character and distribution of borehole breakouts and their relationship to in situ stresses in deep columbia river basalts the character and distribution of borehole breakouts in deeply buried basalts at the hanford site in south central washington state are examined in light of stress indicator data and hydraulic fracturing stress data by means of acoustic televiewer and acoustic waveform logging systems a series of boreholes penetrating the grande ronde basalt of the columbia river basalt group were logged to examine the extent of breakouts at depths near 1000 m breakouts occur discontinuously throughout the interiors of most flows in some boreholes the distribution of borehole wall breakouts closely correlates with the incidence of core disking differences in the distribution of breakouts and disking are attributed to differences in failure mechanisms a thin interval of breakout free basalt occurs near the upper and lower limits of flow interiors with many intervals of breakouts terminating at the intersection of oblique fractures with the borehole hydraulic fracturing stress measurement results obtained from four deep boreholes indicate anisotropic horizontal principal stresses with maximum principal stress along an approximate north trending axis consistent with the east trending orientation of breakouts acoustic waveform logs indicate that there is no measurable difference between the seismic properties of breakout free flow tops and flow interiors the highly coherent waveforms obtained in almost all flow interiors indicate that damage to the borehole wall associated with breakout formation remains confined to the thin annulus of stress concentration

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

mechanical properties of shale gas reservoir rocks part 1 static and dynamic elastic properties and anisotropy abstractunderstanding the controls on the elastic properties of reservoir rocks is crucial for exploration and successful production from hydrocarbon reservoirs we studied the static and dynamic elastic properties of shale gas reservoir rocks from barnett haynesville eagle ford and fort st john shales through laboratory experiments the elastic properties of these rocks vary significantly between reservoirs and within a reservoir due to the wide variety of material composition and microstructures exhibited by these organic rich shales the static young s modulus and dynamic p and s wave moduli elastic parameters generally decrease monotonically with the clay plus kerogen content the variation of the elastic moduli can be explained in terms of the voigt and reuss limits predicted by end member components however the elastic properties of the shales are strongly anisotropic and the degree of anisotropy was found to correlate with the amount of clay and organic content as well as the shale fab

shear wave logging with dipole sources laboratory measurements have verified a novel technique for direct shear wave logging in hard and soft formations with a dipole source as recently suggested in theoretical studies conventional monopole logging tools are not capable of measuring shear waves directly in particular no s waves are recorded in a soft formation with a conventional monopole sonic tool because there are no critically refracted s rays when the s wave velocity of the rock is less than the acoustic velocity of the borehole fluid the present studies were conducted in the laboratory with scale models representative of sonic logging conditions in the field we have used a concrete model to represent hard formations and a plastic model to simulate a soft formation the dipole source operating at frequencies lower than those conventionally used in logging substantially suppressed the p wave and excited a wave train whose first arrival traveled at the s wave velocity as a result one can use a dipole source to log s wave velocity

multipole p wave logging in formations altered by drilling wave trains produced by conventional and multipole sonic logging tools may be expected to depend upon whether the formation has been altered by the drilling process such alteration may include invasion by drilling fluids and or drilling damage we show by theoretical modeling that a dipole or quadrupole tool operating at conventional logging frequencies in the range 10 20 khz detects p waves from the virgin formation with a much higher signal to noise ratio than does a monopole tool this permits the multipole tool to measure formation p wave velocities two to three times farther away from the borehole than a conventional monopole tool this larger radius of investigation typically extends beyond the altered zone for most situations even for sources and receivers spaced several meters apart our conclusion is valid only if the velocity of the altered zone is less than the velocity of the virgin formation if the velocity of the altered zone is larger than that of the virgin formation no appreciable

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

u pb age trace element and hf isotope compositions of zircon in a quartz vein from eclogite in the western dabie mountains constraints on fluid flow during early exhumation of ultrahigh pressure rocks quartz veins in high pressure hp to ultrahigh pressure uhp rocks are the products of fluid rock interaction and thus provide insight into fluid processes in subduction zones in this paper we report an integrated study of mineral inclusion trace element u pb age and lu hf isotope compositions of hydrothermal zircon grains from a quartz vein within an uhp eclogite outcrop from the hong an area western dabie mountains these data are used to decipher the age conditions of formation and source of fluid for zircon formation during the exhumation of uhp rocks zircon grains from the vein have perfect euhedral shape and show sector zoning or weak zoning indicating that they precipitated from the aqueous fluid responsible for the vein formation raman spectroscopy analysis reveals that the zircon grains contain inclusions of garnet omphacite rutile quartz and h2o implying that they crystallized from aqueous fluid under hp eclogite facies conditions the zircon grains show low th u and lu hf ratios nearly flat hree patterns absent eu anomalies and low lree contents these characteristics are consistent with their precipitation in the presence of garnet and epidote and absence of feldspar and thus suggest that trace element concentrations in hydrothermal zircon are controlled by co precipitation of mineral assemblages crystallization temperatures of 670 to 712 c which were calculated using the ti content of zircon are consistent with their formation under eclogite facies conditions and may correspond to the temperature of the infiltrating fluid the weighted mean 206pb 238u age of 224 7 1 3 ma is taken as the best estimate for the age of quartz vein formation and records aqueous fluid flow during the early exhumation stage of uhp rocks the zircon grains in the quartz vein have hf compositions similar to those in the host eclogite which demonstrates isotopic equilibrium between fluid and rocks and that the fluid rock ratio was likely low

ultrahigh pressure minerals and metamorphic terranes the view from china abstract ultrahigh pressure uhp metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths associated mafic ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere but which subsequently have experienced p t conditions within or above the coesite stability field 2 7 gpa 700 c typical products include eclogite garnet peridotite and uhp varieties of metapelite quartzite marble paragneiss and orthogneiss uhp metamorphic assemblages require relatively cold lithospheric subduction to mantle depths some recrystallization even occurs under forbidden p t conditions characterized by a geotherm of the current explosion of research on continental uhp terranes reflects their significance for mantle dynamics and the tectonics of continental subduction collision exhumation mantle slab interactions and geochemical recycling a further characterization of uhp phases and positive identification of uhp minerals requires new experimental studies coupled with state of the art analyses for example the very rare occurrence of microdiamond inclusions in zircons from dabie sulu uhp rocks may reflect high f o 2 attending recrystallization inasmuch as epidote is rather common rutile needles within garnets from sulu uhp eclogitic rocks may not be the result of exsolution so in such cases the apparent uhp pressure may have been over estimated hadean igneous microdiamond inclusions in jack hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from new south wales might have been derived from unexposed uhp metamorphic terranes micro mineral intergrowth and nano size minerals may hold important key to deciphering the actual p t paths of subduction and mantle return flow although most exhumed terranes have returned surfaceward relatively rapidly after short time of uhp condition the long duration of storage at great depth and slow exhumation for the largest uhp terranes remain as major problems

acoustic wave propagation in a fluid filled borehole surrounded by a formation with stress relief induced anisotropy the stress relief associated with the drilling of a borehole may lead to an anisotropic formation in the vicinity of the borehole where the properties in the radial direction differ from those in the axial and tangential directions thus axial and radial compressional acoustic velocities are different and similarly the velocity of an axial shear wave depends on whether the polarization is radial or tangential a model was developed to describe acoustic wave propagation in a borehole surrounded by a formation with stress relief induced radial transverse isotropy rti acoustic full waveforms due to a monopole source are computed using the real axis integration method and dispersion relations are found by tracing poles in the kz plane an analytic expression for the low frequency stoneley wave is developed the numerical results confirm the expectations that the compressional refraction is mainly given by the axial compressional velocity while the shear refraction arrival is due to the shear wave wit

two stage exhumation of ultrahigh pressure metamorphic rocks from the western dabie orogen central china abstract zircon trace element u pb age and lu hf isotope composition were determined for two eclogite samples from the xinxian ultrahigh pressure uhp unit in the western dabie orogen to constrain its exhumation processes cathodoluminescence imaging reveals that most zircons are metamorphic while some have inherited magmatic cores in one sample the magmatic cores have high th u and lu hf ratios high trace element contents and a clear negative eu anomaly consistent with their igneous genesis they yield an upper intercept age of ma and a positive ehf t value of 8 0 arguing for reworking of early mesoproterozoic crust during the middle neoproterozoic in the western dabie orogen the metamorphic zircons contain mineral inclusions of garnet omphacite rutile and quartz they are characterized by low nb ta y and heavy rare earth elements hree contents a nearly flat hree pattern and an insignificant negative eu anomaly these indicate that the metamorphic zircons formed under high pressure

first record and timing of uhp metamorphism from zircon in the xitieshan terrane implications for the evolution of the entire north qaidam metamorphic belt the xitieshan terrane is one of four metamorphic terranes in the north qaidam metamorphic belt which is an early paleozoic high pressure to ultrahigh pressure hp uhp metamorphic belt in nw china however conclusive evidence and precise timing of uhp metamorphism in the xitieshan terrane have not been well documented in this study we report an integrated study of zircon grains from an amphibolite in the xitieshan terrane in terms of mineral inclusions trace elements and u pb age systematics the first record of coesite is reported as an inclusion in a metamorphic zircon which provides unambiguous evidence for the uhp metamorphism of the xitieshan terrane the metamorphic zircon domains have weak or no zoning very low th u ratios insignificant eu anomalies and flat hree patterns zircon grains contain inclusions of garnet omphacite and rutile in addition to the coesite inclusion these inclusions indicate that the metamorphic zircon grains formed under uhp metamorphic conditions the metamorphic zircon grains were dated by the sims and la icpms methods and yield weighted mean ages of 432 14 and 441 9 ma respectively combined with previous results the hp uhp metamorphism of the xitieshan terrane may have lasted 460 440 ma with the peak uhp metamorphism at 441 9 ma a compilation of the reported geochronological data reveals that all four terranes of the north qaidam metamorphic belt might have experienced coeval uhp metamorphism during the early paleozoic 420 450 ma and thus may have suffered a coherent subduction uhp metamorphism and exhumation cycle

dehydration induced damage and deformation in gypsum and implications for subduction zone processes 1 experimental heating tests were performed on volterra gypsum to study the micromechanical consequences of the dehydration reaction the experimental conditions were drained at 5 mpa fluid pressure and confining pressures ranging from 15 to 55 mpa one test was performed with a constantly applied differential stress of 30 mpa the reaction is marked by 1 a porosity increase and homogeneous compaction 2 a swarm of acoustic emissions 3 a large decrease in p and s wave velocities and 4 a decrease in vp vs ratio wave velocity data are interpreted in terms of crack density and pore aspect ratio which modeling pores as spheroids is estimated at around 0 05 crack like spheroid complementary tests performed in an environmental scanning electron microscope indicate that cracks first form inside the gypsum grains and are oriented preferentially along the crystal structure of gypsum most of the visible porosity appears at later stages when grains shrink and grain boundaries open extrapolation of our data to serpentinites in subduction zones suggest that the signature of dehydrating rocks in seismic tomography could be a low apparent poisson s ratio although this interpretation may be masked by anisotropy development due to preexisting crystal preferred orientation and or deformation induced cracking the large compaction and the absence of strain localization in the deformation test suggests that dehydrating rocks maybe seen as soft inclusions and could thus induce ruptures in the surrounding nonreacting rocks

effect of undrained creep on instability behaviour of loose sand prefailure instability in the form of a runway deformation can occur for loose sand under undrained conditions the effect of creep on the instability behaviour of loose sand is studied experimentally in this note previous studies have established that instability can only occur when the stress state is above the instability line however if creep is allowed to develop under undrained conditions instability can still possibly occur even when the stress state at which creep develops is below the instability line a boundary that specifies the condition in which instability may be induced by undrained creep is determined based on the experimental data key words laboratory test liquefaction sands shear strength

time dependent deformation of shale gas reservoir rocks and its long term effect on the in situ state of stress abstract laboratory testing of shale gas reservoir rocks reveal varying amounts of time dependent viscous deformation in response to applied differential stress the time dependent deformation is an inherent property of the dry rock as it occurs in the absence of pore fluid the contribution of the time dependent deformation is generally larger for rocks with more clay and organic content the time dependent behavior can be modeled as a power law function of time its magnitude is approximately linear with the magnitude of the applied differential stress and nearly insensitive to the confining pressure by applying linear viscoelastic theory and using laboratory constrained constitutive parameters we evaluated the effect of the time dependent deformation in modifying the in situ differential stress over time modeling suggests that a significant proportion of a differential stress change would be relaxed over time scales on the order of days because of this short time scale the composition of the rock as it influences the time dependent behavior may influence the in situ differential stress magnitudes stronger than the formation s geological loading history for these reservoirs

metamorphic evolution of the coesite bearing ultrahigh pressure terrane in the north qaidam northern tibet nw china widespread evidence for ultrahigh pressure uhp metamorphism is reported in the dulan eclogite bearing terrane the north qaidam altun hp uhp belt northern tibet this includes 1 coesite and associated uhp mineral inclusions in zircon separates from paragneiss and eclogite identified by laser raman spectroscopy 2 inclusions of quartz pseudomorphs after coesite and polycrystalline k feldspar quartz in eclogitic garnet and omphacite and 3 densely oriented sio2 lamellae in omphacitic clinopyroxene these lines of evidence demonstrate that the dulan region is a uhp metamorphic terrane in the north dulan belt ndb eclogites are characterized by the peak assemblage grt omp rt phn coe pseudomorph and retrograde symplectites of cpx ab and hbl pl the peak conditions of the ndb eclogites are p 2 9 3 2 gpa and t 631 687 c the eclogite shows a near isothermal decompression p t path suggesting a fast exhumation in the south dulan belt sdb three metamorphic stages are recognized in eclogites 1 a peak eclogite facies stage with the assemblage grt omp ky rt phn at p 2 9 3 3 gpa and t 729 746 c 2 a high pressure granulite facies stage with grt cpx jd 30 pl an24 29 scp at p 1 9 2 0 gpa t 873 948 c and 3 an amphibolite facies stage with the assemblage hbl pl ep czo at p 0 7 0 9 gpa and t 660 695 c the clockwise p t path of the sdb eclogites is different from the near isothermal decompression p t path from the ndb eclogites which suggests that the sdb was exhumed to a stable crustal depth at a slower rate in essence these two sub belts formed in different tectonic settings they both subducted to mantle depths of around 100 km but were exhumed to the earth s surface separately along different paths this uhp terrane plays an important role in understanding continental collision in north western china

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

high temperature geochronology constraints on the tectonic history and architecture of the ultrahigh pressure dabie sulu orogen 1 new u pb zircon and th pb monazite ages are presented from the giant sulu ultrahigh pressure uhp terrane combined with sm nd ages rb sr ages inclusion relationships and geologic relationships they help define the timing of peak recrystallization the timing of subsequent amphibolite facies metamorphism and the architecture of the dabie sulu suture zone between the collided sino korean and yangtze cratons the data indicate a 15 myr record of uhp recrystallization the first clearly documented for a giant uhp terrane this requires that continental subduction in the dabie sulu orogen involved multiple uhp tectonic or recrystallization events a 244 236 ma precursor uhp event seen only in the dabie shan was followed by a second 230 220 ma main uhp event which was itself terminated by a 220 205 ma amphibolite facies overprint older eclogite facies events seen in the qinling segment of this orogenic belt raise the possibility that these rocks have undergone u hp metamorphism three or four times but at present there is no geochronological evidence in the dabie sulu area to support this the subduction of the lower yangtze plate did not proceed in a simple fashion the ages of inherited zircon cores demonstrate that a ribbon continent of yangtze affinity escaped subduction and became wedged against the sino korean plate hanging wall

brittle creep in basalt and its application to time dependent volcano deformation time dependent brittle deformation is a fundamental and pervasive process operating in the earth s upper crust its characterization is a pre requisite to understanding and unraveling the complexities of crustal evolution and dynamics the preferential chemical interaction between pore fluids and strained atomic bonds at crack tips a mechanism known as stress corrosion allows rock to fail under a constant stress that is well below its short term strength over an extended period of time a process known as brittle creep here we present the first experimental measurements of brittle creep in a basic igneous rock a basalt from mt etna volcano under triaxial stress conditions results from conventional creep experiments show that creep strain rates are highly dependent on the level of applied stress and can be equally well fit by a power law or an exponential law with a 20 increase in stress producing close to three orders of magnitude increase in creep strain rate results from stress stepping creep experiments show that creep strain rates are also influenced by the imposed effective confining pressure we show that only part of this change can be attributed to the purely mechanical influence of an increase in effective pressure with the remainder interpreted as due to a reduction in stress corrosion reactions the result of a reduction in crack aperture that restricts the rate of transport of reactive species to crack tips overall our results also suggest that a critical level of crack damage is required before the deformation starts to accelerate to failure regardless of the level of applied stress and the time taken to reach this point the experimental results are discussed in terms of microstructural observations and fits to a macroscopic creep law and compared with the observed deformation history at mt etna volcano c 2011 elsevier b v all rights reserved

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

sm nd and rb sr isotopic chronology and cooling history of ultrahigh pressure metamorphic rocks and their country rocks at shuanghe in the dabie mountains central china abstract ultrahigh pressure metamorphic uhpm rocks at shuanghe area in the dabie mountains occur as a uhpm block within the regional granitic gneiss a sm nd isochron defined by garnet omphacite rutile from coesite bearing eclogite yield an age of 226 3 3 2 ma another sm nd isochron defined by garnet two phengites form uhp gneiss yield a similar age of 226 5 2 3 ma the two consistent sm nd ages defined by three uhpm minerals suggest that nd isotopic equilibrium between uhpm minerals in these rocks during uhp metamorphism has been achieved this may correspond to the age of peak metamorphism with an average metamorphic temperature of 800 c it is demonstrated that the retrograde metamorphism of uhpm rocks occurred in an open chemical system whereas the nd and sr isotopic systematics of uhpm minerals may have remained closed nd and sr isotopic disequilibrium between uhpm minerals and retrograde metamorphic minerals has been observed therefore the tie line of garnet or phengite and whole rock containing retrograde metamorphic minerals gives an old sm nd age and a young rb sr age respectively with no geologic significance however a rb sr ages of 219 0 6 6 ma defined by phengite garnet from a uhpm gneiss indicates that the uhpm rocks at shuanghe cooled down to 500 c at that time it suggests that the uhpm rocks at shuanghe experienced the first rapid cooling during 226 219 ma on the other hand the rb sr ages of 174 7 8 to 169 2 3 3 ma defined by retrograde minerals amphibole or biotite with closure temperature ranging from 450 to 300 c and intensely retrograded metamorphic rocks reflect a second rapid cooling during this time interval this is consistent with the rapid cooling time of 190 170 ma of the orthogneiss in the dabie su lu uhpm belt obtained by ar ar dating method in contrast to uhpm rocks the nd isotopic composition of the garnet in granitic gneiss has been reset during retrograde metamorphism and is in equilibrium with those of retrograde epidote and biotite which yields a sm nd isochron age of 213 5 ma indicating the retrometamorphic time corresponding to amphibolite facies in addition the biotites from the granitic gneiss yield rb sr ages of 171 173 ma similar to those of the uhpm rocks these data suggest that the country rocks granitic gneiss may have a similar cooling history to the uhpm rocks at shuanghe two stages of rapid cooling of uhpm rocks at shuanghe may correspond two stages of fast uplift the initial rapid uplifting and cooling of uhpm rocks during 226 219 ma may be caused by compression tectonics during subducting time of the continental crust whereas the later rapid cooling may reflect the exhumation of the entire subducted continental crust by extension during the early middle jurassic

sodic amphibole exsolutions in garnet from garnet peridotite north qaidam uhpm belt nw china implications for ultradeep origin and hydroxyl defects in mantle garnets two types of amphibole exsolution lamellae were discovered in garnet from a few garnet peridotites from the north qaidam ultrahigh pressure metamorphic belt northern tibet nw china the amphibole lamellae are strictly oriented in four directions corresponding to the isometric form 111 i e octahedron planes of garnet observations by transmission electron microscopy tem show that amphibole lamellae are topotaxtially concordant with the host garnet and possess 9 8 to 10 a lattice c spacing electron microprobe analysis reveals that these exsolved amphibole lamellae are high in na and ti the tem observations and recalculated compositions indicate that the oriented amphibole lamellae exsolved from original supersilicic majorites with high concentrations of na2o 0 3 wt and hydroxyl up to 1000 ppm by weight these results imply that the host garnet peridotites were formed at depths greater than 200 km and that garnet can be an important reservoir of water at such depths in the mantle

coupled reservoir geomechanical analysis of the potential for tensile and shear failure associated with co2 injection in multilayered reservoir caprock systems coupled reservoir geomechanical simulations were conductedto study the potential for tensile and shear failure e g tensilefracturing and shear slip along pre existing fractures associated withunderground co2 injection in a multilayered geological system thisfailure analysis aimed to study factors affecting the potential forbreaching a geological co2 storage system and to study methods forestimating the maximum co2 injection pressure that could be sustainedwithout causing such a breach we pay special attention to geomechanicalstress changes resulting from upward migration of the co2 and how theinitial stress regime affects the potential for inducing failure weconclude that it is essential to have an accurate estimate of thethree dimensional in situ stress field to support the design andperformance assessment of a geological co2 injection operation moreover we also conclude that it is important to consider mechanical stresschanges that might occur outside the region of increased reservoir fluidpressure e g in the overburden rock between the co2 injectionreservoir and the ground surface

shrimp u pb dating trace elements and the lu hf isotope system of coesite bearing zircon from amphibolite in the sw sulu uhp terrane eastern china abstract in this study we link mineral inclusion data trace element analyses u pb age and hf isotope composition obtained from distinct zircon domains of complex zircon to unravel the origin and multi stage metamorphic evolution of amphibolites from the sulu ultrahigh pressure uhp terrane eastern china zircon grains separated from amphibolites from the ccsd mh drill hole g12 and niushan outcrop g13 were subdivided into two main types based on cathodoluminescence cl and laser raman spectroscopy big dusty zircons with inherited cores and uhp metamorphic rims and small clear zircons weakly zoned grey white luminescent inherited cores preserve mineral inclusions of cpx pl ap qtz indicative of a mafic igneous protolith dark grey luminescent overgrowth rims contain the coesite eclogite facies mineral inclusion assemblage coe grt omp phe ap and formed at t 732 839 c and p 3 0 4 0 gpa in contrast white luminescent small clear zircons preserve mineral inclusions formed during retrograde hp quartz eclogite to lp amphibolite facies metamorphism t 612 698 c and p 0 70 1 05 gpa inherited zircons from both samples yield shrimp 206 pb 238 u ages of 695 520 ma with an upper intercept age of 800 31 ma the uhp rims yield consistent triassic ages around 236 225 and 239 225 ma for g12 and g13 with weighted means of 229 3 and 231 3 ma respectively small clear zircons from both samples give 206 pb 238 u ages around 219 210 ma with a weighted mean of 214 3 ma interpreted as the age of retrograde quartz eclogite facies metamorphism matrix amphibole from both samples indicate ar ar ages of 209 0 7 and 207 0 7 ma respectively probably dating late amphibolite facies retrogression the data suggest subduction of neoproterozoic mafic igneous rocks to uhp conditions in middle triassic 230 ma times and subsequent exhumation to an early hp 214 ma and a late lp stage 208 ma over a period of 16 and 6 myr respectively thus early exhumation from a mantle depth of 120 100 km to about 60 km occurred at an average rate of 0 3 cm y while subsequent exhumation to a middle crustal level took place at approximately 0 54 cm y these exhumation rates are considerably slower than those obtained for uhp rocks in the dora maira and kokchetav massifs 2 3 cm y based on similar p t estimates and trace element and hf isotope compositions sulu amphibolites can be identified as retrograde uhp eclogites the ehf 800 of 8 implies a significant input from the depleted mantle to the sulu dabie terrane during the middle neoproterozoic overgrown rims are characterized by a distinct trace element composition with low lu hf and th u and significantly higher 176 hf 177 hf ratios than inherited cores consistent with formation during after garnet re crystallization and fractionation of the lu hf system during uhp metamorphism the combined dataset suggests homogenization of the 176 hf 177 hf ratio within the metamorphic mineral assemblage and during protolith formation observed variations are explained by mixing of material from both domains during laser ablation e g due to partial recrystallization of inherited cores

the importance of a small wall deformation in the three dimensional acoustic logging results summary a variety of seismic techniques use fluid filled boreholes to host detectors and seismic sources the complexity of the wavefield generated depends on the position of the receivers and the source frequency excited type of formation and type of source in the present paper the boundary element method is used to study the wavefield elicited by monopole or dipole sources within a fluid filled cylindrical cavity drilled through an unbounded elastic formation this model is used to assess the effects of the existence of a small localized wall deformation on the response such a deformation may occur through the mechanical action of the drill string rock failure adjacent to a drilled borehole plastic deformation and washing out of the borehole in soft or poorly consolidated rocks in addition the effect of a source receiver tool placed in an off centre and or tilted position inside the fluid filled borehole on the propagation of both axisymmetric and non axisymmetric wave modes are studied

elastic anisotropy reversal during brittle creep in shale we conducted two brittle creep experiments on shale samples under upper crustal conditions confining pressure of 80 mpa at 26 c and 75 c we deformed the samples to failure with bedding oriented perpendicular to the maximum compressive stress direction using the stress stepping methodology in both experiments the failure stress was 64 higher than the short term peak strength throughout each differential stress step ultrasonic wave velocities initially decreased and then gradually increased with deformation time the magnitude of these variations depends both on the direction of measurement with respect to the bedding and the temperature and it is largest for velocities measured parallel to the bedding and at high temperature elastic wave anisotropy was completely reversed at 75 c following a limited amount of axial strain 0 6 sem investigation confirmed evidence of a time dependent pressure solution localized compaction crack sealing healing and mineral rotation our observations reveal that elastic anisotropy can evolve rapidly in both time and space which has implications on the stress state and its rotation near fault zones

geochemistry of high grade metamorphic rocks of the north qaidam mountains and their geological significance the high grade metamorphic basement in the north qaidam mountains nqd is composed mainly of jinning age metasedimentary rocks and crustally derived granites possibly accompanied by minor paleoproterozoic and archaean material based on their geochemistry the metasedimentary rocks are divided into two groups type i which makes up the bulk of the high grade metamorphic basement in the nqd is characterized by high k2o low na2o relatively high total rare earth elements treez114 357 ppm and weak fractionation of light lree and heavy hree rare earth elements la yb nz8 27 these rocks also have strong negative eu anomalies eu eu z0 3 0 6 type ii rocks crop out mainly in the wulan area and are characterized by relatively high mgo 2 2 4 1 wt lower total ree and strong fractionation of lree and hree la yb nz29 48 both types of metasedimentary rocks have similar nd isotope compositions with tdmz1 9 2 2 ga and 3ndzk4 1 to k8 5 the granites are compositionally similar to the type i metasedimentary rocks the high grade metamorphic basement of the nqd is similar to that in the qilian orogenic belt and the yangtze platform the central qilian nqd terrane was separated from the dunhuang alxa terrane before the jinning period and the two terranes belong to the south china block and north china craton respectively q 2006 elsevier ltd all rights reserved

coupled multiphase flow and poromechanics a computational model of pore pressure effects on fault slip and earthquake triggering the coupling between subsurface flow and geomechanical deformation is critical in the assessment of the environmental impacts of groundwater use underground liquid waste disposal geologic storage of carbon dioxide and exploitation of shale gas reserves in particular seismicity induced by fluid injection and withdrawal has emerged as a central element of the scientific discussion around subsurface technologies that tap into water and energy resources here we present a new computational approach to model coupled multiphase flow and geomechanics of faulted reservoirs we represent faults as surfaces embedded in a three dimensional medium by using zero thickness interface elements to accurately model fault slip under dynamically evolving fluid pressure and fault strength we incorporate the effect of fluid pressures from multiphase flow in the mechanical stability of faults and employ a rigorous formulation of nonlinear multiphase geomechanics that is capable of handling strong capillary effects we develop a numerical simulation tool by coupling a multiphase flow simulator with a mechanics simulator using the unconditionally stable fixed stress scheme for the sequential solution of two way coupling between flow and geomechanics we validate our modeling approach using several synthetic but realistic test cases that illustrate the onset and evolution of earthquakes from fluid injection and withdrawal

the effect of the invaded zone on full wavetrain acoustic logging most theoretical studies of acoustic borehole logging have employed the simple model of a fluid borehole in an infinite solid this work attempts to account for the invaded zone using a more sophisticated model that additionally includes finite concentric shells surrounding the borehole by appropriately choosing the physical parameters of these cylindrical shells one can study the effect of the invaded zone mudcake or a cased hole on acoustic wavetrain components the model accounts for geometric attenuation but it assumes the formation is perfectly elastic a key consideration is the distance that the acoustic log sees into the formation the body of this report is devoted to studying the effect of the invaded zone mudcake or steel casing on the components of the full wavetrain p wave s wave reflected modes and stoneley mode i conclude that a sonic logging tool has a very shallow depth of investigation this depth for p and s waves depends upon the spacing between the source and receive

nonlinear viscoelastic behavior of sedimentary rocks part ii hysteresis effects and influence of type of fluid on elastic moduli uniaxial stress cycling experiments were conducted on dry brine saturated and hexadecane saturated berea sandstone samples to observe in detail the hysteresis in stress strain diagrams and to understand the influence of different fluids on the strain amplitude dependence of elastic moduli and attenuation cycling experiments were also conducted with sandstone samples saturated with ctab a cationic surfactant that renders the mineral surfaces hydrophobic hexadecane and ctab were selected so as to investigate the relative contributions of adhesion hysteresis and stick slip sliding on attenuation in sedimentary granular rocks young s moduli and poisson s ratios obtained from the cycling tests show a significant dependence on strain amplitude on dry as well as water and hexadecane saturated samples bow tie shaped diagrams are obtained when loading and unloading tangent moduli are plotted against strain the type of fluid in the pore space and at the grain contacts has a large influence on the hysteresis observed in the stress strain diagrams

downhole geophysical logging downhole geophysical logging is the method of making geophysical measurements in drill holes and interpreting these measurements for evaluation of geological properties of the subsurface the downhole measurements can be made with great spatial resolution with a single sonde containing both source and receivers which are separated by 10 cm to tens of meters measurements can be repeated every few centimeters as the sonde moves upward or downward in the borehole this technique is called well logging it is also possible to make downhole measurements with the sensor in the borehole at depth and the source at the surface when the sensor package is moved to a different depth the source signal is repeated when these measurements are made with seismic waves the process is called vertical seismic profiling vsp in vsp the surface source can be any appropriate system that would generate a seismic pulse dynamite air gun or a sinusoidal wave vibroseis and the downhole sensor can be a hydrophone or borehole geophone vertical seismic profiling is a rapidly expanding technique and some of its applications are described in a series of recent books gal perin 1974 hardage 1983 balch lee 1984 toksoz stewart 1984 another method of downhole measurement is possible when closely spaced drill holes are available with the source placed in one drill hole and the detector receiver in another this technique called cross

magmatic history of the northeastern tibetan plateau 1 the northeastern margin of the tibetan plateau is underlain by the qaidam and qilian terranes which consist primarily of mid proterozoic through lower paleozoic oceanic and arc type assemblages that have been accreted to the southern margin of the tarim sino korean craton most previous models suggest that these assemblages formed along a northeast dipping subduction system constructed along the margin of the tarim sino korean craton during early paleozoic time the main components are interpreted to have formed either as an archipelago of volcanic arcs and back arc basins or as a broad expanse of accretionary complexes our geochronologic data support a model suggested by sobel and arnaud 1999 in which the qaidam and qilian terranes are separated from the tarim sino korean craton by a mid paleozoic suture that closed along a southwest dipping subduction zone the basement to these terranes consists of oceanic assemblages that were amalgamated into a coherent crustal fragment prior to emplacement of 920 930 ma granitoids early paleozoic arc type magmatism occurred between 480 and 425 ma apparently sweeping southwestward across much of the qilian and qaidam terranes accretion related magmatism along the inboard margin of the qilian terrane occurred between 423 ma and 406 ma following silurian devonian accretion the region has experienced late paleozoic and mesozoic uplift and erosion and has been severely overprinted by tertiary thrusting uplift and strike slip motion along the altyn tagh fault correlation of geologic features and magmatic histories between the altun shan and the nan shan suggests that the eastern altyn tagh fault has a total left lateral offset of 375 km

trace element signature and u pb geochronology of eclogite facies zircon bergen arcs caledonides of w norway secondary ion mass spectrometry sims u pb and trace element data are reported for zircon to address the controversial geochronology of eclogite facies metamorphism in the lindas nappe bergen arcs caledonides of w norway caledonian eclogite facies overprint in the nappe was controlled by fracturing and introduction of fluid in the proterozoic sveconorwegian granulite facies meta anorthosite norite protolith zircon grains in one massive eclogite display a core rim structure sveconorwegian cores have trace element signatures identical with those of zircon in the granulite protolith i e 0 31 th u 0 89 heavy rare earth element hree enrichment and negative eu anomaly weakly zoned to euhedral oscillatory zoned caledonian rims are characterized by th u 0 13 low lree content minimum normalized abundance for pr or nd variable enrichment in hree and no eu anomaly a decrease of ree towards the outermost rim especially hree is documented this signature reflects co precipitation of zircon with garnet and clinozoisite in a feldspar absent assemblage and consequently links zircon to the eclogite facies overprint the rims provide a mean 206pb 238u crystallization age of 423 4 ma this age reflects eclogite forming reactions and fluid rock interaction this age indicates that eclogite facies overprint in the lindas nappe took place at the onset of the scandian silurian collision between laurentia and baltica

simulation of borehole sonic waveforms in dipping anisotropic and invaded formations a numerical simulation study has been made of borehole sonic measurements that examined shoulder bed anisotropy and mud filtrate invasion effects on frequency dispersion curves of flexural and stoneley waves numerical simulations were considered for a range of models for fast and slow formations computations are performed with a cartesian 3d finite difference time domain code simulations show that presence of transverse isotropy ti alters the dispersion of flexural and stoneley waves in slow formations the flexural wave becomes less dispersive when the shear modulus c44 governing wave propagation parallel to the ti symmetry axis is lower than the shear modulus c66 governing wave propagation normal to the ti symmetry axis conversely the flexural wave becomes more dispersive when c44 c66 dispersion decreases by as much as 30 at higher frequencies for the considered case where c44 c66 and decrease

slow exhumation of uhp terranes titanite and rutile ages of the western gneiss region norway abstract u pb ages of titanite and rutile were obtained from the central western gneiss region norway to assess the style and timing of exhumation and cooling of the western gneiss uhp terrane approximately half of the titanite ages are concordant the majority of which yield a limited age range from 393 to 390 ma the other titanite data are discordant and define discordia arrays with upper intercept ages of either 938 ma or 1 6 ga and a lower intercept of 389 ma concordant rutile analyses range from 385 to 392 ma both titanite and rutile ages young wnw toward the core of the orogen and are 4 ma older than 40 ar 39 ar muscovite ages corresponding to a cooling rate of 90 c ma a well defined boundary between concordant and discordant titanite ages in combination with the wnw increasing p t gradient and the similarity between muscovite cooling ages in the east and eclogite ages in the west suggests that the wgr remained coherent throughout its exhumation history and was progressively unroofed from east to west a 390 2 0 8 ma titanite in the soroyane uhp domain indicates that exhumation occurred at a vertical rate of 7 mm yr for 12 ma these rates are slower than estimates from smaller uhp terranes but similar to other large uhp terranes suggesting that there may be fundamental differences in the mechanisms controlling the evolution of large uhp terranes that undergo protracted subduction and exhumation and smaller uhp terranes that undergo rapid subduction and exhumation

a numerical investigation of head waves and leaky modes in fluid filled boreholes although synthetic borehole seismograms can be computed for a wide range of borehole conditions the physical nature of shear and compressional head waves in fluid filled boreholes is poorly understood this paper presents a series of numerical experiments designed to explain the physical mechanisms controlling head wave propagation in boreholes these calculations demonstrate the existence of compressional normal modes equivalent to shear normal modes or pseudo rayleigh waves with sequential cutoff frequencies spaced between the cutoff frequencies for the shear normal modes major contributions to head wave spectra occur in discrete peaks at frequencies just below mode cutoff for both compressional and shear modes this result is confirmed by calculations with synthetic waveforms at frequencies corresponding to mode cutoff and by branch cut integrals designed to yield independent spectra for the compressional mode for soft formations where shear velocity falls below acoustic velocity in the borehole

excess silica in omphacite and the formation of free silica in eclogite silica lamellae in eclogitic clinopyroxene are widely interpreted as evidence of exsolution during decompression of eclogite however mechanisms other than exsolution might produce free silica and the possible mechanisms depend in part on the nature and definition of excess silica excess silica may occur in both stoichiometric and non stoichiometric pyroxene although the issue has been debated we show that all common definitions of excess silica in non stoichiometric clinopyroxene are internally consistent interchangeable and therefore equivalent the excess silica content of pyroxene is easily illustrated in a three component condensed composition space and may be plotted directly from a structural formula unit or recalculated end members in order to evaluate possible mechanisms for the formation of free silica in eclogite we examined the net transfer reactions in model eclogites using a thompson reaction space we show that there are at least three broad classes of reactions that release free silica in eclogite i vacancy consumption in non stoichiometric pyroxene ii dissolution of ti phases in pyroxene or garnet iii reactions between accessory phases and either pyroxene or garnet we suggest that reliable interpretation of the significance of silica lamellae in natural clinopyroxene will require the evaluation not only of silica solubility but also of titanium solubility and the possible roles of accessory phases and inclusions on the balance of free silica

slow subduction of a thick ultrahigh pressure terrane 1 high precision lu hf and sm nd ages reported here cover a large portion of the western gneiss region wgr ultrahigh pressure uhp terrane norway and range from 413 9 3 7 to 397 1 4 8 ma collectively new and existing geochronologic data for eclogites demonstrate that eclogite facies metamorphism occurred over a large area 60 000 km2 for an unexpectedly long period of 20 ma a thermal model using slow subduction 2 4 mm a 1 of a relatively warm and thick slab can reproduce the observed p t t history of the wgr uhp terrane indicating that large uhp terranes with protracted peak or near peak histories probably reflect slow subduction of relatively thick crustal sections

compressional and shear wave logging in open and cased holes using a multipole tool we have found in field observations that the multipole sonic logging tool can effectively measure formation p wave and s wave data in a cased hole the multipole tool containing both monopole and quadrupole source receiver systems was originally designed to log p and s waves directly in all lithologies for an open borehole the monopole source receiver system p wave operates at 7 10 khz as compared with 15 25 khz for a conventional sonic tool while the quadrupole source receiver system s wave operates at 3 7 khz these lower operating frequencies enable the signals to penetrate the well casing and cement more effectively than signals from a conventional sonic tool as a result the formation p and s waves recorded by the multipole tool are generally much stronger than the unwanted waves which travel along the well casing and cement formation arrivals can be easily identified and separated from the casing arrivals for a wide range of lithologies logs run before and after the wells were cased

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

well tie fluid substitution and avo modelling a north sea example accurate well ties are essential to practical seismic lithological interpretation as long as the geology in the vicinity of the reservoir is not unduly complex the main factors controlling this accuracy are the processing of the seismic data and the construction of the seismic model from well logs this case study illustrates how seismic data processing to a near offset stack quality control of logs and petrophysical modelling improved a well tie at an oil reservoir we demonstrate the application of a predictive petrophysical model in the preparation and integration of the logs before building the seismic model and we quantify our improvements in well tie accuracy the data for the study consisted of seismic field data from a 3d sail line through a well in a north sea oilfield and a suite of standard logs at the well a swathe of fully processed 3d data through the well was available for comparison the well tie in the shallow section from first pass seismic data processing and a routinely edited sonic log was excellent the tie in a deeper interval containing the reservoir was less satisfactory the phase errors within the bandwidth of the seismic wavelet were of the order of 20 which we consider too large for subsequent transformation of the data to seismic impedance reprocessing the seismic data and revision of the well log model reduced these phase errors to less than 10 and improved the consistency of the deep and shallow well ties the reprocessing included densely picked iterative velocity analysis prestack migration beam forming multiple attenuation stacking the near offset traces and demigration and remigration of the near offset data the petrophysical model was used to monitor and where necessary replace the p wave sonic log with predictions consistent with other logs and to correct the sonic log for mud filtrate invasion in the hydrocarbon bearing sand this editing and correction of the p wave transit times improved the normal incidence well tie significantly the recordings from a monopole source severely underestimated the s wave transit times in soft shale formations including the reservoir seal where the s wave velocity was lower than the p wave velocity in the drilling mud the petrophysical model predicted an s wave log that matched the valid recordings and interpolated between them the subsequent seismic modelling from the predicted s wave log produced a class ii avo anomaly seen on the cdp gathers around the well

u pb zircon geochronology of coesite bearing eclogites from the southern dulan area of the north qaidam uhp terrane northwestern china spatially and temporally extensive uhp metamorphism during continental subduction coesite bearing eclogites from 100 km 2 in the southern dulan area north qaidam mountains nqm of western china contain zircon that records protolith crystallization and ultra high pressure uhp metamorphism sensitive high resolution ion microprobe mass spectrometer and laser ablation inductively coupled plasma mass spectrometry u pb analyses from cathodoluminescence cl dark zircon cores in a coesite bearing eclogite yield an upper intercept age of 838 50 ma and oscillatory zoned cores in a kyanite bearing eclogite gave a weighted mean 206 pb 238 u age of 832 20 ma these zircon cores yield steep heavy rare earth element hree slopes and negative eu anomalies that suggest a magmatic origin thus c 835 ma is interpreted as the eclogite protolith age unzoned cl grey or bright zircon and zircon rims from four samples yield weighted mean ages of 430 4 438 2 446 10 and 446 3 ma flat hree patterns without eu anomalies and contain inclusions of garnet omphacite rutile phengite and rare coesite these ages are interpreted to record 16 5 myr of uhp metamorphism these new uhp ages overlap the age range of both eclogite and paragneiss from the northern dulan area suggesting that all uhp rock types in the dulan area belong to the same tectonic unit our results are consistent with slow continental subduction but do not match oceanic subduction and diapiric exhumation uhp model predictions these new data suggest that similar to eclogites in other hp uhp units of the nqm and south altyn tagh protoliths of the eclogites in the dulan area formed in a continental setting during the neoproterozoic and then subducted to mantle depth together with continental materials during the early palaeozoic