum scattering attenuation is independent of axial stresses but varies in different directions of an individual rock with different heterogeneity and anisotropy scales the peak frequency of s coda is smaller and its peak scattering attenuation is larger than p coda in conclusion the stress and frequency dependence of ultrasonic attenuations in shales differ largely in various directions indicating significant anisotropy and heterogeneity

seismic anisotropy of shales the seismic properties and preferred clay mineral orientation of a suite of shales are investigated using laboratory velocity measurements as a function of confining pressure x ray diffraction techniques and electron microprobe backscatter bse imaging the velocity measurements indicate that these shales are transversely isotropic with the main symmetry axis perpendicular to bedding anisotropy at elevated pressures caused mainly by preferred orientation of clays illite parallel to bedding ranges from 20 vp and 19 vs for a sample of new albany shale to 30 vp and 35 vs for a sample of chattanooga shale the degree of clay mineral alignment in the shales is constrained by orientation indices produced using simple x ray diffraction techniques a strong positive correlation is found between the degree of preferred orientation as expressed in the orientation indices and seismic anisotropy bse images of the shale fabrics confirm in a qualitative manner the results of the x ray study to investigate wave propagation in the shales elastic constants of each sample are calculated and used to produce phase and group wave velocity surfaces which describe variation in velocity as a function of angle to the bedding normal the calculated velocity surfaces constrained by independent velocity measurements display a lack of shear wave splitting at near normal incidence in even the most anisotropic shales for the highly anisotropic chattanooga shales group velocity surfaces differ significantly from corresponding phase velocity surfaces

geochemical and petrophysical characteristics of permian shale gas reservoirs of raniganj basin west bengal india abstract thick shale horizons of permian age emerged as the potential source of gas through primary investigation and resulted into the first indian pilot scale gas recovery demonstration project in the raniganj basin however the relation of geochemical properties with shale pore matrix porosity and permeability mechanism are yet to be evaluated for better dealing the phenomena of diffusion transport and gas recovery in this study the analyses like proximate rock eval toc vro low pressure n2 sorption porosity permeability sem edx and image processing have been carried out the values of s1 s2 toc and tmax indicated fair to excellent source rock potential of the shales having type iii iv kerogens prone to thermal gas genesis the low pressure n2 desorption curve of raniganj formation shales shows the type h3 hysteresis comprising slit shaped pores the barren measures shales have type h2 and h3 hysteresis patterns specifying to ink bottle shaped pores and slit shaped pores whereas the oldest barakar formation shales are having ink bottle shaped pores caused due to blocking effect the slit shaped pores favour the pore network and characteristically excellent for the flow of gas the results of sem edx are indicating alteration stability in the order of o the increasing tmax values with a centric decrease in porosity and permeability specifying the role of devolatilization disintegration and blocking of pore spaces openings is a function of the thermal gradient the linear evolution of multipoint bet surface area with increasing porosity suggests that the porosity values from 2 0 to 6 5 mainly corresponds to pore size of 3 0 11 0 nm similarly the inverse relationship between average pore size and porosity attributed to a greater contribution of smaller pores in total porosity the pore network model derived through the sem image processing has shown two types of connectivity i various pore sizes of diverse pore throats with dual opening directions and ii interlinked large and small pores obeying similar normal distributions the barren measures shales have shown higher pore connectivity than the raniganj and barakar formation shales

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

effective elasticity of fractured rocks a snapshot of the work in progress exploration and development of naturally fractured reservoirs rely on understanding and interpretation of certain signatures associated with seismic waves propagating through cracked rocks this understanding comes primarily from the effective media theories that predict an overall elastic behavior of a solid containing many inhomogeneities cracks in particular whose sizes are too small to be seen individually by the waves to model seismic responses of fractured formations a geophysicist typically has a choice between the effective media schemes of hudson and schoenberg while the two predictions usually deviate slightly for liquid filled cracks the differences are significant when the fractures are dry explaining the origin of these differences and selecting a more accurate scheme is the first goal of this tutorial our second more challenging task is to prove that simply adding the compliance contributions of cracks as if they were isolated and noninteracting remains sufficiently accurate even for fractures that grossly violate the basic theoretical assumption of penny shaped cracks real fractures have notoriously irregular shapes might be partially closed and often form interconnected networks yet these details of fracture microgeometry turn out to be unimportant for the effective elasticity given a typical noise level in seismic data no closed form theory exists for irregular fracture shapes however take into account finite element simulations on so called digital rocks demonstrate which features of the crack geometry have to be taken into account because they influence propagation of long compared to the size of fractures seismic waves and which features can be ignored

stress dependent seismic anisotropy of shales a simple theory for the stress dependent seismic anisotropy of shales can be obtained in terms of a second rank tensor and a fourth rank tensor that depend on the orientation distribution of contacts between clay platelets the theory allows the normal and shear stiffness of the contact regions between clay platelets to be obtained as a function of stress from measurements of seismic p and s wave velocities for shales the ratio of the normal to shear compliance bn bt of the contact regions between clay particles is found to be sensitive to the saturation state of the shale inversion of velocity measurements for fully saturated shales shows a low value of bn bt when compared with measurements on air dry shales consistent with the expected reduction in normal compliance in a fluid saturated low permeability rock for all shales considered bn bt is found to be less than unity the contacts between clay particles are therefore more compliant in shear than in compression

multi scale approach for modeling the transversely isotropic elastic properties of shale considering multi inclusions and interfacial transition zone multiscale approach based explicit analytic predictions are obtained for the transversely isotropic properties of shale rock considering the multi inclusion and interfacial transition zone itz effects representative volume elements rves are utilized to describe the material s hierarchical microstructures from the nanoscale to the macroscale a new multilevel micromechanical homogenization scheme is presented to quantitatively estimate the material s transversely isotropic properties with the multi inclusion and itz effects the itz is characterized by the interphase material whose effects are calculated by modifying the generalized self consistent model furthermore the explicit form solutions for the transversely isotropic properties are obtained by utilizing the hill polarization tensor without numerical integration and the standard tensorial basis with the analytic inversions of fourth rank tensors to verify the proposed multiscale framework predictions obtained via the proposed model are compared with experimental data and results estimated by the previous work which show that the proposed multi scaling approaches are capable of predicting the macroscopic behaviors of shale rocks with the multi inclusion and itz effects finally the influences of itz and inclusion properties on the material s macroscopic properties are discussed based on the proposed multiscale framework

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

anisotropy estimate for the horn river basin from sonic logs in vertical and deviated wells abstract shale anisotropy must be quantified to obtain reliable information about reservoir fluids lithology and pore pressure from seismic data failure to account for anisotropy in seismic processing can lead to errors in normal moveout nmo dip moveout dmo migration time to depth conversion and amplitude variation with offset avo analysis kriged estimates of density vertical compressional wave and shear wave velocities were derived from vertical wells in an area of interest in the horn river resource play in northeastern british columbia the kriged predictions were compared with measured logs along the trajectory of a deviated well whereas the density comparison provided a blind test of kriging accuracy because density is scalar and independent of well deviation the same comparison for sonic velocities revealed that they are systematically higher than the kriged vertical velocities this difference was used to estimate anisotropy parameters at the location of the deviated well the fac

pyrolysis induced p wave velocity anisotropy in organic rich shales abstractthe sources of elastic anisotropy in organic rich shale and their relative contribution therein remain poorly understood in the rock physics literature given the importance of organic rich shale as source rocks and unconventional reservoirs it is imperative that a thorough understanding of shale rock physics is developed we made a first attempt at establishing cause and effect relationships between geochemical parameters and microstructure rock physics as organic rich shales thermally mature to minimize auxiliary effects e g mineralogical variations among samples we studied the induced evolution of three pairs of vertical and horizontal shale plugs through dry pyrolysis experiments in lieu of traditional samples from a range of in situ thermal maturities the sensitivity of p wave velocity to pressure showed a significant increase post pyrolysis indicating the development of considerable soft porosity e g microcracks time lapse high resolution backscattered electron scanning electron

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

elastic and resistivity anisotropy of shale during compaction and diagenesis joint effective medium modeling and field observations abstractshales are anisotropic materials which have been observed to have both electrical and elastic anisotropy shales also comprise most of the sedimentary column in clastic basins and thus the ability to model shale response to electrical and seismic fields may improve our ability to better resolve anisotropy by using different measurements compacting shale effective media modeling which captures the geological process of porosity reduction and geochemical compaction was formulated in this research in microstructural parameters associated with particle alignment and pore shape deformation the state of compaction captured by the porosity of the soft sediment governs the pore aspect ratio and the amount of particle alignment diagenetic processes are captured using temperature dependent mineral elasticity and conductivity the model uses the same microstructural parameters and equivalent effective medium approximation scheme to predict electrical and elastic anisotropy in situ measurements of e

a rock physics driven bayesian inversion for toc in the fuling shale gas reservoir abstract total organic content toc is an important indicator in the evaluation of the organic abundance and production potential of shale reservoirs the conventional approach to toc estimation is to correlate toc with inverted elastic parameters in a linearized empirical equation however this approach ignores complex rock physics properties such as complex mineralogy and pore space in the shale while generating smooth low resolution results this paper proposes a rock physics based method to forward model the seismic and elastic response of the shale reservoir and then invert using a bayesian approach to derive key parameters such as the toc total porosity and the fractional volume of brittle minerals and to delineate the sweet spots in an unconventional shale reservoir

preferred orientation and elastic anisotropy in shales anisotropy in shales is becoming an important issue in exploration and reservoir geophysics in this study the crystallographic preferred orientation of clay platelets that contributes to elastic anisotropy was determined quantitatively by hard monochromatic x ray synchrotron diffraction in two different shales from drillholes off the coast of nigeria to analyze complicated diffraction images with five different phases illite smectite kaolinite quartz siderite feldspar and many overlapping peaks we applied a methodology based on the crystallographic rietveld method the goal was to describe the intrinsic physical properties of the sample phase composition crystallographic preferred orientation crystal structure and microstructure and compute macroscopic elastic properties by averaging single crystal properties over the orientation distribution for each phase our results show that elastic anisotropy resulting from crystallographic preferred orientation of the clay particles can be determined quantitatively this provides a possible way to compare measured seismic anisotropy and texture derived anisotropy and to estimate the contribution of the low aspect ratio pores aligned with bedding