empirical relationships among seismic velocity effective pressure porosity and clay content in sandstone we use a multivariate analysis to investigate the influence of effective pressure pe porosity and clay content c on the compressional velocity vp and shear velocity vs of sandstones laboratory measurements on water saturated samples of 64 different sandstones provide a large data set that was analyzed statistically for each sample relationships between effective pressure and vp and vs have been determined all samples were well fit by relationships that have an exponential increase in velocity at low pe tapering to a linear increase with pe for pe greater than 0 2 kbar there are differences in the pressure dependences of velocity for different rocks particularly at very low pressures however the differences cannot be attributed to or c for the combined set of measurements from all samples the best fitting formulations are vp 5 77 6 94 1 73c 0 446 pe e 16 7pe and vs 3 70 4 94 1 57c 0 361 pe e 16 7pe while this is admittedly a very simplified parameterization it is remarkable how well

elastic wave velocity evolution of shales deformed under uppermost crustal conditions conventional triaxial tests were performed on a series of samples of tournemire shale along different orientations relative to bedding 0 90 experiments were carried out up to failure at increasing confining pressures ranging from 2 5 to 80 mpa and at strain rates ranging between 3 10 7s 1 and 3 10 5s 1 during each experiment p and s wave elastic velocities were continuously measured along many raypaths with different orientations with respect to bedding and maximum compressive stress this extensive velocity measurement set up allowed us to highlight the presence of plastic mechanisms such as mineral reorientation during deformation the evolution of elastic anisotropy was quantified using thomsen s parameters wich were directly inverted from measurement of elastic wave velocity brittle failure was preceded by a change in p wave anisotropy due to both crack growth and mineral re orientation anisotropy variations were largest for samples deformed perpendicular to bedding at the onset of rupture anisotropy reversal was observed at the highest confining pressures for samples deformed parallel to bedding the p wave anisotropy change is weaker

x ray micro computed tomography and ultrasonic velocity analysis of fractured shale as a function of effective stress abstract ultrasonic velocity is a key shale gas reservoir property especially in the context of gas production or co 2 injection for geo sequestration this ultrasonic velocity reflects the dynamic elastic properties of the rock and it thus depends on the fracture morphology which varies significantly with effective stress however the precise relationship between ultrasonic velocity and fractured shale morphology is only poorly understood we thus measured p and s wave velocities of fractured shale in two orthogonal directions and imaged the shale with x ray micro computed tomography as a function of applied effective stress and investigated how fracture morphology p and s wave velocity young s modulus shear velocity and poisson s ratio are interconnected with effective stress clearly most of the small fractures the width is around 0 1 mm closed with increasing effective stress resulting in a different fracture size distribution which again had a dramatic effect on the elastic rock properties furthermore with increasing effective stress p and s wave velocities increased significantly such that the orthogonal waves gave a similar response at 2000 psi effective stress despite significant sample heterogeneity we conclude that the fracture aperture direction and network characteristics severely influence wave propagation and thus elastic properties these results can be used to assess natural fracture networks monitor fracture development during hydraulic fracturing and predict fracture closure scenarios during production

mechanical and petrophysical properties of north sea shales one of the problems when evaluating potential borehole stability problems in the drilling of hydrocarbon exploration and production wells is the lack of relevant data to describe the lithologies this relates in particular to the shales since these constitute most of the overburden and are also the most troublesome this paper presents a set of data from a variety of deeply cored shales from the north sea the shales have been extensively characterised and span a wide range of both petrophysical and mechanical properties depending on for example burial history and diagenesis special characteristics of shales have been tested and described including pore size distribution permeability anisotropy and temperature dependence appropriate procedures for sample handling testing and interpretation have been developed or modified from other related sciences

time lapse seismic tomography of an underground mining zone abstract in this study we determined p wave tomographic images of the yongshaba deposit an underground mining zone in guizhou province china by inverting arrival time data of micro seismic events and blasts recorded by a passive seismic array consisting of 28 sensors during january to april 2014 using an event location technique and a travel time tomography method that can handle complex seismic discontinuities the damping parameter used in the damped least squares method is determined with the help of trade off curve to reveal internal p wave velocity changes of the study area under severe mining activities a time lapse data partition scheme is used to assess the human influence on the underground structure we introduce a parameter to measure the relative velocity changes in different periods the resolution of the tomographic images and the robustness of the obtained features are examined by conducting a series of checkerboard resolution tests and a restoring resolution test our tomographic results obtained from the entire data set reveal a prominent low velocity low v zone and many obvious high velocity high v zones these features match well with the geological setting and the excavation plan carried out in the mine according to in site surveys the low v zone may reflect empty volumes stress releases and rock breaking and cracking caused by mining activities whereas the high v zones are probably the consequences of local stress concentrations caused by regional stress redistribution the time lapse tomographic images may reveal the process of stress concentrations caused by rock breaking due to the continuing excavations in the entire observation period indicating that the mining activities influenced the rock property and caused complex changes of the underground structure

understanding of elastic anisotropy of shale under triaxial loading porosity deformation approach abstracttheoretical models for the description of stress dependencies for anisotropic rocks are important for building velocity models and the interpretation of seismic data with respect to stress lithology and pore content especially important is the accurate description of the change of anisotropy due to applied anisotropic stress by means of a quantitative model to aid the interpretation the porosity deformation approach pda originally introduced as the piezosensitivity theory describes stress induced changes of elastic properties without any knowledge about the geometry and orientation of the microcracks we have developed a formulation of the theory for a transverse isotropic rock which was loaded parallel to the bedding plane and we found that it can be described theoretically as an orthorhombic rock we also found out how properties that described the velocity change due to hydrostatic loading can be used to predict elastic anisotropy under additional axial loading the number of free para

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

anisotropy rather than transverse isotropy in longmaxi shale and the potential role of tectonic stress abstract in the last few decades the recovery of and exploration for shale gas has increased this has led to increased interest in the mechanical characterization of shale formations as relates to enhancing the conductivity of such formations characterization and modeling of these materials typically assumes transverse isotropy associated with the direction of bedding despite some studies showing anisotropy in these materials in this work we evaluate the validity of transverse isotropy for the longmaxi shale formation which has been targeted for shale gas exploration in china this evaluation is done through analysis of the macro and microstructure distribution confined triaxial tests and compression and shear wave velocity measurements these tests were performed parallel and perpendicular to the bedding and in the directions interpreted as the tectonic principal stresses the effect of tectonic stress on shale anisotropy is also discussed we found that the assumption of transverse isotropy for the mechanical behavior of the longmaxi shale might not be adequate under low confinement stresses the trends in the measured behavior suggest that transverse isotropy might provide adequate estimations as confinement increases the data also suggest that tectonic stresses affected the longmaxi shale rock mass characteristics at the study site in terms of macro and microstructure as well as mechanical properties the results also indicate that isotropic rocks may show anisotropy in subsurface engineering directing more attention to the possible mechanical differences in the field may provide implications for engineering safety

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

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

impact of fabric microcracks and stress field on shale anisotropy summary few data are available on shales in terms of seismic to ultrasonic properties and anisotropy although all are important with regards to imaging problems often encountered in such lithologies additionally mechanisms causing changes in these properties are not well documented due to the fine grain size of such materials and time required for testing under controlled pore pressure conditions the results presented here are derived from a set of experiments run on muderong shale with pore pressure control in order to evaluate the effect of stress magnitude and stress anisotropy on ultrasonic response this shale was noted to have a linear velocitymean effective stress response and extremely high anisotropy both likely the result of the presence of fluid filled cracks in a low permeability porous medium changes in velocity and vp vs ratios are dependent on both stress and smectite content s wave velocity is significantly affected by the presence of smectite in this and other shales and at low stress 20 mpa is less sensitive to stress change than p wave velocity vp vs ratios are noted to increase in this shale up to 20 mpa effective stress then decrease slightly due to stress induced loss of interlayer water in smectite intrinsic anisotropy comes from composition a strong compaction fabric and the presence of microfractures changes to ultrasonic anisotropy are the result of the magnitude of the stresses their orientation with respect to the fractures and the degree of stress anisotropy

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

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

stress anisotropy and velocity anisotropy in low porosity shale abstract shales are known for often marked intrinsic anisotropy of many of their properties including strength permeability and velocity for example in addition it is well known that anisotropic stress fields can also have a significant impact on anisotropy of velocity even in an isotropic medium this paper sets out to investigate the ultrasonic velocity response of well characterised low porosity shales from the officer basin in western australia to both isotropic and anisotropic stress fields and to evaluate the velocity response to the changing stress field during consolidated undrained multi stage triaxial tests on core plugs cut normal to bedding vpv increases monotonically with increasing effective stress and vs1 behaves similarly although with some scatter vph and vsh remain constant initially but then decrease within each stage of the multi stage test although velocity from stage to stage at any given differential stress increases this has the impact of decreasing both p wave e and s wave anisotropy through application of differential stress within each loading stage however increasing the magnitude of an isotropic stress field has little effect on the velocity anisotropies the intrinsic anisotropy of the shale remains reasonably high at the highest confining pressures the results indicate the magnitude and orientation of the stress anisotropy with respect to the shale microfabric has a significant impact on the velocity response to changing stress fields

compaction driven evolution of porosity and permeability in natural mudstones an experimental study this paper describes a series of experiments designed to investigate the influence of lithology on the compactional loss of porosity and permeability in mudstones two intact samples of london clay with clay fractions of 40 and 67 were compacted to 33 mpa effective stress clay fraction permeability porosity pore size distribution and specific surface area were measured and their evolution was monitored throughout the compaction process electron microscopy was combined with mercury porosimetry to trace the collapse of the pore structure with increasing effective stress in both cases porosity loss occurred primarily by the collapse of large pores this process is more obvious in the coarser grained sample because throughout the compaction process it has a much broader range of pore radii and a much greater mean pore radius consistent with the pore size distributions the permeability of the coarser sample ranges from 10 10 m s 1 to 10 12 m s 1 while that of the finer grained sample ranges from 4 10 12 m s 1 to 5 10 14 m s 1 during progressive compaction from 2 to 33 mpa the compressibility of the finer grained sample is greater than that of the coarser grained sample 0 15 as opposed to 0 07 however in both cases the compressibility is much lower than that inferred for lithologically similar samples compacted over geological timescales the demonstration that both porosity and lithology clay fraction influence the permeability of mudstones should allow the development of more realistic porosity permeability relationships which take into account lithological variations exhibited by mudstones

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 anisotropy of source rocks implications for hydrocarbon generation and primary migration we combined acoustic velocities and anisotropy measurements with x ray diffraction mineralogy rock eval organic geochemical analyses vitrinite reflectance and scanning electron microscope observations on 69 cores of hydrocarbon source rocks collected from eight sedimentary basins the velocity data indicate the lamellar distribution of solid organic matter kerogen in compacted black shales and show a strong correlation between their elastic anisotropy and organic richness the experimental results are consistent with the two component anisotropic model of black shales with a continuous kerogen network in both bedding parallel and bedding normal directions beginning from relatively low toc values the strong elastic anisotropy of tight thermally mature source rocks i further enhanced by overpressure induced bedding subparallel microcrack development during the main stage of hydrocarbon hc generation and primary migration at r0 greater than 0 75 in agreement with published petrographic observations these microcracks significantly aid primary hc migration particularly horizontally elastic anisotropy of medium to high porosity illite smectite shales is weaker than in lower porosity predominantly illite shales however the overall positive trend of elastic anisotropy with kerogen content remains unchanged

geomechanical and ultrasonic characterization of a norwegian sea shale anisotropy of velocity in shaly overburden is known to cause significant problems for geophysical interpretation including depth conversion and fluid identification in addition mechanical and dynamic elastic shale behavior is not well understood because few tests have been performed on well preserved samples multiple stage triaxial tests were performed upon horizontal core plugs of a shale from the norwegian sea with a view to evaluating rock strength and the evolution of ultrasonic response during rock deformation in addition standard rock physical properties were characterized as well as composition the shale microfabric is seen to be strongly laminated with alternating thick clay rich laminae and thin silt rich laminae occasional microfractures are also noted parallel to these laminations the shale has low friction coefficient and cohesive strength and shows anisotropy of these parameters when the maximum principal stress is oriented parallel to and at 45 to the microfabric the orientation of the maximum principal stress parallel to the intrinsic fabric and microcracks was seen to significantly impact on velocity normal to the fabric as stress parallel to the fabric increased s wave anisotropy was significantly affected by the increasing stress anisotropy stress orientation with respect to fabric orientation was therefore found to be an important control on the degree of anisotropy of dynamic elastic properties in this shale

natural fractures in shale a review and new observations natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low porosity and low permeability shale host rock many shale outcrops cores and image logs contain fractures or fracture traces and microseismic event patterns associated with hydraulic fracture stimulation have been ascribed to natural fracture reactivation here we review previous work and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization orientation and size patterns a wide range of shales have a common suite of types and configurations of fractures those at high angle to bedding faults bed parallel fractures early compacted fractures and fractures associated with concretions these fractures differ markedly in their prevalence and arrangement within each shale play however constituting different fracture stratigraphies differences that depend on interface and mechanical properties governed by depositional diagenetic and structural setting several mechanisms may act independently or in combination to cause fracture growth including differential compaction local and regional stress changes associated with tectonic events strain accommodation around large structures catagenesis and uplift fracture systems in shales are heterogeneous they can enhance or detract from producibility augment or reduce rock strength and the propensity to interact with hydraulic fracture stimulation burial history and fracture diagenesis influence fracture attributes and may provide more information for fracture prediction than is commonly appreciated the role of microfractures in production from shale is currently poorly understood yet potentially critical we identify a need for further work in this field and on the role of natural fractures generally

a rock physics model for characterizing the total porosity and velocity of shale a case study in fuling area china abstract the total porosity in a shale gas reservoir is the key parameter to evaluate its potential production capability petroleum geologists have been pursuing an accurate way of predicting total porosity in shale gas reservoirs from logging curves in the fuling area near chongqing in china we use core test data and well logging data in order to establish a logging response model for porosity through the analysis of the data we found that the porosity and the p sonic p velocity have a strong correlation by combining our results with previous research i e physical rock data we establish a new model which better predicts the total porosity of a given a shale reservoir than the models established in previous research in addition to total porosity mineral composition pore pressure pore fluid type and confining pressure affect p velocity by taking a series of artificial cores as test objects and systematic analysis of total porosity mineral composition and content pore fluid type and pressure on the p velocity we establish response templates between every individual parameter and p velocity to this end we further establish a multi parameter rock physical characterization equation for the total porosity and p velocity and analyze its applicable conditions using the rich geological and geophysical data of the lower silurian series in the fuling area we gradually improve the model for the total porosity and verify the results using well logging data finally a set of accurate methods are established to predict the total porosity in shale gas reservoirs by using well logging data

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

empirical strength prediction for preserved shales abstract clay rich shales form the overburden in many sedimentary basins and the seals to hydrocarbon accumulations in the subsurface their strength properties are not well understood and they are not often cored or carefully preserved to allow laboratory measurement of their properties this paper presents the results of geomechanical tests carried out on a global suite of temporally and spatially diverse shales in terms of unconfined compressive strengths cohesive strengths and friction coefficients these shales were also extensively characterised in terms of their composition porosity physicochemical properties grain size and rock physics response in order to derive empirical correlations to shale strength the dataset was expanded using measurements published in the open literature along with some proprietary measurements what might have been presumed to be good relationships such as strength with velocity or friction coefficient with clay content porosity were found not to be the case the best correlations were between strength and porosity or bulk cation exchange capacity a correlation between friction coefficient and rigid grains was noted although the variables involved were not strictly independent the relationship with cation exchange capacity may provide the opportunity of strength prediction from dielectric logs as dispersion of the dielectric constant is directly related to cation exchange capacity

a horizontal buckle model as a dynamic mechanism for back arc spreading of the japan sea a horizontal buckle model is proposed for the geodynamics of neogene clockwise rotation of the sw japan arc and the back arc spreading of the japan sea the model explains the dynamics and kinematics of the formation of megakink bands developed extensively in the shimanto belt of the outer zone of sw japan yanai 1986a 1986b using the megakink bands as indicators of stress orientation and shortening magnitude the regional sigma 1 axial trajectory sigma 1 sigma 2 sigma 3 is longitudinal and almost parallel to the long axis of the sw japan arc and the sigma 2 axis is directed nearly vertical the shortening magnitude increases up to 22 crossing sw japan from japan sea to the philippine sea these data suggest that the rotation mode of sw japan is analogous to buckle folding with vertical rotation axes in which the external compressional force is exerted parallel to the arc trend the sw japan wing was moved with reference to the nearly fixed ryukyu wing forming the inner

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

shallow crustal structure of chicxulub impact crater imaged with seismic gravity and magnetotelluric data inferences about the central uplift summary the structure of the onshore portion of the chicxulub impact crater yucatan mexico has been studied with seismic gravity and magnetotelluric mt exploration methods a dispersion analysis of rayleigh waves along a 150 km long east west profile permitted the shallow tertiary cover to be imaged to a depth of 400 m three layers were mapped the thickness of the first two layers increase as we approach the sinkhole ring from the exterior of the crater consistent with the existence of a central basin those thicknesses change from 100 150 m outside the crater rim to more than 200 m in the terrace zone at the crater centre the first layer is again approximately 100 m thick and has a low shear wave velocity approximately 1 km s 1 its velocity is slightly larger in the rest of the profile outside the central basin we observed a third layer with a shear wave velocity larger than 1 8 km s 1 but its thickness could not be determined the velocity distribution along our profile increases monotonously with depth without low velocity layers the inferred inward slope of the two shallow layers immediately outside the central basin correlates well with a smooth gravity gradient in addition we present results of detailed gravity measurements along a profile that coincides in part with the line covered by the seismic experiment these gravity measurements were inverted to obtain a density model in this model which is of course non unique the steep gravity gradients are explained by the faults along which the slumping of the terrace blocks took place two of these may correspond to the faults observed at radial distances of 85 and 96 km in offshore seismic data finally we show results obtained using mt soundings they provide additional independent constraints on the structure of the chicxulub crater mt soundings at the centre of the crater show a sharper increase in resistivity than elsewhere in the impact structure this sharper increase in resistivity may be related to the uplifted basement of the structural high at a depth of 4 km only the mt soundings located over the central gravity high display this feature our three different data sets are independently consistent with the existence of a central structural high in the crater and support a twin peak shape for this high

saturation pore pressure and effective stress from sandstone acoustic properties 1 acoustic velocity and vp vs ratio show different responses in saturated and dry rocks under increasing differential pressures velocity is also shown to be sensitive to pore pressure not just differential pressure under identical low differential pressure conditions velocity is higher in sandstones when pore pressure is high during simulation of inflationary overpressure conditions vp vs ratios of sandstones with high fluid pressures are shown to lie between dry conditions and saturated conditions with constant and low pore pressure thus the velocity differential pressure relationship is not unique however the use of the measured biot effective stress coefficient results in a unique relationship it is postulated that higher velocity and different vp vs ratios are caused by loss of compliance in microfractures due to stiffening by increasing fluid pressure

anisotropy of elastic wave velocities in deformed shales part 1 experimental results elastic wave velocity measurements in the laboratory are used to assess the evolution of the microstructure of shales under triaxial stresses which are representative of in situ conditions microstructural parameters such as crack aperture are of primary importance when permeability is a concern the purpose of these experiments is to understand the micromechanical behavior of the callovo oxfordian shale in response to external perturbations the available experimental setup allows for the continuous simultaneous measurement of five independent elastic wave velocities and two directions of strain axial and circumferential performed on the same cylindrical rock sample during deformation in an axisymmetric triaxial cell the main results are 1 identification of the complete tensor of elastic moduli of the transversely isotropic shales using elastic wave velocity measurements 2 assessment of the evolution of these moduli under triaxial loading and 3 assessment of the evolution of the elastic ani

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

anisotropic approximations for mudrocks a seismic laboratory study an experimental technique for determining the in situ elastic properties of mudrocks with horizontal alignments in the microstructure is used to study the accuracy of a set of three nested scalar anisotropic approximations for transversely isotropic ti media each subsequent approximation adds one more velocity parameter and includes the previous as a special case these approximations are convenient and robust because of their close relationship to standard geophysical measurements there exists no good theory to predict the effects of an imposed stress on the elasticity of mudrocks in this study the tensor of elastic moduli of a single test specimen of mudrock subjected to an anisotropic stress field is determined from ultrasonic group velocity measurements involving pointlike transducers the mechanical characterization performed at constant pore pressure is accompanied by detailed microscopic observations and analysis the method was used to obtain accurate elastic constants for five well defi

experimental laboratory determination of the dynamic elastic properties of wet drained shales the ultrasonic anisotropic elastic properties of drained saturated shales were measured as a function of confining pressure two shales were characterized in this study a jurassic outcrop shale retrieved under the sea in a saturated state and a kimmeridge clay shale cut from a north sea borehole both shales were highly anisotropic both texturally as revealed by scanning electron microscopy analysis and elastically as measured ultrasonically in the laboratory the strongest anisotropy was seen in the kimmeridge clay shale where up to 38 compressional wave anisotropy thomsen s parameter e and up to 58 shear wave anisotropy thomsen s parameter were observed in addition for both shales e and were found to decrease as a function of increasing confining pressure with the pore pressure drained to atmosphere while the anellipticity deviation of the slowness surfaces from ellipses was found to be positive and decreased as a function of increasing confining pressure accompanying these changes in elastic properties was a decrease in porosity with increasing confining pressure from 10 5 to 8 5 for the jurassic shale the decrease in overall anisotropy of the shales with increasing confining pressure was found to be consistent with theoretical modeling of shale properties where the shale anisotropy and anellipticity were predicted to decrease as a function of decreasing fluid filled porosity