the effects of stress paths on acoustic velocities and 4d seismic imaging time shifts on 4d seismic images result from two basic causes 1 acoustic velocity changes due to fluid migration fluid substitution fluid composition and 2 velocity changes resulting from changes in total stresses i e the vertical and horizontal stresses during depletion this paper is concerned only with the latter the effect of changing total stress on rock velocity

intercalibration of astronomical and radioisotopic time the 40ar 39ar radioisotopic dating technique is one of the most precise and versatile methods available for dating events in earth s history but the accuracy of this method is limited by the accuracy with which the ages of neutron fluence monitors dating standards are known calibrating the ages of standards by conventional means has proved difficult and contentious the emerging astronomically calibrated geomagnetic polarity time scale apts offers a means to calibrate the ages of 40ar 39ar dating standards that is independent of absolute isotopic abundance measurements seven published 40ar 39ar dates for polarity transitions nominally ranging from 0 78 to 3 40 ma are based on the fish canyon sanidine standard and can be compared with apts predictions solving the 40ar 39ar age equation for the age of the fish canyon sanidine that produces coincidence with the apts age for each of these seven reversals yields mutually indistinguishable estimates ranging from 27 78 to 28 09 ma with an inverse variance weighted mean of 27 95 0 18 ma normalized residuals are minimized at an age of 27 92 ma indicating the robustness of the solution

sensitivity of time lapse seismic to reservoir stress path the change in reservoir pore pressure due to the production of hydrocarbons leads to anisotropic changes in the stress field acting on the reservoir reservoir stress path is defined as the ratio of the change in effective horizontal stress to the change in effective vertical stress from the initial reservoir conditions and strongly influences the depletion induced compaction behaviour of the reservoir seismic velocities in sandstones vary with stress due to the presence of stress sensitive regions within the rock such as grain boundaries microcracks fractures etc since the response of any microcracks and grain boundaries to a change in stress depends on their orientation relative to the principal stress axes elastic wave velocities are sensitive to reservoir stress path the vertical p and s wave velocities the small offset p and sv wave normal moveout nmo velocities and the p wave amplitude versus offset avo are sensitive to different combinations of vertical and horizontal stress the relationships between these quantities and the change in stress can be calibrated using a repeat seismic sonic log checkshot or vertical seismic profile vsp at the location of a well at which the change in reservoir pressure has been measured alternatively the variation of velocity with azimuth and distance from the borehole obtained by dipole radial profiling can be used having calibrated these relationships the theory allows the reservoir stress path to be monitored using time lapse seismic by combining changes in the vertical p wave impedance changes in the p wave nmo and avo behaviour and changes in the s wave impedance

simulation of two phase fluid flow through compactible reservoirs fluid flow simulators used in the oil industry model the movement of fluids through a porous reservoir rock these simulators either ignore coupling between the flow and concurring deformation of the solid rock frame or take it into account approximately in the so called loose or staggered in time mode in contrast to existing simulators the one we describe here fully couples two phase oil and water flow to subsurface deformation and simultaneously accounts for all relevant physical phenomena as such our flow simulator inherently links time dependent fluid pressures saturations permeabilities and flow velocities to stresses in the whole subsurface these stresses relate to strains through the non linear theory of elasticity allowing us to model time lapse changes in seismic velocities and anisotropy the velocity variations manifest themselves in time shifts and reflection amplitudes that are conventionally measured from 4d seismic data changes in anisotropy produce time dependent shear wave splitting that can be used for monitoring the horizontal stresses

4 d constrained depth conversion for reservoir compaction estimation application to ekofisk field ekofisk field in the norweigian sector of the north sea is a remarkable example of a strongly compacted chalk reservoir since the start of production in 1971 the water bottom has subsided by over 8 m as a result of reservoir compaction hermansen et al the field wide waterflood begun in 1989 was expected to halt pressure depletion and reservoir compaction after a few years of waterflooding ekofisk reservoir pressure began increasing but compaction and subsidence continued laboratory studies on chalk cores indicated that high porosity oil filled chalk compacts strongly when flushed with seawater these studies along with reservoir flow simulation and geomechanic modeling led to a model of a water weakening compaction sylte et al ongoing efforts deployed to monitor the reservoir compaction and seabed subsidence include twice a year compaction logging in a monitoring well daily gps measurements at the platforms and periodic bathymetry surveys nine surveys from 1970 to 1999

elliptical anisotropic eikonal phase velocity tomography we formulated an anisotropic eikonal tomography approach for phase velocities based on a two dimensional elliptical anisotropic wave equation we can fit the parameters of the ellipse directly from measured first order traveltime surface gradients and constrain these parameters to vary smoothly over space the method is applied to scholte waves in virtual seismic sources from stations in the life of field seismic ocean bottom cable array installed over the ekofisk field the fast directions of the azimuthally anisotropic scholte wave velocities form a large circular pattern over the ekofisk field this pattern dominates the scholte wave phase velocities at ekofisk between 0 7 and 1 1 hz it results from the overburden stress state and from seafloor subsidence induced by decades of hydrocarbon extraction

effects of stress reduction on geomechanical and acoustic relationship of overconsolidated sands abstract relationship between different geomechanical and acoustic properties measured from seven laboratory tested unconsolidated natural sands with different mineralogical compositions and textures were presented the samples were compacted in the uniaxial strain configuration from 0 5 to 30 mpa effective stress each sand sample was subjected to three loading unloading cycles to study the influence of stress reduction geomechanical elastic and acoustic parameters are different between normal compaction and overconsolidation unloaded and reloaded stress path k0 data differ between normal consolidated and overconsolidated sediments the k0 value of approximately 0 5 is founded for most of the normal consolidated sands but varies during unloading depending on mineral compositions and textural differences the k0 and overconsolidation ratio relation can be further simplified and can be influenced by the material compositions k0 can be used to estimate horizontal stress for drilling applications the relationship between acoustic velocity and geomechanical is also found to be different between loading and unloading conditions the static moduli of the overconsolidated sands are much higher than normal consolidated sands as the deformation is small small strain the correlation between dynamic and static elastic moduli is stronger for an overconsolidation stage than for a normal consolidation stage the results of this study can contribute to geomechanical and acoustic dataset which can be applied for many seismic geomechanics applications in shallow sands where mechanical compaction is the dominant mechanism

voluminous lava like precursor to a major ash flow tuff low column pyroclastic eruption of the pagosa peak dacite san juan volcanic field colorado abstract the pagosa peak dacite is an unusual pyroclastic deposit that immediately predated eruption of the enormous fish canyon tuff 5000 km3 from the la garita caldera at 28 ma the pagosa peak dacite is thick to 1 km voluminous 200 km3 and has a high aspect ratio 1 50 similar to those of silicic lava flows it contains a high proportion 40 60 of juvenile clasts to 3 4 m emplaced as viscous magma that was less vesiculated than typical pumice accidental lithic fragments are absent above the basal 5 10 of the unit thick densely welded proximal deposits flowed rheomorphically due to gravitational spreading despite the very high viscosity of the crystal rich magma resulting in a macroscopic appearance similar to flow layered silicic lava although it is a separate depositional unit the pagosa peak dacite is indistinguishable from the overlying fish canyon tuff in bulk rock chemistry phenocryst compositions and 40ar 39ar age the unusual characteristics of this deposit are interpreted as consequences of eruption by low column pyroclastic fountaining and lateral transport as dense poorly inflated pyroclastic flows the inferred eruptive style may be in part related to synchronous disruption of the southern margin of the fish canyon magma chamber by block faulting the pagosa peak eruptive sources are apparently buried in the southern la garita caldera where northerly extensions of observed syneruptive faults served as fissure vents cumulative vent cross sections were large leading to relatively low emission velocities for a given discharge rate many successive pyroclastic flows accumulated sufficiently rapidly to weld densely as a cooling unit up to 1000 m thick and to retain heat adequately to permit rheomorphic flow explosive potential of the magma may have been reduced by degassing during ascent through fissure conduits leading to fracture dominated magma fragmentation at low vesicularity subsequent collapse of the 75 35 km 2 la garita caldera and eruption of the fish canyon tuff were probably triggered by destabilization of the chamber roof as magma was withdrawn during the pagosa peak eruption

late pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at crater lake oregon abstract rhyodacite tephra and three lavas erupted 27 ka interpreted to be early leaks from the climactic magma chamber of mount mazama contain ubiquitous resorbed crystals antecrysts that were recycled from young granodiorite and related plutonic rocks of the same magmatic system the shallow composite pluton is represented by blocks ejected in the 7 7 ka climactic eruption that formed crater lake caldera plagioclase crystals in both rhyodacite and granodiorites commonly have cores with crystallographically oriented fe oxide needles exsolved at subsolidus conditions at least 80 of plagioclase crystals in the rhyodacite are antecrysts derived from plutonic rocks other crystals in the rhyodacite notably zircon also were recycled sims 238 u 230 th dating indicates that zircons in 4 granodiorite blocks crystallized at various times between 20 ka and 300 ka with concentrations of analyses near 50 70 110 and 200 ka that correspond to periods of dacitic volcanism dated by k ar u th ages of zircon from a rhyodacite sample yield similar results no analyzed zircons from the granodiorite or rhyodacite are pre quaternary zircon minimum ages in blocks from different locations around the caldera reflect ages of nearby volcanic vents and may map the distribution of intrusions within a composite pluton survival of zircon in zircon undersaturated hydrous magma and of fe oxide needles in plagioclase suggests that little time elapsed from entrainment of antecrysts to the 27 ka eruption of the rhyodacite the 27 ka rhyodacite is an example of young silicic magma that preserved unstable antecrysts from a known source early during growth of a large high level magma chamber in contrast the voluminous 7 7 ka climactic rhyodacite pumice is virtually lacking in zircon indicating dissolution of any granodioritic debris in the intervening period mineralogical evidence of assimilation may be destroyed in hot vigorously growing silicic magma bodies such as ultimately produced the climactic eruption of mount mazama

predicting stress induced velocity anisotropy in rocks a simple transformation using measured isotropic vp and vs versus hydrostatic pressure is presented for predicting stress induced seismic velocity anisotropy in rocks the compliant crack like portions of the pore space are characterized by generalized compressional and shear compliances that are estimated from the isotropic vp and vs the physical assumption that the compliant porosity is crack like means that the pressure dependence of the generalized compliances is governed primarily by normal tractions resolved across cracks and defects this allows the measured pressure dependence to be mapped from the hydrostatic stress state to any applied nonhydrostatic stress predicted p and s wave velocities agree reasonably well with uniaxial stress data for barre granite and massillon sandstone while it is mechanically similar to methods based on idealized ellipsoidal cracks the approach is relatively independent of any assumed crack geometry and is not limited to small crack densities

linking reservoir geomechanics and time lapse seismics predicting anisotropic velocity changes and seismic attributes seismic technology has been used successfully to detect geomechanically induced signals in repeated seismic experiments from more than a dozen fields to explain geomechanically induced time lapse 4d seismic signals we use results from coupled reservoir and geomechanical modeling the coupled simulation yields the 3d distribution over time of subsurface deformation and triaxial stress state in the reservoir and the surrounding rock predicted changes in triaxial stress state are then used to compute changes in anisotropic p and s wave velocities employing a stress sensitive rock physics transform we predict increasing vertical p wave velocities inside the reservoir accompanied by a negative change in p wave anisotropy e 0 a stress sensitive rock physics transform that predicts a

comparison of geomechanical deformation induced by megatonne scale co2 storage at sleipner weyburn and in salah geological storage of co2 that has been captured at large point source emitters represents a key potential method for reduction of anthropogenic greenhouse gas emissions however this technology will only be viable if it can be guaranteed that injected co2 will remain trapped in the subsurface for thousands of years or more a significant issue for storage security is the geomechanical response of the reservoir concerns have been raised that geomechanical deformation induced by co2 injection will create or reactivate fracture networks in the sealing caprocks providing a pathway for co2 leakage in this paper we examine three large scale sites where co2 is injected at rates of 1 megatonne y or more sleipner weyburn and in salah we compare and contrast the observed geomechanical behavior of each site with particular focus on the risks to storage security posed by geomechanical deformation at sleipner the large high permeability storage aquifer has experienced little pore pressure increase over 15 y of injection implying little possibility of geomechanical deformation at weyburn 45 y of oil production has depleted pore pressures before increases associated with co2 injection the long history of the field has led to complicated sometimes nonintuitive geomechanical deformation at in salah injection into the water leg of a gas reservoir has increased pore pressures leading to uplift and substantial microseismic activity the differences in the geomechanical responses of these sites emphasize the need for systematic geomechanical appraisal before injection in any potential storage site

ultrasonic velocity and shear wave splitting behavior of a colton sandstone under a changing triaxial stress ultrasonic experiments on a dry colton sandstone placed in a triaxial pressure machine show that effective stress changes lead to distinct anisotropic velocity changes in compressional waves and shear waves the stress imprint can be recognized from the associated velocity pattern by relating the velocities to the three normal stress directions the ultrasonic velocities indicate that the sensitivity of the different waves to stress predominantly depends on stresses applied in the polarization and propagation directions of the particular wave mode also stress induced changes in shear wave splitting are observed

theory of traveltime shifts around compacting reservoirs 3d solutions for heterogeneous anisotropic media time lapse traveltime shifts of reflection events recorded above hydrocarbon reservoirs can be used to monitor production related compaction and pore pressure changes existing methodology however is limited to zero offset rays and cannot be applied to traveltime shifts measured on prestack seismic data we give an analytic 3d description of stress related traveltime shifts for rays propagating along arbitrary trajectories in heterogeneous anisotropic media the nonlinear theory of elasticity helps to express the velocity changes in and around the reservoir through the excess stresses associated with reservoir compaction because this stress induced velocity field is both heterogeneous and anisotropic it should be studied using prestack traveltimes or amplitudes then we obtain the traveltime shifts by firstorder perturbation of traveltimes that accounts not only for the velocity changes but also for 3d deformation of reflectors the resulting closed form expression can be used efficiently for numerical modeling of traveltime shifts and ultimately for reconstructing the stress distribution around compacting reservoirs the analytic results are applied to a 2d model of a compacting rectangular reservoir embedded in an initially homogeneous and isotropic medium the computed velocity changes around the reservoir are caused primarily by deviatoric stresses and produce a transversely isotropic medium with a variable orientation of the symmetry axis and substantial values of the thomsen parameters and the offset dependence of the traveltime shifts should play a crucial role in estimating the anisotropy parameters and compaction related deviatoric stress components

timing of the permian triassic biotic crisis implications from new zircon u pb age data and their limitations abstract the most profound biotic crisis in the earth s history causing the near extinction of both terrestrial and marine life occurred at the end of the permian period about 253 myr ago and marks the paleozoic mesozoic era boundary the cause of this event is still a matter of vigorous debate with both brief and catastrophic as well as gradual mechanisms having been proposed similar to a recent landmark study this study uses the u pb method on zircons from the uppermost permian lowermost triassic ash fall deposits at meishan zhejiang province se china in order to examine time and rate constraints for these events the results of both this study and previous work show that for these ash layers the effects of pb loss are combined with varying amounts and sources of inheritance resulting in an age scatter which prohibits the extraction of a statistically robust age in many cases though the effects of pb loss on the zircons analyzed in this study were reduced by leaching the grains in hydrofluoric acid as opposed to commonly applied air abrasion prior to analysis the presence within a single ash layer of multiple generations of older xenocrysts in many cases only slightly older than the depositional age has made quantitative interpretation even more difficult when these combined phenomena bias individual zircon ages by less than a percent they are extremely difficult to deconvolute and if multi grain analyses are used can become impossible to recognize because of the resulting age averaging monte carlo simulations using actual measurements of individual zircon crystals show that age excursions due to pb loss and xenocrystic contamination for the meishan bentonites are easily homogenized to the point of undetectability when replicate analyses of multi grain zircon samples are compared thus this study uses only high precision analyses of single crystals whether from our work or that of previous studies three main conclusions have emerged first our data require a significant increase in the age of the permian triassic boundary by more than 2 myr compared to the previous study which shifts the age to a value older than 253 ma second neither our data nor those from previous work can confirm or negate the possibility of a very abrupt biotic crisis third even large suites of very high quality single zircon u pb analyses for these tuffs cannot in most cases yield objective reliable and robust dates with accuracies at the sub myr level though the temptation to perform arbitrary selection of subsets of the analyses for that purpose is almost irresistible the last conclusion is not an indictment of zircon u pb dating in general other rocks and other zircon populations can and do behave very differently and further technical advances will likely improve our ability to prepare grains or sub grains of adequately enhanced quality for analysis consequently the results of the present study strongly suggest that for problems requiring time scale accuracy inferences from zircon u pb dating must be based on sufficiently large suites of single crystal or crystal domain high precision analyses

rocks under strain strain induced time lapse time shifts are observed for depleting reservoirs pressure depletion as a result of oil and gas production creates changes in the stress and strain fields of the rock material both inside and outside the reservoir seismic waves propagating through these rocks have different traveltimes before and after depletion because the layer thicknesses change and the seismic velocity is altered by the changes in the stress and strain fields the cumulative change in two way traveltimes with depth is referred to as time lapse time shift

monitoring pressure depletion and improving geomechanical models of the shearwater field using 4d seismic dedicated 4d seismic data have been used to investigate horizontal and vertical compartmentalization in the high pressure high temperature hpht shearwater field in the u k central north sea geomechanical modeling and conversion of model results to synthetic seismic time shifts allow quantitative comparison of models with real 4d seismic time shifts drilling in and around heavily depleted reservoirs is very challenging and matching the geomechanical model to all data sets including 4d provides the best possible constraints on the likely stress fields an infill well will encounter in the overburden and the reservoir

analysis of time lapse travel time and amplitude changes to assess reservoir compartmentalization fluid depletion within a compacting reservoir can lead to significant stress and strain changes and potentially severe geomechanical issues both inside and outside the reservoir we extend previous research of time lapse seismic interpretation by incorporating synthetic near offset and full offset common midpoint reflection data using anisotropic ray tracing to investigate uncertainties in time lapse seismic observations the time lapse seismic simulations use dynamic elasticity models built from hydro geomechanical simulation output and a stress dependent rock physics model the reservoir model is a conceptual two fault graben reservoir where we allow the fault fluid flow transmissibility to vary from high to low to simulate non compartmentalized and compartmentalized reservoirs respectively the results indicate time lapse seismic amplitude changes and travel time shifts can be used to qualitatively identify reservoir compartmentalization due to the high repeatability and good quality of the time lapse synthetic dataset the estimated travel time shifts and amplitude changes for near offset data match the true model subsurface changes with minimal errors a 1d velocity strain relation was used to estimate the vertical velocity change for the reservoir bottom interface by applying zero offset time shifts from both the near offset and full offset measurements for near offset data the estimated p wave velocity changes were within 10 of the true value however for full offset data time lapse attributes are quantitatively reliable using standard time lapse seismic methods when an updated velocity model is used rather than the baseline model

estimation of layer thickness and velocity changes using 4d prestack seismic data in some hydrocarbon reservoirs severe compaction of the reservoir rocks is observed this compaction is caused by production and is often associated with stretching and arching of the overburden rocks time lapse seismic data can be used to monitor these processes since compaction and stretching cause changes in layer thickness as well as seismic velocities it is crucial to develop methods to distinguish between the two effects we introduce a new method based on detailed analysis of time lapse prestack seismic data the equations are derived assuming that the entire model consists of only one single layer with no vertical velocity variations the method incorporates lateral variations in relative velocity changes by utilizing zero offset and offset dependent time shifts to test the method we design a 2d synthetic model that undergoes severe reservoir compaction as well as stretching of the overburden rocks finally we utilize the method to analyze a real 2d prestack time lapse seismic line from t

monitoring primary depletion reservoirs using amplitudes and time shifts from high repeat seismic surveys in the high porosity poorly consolidated turbidites of the deepwater gulf of mexico production induced compaction is the main production drive mechanism when aquifer support is weak and prior to pressure support by secondary recovery water injection time lapse 4d seismic monitoring of this class of reservoirs has provided several new learning opportunities the time lapse amplitude response of these fields can be complicated due to saturation changes water replacing oil inside the reservoir rock compaction causing density and velocity changes inside the reservoir stress relief and associated deformation of the rock outside the reservoir and changes in reservoir fluid pressures due to pore pressure decrease methods that rely on time lapse amplitude changes with offset to discriminate pressure and saturation changes can help separate and thus simplify the interpretation of some of these effects tura and lumley 1999 landro 2001

asymmetry in the time lapse seismic response to injection and depletion large changes in seismic reflection amplitude have been observed around injectors and result from the decrease in elastic wave velocity due to the increase in pore pressure in the reservoir in contrast the velocity change resulting from the decrease in pore pressure in depleting reservoirs is observed to be smaller in magnitude elastic wave velocities in sandstones vary with stress due to the presence of stress sensitive grain boundaries within the rock grain boundary stiffness increases non linearly with increasing compressive stress due to increased contact between opposing faces of the boundary this results in a change in velocity due to a decrease in pore pressure that is smaller than the change in velocity caused by an increase in pore pressure in agreement with time lapse seismic observations the decrease in porosity resulting from depletion is not fully recovered upon re pressurization and this leads to an additional steepening of the velocity vs effective stress curve for injection relative to depletion this difference is enhanced by any breakage of cement or weakening of grain contacts that may occur during depletion and by the reopening or formation of fractures or joints and dilation of grain boundaries that may occur during injection

stress induced temporal variations in seismic anisotropy observed in microseismic data summary in situ stress monitoring of crustal rocks is desirable as it yields insights into earthquake mechanisms volcanic eruptions and changes in hydrocarbon reservoirs shear wave splitting induced by stress controlled cracks in the shallow crust provides a way to infer this stress temporal variations in these observations can be difficult to quantify due to scatter in the data and discontinuous observations here we present evidence of temporal variations in shear wave splitting from a continuous time series with a high occurrence of microseismic events recorded in a borehole we interpret these observations in terms of stress controlled cracks and are able to infer changes in stress and via modelling suggest the cause of the anisotropy possible origins of the temporal variation in per cent anisotropy are tidal or oil field production processes our results suggest that shear wave splitting is a viable probe for inferring changes in crustal stress in cracked rock

influence of background heterogeneity on traveltime shifts for compacting reservoirs compaction induced by pore pressure decrease inside a reservoir can be monitored by measuring traveltime shifts of reflection events on time lapse seismic data recently we introduced a perturbation based formalism to describe traveltime shifts caused by the 3d stress induced velocity field around a compacting reservoir application of this method to homogeneous background models showed that the offset variation of traveltime shifts is controlled primarily by the anisotropic velocity perturbations and can provide valuable information about the shear and deviatoric stresses here we model and analyse traveltime shifts for compacting reservoirs whose elastic properties are different from those of the surrounding medium for such models the excess stress is influenced primarily by the contrast in the rigidity modulus across the reservoir boundaries synthetic examples demonstrate that a significant 25 or more contrast in enhances the isotropic velocity perturbations outside the reservoir nevertheless the influence of background heterogeneity is mostly confined to the reservoir and its immediate vicinity and the anisotropic velocity changes are still largely responsible for the offset dependence of traveltime shifts if the reservoir is stiffer than the host rock the background heterogeneity reduces anisotropic velocity perturbations inside the reservoir but increases them in the overburden as a result in this case the magnitude of the offset variation of traveltime shifts is generally higher for reflections from interfaces above the reservoir we also study compaction induced stress strain and traveltime shifts for a stiff reservoir embedded in a softer layered model based on velocity profiles from the valhall field in the north sea despite producing discontinuities in strain across medium interfaces horizontal layering does not substantially alter the overall behaviour of traveltime shifts the most pronounced offset variation of traveltime shifts is observed for overburden events recorded at common midpoints close to the reservoir edges on the whole prestack analysis of traveltime shifts should help better constrain compaction induced velocity perturbations in the presence of realistic background heterogeneity

pb u and th diffusion in natural zircon zircon zrsio4 is one of the most widely used minerals for determining the age origin and thermal history of rocks by u th pb geochronology but the parameters describing the solid state volume diffusion rates of these elements in natural zircon which are themselves important for establishing the limits on the applicability of zircon for geochronological studies have remained poorly quantified1 this is because of the measurement difficulties associated with the low p p m concentrations and low diffusion rates of these elements in natural zircon and the chemical and physical heterogeneity present in most crystals here we present direct measurements of the uranium thorium and lead loss from a thermally treated gem quality natural zircon and show that lead diffuses much faster than uranium or thorium we find that the u th pb isotopic system in natural zircon will typically have a closure temperature greater than 900 c which explains why zircon is apparently such a robust geochrometer and is capable of remaining isotopically closed through extended periods of high grade metamorphism and partial melting of the host rock

the importance of early crustal architecture for subsequent basin forming magmatic and fluid flow events the granites tanami orogen example abstract the paleoproterozoic granites tanami orogen gto consists of tightly folded predominantly greenschist facies rocks the orogen a component of the north australian craton includes a ca 1864 ma succession of turbiditic and pelagic sedimentary rocks interbedded with volcanic rocks and abundant dolerite sills in a back arc basin setting which is intruded by collision related ca 1800 ma granitic rocks integrated 4d interpretation of deep seismic profiles gravity and magnetic field observations associated with available surface geology and igneous isotope geochemistry suggest that the gto forms an imbricated crust developed on a partially inverted s dipping rifted archean basement the rifting took place in the paleoproterozoic around 1864 ma and was followed by e w orientated compressional deformation ca 1850 ma with the development of northerly trending isoclinal folds and associated faults orthogonal collisional tectonics followed at ca 1800 ma which is associated with s verging kilometre scale folds faults and orogenic lode gold deposits in the region recognition of the preserved early rift architecture in the gto has major implications for the subsequent evolution of the orogen the early architecture has influenced all subsequent deposition deformation magmatic and fluid events including spatial control of the ca 1864 ma rock succession to the location of intrusive bodies and fluid transfer during formation of ca 1800 ma orogenic gold mineralization

modeling combined fluid and stress change effects in the seismic response of a producing hydrocarbon reservoir editor s note a fuller version of this article can be downloaded in pdf format from the gumpa project web site at the following url http www pet hw ac uk research gumpa index html the exploration and production of hydrocarbons are generally accomplished with the aid of 3 d seismic to image reservoir structure and in some instances reservoir properties and direct hydrocarbon indicators repeated seismic surveys over a period are termed time lapse seismic and sometimes 4 d seismic changes observed in the seismic character with time have been attributed to impedance changes as a result of production e g gawith and gutteridge 1996 these changes have been used in a few producing fields to monitor reservoir performance identifying observed differences in repeat 3 d surveys and relating these to either in situ saturation or stress state changes or both has been difficult because of the lack of control data it has been noted that in some fields watts et al 1995 the sensitivity to stress changes can be very much greater than the sensitivity to saturation changes in other fields landro et al 1999 the saturation changes are thought to be more significant to aid understanding a need for greater integration of geophysics and reservoir engineering has been noted and was the motivation for this study jack 2001 there is a limited window of opportunity in a field s producing life when there are sufficient changes saturation or stress in the subsurface to show a surface seismic response these changes have to be monitored before the field has reached significant decline for the observed changes to be exploited for reservoir management through in fill drilling for by passed compartmentalized or attic oil reservoir modeling is an essential tool for managing the development of and production from hydrocarbon reservoirs many technical issues however surround the realism and validity of the models on which management decisions are based

shear wave polarizations and subsurface stress directions at lost hills field 2 2 s wave data matrix accomplished by computationally rotating sources and receivers although polarization directions obtained by assuming a homogeneous subsurface were moderately consistent with depth considerable improvement in consistency resulted from analytically stripping off a thin near surface layer whose fast s wave polarization direction was about n 6 e s wave birefringence for vertical travel averaged 3 percent in two zones 200 700 ft and 1200 2100 ft 60 210 m and 370 640 m which had closely similar s wave polarizations between those zones the polarization direction changed and the birefringence magnitude was not well defined s wave polarizations from two concentric rings of offset vsps were consistent in azimuth with one another and with polarizations of the near offset vsp this consistency argues strongly for the robustness of the s wave polarization technique as applied in this area the s wave polarization pattern in offset data fits a model of vertical cracks striking n 55 e

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

seismic velocity changes caused by an overburden stress abstractan increase in seismic velocity with depth is a common rock property one that can be encountered practically everywhere overburden pressure increases vertical stress producing a nonlinear elastic response application of a conventional nonlinear theory to this problem leads to transverse isotropy with explicit relationships between nonlinear constants and elastic anisotropy parameters these relationships can be used in velocity depth trend removal and in computing offset dependent corrections for stacking and migration assumptions about small static stress and the use of linearized solutions for its evaluation are invalid for overburden problems more accurate approximations are required realistic tomography models should account for elastic anisotropy as a basic feature our theory gives an accurate fit to well and stacking velocity data for the los angeles basin overburden stress is a likely cause of shear wave generation by underground explosions

in situ calibrated velocity to stress transforms using shear sonic radial profiles for time lapse production analysis abstract borehole acoustic waves are affected by near and far field stresses within rocks that exhibit stress sensitivity typically in medium to high porosity formations nonlinear or third order elastic constants are obtained from the inversion of borehole sonic shear radial profiles with an elastic wellbore stress model the stress to velocity relationship determined from these profiles in the elastic region surrounding the wellbore is used for calibration to compare with empirical laboratory data traditionally used in time lapse seismic feasibility studies to assess simulated production this analysis enables rock physicists to use the wellbore as a laboratory and to examine the stress dependence of the acoustic velocities from in situ field data in their zone of interest laboratory experiments on core samples can yield both empirical and mathematical rock physics models to describe the relationship between stress and velocity to link rock properties to in situ measurements of acoustic data seis

characterizing reservoir parameters by integrating seismic monitoring and geomechanics the cere la ronde underground gas storage reservoir in the paris basin a test site to study and enhance reservoir seismic monitoring is a water bearing sandstone reservoir in a faulted anticline structure seismic data acquired so far have generated a qualitative interpretation of the location of the gas bubble by studying fluid saturation meunier 1998 however between 1994 and 1997 two sonic logs showed subtle differences in vp not explained solely by saturation variations changes in pore pressure and stresses also influence reservoir elastic properties hence we used geomechanical modeling to evaluate quantitatively how exploiting the gas reservoir impacts seismic measurements

predicting time lapse stress effects in seismic data time lapse changes in seismic data are commonly evaluated in terms of changes in reservoir properties such as pressure saturation or temperature traditionally the evaluation of time lapse seismic data has focused on changes of seismic signatures within the reservoir interval recent studies however have shown convincingly that time lapse seismic changes occur not only in the reservoir but also in the overburden and generally in the rock mass surrounding the reservoir these time lapse changes can be explained by production induced stress changes in the rocks surrounding the reservoir

effect of mining subsidence on seismic velocity monitored by a repeated reflection profilel1 we have monitored changes in seismic velocity due to longwall coal mining in the selby coalfield yorkshire england by ten repeated surveys of a surface seismic reflection profile the direction of face advance in the barnsley seam at 550 m depth was parallel to the orientation of the profile the traveltime of a strong reflection event from an anhydrite bed at 150 m depth was measured after processing the data with standard techniques as the face advanced the traveltime increased by about 4 overall in detail the progressive increase in traveltime correlates well with empirical calculations of differential subsidence between the surface and the anhydrite however the magnitude of the change must principally be accounted for by a decrease in seismic velocity which we attribute to a reduction in the vertical effective stress

quantitative estimation of compaction and velocity changes using 4d impedance and traveltime changes in some hydrocarbon reservoirs severe compaction of the reservoir rocks is observed this compaction is caused by production and it is often associated with changes in the overburden time lapse or 4d seismic data are used to monitor this compaction process since the compaction causes changes in both layer thickness and seismic velocities it is crucial to distinguish between the two effects two new seismic methods for monitoring compacting reservoirs are introduced one based on measured seismic prestack traveltime changes and the other based on poststack traveltime and amplitude changes in contrast to earlier methods these methods do not require additional empirical relationships such as for instance a velocity porosity relationship the uncertainties in estimates for compaction and velocity change are expressed in terms of errors in the traveltime and amplitude measurements these errors are directly related to the quality and repeatability of time lapse seismic data for a reservoir at 3000 m depth wit h9mo fcompaction and assuming a 4d timeshift error of 0 5 ms at near offset and 2 ms at far offset we find relative uncertainty in the compaction estimate of approximately 50 60 using traveltime information only

temporal changes in shear wave splitting during an earthquake swarm in arkansas shear wave splitting has been observed in seismograms of two aftershock and one foreshock sequence recorded over 12 days by a local network of nine three component digital seismometers during the 1982 enola arkansas earthquake swarm polarizations of the faster split shear wave show alignments within the shear wave window which correlate with the regional stress field the time delay between split shear waves appears to increase before and to decrease at or after the time of each main shock this behavior is similar to observations before and after a m 6 0 event near the anza seismic gap southern california and may correspond to variations in crack geometry caused by changes in the stress field before and after the earthquakes there are indications that the decrease in delay may occur a few hours before the main shock and could be a possible precursor for earthquake prediction

a multimethod global sensitivity analysis to aid the calibration of geomechanical models via time lapse seismic data time lapse seismic attributes are used extensively in the history matching of production simulator models however although proven to contain information regarding production induced stress change it is typically only loosely i e qualitatively used to calibrate geomechanical models in this study we conduct a multimethod global sensitivity analysis gsa to assess the feasibility and aid the quantitative calibration of geomechanical models via near offset time lapse seismic data specifically the calibration of mechanical properties of the overburden via the gsa we analyse the near offset overburden seismic traveltimes from over 4000 perturbations of a finite element fe geomechanical model of a typical high pressure high temperature hpht reservoir in the north sea we find that out of an initially large set of material properties the near offset overburden traveltimes are primarily affected by young s modulus and the effective stress i e biot coefficient the unexpected significance of the biot coefficient highlights the importance of modelling fluid flow and pore pressure outside of the reservoir the fe model is complex and highly nonlinear multiple combinations of model parameters can yield equally possible model realizations consequently numerical calibration via a large number of random model perturbations is unfeasible however the significant differences in traveltime results suggest that more sophisticated calibration methods could potentially be feasible for finding numerous suitable solutions the results of the time varying gsa demonstrate how acquiring multiple vintages of time lapse seismic data can be advantageous however they also suggest that significant overburden near offset seismic time shifts useful for model calibration may take up to 3 yrs after the start of production to manifest due to the nonlinearity of the model behaviour similar uncertainty in the reservoir mechanical properties appears to influence overburden traveltime to a much greater extent therefore reservoir properties must be known to a suitable degree of accuracy before the calibration of the overburden can be considered

azimuthal anisotropy from the valhall 4c 3d survey recording and processing both the converted shear waves and the compressional waves from multicomponent ocean bottom seismic obs data provide additional opportunities to understand the subsurface the converted waves are generally used to provide complementary images and velocity information to support the conventional seismic model however they also provide a new opportunity to measure and understand shear wave azimuthal anisotropy this paper describes the results from an azimuthal anisotropy study on 3d 4c obs data from valhall field

on the importance of the heterogeneity assumption in the characterization of reservoir geomechanical properties the geomechanical analysis of a highly compartmentalized reservoir is performed to simulate the seafloor subsidence due to gas production the available observations over the hydrocarbon reservoir consist of bathymetric surveys carried out before and at the end of a 10 yr production life the main goal is the calibration of the reservoir compressibility cm that is the main geomechanical parameter controlling the surface response two conceptual models are considered in one i cm varies only with the depth and the vertical effective stress heterogeneity due to lithostratigraphic variability in another ii cm varies also in the horizontal plane that is it is spatially distributed within the reservoir stratigraphic units the latter hypothesis accounts for a possible partitioning of the reservoir due to the presence of sealing faults and thrusts that suggests the idea of a block heterogeneous system with the number of reservoir blocks equal to the number of uncertain parameters the method applied here relies on an ensemble based data assimilation da algorithm i e the ensemble smoother es which incorporates the information from the bathymetric measurements into the geomechanical model response to infer and reduce the uncertainty of the parameter cm the outcome from conceptual model i indicates that da is effective in reducing the cm uncertainty however the maximum settlement still remains underestimated while the areal extent of the subsidence bowl is overestimated we demonstrate that the selection of the heterogeneous conceptual model ii allows to reproduce much better the observations thus removing a clear bias of the model structure da allows significantly reducing the cm uncertainty in the five blocks out of the seven characterized by large volume and large pressure decline conversely the assimilation of land displacements only partially constrains the prior cm uncertainty in the reservoir blocks marginally contributing to the cumulative seafloor subsidence that is blocks with low pressure

pressure dependent seismic velocities based on effective compliance theory and an asperity deformation model abstractseismic velocities of rocks depend strongly on confining pressure which is often explained by the fracture compliances changes within the rocks it is important to have an accurate model describing the relations between confining pressure and seismic velocities for applications such as time lapse reservoir characterization we propose a new model to address this problem by combining the existing effective compliance theory with new solutions for the pressure dependence of fracture compliances specifically we assume the fracture contact surface can be represented by a set of elastic hemispheres with radii following power law distribution and the pressure dependence of seismic velocities can be expressed through pressure dependent normal and tangential fracture compliances that are derived from hertzian contact theory joint data fittings of p and s wave velocity laboratory data show that our model is accurate we also implement fluid substitution using our model to explain the similar stress i

seismic time lapse effects and stress changes examples from a compacting reservoir compaction of unconsolidated reservoir rock represents a major drive mechanism in many oil and gas reservoirs significant volumes of produced hydrocarbons may be credited to this effect however compaction might also result in surface subsidence a dynamic overburden and local permeability losses in the reservoir

isotropic and anisotropic velocity model building for subsalt seismic imaging basin simulators have been used previously for deriving subsalt velocity models with the use of a correlation to relate effective stress to velocity we build on this and the work of others to use physical models to relate porosity to velocity for migrating seismic data this process yields a physically realizable isotropic velocity model that is consistent with the geologic model and matches the tomographic velocity model above salt and in regions where the tomographic velocity estimate is accurate we then use a geomechanical simulator to model the stress distribution in and around allochthonous salt where material properties between salt and sediment change our stress model is the basis for an anisotropic velocity model using murnaghan s theory for finite elastic deformation this formulation with bounds placed on the elastic coefficients leads to significant imaging improvements adjacent to salt

modelling microseismicity of a producing reservoir from coupled fluid flow and geomechanical simulation in this paper we investigate production induced microseismicity based on modelling material failure from coupled fluid flow and geomechanical simulation the model is a graben style reservoir characterized by two normal faults subdividing a sandstone reservoir into three compartments the results are analysed in terms of spatial and temporal variations in distribution of material failure we observe that material failure and hence potentially microseismicity is sensitive to not only fault movement but also fluid movement across faults for sealing faults failure is confined to the volume in and around the well compartment with shear failure localized along the boundaries of the compartment and shear enhanced compaction failure widespread throughout the reservoir compartment for non sealing faults failure is observed within and surrounding all three reservoir compartments as well as a significant distribution located near the surface of the overburden all shear enhanced compaction failures are localized within the reservoir compartments fault movement leads to an increase in shear enhanced compaction events within the reservoir as well as shear events located within the side burden adjacent to the fault we also evaluate the associated moment tensor mechanisms to estimate the pseudo scalar seismic moment of failure based on the assumption that failure is not aseismic the shear enhanced compaction events display a relatively normal and tight pseudo scalar seismic moment distribution centred about 106 pa whereas the shear events have pseudo scalar seismic moments that vary over three orders of magnitude overall the results from the study indicate that it may be possible to identify compartment boundaries based on the results of microseismic monitoring

porosity and elastic anisotropy of rocks under tectonic stress and pore pressure changes elastic properties of rocks depend on tectonic stress using the theory of poroelasticity as a constraint we analyze features of these dependencies related to changes in rock pore space geometry we develop a formalism describing elastic moduli and anisotropy of rocks as nonlinear functions of confining stress and pore pressure this formalism appears to agree with laboratory observations to a first approximation elastic moduli and seismic velocities as well as porosity depend only on the difference between the confining tectonic stress and pore pressure however in general both the confining stress tensor and the pore pressure must be taken into account as independent variables the stress dependent geometry of the pore space fully controls the stress induced changes in elastic moduli and seismic velocities specifically the compliant porosity plays the most important role despite the fact that in many rocks the compliant porosity is a very small part of total porosity changes in compliant porosi

simulation of seismic events induced by co2 injection at in salah algeria a b s t r a c t article history carbon capture and storage technology has the potential to reduce anthropogenic co2 emissions however the geomechanical response of the reservoir and sealing caprocks must be modelled and monitored to ensure that injected co2 is safely stored to ensure confidence in model results there is a clear need to develop ways of comparing model predictions with observations from the field in this paper we develop an approach to simulate microseismic activity induced by injection which allows us to compare geomechanical model predictions with observed microseismic activity we apply this method to the in salah ccs project algeria a geomechanical reconstruction is used to simulate the locations orientations and sizes of pre existing fractures in the in salah reservoir the initial stress conditions in combination with a history matched reservoir flow model are used to determine when and where these fractures exceed mohr coulomb limits triggering failure the sizes and orientations of fractures and the stress conditions thereon are used to determine the resulting micro earthquake focal mechanisms and magnitudes we compare our simulated event population with observations made at in salah finding good agreement between model and observations in terms of event locations rates of seismicity and event magnitudes 2015 the authors published by elsevier b v this is an open access article under the cc by license

simulation driven seismic modeling applied to the design of a reservoir surveillance system for ekofisk field results from time lapse seismic data on ekofisk field have been successfully used in monitoring reservoir compaction and well planning for more than five years in the past the time between repeat surveys was up to 10 years 1989 99 and these surveys observed large time lapse signals dominated by time shifts attributed to reservoir compaction and overburden subsidence in addition to continued use of time shifts joint interpretation of three streamer surveys 1989 1999 2003 has demonstrated a useful though quite noisy time lapse amplitude signal at the reservoir level

nonlinear rock physics model for estimation of 3d subsurface stress in anisotropic formations theory and laboratory verification we develop a rock physics model based on nonlinear elasticity that describes the dependence of the effective stiffness tensor as a function of a 3d stress field in intrinsically anisotropic formations this model predicts the seismic velocity of both p and s waves in any direction for an arbitrary 3d stress state therefore the model overcomes the limitations of existing empirical velocity stress models that link p wave velocity in isotropic rocks to uniaxial or hydrostatic stress to validate this model we analyze ultrasonic velocity measurements on stressed anisotropic samples of shale and sandstone with only three nonlinear constants we are able to predict the stress dependence of all five elastic medium parameters comprising the transversely isotropic stiffness tensor we also show that the horizontal stress affects vertical s wave velocity with the same order of magnitude as vertical stress does we develop a weakanisotropy approximation that directly links commonly measured anisotropic thomsen parameters to the principal stresses each thomsen parameter is simply a sum of corresponding background intrinsic anisotropy and stressinduced contribution the stress induced part is controlled by the difference between horizontal and vertical stresses and coefficients depending on nonlinear constants thus isotropic rock stays isotropic under varying but hydrostatic load whereas transversely isotropic rock retains the same values of dimensionless thomsen parameters only unequal horizontal and vertical stresses alter anisotropy since thomsen parameters conveniently describe seismic signatures such as normal moveout velocities and amplitudevariation with offset gradients this approximation is suitable for designing new methods for the estimation of 3d subsurface stress from multicomponent seismic data

modeling and analysis of compaction induced traveltime shifts for multicomponent seismic data abstractmodeling of time shifts associated with time lapse 4d seismic surveys is helpful in evaluating reservoir depressurization and inverting for subsurface stress using coupled geomechanical and full waveform seismic modeling we study the influence of compaction induced stress and strain around a simplified reservoir on compressional p shear s and mode converted ps waves we estimate compaction induced time shifts and analyze their dependence on reflector depth and pressure drop inside the reservoir time shifts between synthetic baseline and monitor surveys are obtained by processing techniques that are potentially applicable to field data although p wave time shift lags for reflectors in the overburden are indicative of induced anisotropy they are two to three times smaller than s wave time shift leads for reflectors beneath the reservoir we also investigate the contributions of the deviatoric and volumetric stains to the time shifts for all three modes time shifts for s and ps waves

the effect of microstructure and nonlinear stress on anisotropic seismic velocities recent work in hydrocarbon reservoir monitoring has focused on developing coupled geomechanical fluid flow simulations to allow production related geomechanical effects such as compaction and subsidence to be included in reservoir models to predict realistic time lapse seismic signatures generation of appropriate elastic models from geomechanical output is required these elastic models should include not only the fluid saturation effects of intrinsic shape induced and stress induced anisotropy but also should incorporate nonlinear stress dependent elasticity to model nonlinear elasticity we use a microstructural effective medium approach in which elasticity is considered as a function of mineral stiffness and additional compliance is caused by the presence of low aspect ratio displacement discontinuities by jointly inverting observed ultrasonic p and s wave velocities to determine the distribution of such discontinuities we assessed the appropriateness of modeling them as simple planar penny shaped features by using this approximation we developed a simple analytical approach to predict how seismic velocities will vary with stress we tested our approach by analyzing the elasticity of various sandstone samples from a united kingdom continental shelf ukcs reservoir some of which display significant anisotropy as well as two data sets taken from the literature

production induced stresses from time lapse time shifts a geomechanics case study from franklin and elgin fields franklin and elgin fields were discovered in 1986 and 1991 respectively within the u k north sea central graben blocks 22 30 and 29 5 the producing reservoirs are contained in the jurassic fulmar shallow marine and pentland fluvial formations at depths of 5100 5600 m subsea the fields presented significant development challenges both in terms of seismic imaging complexity and being in a state of exceptionally high pressure high temperature hpht addressing these challenges meant that production could not commence until 2001