climate instability during the last interglacial period recorded in the grip ice core isotope and chemical analyses of the grip ice core from summit central greenland reveal that climate in greenland during the last interglacial period was characterized by a series of severe cold periods which began extremely rapidly and lasted from decades to centuries as the last interglacial seems to have been slightly warmer than the present one its unstable climate raises questions about the effects of future global warming

predicting porosity from acoustic velocities in siliciclastics a new look two principally different mechanisms of porosity reduction induced elastic stiffening of siliciclastic rocks are recognized which in the domain of grain supported siliciclastics with less than 15 volume clay result in two linear trends with different slopes in the acoustic velocity porosity plane transition between the two mechanisms occurs at about 30 porosity and roughly coincides with the transition from 1 early diagenesis characterized by mechanical compaction and initial intergranular bonding to 2 advanced chemical diagenesis caused by pressure solution and cementation peculiarities of velocity stress relations in each of these diagenetic domains suggest that while the stress factor is important in loose sandstones it has practically very little effect on the frame moduli of consolidated less than about 30 porosity sandstones at stress magnitudes beyond the closure stress of microcracks introduced by natural or coring related processes appropriate grain contact and effective medium

predicting horizontal velocities from well data the imperial college borehole test site consists of four boreholes with depths lying between 260 and 280m the boreholes intersect several cyclical sequences of sandstones mudstones and limestones the formations are highly laminated and ultrasonic measurements on preserved core have shown that the mudstones are intrinsically anisotropic little or no anisotropy is associated with the sandstones and limestones a scheme is proposed to predict synthetic vertical and horizontal p and s wave logs combining an isotropic effective medium theories the gassmann equation and backus averaging the scheme extends previous sand shale models to transversely isotropic rock formations the model assumes that the anisotropy is due to layering and due to the preferred horizontal orientation of the clay minerals pores and cracks within the mudstones the pores and cracks within the sandstones and limestones are randomly orientated after fitting the model to the ultrasonic data to obtain the unknown parameters the model successfully predicts the sonic log and the direct arrival times from a cross hole survey

palaeolimnological approaches to climate change with special regard to the biological record abstract lakes systems respond physically chemically and biologically to changes in climate and these responses are registered in many various ways in lake sediment records this paper focuses principally on the biological record and describes recent research that aims to reconstruct i past variability in effective moisture and ii past variability in temperature changes in effective moisture are manifested especially by changes in lake water level in closed lake basins water level change can cause significant changes in water salinity that is registered in sediments by changes in the composition of fossil assemblages especially diatoms in open freshwater lake basins changes in water level are best reconstructed from the aquatic macrofossil record and potentially from the ratio of open water to littoral species of cladocera ostracods and diatoms the most promising biological method for reconstructing past temperature is chironomid analysis and a range of new chironomid temperature transfer functions are being developed however biological responses to changing temperature are usually indirect as a result of temperature effects on water column stratification nutrient cycling and alkalinity generation and in most cases a correct interpretation of the climatic significance of changes in the fossil record requires an understanding of the processes that are operating in the water column a multi proxy whole lake approach is advocated as the best way forward not only as a means of reconstructing past climate but also as a means of assessing the impact of climate change on lake systems

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

joint elastic electrical effective medium models of reservoir sandstones improvements in the joint inversion of seismic and marine controlled source electromagnetic data sets will require better constrained models of the joint elastic electrical properties of reservoir rocks various effective medium models were compared to a novel laboratory data set of elastic velocity and electrical resistivity obtained on 67 reservoir sandstone samples saturated with 35 g l brine at a differential pressure of 8 mpa with mixed results hence we developed a new three phase effective medium model for sandstones with pore filling clay minerals based on the combined self consistent approximation and differential effective medium model we found that using a critical porosity of 0 5 and an aspect ratio of 1 for all three components the proposed model gave accurate model predictions of the observed magnitudes of p wave velocity and electrical resistivity and of the divergent trends of clean and clay rich sandstones at higher porosities using only a few well constrained input parameters the new model offers a practical way to predict in situ porosity and clay content in brine saturated sandstones from co located p wave velocity and electrical resistivity data sets

holocene climatic instability a prominent widespread event 8200 yr ago the most prominent holocene climatic event in greenland ice core proxies with approximately half the amplitude of the younger dryas occurred 8000 to 8400 yr ago this holocene event affected regions well beyond the north atlantic basin as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane widespread proxy records from the tropics to the north polar regions show a short lived cool dry or windy event of similar age the spatial pattern of terrestrial and marine changes is similar to that of the younger dryas event suggesting a role for north atlantic thermohaline circulation possible forcings identified thus far for this holocene event are small consistent with recent model results indicating high sensitivity and strong linkages in the climatic system

the effect of clay distribution on the elastic properties of sandstones the shape and location of clay within sandstones have a large impact on the p wave and s wave velocities of the rock they also have a large effect on reservoir properties and the interpretation of those properties from seismic data and well logs numerical models of different distributions of clay structural laminar and dispersed clay can lead to an understanding of these effects clay which is located between quartz grains structural clay will reduce the p wave and s wave velocities of the rock if the clay particles become aligned or form layers the velocities perpendicular to the alignment will be reduced further s wave velocities decrease more rapidly than p wave velocities with increasing clay content and therefore poisson s ratios will increase as the velocities decrease these effects are more pronounced for compacted sandstones small amounts of clay that are located in the pore space will have little effect on the p wave velocity due to the competing influence of the density effect and pore fluid stiffening the s wave velocity will decrease due to the density effect and thus the poisson s ratio will increase when there is sufficient clay to bridge the gaps between the quartz grains p wave and s wave velocities rise rapidly and the poisson s ratios decrease these effects are more pronounced for under compacted sandstones these general results are only slightly modified when the intrinsic anisotropy of the clay material is taken into account numerical models indicate that there is a strong nearly linear relationship between p wave and s wave velocity which is almost independent of clay distribution s wave velocities can be predicted reasonably accurately from p wave velocities based on empirical relationships however this does not provide any connection between the elastic and petrophysical properties of the rocks numerical modelling offers this connection but requires the inclusion of clay distribution and anisotropy to provide a model that is consistent with both the elastic and petrophysical properties if clay distribution is ignored predicting porosities from p wave or s wave data for example can result in large errors estimation of the clay distribution from p wave and s wave velocities requires good estimates of the porosity and clay volume and verification from petrographic analyses of core or cuttings for a real data example numerical models of the elastic properties suggest the predominance of dispersed clay in a fluvial sand from matching p wave and s wave velocity well log data using log based estimates of the clay volume and porosity this is consistent with an interpretation of other log 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

a comparison of laboratory and field measurements of p wave anisotropy1 p wave velocity anisotropy determined from a cross hole survey at the imperial college borehole test site compares favorably with that measured in the laboratory on core from the holes the holes penetrate a layered sequence of sandstones shales and carbonates of the namurian upper limestone group the laboratory measurements of the vertical and horizontal velocities of core samples indicate that the shales exhibit p wave anisotropies of over 20 but that the sandstones and limestones are only slightly anisotropic these discrete measurements have been used in combination with wireline data to produce a log of p wave anisotropy including the anisotropic information vastly improves the match between observed and synthetic traveltimes from the cross hole data set this implies that there is little frequency dependence of intrinsic p wave anisotropy inversion of the cross hole traveltimes highlights the need for good angular coverage in order to resolve the anisotropy parameters the observed p wave anisotropy of the field data is due to the combined effect of sedimentary layering and the intrinsic anisotropy of the rocks the intrinsic anisotropy is found to be the dominant factor

elastic wave velocities and attenuation in an underground granitic repository for nuclear waste the behavior of a quartz monzonite rock mass subjected to a thermal load from emplaced canisters with electric heaters simulating high level nuclear waste was monitored by a cross hole seismic technique in a drift 340 m below the surface in the stripa mine facility in sweden traveltimes and amplitudes of 20 to 60 khz ultrasonic compressional p and shear s waves were measured over the experiment duration of 750 days on 2 to 4 m paths between four diamond drilled boreholes around a heater the signals were transmitted between the boreholes in six different directions and at different depths path averaged p and s wave velocities were obtained from the times of flight of pulses of acoustic waves between separate p and s wave piezoelectric transmitter and receiver crystals the attenuation q super 1 was obtained by a spectral ratio technique when the heater was turned on the p and s wave velocities increased to 4 and 10 percent respectively and stabilized at the elevated values the p wave velocities along a particular profile were found to increase linearly with the mean temperatures in the profiles sampled these mean temperatures increased 25 degrees to 55 degrees c during the course of the experiment when the heater was turned off after 398 days of heating the velocities decreased with temperature and finally reached levels in most cases below those observed prior to heating the rock the highest thermal stress close to the heater was estimated at 55 mpa q sub alpha values increased throughout the heating interval reaching changes of up to 60 percent shortly after turning off the heater the q values reveal no direct correlation with temperature or the closely associated thermal stress although these phenomena clearly are driving the variation in q there is strong evidence relating attenuation properties to fracture closure and pore pressure changes associated with draining of the rock mass

t matrix approach to shale acoustics summary the t matrix approach of quantum scattering theory is used here to place many long wavelength equivalent medium approximations for porous composites polycrystals and cracked media on a common footing and to indicate their limitations but also to derive some new results based on two point statistics in this way we have obtained an insight into the difficult problem of elastic inclusions at finite concentrations which is of foremost relevance when estimating the effective material parameters of porous cracked shales involving stacks of more or less horizontally aligned clay platelets mixed together with more rounded silt minerals and with fluid filling the spaces a rather involved perturbative analysis of the effects of interactions between or structural correlations among the various inclusions minerals and cavities making up a real shale of hexagonal symmetry was performed in an attempt to obtain a better match between theoretical predictions based on a combination of coherent and optical potential approximations and experimental results recovered from ultrasonic wave speeds for the effective elastic stiffness tensor for the particular data set considered in this study the t matrix approach was able to match the data better than the approach of hornby et al but the match was not completely satisfactory further progress in theoretical shale modelling may come from a better knowledge of the elastic properties of pure clay minerals a more detailed knowledge of the microstructure of shales the incorporation of constraints obtained from comparisons between theoretical predictions and experimental results as well as a continuing development of the t matrix approach numerical results also for the effect of bedding parallel microcracks on the elasticity of such a real shale have value in illustrating the importance of taking into account the effects of spatial distribution when trying to deal with non dilute mixtures of highly contrasting material properties

a new velocity model for clay sand mixtu res1 none of the standard porosity velocity models e g the time average equation raymer s equations is satisfactory for interpreting well logging data over a broad depth range clays in the section are the usual source of the difficulty through the bias and scatter that they introduce into the relationship between porosity and p wave transit time because clays are composed of fine sheet like particles they normally form pores with much smaller aspect ratios than those associated with sand grains this difference in pore geometry provides the key to obtaining more consistent resistivity and sonic log interpretations a velocity model for clay sand mixtures has been developed in terms of the kuster and toksoz effective medium and gassmann theories in this model the total pore space is assumed to consist of two parts 1 pores associated with sand grains and 2 pores associated with clays including bound water the essential feature of the model is the assumption that the geometry of pores associated with sand grains is significantly different from that associated with clays because of this porosity in shales affects elastic compliance differently from porosity in sand stones the predictive power of the model is demonstrated by the agreement between its predictions and laboratory measurements and by its ability to predict sonic logs from other logs over large depth intervals where formations vary from unconsolidated to consolidated sandstones and shales

effects of sand shale anisotropy on amplitude variation with angle ava modelling the sawan gas field pakistan as a key case study for south asia s sedimentary basins amplitude variation with angle ava is a technique widely used in the characterisation of hydrocarbon reservoirs and assumes the earth s crust to be an isotropic medium yet anisotropy is ubiquitous in stratigraphic sequences and has first order effects on seismic ava responses when investigating subsurface prospects this work analyses the effects of anisotropic strata on ava responses using the lower goru formation middle indus basin pakistan as a case study in the study area shale intervals are interbedded with reservoir sands of the sawan gas field shales in this field form laminae or are dispersed within reservoir sands making the lower goru formation an example of a vertically transversely isotropic vti medium in this work we calculate the effective saturated mechanical properties of the lower goru formation based on rock physics templates the backus 1962 average typically designed for layered media combined with the empirical relations of brown and korringa 1975 and wood 1955 the input data used in our rock physics modelling is based on detailed petrophysical analyses of well data using the saturated effective mechanical properties of the lower goru formation we generate angle dependent reflection coefficient curves and seismic ava responses based on exact and approximate solutions for both isotropic and anisotropic reservoir scenarios our results suggest that the effects of lithological anisotropy are more pronounced in places with thick shale beds within reservoir sands conversely angle dependent reflection curves and seismic ava responses based on isotropic or anisotropic cases give similar solutions in the presence of thin shale beds as a corollary of this work we present a bayesian inversion method for the estimation of porosity in vti media

prediction of elastic wave velocities in sandstones using structural models elastic wave propagation in fluid saturated sandstones depends upon two sets of rock features 1 the volume fractions and elastic constants of the rock constituents quartz clay water etc and 2 microstructural geometry grain contacts pore aspect ratios while the former data are usually obtainable the latter are relatively inaccessible we present a new method for determining microstructural data using idealized but physically representative models of sandstone the key to the method is the simulation of certain depositional and diagenetic processes in a manner that completely specifies the geometry of the resulting models hence the geometric features of the grain space and void space required for various theories of elastic propagation can be calculated directly from the models we find good agreement between predictions and measurements of compressional and shear wave velocities in both clean and clay bearing saturated sandstones in contrast with previous efforts at predicting velocitie

modulus decomposition of compressional and shear velocities in sand bodies the advent of borehole shear slowness measurements in sonically slow formations has lead to breakthroughs in the subsurface profiling of geological bodies in sand bodies compressional and shear velocities depend predictably on porosity mineralogy grain contacts and fluid saturation an interpretation is best performed by decomposing the velocities into moduli that are intrinsic measures of the rock frame and pore fluid compressibilities careful experiments on pure materials i e pure quartz sandstones demonstrate two simplifying constitutive relationships first the bulk and shear frame moduli are simple functions of the porosity a comparison of the measured shear frame modulus to the prediction for the pure material distinguishes sand from shale second the ratio of the bulk and shear frame moduli is a constant 0 9 independent of the porosity the measured velocities are directly inverted to yield the bulk modulus of the pore fluid the fluid saturation effects are so dramatic at high porosit

rock physics modeling of a dispersed clay sandstone and dispersed quartz shale sequence abstract the elastic and petrophysical properties of the fluvial rocks of the miocene in the western basin of the gulf of thailand display typical signatures of dispersed clay sandstones and dispersed quartz shales that is the relationships between various properties exhibit two strong trends associated with a grain supported domain and a matrix supported domain the minimum porosity and maximum p velocity occur at the transition between these domains at a clay volume of approximately 40 50 this means that the hardest rocks occur at this transition a simple rock physics model has been developed that quantifies the elastic properties over a large depth range but does not explicitly account for the dispersed nature of the clay and quartz it employs a single aspect ratio at each depth within an inclusion based modeling scheme the required aspect ratio at each depth can be estimated from the depth the porosity and the clay volume of the rock

paleomagnetically derived folding rates southern pyrenees spain well dated syntectonic unconformities with the spanish pyrenees allow characterization of sedimentation and deformation within an orogenic foreland the spanish pyrenees are composed of a south vergent thrust belt and foreland basin in which folds developed both parallel and perpendicular to tectonic transport unconformity geometries and magnetostratigraphic data from synorogenic strata provide a precise temporal framework for the quantitative analysis of the mediano anticline the largest of the transverse folds folding began in the early eocene and ended by 42 ma at which time an anticline 20 km long and 5 km in wavelength had been produced angular and progressive unconformities within syntectonic sedimentary units preserve the history of fold development fold growth resulted from west ward salt movement driven by prograding sediments and was characterized by 2 3 m y intervals of slow continuous limb tilting 2 2 4 2 m y preserved as progressive unconformities within syntectonic strata because the emplacement of the cotiella montsec thrust sheet was episodic intervals of slow fold growth were punctuated by intervals lasting

a physical model for shear wave velocity prediction the clay sand mixture model of xu and white is shown to simulate observed relationships between s wave velocity or transit time porosity and clay content in general neither s wave velocity nor s wave transit time is a linear function of porosity and clay content for practical purposes clay content is approximated by shale volume in well log applications in principle the model can predict s wave velocity from lithology and any pair of p wave velocity porosity and shale volume although the predictions should be the same if all measurements are error free comparison of predictions with laboratory and logging measurements show that predictions using p wave velocity are the most reliable the robust relationship between s and p wave velocities is due to the fact that both are similarly affected by porosity clay content and lithology moreover errors in the measured p wave velocity are normally smaller than those in porosity and shale volume both of which are subject to errors introduced by imperfect models and imperfect parameters when estimated from logs because the model evaluates the bulk and shear moduli of the dry rock frame by a combination of kuster and toksoz theory and differential effective medium theory using pore aspect ratios to characterize the compliances of the sand and clay components the relationship between p and s wave velocities is explicit and consistent consequently the model sidesteps problems and assumptions that arise from the lack of knowledge of these moduli when applying gassmann s theory to this relationship making it a very flexible tool for investigating how the v p v s relationship is affected by lithology porosity clay content and water saturation numerical results from the model are confirmed by laboratory and logging data and demonstrate for example how the presence of gas has a more pronounced effect on p wave velocity in shaly sands than in less compliant cleaner sandstones

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

allerod younger dryas lake temperatures from midge fossils in atlantic canada remains of freshwater midges are abundant in lake sediments and their species distributions are closely related to the surface water temperature of lakes their distributions thus provide a powerful tool for paleoclimatology the distribution of species in a core from splan pond in atlantic canada indicates that there were abrupt transitions in late glacial temperatures between warm and cold states the transitions are correlative with the well known warm allerod and cold younger dryas events in europe these data thus confirm the inference from palynological data that these events affected regions on both sides of the atlantic

a chironomid based holocene summer air temperature reconstruction from the swiss alps we developed a quantitative chironomid july air temperature inference model based on surface sediments from 81 swiss lakes and applied it to the holocene subfossil chironomid record of hinterburgsee a small subalpine lake in the northern swiss alps present day mean july air temperature of 11 3 c after smoothing to reduce the high between sample variability of inferred temperatures the reconstruction indicates july air temperatures of 10 4 10 9 c at the end of the younger dryas of 11 9 12 8 c during the early and mid holocene 11500 4000 cal bp and slightly lower temperatures of 11 5 12 0 c during the late holocene 3500 1000 cal bp a warming trend inferred for the past millennium is most likely an artifact of human impact on hinterburgsee s chironomid fauna rather than a genuine temperature signal the most prominent climatological events during the holocene were two periods of lower temperatures at c 10700 10500 cal bp and 8200 7700 cal bp and an abrupt shift to a cooler late holocene cl

effective medium theory and multiple linear regression based velocity estimation in syn rift clastic sequence of the krishna godavari basin india abstract an inclusion model based on the kuster toksoz effective medium theory along with gassmann theory is tested to forward model velocities for fluid saturated rocks a simulated annealing algorithm along with the inclusion model effectively inverts measured compressional velocity vp to achieve an effective pore aspect ratio at each depth in a depth variant manner continuously along with depth early cretaceous syn rift clastic sediments at two different depth intervals from two wells well a 2160 2274 m and well b 5222 5303 m in the krishna godavari basin india are used for this study shear velocity vs estimated using modelled pore aspect ratio offers a high correlation coefficient 0 95 for both the wells with measured data the modelled pore aspect ratio distribution suggests the decrease in pore aspect ratio for the deeper interval mainly due to increased effective vertical stress the pore aspect ratio analysis in relation to total porosity and volume of clay reveals that the clay volume has insignificant influence in shaping the pore geometry in the studied intervals an approach based on multiple linear regression method effectively predicts velocity as a linear function of total porosity the volume of clay and the modelled pore space aspect ratio of the rock we achieved a significant match between measured and predicted velocities the correlation coefficients between measured and modelled velocities are considerably high approximately 0 85 and 0 8 for vp and vs respectively this process indicates the possible influence of pore geometry along with total porosity and volume of clay on velocity

joint elastic electrical properties of reservoir sandstones and their relationships with petrophysical parameters we measured in the laboratory ultrasonic compressional and shear wave velocity and attenuation 0 7 1 0 mhz and low frequency 2 hz electrical resistivity on 63 sandstone samples with a wide range of petrophysical properties to study the influence of reservoir porosity permeability and clay content on the joint elastic electrical properties of reservoir sandstones p and s wave velocities were found to be linearly correlated with apparent electrical formation factor on a semi logarithmic scale for both clean and clay rich sandstones p and s wave attenuations showed a bell shaped correlation partial for s waves with apparent electrical formation factor the joint elastic electrical properties provide a way to discriminate between sandstones with similar porosities but with different clay contents the laboratory results can be used to estimate sandstone reservoir permeability from seismic velocity and apparent formation factor obtained from co located seismic and controlled source electromagnetic surveys

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

the stress sensitivity of shaley sandstones the link between the stress sensitivity of shaley sandstones and their porosity and clay content is investigated this is achieved by firstly fitting a compliance based stress sensitivity law to laboratory measurements of ultrasonic velocity taken from four sets of reservoir sandstones extracted from a variety of depositional settings correlations are then sought between the independent parameters of this law and the porosity or clay fraction of the rocks which are then subsequently interpreted in terms of framework or pore space related microstructural clay models the general conclusion drawn from the results is that both of the parameters defining the stress sensitivity law the asymptotic modulus and the stress dependent excess compliance clearly vary with porosity however only the asymptotic modulus shows a convincing trend with clay and there is little observed variation of the stress dependent compliance with clay there is therefore a resultant variation of stress sensitivity with clay but it is controlled only by the asymptotic modulus the analysis also concludes that all four data sets fall into a framework related category of clay model

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

pore filling material and its effect on velocity in rocks a new method estimates the dependence of p and s velocities in rocks on elastic properties of the pore filling material by going beyond gassmann s relations and accounting for pore filling materials with nonzero shear stiffness the resulting estimates are in good agreement with experimental data and gassmann s relations for values of velocity in water saturated rocks predicted from velocity measured on dry rocks the method successfully predicts the temperature dependence of velocity 1 in frozen rocks at the ice water phase transition and 2 in rocks saturated with heavy hydrocarbon undergoing solid liquid phase transition

compressional velocity and porosity in sand clay mixtures laboratory measurements of porosity and compressional velocity were conducted on unconsolidated brine saturated clean ottawa sand pure kaolinite and their mixtures at various confining pressures a peak in p velocity versus clay content in unconsolidated sand clay mixtures at 40 percent clay by weight was found the peak in velocity is 20 30 percent higher than for either pure clay or clean sand a minimum in porosity versus clay content at 20 40 percent clay by weight is also observed such behavior is explained using a micro geometrical model for mixtures of sand and clay in which two classes of sediments are considered 1 sands and shaley sands in which clay is dispersed in the pore space of load bearing sand and thus reduces porosity and increases the elastic moduli of the pore filling material and 2 shales and sandy shales in which sand grains are dispersed in a clay matrix for these sediments the model reproduces the extrema in velocity and porosity and accounts for much of the scatter in