exceptional reservoir quality in hpht reservoir settings examples from the skagerrak formation of the heron cluster north sea uk abstract as the exploration of hydrocarbons moves into more complex and deeper basinal settings the need to understand the effect of high temperatures and high pressures on reservoir quality and rock properties becomes even more important the fluvial channel sandstone reservoirs of the skagerrak formation in the central north sea exhibit anomalously high porosities and permeabilities considering their depth of burial 3500 m below sea floor despite the complex depositional setting diagenetic history high overpressure and temperatures encountered in the skagerrak formation hydrocarbons are currently being produced the skagerrak formation reservoirs used in this study have encountered overpressures of 40 mpa and temperatures up to 185 c at present day maximum burial to identify the role played by the high pressure and high temperature encountered in the reservoir sandstones a multidisciplinary approach involving petrographic fluid inclusion and burial history modelling studies has been adopted our interpretation of the results is that the generation of shallow overpressure in these fields limited mechanical compaction and also played an important role in minimizing pressure solution in the chemical compaction regime as evidenced by reduced quartz cementation fluid inclusions found in the quartz overgrowths indicate late stage development with temperatures of formation in the range 130 to 170 c coincident with late stage deeper burial hydrocarbon emplacement occurred after quartz cementation and has had little to no effect on porosity preservation the formation of well developed authigenic chlorite 70 surface coating and to a lesser extent illite clay coats with burial had a positive effect on porosity preservation even though permeability was marginally reduced in the illite rich sandstones a schematic porosity and quartz cement evolution model has been developed which allows for pre drill prediction of reservoir quality in the heron cluster and provide valuable insights for other complex high pressure high temperature reservoirs

sharpness of upper mantle discontinuities determined from high frequency reflections an understanding of the nature of seismic discontinuities in the earth s upper mantle is important for understanding mantle processes in particular the amplitude and sharpness of these discontinuities are critical for assessing models of upper mantle phase changes and chemical layering so far seismic studies aimed at determining the thickness and lateral variability of upper mantle discontinuities have yielded equivocal results particularly for the discontinuity at 410km depth1 2 here we present short period 0 8 2 0 s recordings of upper mantle precursors to the seismic phase p p pkppkp from two south american earthquakes recorded by the 700 station short period array in california our results show that the 410 and 660 km discontinuities beneath the indian ocean are locally simple and sharp corresponding to transi tion zones of 4 km or less these observations pose problems for mineral physics models3 5 which predict a transitional thickness greater than 6 km for the peridotite to spinel phase transition in contrast to the results of long period studies6 7 we observe no short period arrivals from near 520 km depth

axisymmetric circulation driven by marginal heating in ice covered lakes below the temperature of maximum density tmd in freshwater lakes heating at the lateral margins produces gravity currents along the bottom slope akin to katabatic winds in the atmosphere and currents on continental shelves we describe axisymmetric basin scale circulation driven by heat flux at the shorelines in polar lake kilpisjarvi a dense underflow originating near the shore converges toward the lake center where it produces warm upwelling and return flow across the bulk of lake water column the return flow being subject to coriolis force creates a lake wide anticyclonic gyre with velocities of 2 4 cm s 1 while warm underflows are common on ice covered lakes the key finding is the basin scale anticyclonic gyre with warm upwelling in the core this circulation mechanism provides a key to understanding transport processes in semi enclosed basins subject to negative buoyancy flux due to heating or cooling at temperatures above tmd at their lateral boundaries

evidence for reduced quartz cementation rates in oil filled sandstones when quartz rich sands are buried and heated pore space is gradually filled by precipitation of quartz cement from aqueous formation fluids here we examine whether the presence of oil in the pore space can retard or halt this loss of porosity by slowing or stopping quartz cementation the effect of oil fill on quartz cementation is examined by using the distribution of quartz cement in the brae formation deep water sandstone reservoir of the miller oil field north sea petrographic data demonstrate that sandstones from the oil zone have much less quartz cement and more porosity than sandstones from the water zone sandstones in both oil and water zones are compositionally and texturally identical and have been affected by a similar burial history kinetic modeling of the cementation process suggests that progressive oil charging has slowed quartz cement growth rates by at least two orders of magnitude halting it completely in the most extreme cases our data demonstrate that early oil charging in the crestal part of an anticline can preserve porosity in deeply buried sandstones this knowledge is especially relevant to porosity prediction for petroleum exploration in deeply buried sandstones

the effect of water on the 410 km discontinuity an experimental study 1 in order to evaluate the effect of h on the pressure and transition interval of the olivine wadsleyite transformation synthesis experiments were carried out in hydrous and anhydrous peridotite compositions at 1400 c under hydrous near saturated conditions the pressure of the transition is 1 0 0 5 gpa lower than under anhydrous conditions in addition the two phase interval appears to broaden from about 0 4 gpa 12 km in the anhydrous system to as much as 1 3 gpa 40 km in the hydrous system under hydrous conditions the spatial distribution of olivine and wadsleyite appears to be controlled by h diffusion extrapolating previously measured diffusion coefficients for h in olivine and in liquids it appears that h diffusion driven by gravitational equilibration may serve to narrow the transition interval between olivine and wadsleyite

sandstone diagenesis and reservoir quality prediction models myths and reality models and concepts of sandstone diagenesis developed over the past two decades are currently employed with variable success to predict reservoir quality in hydrocarbon exploration not all of these are equally supported by quantitative data observations and rigorous hypothesis testing simple plots of sandstone porosity versus extrinsic parameters such as current subsurface depth or temperature are commonly extrapolated but rarely yield accurate predictions for lithified sandstones calibrated numerical models that simulate compaction and quartz cementation when linked to basin models have proven successful in predicting sandstone porosity and permeability where sufficient analog information regarding sandstone texture composition and quartz surface area is available analysis of global regional and local data sets indicates the following regarding contemporary diagenetic models used to predict reservoir quality 1 the effectiveness of grain coatings on quartz grains e g chlorite microquartz as an inhibitor of quartz cementation is supported by abundant empirical data and recent experimental results 2 vertical effective stress although a fundamental factor in compaction cannot be used alone as an accurate predictor of porosity for lithified sandstones 3 secondary porosity related to dissolution of framework grains and or cements is most commonly volumetrically minor 2 exceptions are rare and not easily predicted with current models 4 the hypothesis and widely held belief that hydrocarbon pore fluids suppress porosity loss due to quartz cementation is not supported by detailed data and does not represent a viable predictive model 5 heat flow perturbations associated with allochthonous salt bodies can result in suppressed thermal exposure thereby slowing the rate of quartz cementation in some subsalt sands

short period constraints on the proposed transition zone discontinuity short period p wave data from several events which occurred off the coast of northern mexico and were recorded on the southern california seismic network are used to investigate the existence and nature of the proposed transition zone discontinuity at 520 km synthetically the 520 discontinuity produces a characteristic signature at 19 to 22 epicentral distance seen as a small triplication which adds constructively to the first arriving phase to produce a large amplitude first arrival convincing evidence for such an amplitude pattern is not present in record sections thus the possibility of a diffuse discontinuity at 520 km is explored several models with 1 2 and 2 5 velocity contrasts distributed over zones from 10 to 30 km are tested again evidence for this feature in our data set is negligible and does not justify its inclusion in our model however a model with a simple sharp density contrast at 520 km produces subcritical angle reflections similar to those observed by other authors and does not violate the short period triplication data

influence of multistage oil emplacement on carbonate cementation in the eocene shahejie formation dongying sag bohai bay basin china abstract the influence of multi stage oil emplacement on carbonate cementation was investigated on the basis of determining the sequence of oil emplacement and carbonate cementation in the clastic reservoirs of the upper part of the fourth member of the eocene shahejie formation es4s in the yanxie 229 area dongying sag the sequence of multistage oil emplacement and carbonate cementation was determined by projecting the homogenization temperatures ths of two phase aqueous inclusions on a burial and thermal history plot the ratio of the number of quartz grains with yellow fluorescent oil inclusions therein to the total number of quartz grains ygoi and the ratio of the number of quartz grains with blue fluorescent oil inclusions therein to the total number of quartz grains bgoi were calculated to analyze the paleo oil saturation of each stage of oil emplacement the ygoi and bgoi values reflect the relative level of the paleo oil saturation of the first stage yellow fluorescent fsyf oil and the second stage blue fluorescent ssbf oil respectively a large number of primary pores were preserved in the oil layers whereas only a small number of dissolution pores and micropores were developed in the oil bearing water layers and dry layers calcite dolomite ferrocalcite and ankerite cementation and two stage oil emplacement occurred in the es4s reservoirs the fsyf oil emplacement occurred earlier than the ankerite cementation and ferrocalcite cementation but later than the calcite cementation and dolomite cementation the ssbf oil emplacement occurred later than all phases of carbonate cementation the dolomite cementation was not influenced by the fsyf oil emplacement because it occurred earlier than the fsyf oil emplacement the ankerite cementation was dominantly inhibited by the fsyf oil emplacement in the oil layers and the degree of inhibition increased with the increase in paleo oil saturation of the fsyf oil the ssbf oil emplacement further increased the paleo oil saturation in the oil layers which enhanced the inhibition of the ankerite cementation and even stopped it the ankerite cementation in the dry layers and oil bearing water layers was slowed down or stopped due to the lack of pores and the low transportation rate of external material which was caused by the intense ankerite cementation prior to the ssbf oil emplacement

the brittle plastic transition in experimentally deformed quartz aggregates deformation experiments have been conducted to provide constraints on the processes responsible for the brittle plastic transition in quartz aggregates a correlation between mechanical behavior and distinctive microstructural characteristics indicates that the brittle plastic transition in nonporous quartzite involves at least three transitions in deformation mechanism that occur with increasing temperature and or pressure first there is a transition from cataclastic faulting to semibrittle faulting microstructural observations indicate that this transition occurs due to the activation of dislocations in addition faulting is more stable in the semibrittle faulting regime due to the blunting of the stresses at the advancing fault tip by dislocation glide second there is a transition from semibrittle faulting to semibrittle flow this transition corresponds to a change from localized to distributed deformation microstructural observations indicate that microcracks nucleate in response to stress concentrations at dislocation pileups in the semibrittle flow regime we conclude that the transition to semibrittle flow occurs when the stress intensity at crack tips is insufficient to allow propagation across grain boundaries third there is a transition from semibrittle flow to dislocation creep microstructural observations suggest that this transition occurs as a result of an increase in grain boundary mobility with increasing temperature in addition microstructural observations indicate that a transition from dominantly mode i axial to mode ii shear microcracking occurs with an increase in confining pressure from 0 4 to 0 8 gpa regardless of temperature the differential stresses supported by the experimentally deformed samples are higher than those expected under geologic conditions however a comparison of the experimentally produced microstructures to those reported from natural fault zones suggests that similar processes are operative in the laboratory and in the earth the results of this study provide further evidence to indicate that the brittle plastic transition in the continental crust occurs over a relatively wide range of conditions

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

anomalous porosity and permeability preservation in deeply buried tertiary and mesozoic sandstones in the cusiana field llanos foothills colombia reply abstract porosity in deeply buried 14000 ft 4200 m sandstones of the lower tertiary mirador formation in the cusiana field llanos foothills colombia is less than 10 porosity units p u however permeability is as much as 1000 md which is believed to be due to the simple quartzose detrital grain composition and large grain size the low porosity has resulted from pervasive quartz cement 10 15 fluid inclusion microthermometry and intergranular grain volumes indicate that the quartz cement precipitated at a temperature of around 100 c during deep burial surprisingly thick 400 ft 120 m sandstones of the underlying upper cretaceous guadalupe group in the cusiana field have porosity as high as 20 p u these sandstones are lithic sandstones composed of quartz and a variety of phosphate grain types and are poorly cemented quartz cement is not abundant and occurs as rims of microcrystalline quartz on quartz and phosphate grains in contrast interbedded quartz arenites within the guadalupe group are heavily cemented by quartz cement and have porosity similar to the overlying mirador formation the major control on porosity in these sandstones appears to be the presence of quartz cement the quartz cement is controlled by grain composition the reasons for the lack of quartz cement in the lithic sandstones of the guadalupe group are uncertain but phosphate poisoning inhibiting quartz cement growth is a possibility the presence of high porosity sweet spots in deeply buried sandstones may be linked to the occurrence of phosphate beds a primary depositional control

the orthorhombic to high p monoclinic phase transition in mg fe pyroxenes can it produce a seismic discontinuity the orthorhombic to high p monoclinic phase transition in mg fe sio 3 pyroxene with a mantle relevant composition x fs 0 1 is expected to occur at 300 km depth woodland and angel 1997 however the divariant nature of the phase transition in the mg fe system leaves the question open as to whether this transition occurs over a narrow enough pressure interval to cause a seismic discontinuity new experimental results with binary mg fe pyroxenes constrain the divariant loop to be 0 2 gpa wide at the composition of x fs 0 4 and on the order of 0 15 gpa for a mantle relevant composition this implies that the phase transition will be complete over a depth interval of about 5 6 km in the mantle and it is concluded that the divariant loop of the orthorhombic to high p monoclinic phase transition in mg fe sio 3 pyroxene is indeed narrow enough to produce a jump in seismic velocities the experimentally observed metastable behavior of orthopyroxene could further reduce the effective depth interval of this phase transition the expected location of this phase transition coincides with a small magnitude seismic discontinuity the x discontinuity occasionally observed in seismic profiles at 300 km depth and thus provides a viable petrologic explanation for the origin of this discontinuity if it truly exists

olivine spinel transitions experimental and thermodynamic constraints and implications for the nature of the 400 km seismic discontinuity the sequence of high pressure phase transitions in olivine is traditionally used as a model for seismic velocity variations in the 200 to 650 km depth interval in a mantle of peridotitic bulk composition it has been proposed that the observed seismic velocity increase at 400 km depth is too sharp and of too small a magnitude to be attributable to the phase change and that the upper mantle must thus be chemically stratified with the 400 km discontinuity being due either to a combination of phase changes in a layer of pyroxene garnet rich piclogite composition or to a chemical boundary between such a piclogite layer and an overlying peridotitic layer using available calorimetric thermoelastic and synthesis data and their associated experimental uncertainties we have derived internally consistent high pressure phase relations for the mg2sio4 fe2sio4 join we find that the divariant transition which is generally regarded as occurring over a broad depth interval for mantle olivine compositions is in fact extremely sharp the seismic discontinuity corresponding to the transition in mg0 9fe0 1 2sio4 should occur over a depth interval isothermal of about 6 km at a depth of approximately 400 km the sharpness of this transition is quite insensitive to uncertainties in the constraining calorimetric thermoelastic and synthesis data in addition we have computed seismic velocity profiles for a model mantle consisting of pure olivine of mg0 9fe0 1 2sio4 composition comparison of these computed profiles to those derived from recent seismic studies indicates that the magnitude of the observed velocity increase at 400 km depth is consistent with a mantle transition zone composed of about 60 70 olivine we conclude that there is no need to infer the existence of pyroxene garnet rich compositions such as eclogite or piclogite in the transition zone since an upper mantle of homogeneous olivine rich peridotitic composition is consistent with the available seismic velocity data

p v vp vs t measurements on wadsleyite to 7 gpa and 873 k implications for the 410 km seismic discontinuity the compressional p and shear wave s velocities for mg2sio4 wadsleyite have been measured to 7 gpa and 873 k using simultaneous ultrasonic interferometry and in situ x ray diffraction techniques from the velocity measurements we obtained the pressure and temperature derivatives for the elastic shear g and adiabatic bulk ks moduli g p t 1 5 1 g t p 0 017 1 gpa k ks 173 2 gpa ks p t 4 2 1 and ks t p 0 012 1 gpa k for the p and s waves we obtained vs p t 0 021 1 km s gpa vs t p 0 035 2 km s k vp p t 0 065 2 km s gpa and vp t p 0 038 2 km s k values in parentheses are standard deviations e g 1 5 1 1 5 1 independent equation of state analysis of p v t data provided an estimation of the temperature dependence for the isothermal bulk modulus of kt t p 0 022 12 gpa k and thermal expansion a bt coefficients of a 2 10 5 k 1 and b 2 5 10 8 k 2 using these data along with elastic properties for other mantle phases a velocity depth profile for a pyrolite model to 670 km depth is constructed using a finite strain method along a 1673 k adiabat in the transition zone the pyrolite model has a smaller gradient between 410 and 660 km than the body wave models from synthetic waveform analyses but converges with the seismic profiles at the bottom of the transition zone just above the 660 km discontinuity the pyrolite model has velocity jumps of 6 9 and 7 9 for p and s waves respectively over a thickness of 10 km for the phase transformation from olivine to wadsleyite which is in good agreement with short period p wave reflection data and a recent fine structure model c4 for both the velocity jumps and the thickness of the 410 km seismic discontinuity

porosity depth trends in reservoir sandstones theoretical models related to jurassic sandstones offshore norway abstract theoretical models of compaction and porosity modification in sandstones during burial are discussed theory predicts that mechanical compaction is the dominant porosity reducing mechanism during burial from 0 to 2 5 3 km porosity depth trends are at this stage mainly controlled by the stability of the grain framework and the effective stress time and temperature also influence the porosity depth gradients at shallow to moderate depths but are probably less important than effective stress and framework grain composition at greater burial depths below 2 5 3 km chemical compaction including pressure solution can explain most of the porosity reduction in well sorted clean arenites theoretical modelling of the intergranular pressure solution suggests that sandstones may lose porosity rapidly by this mechanism below 2 5 3 km and that factors capable of retarding pressure solution and quartz cementation are required to maintain prospective porosities below 4 km this theory and comparisons with empirical data from jurassic sandstones from offshore norway indicate that high porosity is favoured by 1 overpressures approaching the lithostatic pressure gradient 2 a short residence time below 3 km burial 3 coatings on grains of clay or microcrystalline quartz which prevent quartz dissolution and precipitation and 4 a high content of feldspar and early diagenetic secondary pores early hydrocarbon emplacement may retard quartz cementation and preserve porosity but chemical compaction probably continues after oil emplacement except at very high hydrocarbon saturations

mantle discontinuity structure beneath the colorado rocky mountains and high plains analysis of mantle discontinuity structure using converted p to s pas phases beneath colorado from the program for array seismic studies of the continental lithosphere passcal rocky mountain front rmf experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations a stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km respectively imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the pds conversion points and then stacking the receiver functions in each bin to form spatial images analogous to common depth point stacking corrections for lateral velocity heterogeneity are calculated using the local s wave tomographic model of lee and grand 1996 and a constant lnvs lnvp scaling of 1 3 this scaling value is determined from the relative scaling between teleseismic p and s wave travel time residuals measured from the rocky mountain front deployment mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography 30 km of 660 km discontinuity topography and up to 40 km of transition zone thickness variation features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western colorado and a gradual 10 km dip of the 410 km discontinuity beneath the high plains the thickening of the transition zone beneath southwest colorado is consistent with the presence of the subducted farallon slab in this region as imaged by van der lee and nolet 1997 in general our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies

the effects of oil emplacement on diagenetic processes examples from the fulmar reservoir sandstones central north sea geologic note 1 sedimentary petrography and petroleum geochemistry studies have been done on fulmar formation upper jurassic cores from fulmar field central north sea the extent of quartz overgrowths and albitization of k feldspars is lower in oil saturated zones compared with water saturated zones however microthermometric studies of aqueous fluid inclusions in quartz overgrowths show similar temperature ranges for both oil and water saturated zones 85 125 degrees c with maximum recorded temperatures close to the current reservoir temperature about 130 degrees c this suggests that quartz overgrowth development in the oil saturated zones continued after oil emplacement but was retarded compared with development in the water saturated zones the quartz cement therefore mus have been essentially locally derived by pressure solution albitization of k feldspars is significantly less about 50 in the oil saturated zones compared with albitization in the water saturated zones because albitization of k feldspars depends on a supply of sodium and removal of potassium this process is more likely to be terminated by oil emplacement the observed difference in the albitization has been used to suggest that the fulmar reservoir probably was filled during the neogene this age estimate agrees broadly with timing determined independently by modeling of oil generation in adjacent source basins the fulmar sandstones contain little or no kaolinite and show no evidence of substantial leaching of feldspars or carbonates this is probably because the fulmar sandstones being partly turbiditic offshore sand deposits were not exposed to meteoric water their close proximity to the marginally mature to mature kimmeridge clay formation seems to have little effect in terms of developing secondary porosity suggesting that the role of organic acids or co 2 generated in these source rocks is of minor importance

subduction and the depth of convection in the mediterranean mantle 1 we tie together geological data paleotectonic reconstruction plate motion and tomographic analysis to unravel the history of subduction and back arc extension of the eastern and central mediterranean in these two regions extensional processes started contemporaneously around 30 myr ago but with marked differences in the eastern region the aegean basin opened slowly 1 cm yr behind a shallow dipping slab 40 45 the corresponding high velocity anomaly extends inside the upper mantle and can be also followed in the midmantle down to a depth of at least 1500 km its descent into the midmantle initiated most probably during the late cretaceous and the trench moved northeastward following the path of the eurasian plate and under the persistent push of the african plate conversely in the central mediterranean region subduction initiated later and the motion of the subducting slab is confined to the upper mantle causing punctuated and rapid episodes of back arc extension provencal and tyrrhenian basins behind a slab that dips steeply 75 we explore the causes that control how the slab subducted and interacted with the lower more viscous part of the mantle

mantle layering from scs reverberations 2 the transition zone this is the second paper in a four part sequence that examines the nature of mantle layering using the scsn phases and internal reflections observed within the reverberative interval of sh polarized seismograms mantle reflectivity profiles for 18 seismic corridors sampling portions of indonesia australia and the western and central pacific are constructed from low frequency 25 mhz scs reverberations i e scsn type phases reflected once from internal mantle discontinuities modeling the reflectivity profiles using synthetic seismograms yields precise path specific estimates of the travel times to the 410 km and 660 km discontinuities and their reflection coefficients the data are consistent with an olivine dominated model of the transition zone marked by two major phase transitions having average apparent depths of 414 2 km and 660 2 km and impedance contrasts of 0 046 0 010 and 0 072 0 010 respectively the interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km peak to peak and is negatively correlated consistent with an endothermic transition at 660 km whose clapeyron slope magnitude is comparable to that of the 410 km exothermic transition in the case of the 660 km discontinuity we have been able to supplement these long wavelength 1500 5000 km observations with an estimate of intrapath topography inferred from the frequency dependence of the 660 km reflection coefficient which gives peak to peak variations less than 40 km on scale lengths of 500 1500 km for the 660 km discontinuity the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction however we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660 km discontinuity which may involve small compositional variations plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition loss of reflected energy due to local curvature or roughness of the reflector and or extreme topography occurring on a small compositional component to the discontinuity 40 of the total impedance contrast by stripping the 410 km and 660 km peaks from the composite path reflectivity profile we identify three minor reflectors at mean depths of 520 710 and 900 km the shallowest may mark the phase spinel phase transition the 710 km discontinuity can be attributed to the ilmenite gamet perovskite phase transition although changes in perovskite symmetry can potentially provide explanations for both the 710 km and 900 km features

the application of synthetic seismograms to the interpretation of the upper mantle p wave velocity structure in northern australia abstract during the period 1974 to 1977 a long range seismic refraction project was conducted in central australia along a profile extending south from darwin earthquakes from the banda sea region were used as seismic sources for this experiment an analysis by hales and co workers of the resulting data based on travel times and using geometric raytracing techniques has resulted in the construction of an upper mantle velocity model using synthetic seismograms to model amplitudes it is shown that additional constraints can be placed on the derived velocity profile the low velocity zone beneath the 200 km discontinuity is found to have a more abrupt onset than was previously suggested a smaller discontinuity at 325 km depth is now implied the analysis suggests that the 400 km discontinuity is a first order velocity increase whereas all other observed upper mantle discontinuities are more satisfactorily modelled as second order type structures

vertical effective stress as a control on quartz cementation in sandstones abstract temperature controlled precipitation kinetics has become the overwhelmingly dominant hypothesis for the control of quartz cementation in sandstones here we integrate quantitative petrographic data high spatial resolution oxygen isotope analyses of quartz cement basin modelling and a kinetic model for quartz precipitation to suggest that the supply of silica from stress sensitive intergranular pressure dissolution at grain contacts is in fact a key control on quartz cementation in sandstones we present data from highly overpressured sandstones in which despite the current burial temperature of 190 c quartz cement occurs in low amounts 4 6 1 2 of bulk volume in situ oxygen isotope data across quartz overgrowths suggest that cementation occurred over 100 ma and a temperature range of 80 150 c during which time high fluid overpressures resulted in consistently low vertical effective stress we argue that the very low amounts of quartz cement can only be explained by the low vertical effective stress which occurred throughout the burial history and which restricted silica supply as a result of a low rate of intergranular pressure dissolution at grain contacts

410 km discontinuity sharpness and the form of the olivine α β phase diagram resolution of apparent seismic contradictions summary estimates of the thickness of the 410 km seismic discontinuity believed to be due to the olivine modified spinel transformation in olivine are as low as 4 km based on discontinuity reflectivity the seismically estimated thickness is however biased to values narrower than the true transformation interval if linear interpolation of properties is used for modelling a 5 km linear velocity gradient yields an average reflection coefficient identical to that of a 10 km transition interval based on olivine phase diagram features moreover alternative forms of the phase diagram equally consistent with experimentally determined iron magnesium partitioning can yield true transition intervals as narrow as 4 km this reconciles a discrepancy between phase equilibrium and seismic measures of discontinuity thickness in two ways 1 seismic thickness estimates are too narrow and 2 narrow transition intervals are permissible given existing phase equilibrium constraints incorporating recent results on the influence of h2o on discontinuity properties it appears that 410 km discontinuity reflectivity is much more sensitive to varying h2o concentration than to temperature suggesting that discontinuity reflectivity variations reflect changes in mantle chemistry

on the collapse behaviour of oil reservoir chalk oil exploitation in north sea ekofisk oilfield started in 1971 the reservoir is located in a 150 m thick layer of porous chalk n 40 50 at a 3000 m depth enhanced oil recovery procedure by sea water injection waterflooding was initiated in 1987 starting from this date seabed subsidence due to chalk compaction evolves at a fairly constant rate i e 40 cm year nowadays the decrease of the seafloor level is of about 10 m reservoir management and production strategies are at the origin of the growing interest of petroleum industry in disposing of a comprehensive description of the chalk mechanical behaviour in this note the subsidence due to waterflooding is interpreted within a framework taken from the mechanics of unsaturated soils by considering oil as the non wetting fluid and water as the wetting fluid chalk compaction is depicted as a collapse phenomenon due to oil water suction decrease a series of suction controlled tests in the osmotic oedometer cell are presented water weakening effects and chalk compaction collapse seem likely to occur through the lost of strength of the inter granular links existing in the oil saturated sample the nature of these links includes both capillary and physico chemical fluids chalk interactions and is well characterised by the oil water suction

p wave tomography of the mantle under the alpine mediterranean area 1 we study the upper mantle p wave velocity structure below the euro mediterranean area down to 1000 km depth by seismic travel time tomography we invert summary residuals constructed with both regional and teleseismic first arrival data reported by the international seismological centre isc 1964 1995 introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure our high resolution model pm0 5 is parameterized with three dimensional 3 d linear splines on a grid of nodes with 0 5 spacing in both horizontal directions and 50 km vertical spacing we obtain about 26 root mean square rms reduction of residuals by inversion in addition to roughly 31 reduction after summary rays formation and selection sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales the distribution of large scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions the mantle dynamics of western central europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone contrarily in the eastern mediterranean fast velocity material sinks into the lower mantle suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity on a smaller scale the existence of tears in the subducted slab lithospheric detachment all along both margins of the adriatic plate as proposed by some authors is not supported by our tomographic images

seismic evidence for very deep roots of continents abstract a major problem in geodynamics with seismic data is discussed how deeply do the continents penetrate into the mantle differential travel times of underside reflections from mantle discontinuities that appear as precursors to ss in large parts of the globe show a clear correlation with oceans and continents they are significantly larger beneath the asian and north american continents than underneath the neighbouring pacific from this observation we conclude that the asian and north american continents affect the mantle well below 410 km changes in the thickness of the transition zone can explain our observations which are in agreement with the hypothesis of petrological phase changes causing the 410 km and 660 km seismic discontinuities average thickness of the transition zone underneath continents is about 14 kim thicker than beneath oceans moreover our findings imply temperature variations about 100 200 k in the mantle transition zone weak reflections from 520 km depth corresponding to an impedance contrast of about 2 can be observed only in some areas of the earth while observations from other locations definitively show no signal from this depth therefore we propose that the 520 km reflector is only a regional feature

the nature of the 660 kilometer upper mantle seismic discontinuity from precursors to the pp phase global seismic network data were used to image upper mantle seismic discontinuities stacks of phases that precede the pp phase thought to be underside reflections from the upper mantle discontinuities at depths of 410 and 660 kilometers show that the reflection from 410 kilometers is present but the reflection from 660 kilometers is not observed a continuous lame s constant lambda and seismic parameter at the 660 kilometer discontinuity explain the missing underside p reflections and lead to a p wave velocity jump of only 2 percent whereas the s wave velocity and density remain unchanged with respect to previous global models the model deemphasizes the role of lame s constant lambda with regard to the shear modulus and constrains the mineralogical composition across the discontinuity

seismic discontinuities and subduction zones abstract the seismic discontinuities at 410 and 660 km depth in the mantle are generally believed to be due to phase transformations in its major component olivine an increasing observational base of short period regional seismic network data supports this view here we analyse cross sections of discontinuity topography in the northwest pacific subduction zones to infer the thermal and chemical state of the mantle in a subduction environment most of the data is from the southern part of the izu bonin subduction zone penetration of the lower mantle by the steeply dipping subducting slab is clearly indicated by depression of the 660 km discontinuity elevation of the 410 km discontinuity in this region by up to 60 km implies a thermal anomaly of approximately 900 1000 c relative to the ambient mantle observation of the elevated 410 in the seismically active part of the slab indicates that the olivine wadsleyite phase transformation occurs under essentially equilibrium conditions there which argues against the transformational faulting hypothesis as the mechanism for deep earthquakes upgoing p wave reflections from the 410 km discontinuity indicate that it is sharp in the mantle far from subducting slabs reflection coefficient variability in and near slabs indicates either a chemical effect or the consequences of topographic focusing

the effect of grain coating microquartz on preservation of reservoir porosity clay coatings have been widely accepted by many workers as an explanation for preserving high porosity in deeply buried sandstones but few workers have realized that similar effects can be produced by microcrystalline quartz coatings this phenomenon can be expected only under special circumstances but in such cases it can have profound consequences for exploration in the central graben area of the southern north sea unusually high porosity 20 27 and permeability 100 1000 md are found in certain zones in upper jurassic sandstones at depths of 3 4 4 4 km the porosity in these zones is 5 15 higher than expected based on average porosity depth trends from brent and haltenbanken sandstones we propose that the high porosity is due to continuous grain coatings of euhedral microcrystalline quartz crystals that are 0 1 2 m thick the distribution of microcrystalline quartz coatings is controlled by the presence of siliceous sponge spicules rhaxella which implies a sedimentological control on the reservoir quality we present a thermodynamic model showing how continuous microcrystalline quartz coatings inhibit development of no mal macrocrystalline quartz overgrowths sourced mainly from stylolites high porosities in parts of various upper jurassic oil fields ula and gyda have previously been explained by inhibition of quartz cementation by early hydrocarbon charge we suggest that the microcrystalline quartz coatings provide a more plausible explanation

chalk fluid interactions with glycol and brines abstract mechanical properties of high porosity chalks are strongly dependent on the type of fluid in the pores dry chalk is considerably stronger than water saturated chalk and this phenomenon is often referred to as the water weakening effect to address the problem of chalk fluid interactions several series of tests have been performed with glycol and high concentration brines as saturating fluids glycol is a fluid that in many aspects resembles oil except that glycol is miscible with water glycol saturated chalk turns out to have properties very similar to oil saturated chalk compared to dry chalk both oil and glycol make the chalk somewhat weaker but this weakening effect is much less than with water several series of tests with brines with high concentrations of calcium chloride or sodium chloride show that the water weakening effect is considerably reduced in high ionic strength solutions most tests were performed as quasi hydrostatic tests with a constant stress ratio of 0 9 in such tests the yield point marks the onset of accelerated pore collapse and the yield value is close to the hydrostatic yield stress in addition to these compressive tests a series of brazilian tests were performed revealing the same trend the variations in mechanical strength have been correlated with the activity of water in the brines within the experimental accuracy of the compressive tests there is a linear trend between reduction of water activity and the corresponding increase in strength this leads to the hypothesis that the water activity may be a key parameter in the water weakening mechanism but this conclusion also indicates that water weakening may be a special case of general chalk fluid interactions where the degree of weakening depends on the strength of adsorption of the fluid molecules to the calcite surfaces

contrasting diagenetic evolution patterns of platform margin limestones and dolostones in the lower triassic feixianguan formation sichuan basin china abstract deeply buried carbonate reservoirs from the lower triassic feixianguan formation in the sichuan basin of china host extensive natural gas resources these reservoirs are predominantly found in oolitic shoals with the reservoir quality of dolomitized zones being higher than that of undolomitized limestone counterparts here we present a combination of petrographic isotopic fluid inclusion and quantitative porosity data in order to understand and predict the diagenetic processes that have impacted the reservoir quality of dolostones and limestones the porosity of limestones has been reduced to 7 5 due to calcite cementation whereas the porosity in oolitic dolostones is not cemented with calcite and typically has 23 5 porosity dolomitization and concurrent early diagenetic gypsum growth played crucial roles on the development and preservation of high porosity in the oolitic dolostone first by stabilizing the rock fabric to inhibit loss of porosity during burial and secondly through the generation of new porosity by dissolution of carbonate and anhydrite a negative shift of 18 o and salinity values 13 c values 20 vpdb and 18 o 10 vpdb together with elevated calcite precipitation temperatures 110 c it is likely that tsr was responsible for the formation of enlarged dissolution vugs that increased porosity by 2 in dolostones due to i anhydrite dissolution ii production of significant amounts of water resulting in formation water undersaturated with respect to calcite and dolomite iii generation of h 2 s and co 2 and the consequent reaction of h 2 s with the siderite feco 3 component in calcite and dolomite this study demonstrates the importance of diagenesis in the formation of deeply buried high quality reservoirs in ooid dominated grainstones influenced by the presence of evaporites our results should be useful for guiding future exploration and reservoir developments in similar paleogeographic and diagenetic settings

experimental study of the compaction of phyllosilicate bearing sand at elevated temperature and with controlled pore water pressure relative to the time dependent compaction behavior of clean quartz sands little experimental work has been carried out on phyllosilicate rich siliciclastic rocks mudstones compaction is expected to be produced by intracrystalline plastic deformation of the phyllosilicate matrix coupled with pressure solution of embedded quartz grains and with silica reprecipitation in the adjacent pore spaces or loss from the system via an advective flux of pore water time dependence of compaction also arises in nature owing to the generation of excess pore pressure which lowers effective stress and hence compaction rate until such pressure excess becomes dissipated by permeation a suite of experiments was performed on synthetic mixtures of quartz sand in a matrix of fine grained illite muscovite hydrostatically compacted at a range of effective pressures between 10 mpa and 210 mpa with a constant pore water pressure of 70 mpa and at temperatures ranging between 300 and 450 c a small adjacent volume of porous quartz sand was provided as a precipitation sink for silica dissolved from the phyllosilicate quartz mixture volume strains were measured by a direct measurement of sample length and b measurement of amount of pore water expelled at constant pore pressure and interpreted with the aid of quantitative petrography on compacted samples control experiments were performed on i dry clean sand ii dry sand phyllosilicate and iii wet clean sand dry clean sand showed almost no compaction wet clean sand showed a degree of compaction similar to dry phyllosilicate sand but wet phyllosilicate sand showed dramatically more compaction compaction rate for the wet mixed phyllosilicate sand samples is described by formula 1 in which is effective stress mpa is porosity r is the gas constant and t is temperature k petrographic study showed interpenetration of grain contacts formation of quartz overgrowths a decrease in the amount of quartz embedded in the phyllosilicate matrix and an increase in mean grain size of quartz with progressive compaction these effects are attributed to solution and redeposition of quartz the empirical compaction law was integrated over a natural burial history the porosity evolution is extremely sensitive to the evolution of pore fluid pressure and relatively less to temperature gradient and burial rate there is reasonable qualitative agreement when compared with porosity depth relations reported in the literature 1 embed graphic 1 gif

depositional and diagenetic controls on deeply buried eocene sublacustrine fan reservoirs in the dongying depression bohai bay basin china abstract this paper investigates the reservoir potential of deeply buried eocene sublacustrine fan sandstones in the bohai bay basin china by evaluating the link between depositional lithofacies that controlled primary sediment compositions and diagenetic processes that involved dissolution precipitation and transformation of minerals this petrographic mineralogical and geochemical study recognizes a complex diagenetic history which reflects both the depositional and burial history of the sandstones eogenetic alterations of the sandstones include 1 mechanical compaction and 2 partial to extensive non ferroan carbonate and gypsum cementation typical mesogenetic alterations include 1 dissolution of feldspar non ferroan carbonate cements gypsum and anhydrite 2 precipitation of quartz kaolinite and ferroan carbonate cements 3 transformation of smectite and kaolinite to illite and conversion of gypsum to anhydrite this study demonstrates that 1 depositional lithofacies critically influenced diagenesis which resulted in good reservoir quality of the better sorted middle fan but poor reservoir quality in the inner and outer fan lithofacies 2 formation of secondary porosity was spatially associated with other mineral reactions that caused precipitation of cements within sandstone reservoirs and did not greatly enhance reservoir quality and 3 oil emplacement during early mesodiagenesis temperatures 70 c protected reservoirs from cementation and compaction

thermal structure and water ice heat transfer in a shallow ice covered thermokarst lake in central qinghai tibet plateau abstract a large number of lakes and ponds are unevenly distributed over the qinghai tibet plateau qtp little is known about their ice processes and thermal regimes seasonal ice mass balance thermal regime and stratification of under ice water were investigated in a shallow thermokarst lake in central qtp using in situ observations and global reanalysis data congelation ice grew to 60 70 cm depth while continuous surface sublimation caused a total ice loss of over 30 cm the bulk lake temperature below ice remained above 3 0 c through the ice freezing period and rose gradually up to 7 9 c during the melting period the vertical thermal structure of under ice water consisted of a stable strongly stratified interfacial layer il and an underlying convective layer reaching the lake bottom a warm layer existed just beneath the il since the middle equilibrium period and increased in thickness and temperature there was seasonal variation of il depth and temperature gradient in response to the ice thermodynamics and atmospheric conditions the calculated daily water ice heat flux fw was of 10 w m 2 magnitude seasonal variation of fw manifested both diffusive and convective heat transport to the ice water interface this study suggests that strong penetration of solar radiative flux is the dominant contributor to high fw which results in a relatively thin ice compared with other equivalent high latitude climate

spinel elasticity and seismic structure of the transition zone of the mantle there is continuing debate about whether the upper mantle is chemically stratified and whether the seismic discontinuity at 400 km depth represents a chemical boundary a phase change of the mg fe 2sio4 component from the phase olivine to the p phase or a combination of both recent developments in high pressure synthesis1 2 and ultrasonic interferometry3 4 have made possible measurements of elastic wave velocities in small poly crystalline samples of high pressure phases by combining our new acoustic measurements on the spinel phase of mg2 sio4 with existing data for the and phases we present here velocity profiles for the m2sio4 component of the mantle where m represents mg or fe to depths of about 600 km we find it to be unlikely that any seismologically observable velocity discontinuity at about 520 km depth can be attributed to this component although the contrast in impedance the product of density and velocity might be sufficient for the transformation to be observed in long period seismic reflection studies at near normal incidence5 the velocity gradients in the transition zone particularly for shear waves are steeper than would be expected for simple adiabatic compression of likely mantle compositions suggesting that alternative explanations including chemical heterogeneity and anelastic relaxation need to be explored

pressure driven shoreline currents in a perennially ice covered pro glacial lake in antarctica identified from a licl tracer injected into a pro glacial stream the distribution of streamwater within ice covered lakes influences sub ice currents biological activity and shoreline morphology perennially ice covered lakes in the mcmurdo dry valleys antarctica provide an excellent natural laboratory to study hydrologic limnologic interactions under ice cover for a 2 h period on 17 december 2012 we injected a lithium chloride tracer into andersen creek a pro glacial stream flowing into lake hoare over 4 h we collected 182 water samples from five stream sites and 15 ice boreholes geochemical data showed that interflow travelled west of the stream mouth along the shoreline and did not flow towards the lake interior the chemistry of water from andersen creek was similar to the chemistry of water below shoreline ice additional evidence for westward flow included the morphology of channels on the ice surface the orientation of ripple marks in lake sediments at the stream mouth and equivalent temperatures between andersen creek and water below shoreline ice streamwater deflected to the right of the mouth of the stream in the opposite direction predicted by the coriolis force deflection of interflow was probably caused by the diurnal addition of glacial runoff and stream discharge to the eastern edge of the lake which created a strong pressure gradient sloping to the west this flow directed stream momentum away from the lake interior minimizing the impact of stream momentum on sub ice currents it also transported dissolved nutrients and suspended sediments to the shoreline region instead of the lake interior potentially affecting biological productivity and bedform development copyright 2014 john wiley sons ltd

pressure solution compaction of sodium chlorate and implications for pressure solution in nacl sodium chloride nacl has been extensively used as a material to develop test and improve pressure solution ps rock deformation models however unlike silicate and carbonate rocks nacl can deform plastically at very low stresses 0 5 mpa this could mean that nacl is less suitable for use as an analogue for rocks that do not deform plastically at conditions where ps is important in order to test the reliability of nacl as a rock analogue we carried out a series of uniaxial compaction experiments on sodium chlorate naclo3 at room pressure and temperature p t conditions and applied effective stresses of 2 4 and 5 0 mpa naclo3 is a very soluble elastic brittle salt that cannot be deformed plastically at room p t conditions the results were compared with experiments on nacl at similar conditions and show that naclo3 behaves in a strikingly similar way to nacl despite its brittleness like nacl it most likely compacts by a grain boundary diffusion controlled ps mechanism mechanical data were fitted to a power law in the form p n d m with volumetric strain rate p volumetric strain effective stress and grain size d a reasonable fit was obtained with d 2t o 4 nd 1 6 0 5 and md 2 8 0 5 the similarity in mechanical behaviour of the two materials nacl plastic naclo3 brittle suggests that plasticity does not play a key role in ps compaction deformation of nacl this means that its plasticity is not a drawback for its use as a ps analogue for rocks or for deriving ps creep laws for salt from compaction experiments 1999 elsevier science b v all rights reserved

seismic imaging of upper mantle structure with new evidence for a 520 km discontinuity stacked images of long period seismic data reveal many phases some not previously observed which are caused by reflections and conversions at discontinuities in the earth s upper mantle these images support the existence of discontinuities at 410 520 and 660km depth but no evidence is found for a significant interface at 220 km

overpressure preventing quartz cementation a reply chemical compaction and the relative importance of the pressure dissolution and illite mica induced dissolution imid models have remained a contentious issue as is the role played by stress in chemical compaction this paper offers further support and evidence as discussed in stricker et al 2016b focusing on the reservoir quality of the triassic skagerrak formation sandstones in the high pressure high temperature hpht central graben north sea the reply discusses alterative reservoir quality interpretations and comments as raised by maast 2016 a series of theoretical and experimental studies as well as field based evidence is presented providing strong support to the important role of stress e g vertical effective stress during chemical compaction the evidence leads to the conclusion that the process of chemical compaction is stress and temperature driven and significantly enhanced by clay minerals playing a catalytic role by increasing the width of diffusion pathway or by modifying the kinetics of the dissolution process

relationship between reservoir diagenetic evolution and petroleum emplacement in the ula field north sea abstract the diagenetic and petroleum filling histories of the ula field have been studied by analysing aqueous and petroleum inclusions occurring in authigenic cements this study shows that diagenesis continues actively after the arrival of petroleum in the sandstones although the reaction rates and petroleum saturation remain obscure microthermometric measurements of fluid inclusions in authigenic quartz suggest an onset of extensive quartz cementation at temperatures around 110 c i e 2 5 km and that this cementation has continued to the present day synchronously with quartz cementation authigenic albite was formed as inter and intragranular cements large amounts of petroleum inclusions locally occur in the albite and quartz cements these inclusions were trapped in the cements as the main charge of petroleum arrived at the structure diffusion through thin water layers between the petroleum and the mineral surface would probably have dominated mass transfer of solutes for the precipitation of authigenic quartz and albite it is evident from analysis of the petroleum inclusions in early formed k feldspar overgrowths that some secondary migrated petroleum probably arrived in the reservoir at a shallow burial depth before the rapid subsidence in the last 25 my compared with petroleum in the free porosity of the ula formation the petroleum in the k feldspar inclusions is evidently sourced from a different source facies and most maturity parameters testify to the latter being less mature later trapped petroleum inclusions in quartz and albite have characteristics found both in k feldspar and in the ula formation dst oil and thus is likely to reflect the progressive change in the ula field petroleum charge which occurred during the time period of quartz diagenesis

coupling between pressure solution creep and diffusive mass transport in porous rocks received 29 september 2000 revised 25 january 2002 accepted 30 january 2002 published xx month 2002 1 pressure solution is widely regarded as a mechanism of ductile deformation in the upper crust it is driven by stress differences and its rate is affected by temperature grain size and fluid chemistry pressure solution involves dissolution at grain contacts under high stress and precipitation at grain contacts on pore surfaces under low stress leading to porosity reduction by precipitation in the pore space or by grain indentation for a system closed at a grain scale pressure solution is traditionally described by a mechanism involving three steps 1 dissolution at intergranular interfaces 2 diffusion of solutes inside the contact between two grains and 3 precipitation on the surface of the grains in contact with the pore fluid in this paper we propose a model where we have added a fourth step to this process diffusive transport to other open pores to account for the macroscopic diffusion of solutes in pore fluids such that the deformation is not closed at the grain scale in this model differences in mineral solubility due to variations in stress and grain size produce concentration gradients which drive diffusive mass transport the interaction between pressure solution at a grain scale and transport over distances of several grains can lead to the amplification of initial porosity heterogeneities and subsequent localization of deformation regions of intense dissolution compact and form bands in close proximity to regions where the porosity reduction is mainly due to cementation pressure solution augmented by large scale diffusional transport will cause mass transport from fine grained to coarse grained rock volumes we show that such processes are important during both diagenesis of sediments and compaction of fault gouge index terms 3902 mineral physics creep and deformation 5114 physical properties of rocks permeability and porosity 8045 structural geology role of fluids 8159 tectonophysics evolution of the earth rheology crust and lithosphere keywords cementation compaction diagenesis gouge pressure solution transport citation gundersen e f renard d k dysthe k bjorlykke and b jamtveit coupling between pressure solution creep and diffusive mass transport in porous rocks j geophys res 107 0 xxxx doi 10 1029 2001jb000287 2002

constraints on upper mantle discontinuities from observations of long period reflected and converted phases stacked images combining over 5 years of long period global digital seismograph network data reveal many phases associated with reflections and conversions from upper mantle discontinuities these images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking results obtained for p s ss and pp reference arrivals resolve numerous phases from discontinuities near 410 and 660 km while some of the stacks also show evidence for a weaker discontinuity near 520 km phases of particular interest include p and sh multiples resulting from topside reflections precursors to ss from underside reflections and p to sv converted phases these phases can be clearly seen both in the waveform stacks and in cross correlation analysis of individual seismograms travel times for these arrivals are converted to discontinuity depths relative to velocities in the preliminary reference earth model and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle average apparent depths to the discontinuities for the different phases agree to within 3 km for the 410 km discontinuity to within 4 km for the 520 km discontinuity and to within 8 km for the 660 km discontinuity the best global averages are obtained from the ss precursor data which indicate discontinuities at 415 519 and 659 km discontinuity depths obtained from the p to sv converted phases at over 35 individual seismic stations exhibit variations of less than 20 km apparent depths to the 660 km discontinuity consistently show greater variability than depths to the 410 km discontinuity supporting recent laboratory results which indicate that the clapeyron slope for the 660 km discontinuity is significantly larger in magnitude than the slope for the 410 km discontinuity precursors to ss seen between 110 and 180 are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point apparent discontinuity depths computed from ss precursors for different tectonic regions agree to within about 5 km the ss precursors have especially good coverage near the subducting slabs in the northwest pacific analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000 to 1500 km wide region near the slab in which the 660 km discontinuity is depressed by about 20 km measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking relative amplitude analysis suggests that the p and s wave impedance changes at 410 km are about 0 8 1 1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0 3 and 0 6 of the changes at 410 km

global de correlation of the topography of transition zone discontinuities analysis of long period shear wave underside reflections from upper mantle discontinuities shows the shape of the 410 km discontinuity to be neither globally nor locally correlated with that of the 660 km discontinuity the long wavelength pattern of 410 topography is dominated by degree 1 spherical harmonics and has a reasonable correlation with the seismic velocity perturbations in the mantle above it the global topography of 660 dominated by degree 2 spherical harmonics is strongly correlated with shear wave velocity variations in the transition zone the poor correlation between the observed topography of the 410 and 660 km discontinuities is inconsistent with a simple model of large scale whole mantle flow thus implying a more complex pattern which includes at least temporary accumulation of subducted material in the transition zone ringwood and irifune nature 331 1988 131 136 tackley et al nature 361 1993 699 704 the modest seismic discontinuity at 520 km is well observed when the reflections are under mid age oceans but is not observed when they are under continental shields interpretation of this observation in terms of olivine depletion in the deep continental mantle indicates the possible existence of continent ocean chemical heterogeneity at depths exceeding 500 km

elastic properties of hydrous ringwoodite γ phase in mg2sio4 abstract single crystal elastic properties of hydrous ringwoodite are reported for samples that were synthesized at 19 gpa and 1300 c the mg si ratio is determined to be 1 95 by epma which is slightly lower than that of ideal anhydrous ringwoodite the h 2 o content determined by sims is 2 2 wt resulting in the stoichiometry mg 1 89 si 0 97 o 4 h 0 33 the lattice parameter a 8 0786 4 a and elastic constants were determined from a single crystal with x ray diffractometry and brillouin scattering spectroscopy the crystal structure is cubic fd3m with a unit cell volume which is 0 51 larger than that of the anhydrous ringwoodite the adiabatic single crystal elastic moduli of hydrous ringwoodite in gpa are c 11 281 6 c 44 117 4 c 12 92 5 the isotropic properties of hydrous ringwoodite are k vrh 155 4 and g vrh 107 3 which are about 16 and 10 smaller than those of anhydrous ringwoodite thus hydrous ringwoodite if present drastically reduces the seismic velocity in the mantle transition zone relative to anhydrous ringwoodite in addition our results show that the anisotropy is enhanced relative to anhydrous ringwoodite using the present results we discuss the 520 km seismic discontinuity in a wet mantle transition zone

wadsleyite ii a new high pressure hydrous phase in the peridotite h2o system a new hydrous silicate phase with formula mg1 71fe0 177al0 01si0 965h0 332o4 has been synthesized at 1400 c and 17 5 gpa from a hydrous iron rich mantle peridotite composition the space group is imma and unit cell parameters a 5 6884 4 b 28 9238 15 and c 8 2382 6 a so that a and c are approximately those of wadsleyite whereas b is 2 5 times that of wadsleyite the calculated density of this phase is 3 513 g cm3 in addition to five mg fe octahedra and two silicate tetrahedra the structure contains one partially occupied mg fe octahedron and one partially occupied silicate tetrahedron these two partially occupied sites may be the result of decompression of a single mg si octahedron the synthesis of this phase which has not been previously noted or described indicates that the crystal chemistry of h incorporation into high pressure spinelloid structures is more complex than previously thought whereas the transition zone of the earth s mantle may serve as a very large reservoir for hydrogen further work will be required to outline the relative stabilities and physical properties of the various hydrous silicates that may xist there

fluid overpressure as a control on sandstone reservoir quality in a mechanical compaction dominated setting magnolia field gulf of mexico the reservoir quality porosity and permeability of deeply buried hydrocarbon reservoir sandstones in sedimentary basins is significantly affected by burial diagenesis many deep reservoirs develop anomalous fluid overpressures during burial previous studies on the effect of fluid overpressure on reservoir quality in these deep reservoirs have been inconclusive because of the difficulty in constraining the individual contributions of various porosity preserving factors that are simultaneously active in these reservoirs owing to its rapid burial and low burial temperatures the neogene gravity flow sandstone reservoirs from the magnolia field gulf of mexico offers a unique opportunity to investigate in isolation the effect of fluid overpressure on reservoir quality examination of petrography pore pressure and routine core analysis datasets showed a positive correlation between high fluid overpressure and enhanced reservoir quality this study confirms that fluid overpressure preserves reservoir quality in deeply buried sandstone reservoirs in compaction dominated high sedimentation basin settings

lack of inhibiting effect of oil emplacement on quartz cementation evidence from cambrian reservoir sandstones paleozoic baltic basin currently the question of whether or not the presence of oil in sandstone inhibits quartz cementation and preserves porosity is still debated data from a number of cambrian sandstone oil fields and dry fields have been studied to determine the effects of oil emplacement on quartz cementation the data show that the porosity distribution is not affected by the presence of oil in sandstones from oil fields and dry fields with similar porosity distribution from this it can be concluded that oil emplacement does not lead to preservation of primary porosity and silica supply for quartz cementation is derived from internal sources rather in spite of large variation in porosity and quartz cement content a regular pattern of porosity decrease is related to increasing temperature or depth the observed heterogeneity is due to local factors that influence the precipitation of quartz cement including sandstone architecture i e distribution of shales within the sandstone bodies and sandstone thickness heterogeneity is inherent to sandstone architecture and to the fact that silica for quartz cementation is derived from heterogeneously distributed local pressure solution models predicting reservoir properties should encompass facies and architecture as important independent factors

the influence of multiple stage oil emplacement on deeply buried marine sandstone diagenesis a case study on the devonian donghe sandstones tabei uplift tarim basin nw china abstract understanding the oil emplacement influence on diagenesis is one of the most key factors of reservoir quality prediction the devonian donghe sandstones are deeply buried to 5700 6000 m and they are important exploration target in the tarim basin additional the reservoirs experienced multi phase hydrocarbon accumulation and destruction the donghe sandstones contain various diagenetic minerals which are mainly composed of calcite cement a small amount of quartz overgrowth and pyrite trace ankerite and ferrocalctie the current study investigates the influence of multiple stage oil emplacement on the diagenesis with particular attention to calcite and quartz cementation of marine clastic reservoir interval of the donghe sandstones in the tarim basin the samples in the oil leg and water leg have been collected and a series of studies were performed including thin section petrography scanning electron microscopy sem carbon and oxygen isotopes and cathodoluminescence cl analysis the 13cv pdb values of calcite cement from the oil leg and water leg between 4 73 and 1 21 and 3 81 to 0 86 respectively the 18ov pdb values of calcite cement from the oil leg and water leg range from 15 59 to 6 61 and 14 97 to 7 76 respectively both of them display no obviously different in the oil leg and water leg all of the 13cv pdb values in the whole interval of the donghe sandstones show lightly depletion was explained mixing from marine carbon and carbon produced from bacterial sulfate reduction bsr that happens during petroleum degradation associated with sulfate reduced by hydrocarbons bacterially two dull luminescence zones trace fe and mn content and high fe accompanied by low mn content and orange red and bright orange yellow luminescence zones in oil leg indicate four generations of calcite occurred whereas two generations of calcite occurs in water leg which are dull and orange red luminescence zones two origins of calcite cement were clearly identified marine biogenic carbonates and hydrocarbon related from palaeo reservoir bsr during uplift the first generation of calcite cement originated from biogenic carbonate the second and third generations of calcite cement were mixing from predate carbonate cement dissolved and re precipitated and calcite precipitated by bsr directly some quartz grains and k feldspars were dissolved by residual organic acids that remained in the irreducible water and following replaced by the fourth generation of calcite cement some calcite cement in the oil leg precipitated in a temperature higher than the homogeneous temperature of fluid inclusion which indicates oil emplacement has no influence on calcite cementation the quartz overgrowth postdate the first hydrocarbon accumulation and predate the second hydrocarbon accumulation which originated from quartz grains dissolved by organic acids that produced from bsr and re precipitated at a low temperature the current study can promote the understanding of the influence of multiple stage hydrocarbon accumulation on diagenesis and it can also inspire the diagenesis process of other paleozoic hydrocarbon reservoirs in the tarim basin

sharpness of the mantle discontinuities seismic estimates of sharpness of the mantle discontinuities are important for constraining models of composition and temperature in the deep earth but these data are difficult to obtain we explore a possibility to determine sharpness of the major mantle discontinuities those at depths around 400 and 650 km from the broad band records of phases converted from p to s underneath the receiver our estimates are obtained from a comparative analysis of waveforms of the converted phases and those of the p waves in the teleseismic records of brv kazakhstan grf germany nre0 norway and ykw canada the data indicate that the shape of the pulse converted at the 400 km discontinuity is close to that of the p wave whereas the pulse of the 650 km conversion is substantially broader we infer from this comparison that the 400 km discontinuity is sharp width of the transition zone is less than 5 7 km the 650 km transition is modelled by a linear gradient zone 20 30 km thick

note on the importance of hydrocarbon fill for reservoir quality prediction in sandstones oil emplacement retarded the rate of quartz cementation in the brae formation deep water sandstone reservoirs of the miller and kingfisher fields united kingdom north sea thus preserving porosity despite the rocks being buried to depths of 4 km and 120 degreesc quartz precipitation rates were reduced by at least two orders of magnitude in the oil legs relative to the water legs important contrasts in quartz cement abundances and porosities have emerged between the oil and water legs where reservoirs have filled with hydrocarbons gradually over a prolonged period of time 15 m y the earlier the hydrocarbon fill the greater is the degree of porosity preservation failure to consider this phenomenon during field development could lead to overestimation of porosity and permeability in the water leg potentially leading in turn to poor decisions about the need for and placement of downflank water injectors during exploration the retarding effect of oil on quartz cementation could lead to the presence of viable reservoirs situated deeper than the perceived regional economic basement

diagenesis of the palaeo oil water transition zone in a lower pennsylvanian carbonate reservoir constraints from cathodoluminescence microscopy microthermometry and isotope geochemistry abstract oil water transition zones in carbonate reservoirs represent important but rarely studied diagenetic environments that are now increasingly re evaluated because of their potentially large effects on reservoir economics here data from cathodoluminescence and fluorescence microscopy isotope geochemistry microthermometry and x ray tomography are combined to decipher the diagenetic history of a 5 m long core interval comprising the oil water transition zone in a lower pennsylvanian carbonate reservoir the aim is to document the cementation dynamics prior during and after oil emplacement in its context of changing fluid parameters intergrain porosity mean values of 7 are present in the upper two sub zones of the oil water transitions zone but values sharply increase to a mean of 14 in the lower sub zone grading into the water saturated portions of the reservoir and a very similar pattern is observed for permeability values in the top of the water filled zone cavernous porosity with mean values of about 24 is found carbonate cements formed from the earliest marine to the late burial stage five calcite ca 1 through 5 and one dolomite dol phase are recognized with phase ca 4b recording the onset of hydrocarbon migration carbon and oxygen cross plots clearly delineate different paragenetic phases with ca 4 representing the most depleted 13c ratios with mean values of about 21 during the main phase of oil emplacement arguably triggered by far field alpine tectonics carbonate cementation was slowed down and eventually ceased in the presence of hydrocarbons and corrosive fluids with temperatures of 110 140 c and a micro hiatal surface formed in the paragenetic sequence these observations support the oil inhibits diagenesis model the presence an earlier corrosion surface between phase ca 3 and 4 is best assigned to initial pulses of ascending corrosive fluids in advance of hydrocarbons the short lived nature of the oil migration event found here is rather uncommon when compared to other carbonate reservoirs the study is relevant as it clearly documents the strengths of a combined petrographic and geochemical study in order to document the timing of oil migration in carbonate reservoirs and its related cementation dynamics

seismic evidence for a 920 km discontinuity in the mantle analysis of hundreds of seismograms from a seismic array in the japanese islands reveals a seismic discontinuity at a depth of about 920 km in the mantle beneath the tonga subduction zone this discontinuity is also evident beneath subduction zones in the japan and flores seas the discontinuity probably represents either a change in mantle chemical composition or the signature of the subducted slab s garnet layer

basin scale circulation and heat fluxes in ice covered lakes a field and theoretical study of the mean circulation patterns gravity currents and heat fluxes in a mid latitude ice covered lake is presented thermistor chains captured the lake wide pressure gradients and thermal regime acoustic instruments measured bottom and interior velocities and a sediment temperature probe measured sediment temperatures and heat fluxes typical rossby numbers showed that the mean circulation was geostrophic in the form of either an anti cyclonic or a cyclonic basin scale gyre each gyre reached a steady state after 10 15 d circulating at a maximum velocity of 1 mm s 1 the gyres persisted throughout most of the winter and generally decayed with time the cyclonic gyre was generated by the lateral temperature gradients created by sediment heat release it was not possible to determine the driver of the anti cyclonic gyre because most ice covered lakes are controlled by the earth s rotation anti cyclonic and cyclonic circulation is postulated as being the most common mean interior flow pattern the horizontal heat fluxes were of the same order as the vertical heat fluxes which is consistent with the findings of previous research the sediment heat flux decayed with time stabilizing after 60 d a scaling analysis of down sloping advection with dynamic seasonal scale temperatures showed that a warmer lake and an increase in the length to depth geometric aspect ratio both within a given lake and an increase in lake size all increased advection time scales and therefore circulation time scales near the boundaries

influence of hydrocarbon injection on the compaction by pressure solution of a carbonate rock an experimental study under triaxial stresses abstract the preservation of good petrophysical properties high porosity high permeability at great depth in carbonate rocks may lead to the existence of a deeply buried reservoir dbr a target of interest for the oil industry one of the key processes controlling diagenesis of the burial environment is pressure solution creep psc an efficient compaction process responsible for the evolution of porosity and permeability in many carbonate reservoirs in this experimental study we examine the effect of i the presence of oil in the pore space and ii its timing of injection on the psc process and the petrophysical properties of a carbonate rock the experiments were performed using a flow through high pressure cell allowing the simulation of the pressure stresses and temperature conditions of a dbr multi disciplinary data mechanical chemical petrographical and petrophysical demonstrate that without oil in the pore space the main diagenetic process is the psc a process reducing by three the initial porosity but having no influence on intrinsic water permeability an early injection of oil prior to water circulation causes the inhibition of psc by the coating of the grains leading to the preservation of porosity conversely a late injection of oil does not preserve initial porosity the dataset obtained from these experiments show the importance of the timing of oil charging in a reservoir in the preservation of initial porosity at great depth by the inhibition of psc however the coating of grains by hydrocarbons may also inhibit further diagenetic processes leading to a creation of secondary porosity at depth

field investigation of winter thermo and hydrodynamics in a small karelian lake during late winter 18 march 7 april 1994 temperature and current measurements were made in lake vendyurskoe russia including three surveys at six cross sections of the lake also the temperature profile evolution was registered with two thermistor chains at two stations bottom depths of 7 6 and 11 5 m until the time of ice breakup temperature gradients were measured just below the ice cover and in the upper 1 ocm layer of the bottom sediments the isotherms were found to be almost horizontal and evenly spaced vl rtically so no conditions for largescale density induced currents existed the heat flux from sediments to water ranged from 0 6 to 2 0 w m 2 these values were inversely related to the depth the heat flux from water to ice ranged from 0 7 to 1 2 w rn2 when water heating from solar radiation penetration became apparent this flux increased by a factor of two when solar radiation increased convection occurred in the upper layers of the water column when solar radiation heating became significant at the beginning of spring the average net heat flux at the ice water interface during daytime was 7 7 w m 2 weak currents few mm s l with a seiche like character were observed which most likely resulted from ice cover oscillations winter thermo and hydrodynamics in dimictic lakes have only been sparsely studied and are far from being well understood because dimictic lakes can be ice covered for several months the influences of water temperature currents and mixing during this period play an important role for the water quality and ecological state of the lake the dynamic processes are different when the lake is ice covered than when it is not thermal regime the thermal regime is rather stable during winter having small horizontal temperature differences the vertical temperature structure has a continuous increase from 0 c at the ice water interface to 4 c and higher in the bottom layers hutchinson 1957

seismic discontinuities in the mediterranean mantle abstract layering in the upper and lower mantle across the mediterranean has been determined using p to s converted phases identified through receiver function analysis conversion of the receiver function s time axis to depth was based on local 1d s velocity models the 1d velocity model for each station was extracted from a three dimensional s velocity model for the mediterranean region eav03 enhanced with crustal structure derived from receiver function analysis under the mediterranean region we observed a mantle transition zone thickness of 261 10 km on average which agrees with a dominance of high velocities imaged in tomographic models at these depths a thick mantle transition zone 270 km was observed in regions with ongoing or past subduction eastern spain southern italy southern greece and the north western african coast conversions from the 410 km and 660 km discontinuities were clearly observed for most stations amplitudes of both discontinuities are anomalously large throughout the mediterranean region 4 9 and 5 7 respectively we observed significant conversions from the 520 km discontinuity the lower mantle showed strong lateral variations resulting in spatially incoherent converted phases from discontinuities near 860 km between 900 km and 1200 km and 1320 km depth

dynamics of cementation in response to oil charge evidence from a cretaceous carbonate field u a e abstract oil charge is thought to inhibit the growth of cements within subsurface pore systems we explore this phenomenon in a giant cretaceous carbonate field from u a e where the oil filled crest porosity ranges from 10 to 50 and permeability from 0 08 to 830 md but coeval water leg porosity is reduced to 10 to 23 and permeability to 0 1 to 4 md only 5 of primary interparticle pores 30 m diameter in the crest are fully cemented compared to 99 of pores in the water leg syntaxial calcite burial cements 10 m diameter in the oil leg show 12 cathodoluminescence zones with oil inclusions n 27 occurring in four of the five final zones mean in situ ion microprobe 18ovpdb data from the oil leg cements range from 1 2 in the oldest zone decreasing to 10 3 in zone 11 returning to 7 7 in the final zone the oldest distinguishable cement zone in the water leg shows highly variable 18o from 3 6 to 9 3 with a mean of 7 3 and with subsequent zones decreasing to a mean value of 9 4 for the youngest cement zone decreasing 18o values are interpreted as indicating increasing temperature reflecting burial and the evolution of pore water composition broadly similar trends in the oil and water legs suggest precipitation under the same general conditions unlike the oil leg cements the final zone in the water leg occludes nearly all remaining pore space the 18ovpdb of bulk micrite from the water leg shows an average of 7 4 n 9 compared to 6 2 n 10 from the oil leg suggesting the precipitation of further micrite cement at greater burial depths we infer that burial cementation slowed in the presence of oil due to a reduction of potential nucleation sites as well as porewater and solute movement within weakly oil wet pores whereas continued flow and solute movement through all pores including the micropores

postaccumulation sandstone porosity evolution by mechanical compaction and the effect on gas saturation case study of the lower shihezi formation in the bayan aobao area ordos basin china abstract porosity evolution and reservoir quality are strongly affected by mechanical compaction however the compaction induced porosity reduction process in sandstone reservoirs during the postaccumulation period and its effect on gas saturation remain poorly understood by studying the sandstones in the bayan aobao area we were able to quantify the porosity evolution and gas saturation variations caused by mechanical compaction during the postaccumulation period through fluid inclusion analysis and mathematical modeling moreover we verified our findings by compaction experiments and geological statistics fluid inclusion microthermometry was employed to determine the hydrocarbon accumulation periods and the highest homogenization temperature of the two phase aqueous fluid inclusions coexisting with natural gas fluid inclusions i e 115 c with a corresponding geological age of 140 ma was regarded as the beginning of the postaccumulation stages with these data we constructed an inversion evolution model based on the geologic statistical model of sandstone mechanical compaction for the postaccumulation period and determined the gas saturation changes based on compaction porosity evolution the modeling results indicate that the sandstone porosity decreased greatly during postaccumulation subsidence and increased slightly during late stage uplift while the gas saturation increased with both the porosity decrease during the postaccumulation subsidence and the porosity increases during the later uplift the modeling results of subsidence are concordant with the compaction experiments which also show that the larger the initial porosity the greater the porosity reduction the uplift modeling results demonstrate that an erosion induced decrease in burial depth allowed the sandstone porosity to increase which is supported by some scholars findings e g neuzil and pollock 1983 jiang et al 2004 zhang 2013 and by the positive correlation between sandstone porosities and restored geologic unit thicknesses in the study area however the inversion modeling indicates that the reason for the porosity rebound in litharenites is that the increase in elastic porosity slightly exceeds the decrease in viscoplastic porosity which would have been different if the uplift induced erosion rate had been distinctly lower gas production test data and restored geologic unit thicknesses indicate that uplift and erosion may be able to enrich sandstone gas reservoirs to a certain extent consistent with the calculated gas saturation variation results but a full understanding of gas saturation evolution awaits further studies the inversion model can provide a powerful tool for quantitative analysis of the sandstone compaction process during the postaccumulation period which could be used for reference in other similar regions

quantitative evaluation of the oil leg potential in the oliver gas field timor sea australia oil bearing fluid inclusions in sandstone cores and cuttings represent hidden oil shows the frequency of quartz grains containing these inclusions the goi number reflects the maximum paleo oil saturation experienced in a sandstone reservoir irrespective of the present fluid phase samples that have been exposed to high oil saturation have goi numbers at least one order of magnitude greater than samples that have demonstrably low oil saturation in this way these fluid inclusion data can be used to identify paleo oil columns and to map original oil water contacts in wells where oil has been displaced by a later gas charge moreover the use of detailed goi mapping to accurately define the location of the original oil water contact allows the height of the paleocolumn to be determined and an estimate to be made of original oil in place the oliver oil and gas discovery located in the timor sea australia presently contains a hydrocarbon column of 178 5 m composed of 164 m of gas over a 14 5 m oil leg and is filled to spillpoint in well oliver 1 goi mapping has delineated a gross paleo oil column of between 99 and 132 m within the present gas leg this corresponds to original oil in place of up to 200 million bbl considerably greater than the 45 million bbl of oil presently reservoired the displacement of up to 155 million bbl of oil from this structure has high graded the prospectivity of tilted fault blocks updip from the oliver structure goi mapping is an innovative approach to reservoir characterization that can reliably detect paleo oil accumulation in hydrocarbon traps that are presently filled by gas these data allow the oil leg potential of both gas discoveries and nearby untested structures to be addressed in a quantitative manner before additional drilling is commissioned

an analysis of the roles of stress temperature and ph in chemical compaction of sandstones abstract chemical compaction also known as pressure solution involves dissolution along grain contacts and transport of solute to the adjacent pore space by diffusion the driving force for diffusion is a gradient in chemical potential therefore chemical compaction requires persistent gradients in chemical potential along grain contacts in order to drive diffusion continuously such gradients exist in porous rocks because of the heterogeneous distribution of stress over the grain surfaces the rate of chemical compaction depends on stress fluid pressure temperature and the composition of the pore fluid which itself depends on various processes that add or remove solute an alternative model of chemical compaction invokes the effect of sheet silicates on ph to account for localized dissolution of grains adjacent to stylolites or where sheet silicates are present along grain contacts in this model the rate of chemical compaction is independent of stress and fluid pressure depending only on temperature this model is untenable because the proposed mechanism cannot give rise to the persistent gradient in chemical potential required to drive diffusion however sheet silicates may increase the rate of chemical compaction because of their influence on dissolution and diffusion rates the development of anisotropic fabrics as a result of chemical compaction and the widespread occurrence of stylolites in carbonates as well as sandstones are inconsistent with the ph model and provide strong support for the role of stress in chemical compaction

the effect of oil saturation on the mechanism of compaction in granular materials higher oil saturations lead to more grain fracturing and less pressure solution abstract whether addition of oil to sandstones slows stops or has no influence upon pressure solution and quartz cementation has long been disputed despite having major implications for petroleum exploration and appraisal strategies to elucidate the effect of addition of oil to compaction pressure solution and cementation processes this study utilizes an experimental approach simulating isochemical volumetric compaction using granular halite in the presence of variable brine oil mixtures each experiment at 500 kpa effective stress lasted 48 hours and involved repeated measurements of volumetric strain the lithified products of experiments were examined using sem techniques after 30 hours approximately 4 volumetric strain occurs when the brine oil ratio is 100 0 pure brine fractures are rare in samples from experiments undertaken with pure brine so that pressure solution and concomitant cementation must be the compaction mechanism when the brine oil ratio is decreased from 100 0 to 40 60 the volumetric strain remains about 4 but there are more fractures as quantified from sem image analysis these observations suggest that although pressure solution has occurred some of the volumetric strain is the result of fracturing implying that pressure solution has been less effective in the presence of some oil than it is in the presence of pure brine when the brine oil ratio is decreased further from 40 60 to 5 95 the volumetric strain increases up to 7 3 sem image analysis reveals that the strain is predominantly due to grain fracturing this change from pressure solution to grain fracturing is likely due to heterogeneous pressure solution resulting from heterogeneous distribution of stresses brine coated grain contacts undergo pressure solution but oil coated grain contacts experience increasing stress up to the point of failure as the surrounding grain pack compacts when the brine oil ratio is decreased to less than 5 95 volumetric strain decreased to about 1 with few induced fractures and little pressure solution compaction is negligible when the brine oil ratio is 0 100 only oil in the pores and fractures are negligible halite is more water soluble and more hygroscopic than quartz so that water will cling to halite surfaces more tenaciously than to quartz since even halite has its mechanism of volumetric strain affected when the brine oil ratio is less than 40 60 quartz pressure solution and subsequent cementation is even more likely to be affected by oil filling this experimental study suggests that the mechanism and extent of compaction and subsequent cementation in sandstones is strongly affected by the addition of oil

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

variability of p660s phases as a consequence of topography of the 660 km discontinuity abstract records from the broad band nars nl array deployed in the netherlands western germany and belgium have been analyzed for p to s converted phases from the 660 km discontinuity p660s no individual seismogram from this array shows a clear p660s phase stacks of up to 113 crosscorrelograms show p660s energy that is weak in comparison with similar stacks elsewhere very high amplitude observations of p660s in single seismograms from the same region have been made in a previous study paulssen 1985 geophys res lett 12 709 712 paulssen 1988 j geophys res 93 10489 10500 such variations may be due to relief on the 660 km discontinuity in this paper we quantify the effects of different topographies on the time delays amplitudes and waveforms of the direct p wave as well as on the p660s converted wave synthetic p and p660s waveforms are calculated by applying a kirchhoff helmholtz integration over the wave fields at the 660 km discontinuity interface for topography on scales of a few hundreds of kilometers the effects on the p wave are very small compared with those on the p660s phases time delays focused and defocused amplitudes and waveform distortion of p660s phases influenced by such topography diminish the coherence of p660s in synthetic stacks our results show that topography of the 660 km discontinuity with dominant scale lengths of a few hundreds of kilometers and variations in depth of 15 km to 25 km is consistent with the pattern of p660s observations in single seismograms and stacks

p p precursors under africa evidence for mid mantle reflectors observations of precursors to p p from a recent exceptionally large deep earthquake in the fiji islands moment magnitude 7 6 at an array of broadband stations in california revealed mid mantle reflectors near depths of 785 kilometers and 1200 kilometers under the southern african rift such observations previously reported primarily in subduction zones suggest that these reflectors may have a global character our analysis which also indicated a sharp uplifted 670 kilometer discontinuity demonstrates the power of sparse regional broadband arrays for the study of weak frequency dependent features in deep earth structure

flowpath and retention of snowmelt in an ice covered arctic lake the extent to which snowmelt flowing into ice covered lakes spreads horizontally and mixes vertically influences retention of solutes derived from the landscape to quantify these transport processes and retention we combine time series temperature and specific conductance measurements in toolik lake alaska and its major inflow with measurements of discharge and meteorology and profiles of specific conductance temperature fluorescence chlorophyll a and dissolved organic carbon doc in spring of 3 yr during early snowmelt the concentration of doc in the stream was 750 m twice that in the lake during slow melt discharge q 6 m3 s 1 the incoming water partially flushed the inlet basin and the more dilute water flowed over the original intrusion with a preferential flowpath to the outlet penetrative convection was restricted by the increased density gradients from the incoming plume and initially constrained to shallow mixing zones associated with the step changes in density as ice thickness decreased to less than 1 m heating caused density instabilities at the base of the intrusions that mixed solutes 10 m vertically contributing to retention near surface layers enriched with doc persisted for 10 d during a rapid melt and for over 3 weeks when the melt was slow retention of order 10 20 also depended on the rapidity of melt and magnitude of discharge

vertical effective stress and temperature as controls of quartz cementation in sandstones evidence from north sea fulmar and gulf of mexico wilcox sandstones abstract we present quantitative petrographic data high spatial resolution oxygen isotope analyses of quartz cement basin modelling and a kinetic model for quartz precipitation for two paleocene eocene wilcox group sandstones from texas and two jurassic fulmar formation sandstones from the central north sea at each location one sandstone has been buried to ca 145 c and one to ca 185 c a key difference between the wilcox and fulmar burial histories is that the wilcox sandstones are currently at higher vertical effective stresses and from basin modelling studies have been subjected to generally higher vertical effective stresses through their burial history the amounts of quartz cement in the wilcox sandstones are between 12 and 18 and between 2 and 6 in the fulmar sandstones high spatial resolution oxygen isotope data obtained from the quartz cements suggest temperature ranges for quartz precipitation from 60 to 80 c to values approaching maximum burial temperature factors such as grain coatings or the timing of petroleum emplacement cannot explain the differences in the amounts of quartz cement petrographic data show that most of the silica for quartz cement can be derived from intergranular pressure dissolution although the sample set is small we interpret the results to suggest that the differences in quartz cementation in fulmar and wilcox sandstones can be explained better by differences in their vertical effective stress history than their temperature history in this case the supply of silica rather than the precipitation of quartz becomes an important control on the rate and extent of cementation

depth variation of the mid mantle seismic discontinuity short period array seismograms of deep events that occurred in the indonesia japan and izu bonin arcs are stacked and beam formed to identify the near source s p converted waves that result from the mantle transition discontinuities most of the resulting images reveal the existence of a mid mantle seismic discontinuity 920 km discontinuity in these regions of the 15 events analyzed three that occurred at the western end of the indonesia arc show clear s p arrivals observable even in individual seismograms the mid mantle discontinuity is characterized by large depth variation 900 1080 km and velocity contrast variation in different subduction zones especially the depth variation of the mid mantle discontinuity beneath the indonesia arc where the discontinuity deepens from 940 km at the eastern end to 1080 km at the western end appears to be well correlated with the location of the high velocity anomalies in recent tomographic models however the mid mantle discontinuity cannot be simply coincided with the bottom of the high velocity anomalies because a velocity increase at the discontinuity is observed from the waveform analysis

global variability of transition zone thickness 1 we analyze recordings from more than 3000 shallow earthquakes and measure s410s s660s differential travel times on a global scale these differential times are obtained using a novel cross correlation technique that eliminates effects associated with a priori assumptions of crustal thickness and velocity variations above the upper mantle transition zone our measurements show absolute perturbations up to 8 s from the global average 242 km which imply transition zone thickness variations of 20 km on a length scale of 2000 km the transition zone under major subduction zones e g the western pacific and south america is significantly thicker than the global average this could be associated with large scale depressions of the 660 km discontinuity due to the accumulation of cold oceanic lithosphere at the base of the upper mantle a narrow transition zone is observed under the central pacific and northern atlantic oceans we also observe a modest anticorrelation between s410s s660s times and the delay times of shear waves predicted using s12 wm13 su et al 1994 in the transition zone which suggests considerable thermal influence on the large scale topography of the 410 and 660 km discontinuities the lack of a stronger anticorrelation suggests that 1 compositional heterogeneities may be present and 2 a significant fraction of thermal anomalies near the transition zone do not extend through the transition zone and influence both the 410 and the 660 km discontinuities a regional analysis shows that the transition zone is on average 6 8 km thicker under continents than under oceans this implies a temperature difference of 50 70 c however we do not observe a clear correlation between thickness and the age of the crust

compaction creep of quartz sand at 400 600 c experimental evidence for dissolution controlled pressure solution intergranular pressure solution ips is an important compaction and deformation mechanism in quartzose rocks but the kinetics and rate controlling process remain unclear the aim of the present study is to test microphysical models for compaction creep by ips against isostatic hot pressing experiments performed on quartz sand under conditions expected to favor pressure solution confining pressure 300 mpa pore water pressure 150 250 mpa temperature 400 600 c microstructural observations revealed widespread intergranular indentation features and confirmed that intergranular pressure solution was indeed the dominant deformation mechanism under the chosen conditions for porosities down to 15 the mechanical data agree satisfactorily with a microphysical model incorporating a previously determined kinetic law for dissolution of loose granular quartz suggesting that the rate limiting mechanism of ips was dissolution the model also predicts ips rates within one order of magnitude of those measured in previous experiments at 150 350 c and thus seem robust enough to model sandstone compaction in nature such applications may not be straightforward however as the present evidence for dissolution control implies that the compositional variability of natural pore fluids may strongly influence ips rates in sandstones

seismic velocity and density jumps across the 410 and 660 kilometer discontinuities the average seismic velocity and density jumps across the 410 and 660 kilometer discontinuities in the upper mantle were determined by modeling the observed range dependence in long period seismic wave arrivals that reflect off of these interfaces the preliminary reference earth model prem is within the computed 95 percent confidence ellipse for the 410 km discontinuity but outside the allowed jumps across the 660 kilometer discontinuity current pyrolite mantle models appear consistent with the constraints for the 410 kilometer discontinuity but overpredict amplitudes for the 660 kilometer reflections the density jump across the 660 kilometer discontinuity is between 4 and 6 percent below the prem value of 9 3 percent commonly used in mantle convection calculations

pervasive pressure solution transfer in a quartz sand the kinetics of deformation of a quartz aggregate by pervasive pressure solution can be under certain conditions of temperature and grain size strongly dependent upon the diffusivity of silica into the grain to grain contacts an analysis of the factors affecting this key parameter and less well constrained in the analysis of the problem of rock deformation by pressure solution is presented this analysis is based on recent advances on studying silica surfaces and particularly on the existence of a silica gel layer on the silica surfaces undergoing dissolution by reinvestigating the electroviscous effect occurring at the grain to grain contact the present analysis shows that the diffusivity of silica at the grain to grain contacts is likely to be relatively similar to that in the bulk pore water maybe 1 order of magnitude smaller but not more this contradicts the previous work by rutter 1976 which has been the key reference used in many subsequent papers to justify an extremely low value for the diffusivity of silica at the grain to grain contacts 5 orders of magnitude smaller than the diffusivity of silica in free water this finding has dramatic implications concerning the deformation rate of quartz sands and sandstones by pressure solution in sedimentary basins with regard to 1 the limiting step affecting the kinetics of the process diffusion of the solute or dissolution precipitation chemistry and 2 the existence of a thermodynamic equilibrium state when deformation by pressure solution occurs over geological timescales a poroviscoplastic model is used to describe deformation associated with pervasive pressure solution transfer in quartz sands this model is shown to be consistent with the current state of knowledge of the surface chemistry of silica in addition the comparison between this model and both laboratory and field data is rather good

the effect of oil emplacement on quartz cementation in a deeply buried sandstone reservoir quartz is an important porosity occluding cement in sandstone reservoirs that have been subjected to elevated temperature 80 c to 100 c for a substantial period of time the effect of oil emplacement on quartz cementation in reservoir sandstones is controversial some studies have concluded that early oil emplacement can inhibit quartz cementation leading to the preservation of porosity whereas other studies have concluded that quartz cementation appears largely unaffected by oil emplacement here we have studied shallow marine upper jurassic sandstones from ula field norwegian north sea with reservoir temperatures of approximately 150 c to determine whether oil emplacement had a significant impact on diagenesis with particular attention to quartz cementation following sedimentological description of cores samples above and below the oil water contact have been collected adjacent to core analysis plug points these samples then underwent a series of studies including petrographic point counting with a transmitted light microscope scanning electron microscopy sem backscattered electron microscopy sem cathodoluminescence microscopy and fluid inclusion studies these data were integrated with routine core analysis and petrophysical log data density and resistivity log data have been used to determine the precise oil saturation of each sample studied the distributions of all potential controls on porosity and permeability such as grain size sorting matrix clay content and degree of bioturbation the presence of grain coatings and dolomite cement and the amount of quartz cement have been assessed the presence of primary oil inclusions within quartz cement shows that oil ingress into the ula reservoir commenced prior to the onset of quartz cementation very fine grained matrix rich bioturbated microquartz cemented sandstones have uniformly low quartz cement contents irrespective of oil saturation medium grained graded matrix poor microquartz poor sandstones have quartz cement ranging from 1 to greater than 17 associated with core porosities of approximately 22 and 7 respectively higher oil saturations equate to higher porosities and permeabilities in the medium grained graded matrix poor microquartz poor sandstones which cannot be explained by any control other than the amount of quartz cement as a function of pore fluid type oil emplacement therefore appears to have inhibited quartz cementation at high oil saturations and can be viewed as an important control on reservoir quality the significance of this study is that the presence of oil in a sandstone reservoir at the time that quartz cement was growing can have a considerable impact on reservoir quality models that seek to predict quartz cement and reservoir quality in sandstones need to account for the timing of oil emplacement compared with other diagenetic processes

packing states of ideal reservoir sands insights from simulation of porosity reduction by grain rearrangement abstract the question being tackled in this study is to which extent grain rearrangement contributes to porosity reduction in very well sorted quartzose sands ideal reservoir sands a numerical model rampage an acronym of random packing generator has been developed to address this long standing problem rampage represents a synthesis of various algorithms designed to simulate packing of equal sized spheres which have been used to represent ideal solids liquids and gases as well as natural porous media the results of rampage simulations compare favourably to theoretical and experimental data from various disciplines and allow delineation of the field of gravitationally stable random packing of equal sized spheres in the 2 d state space of porosity p versus mean coordination number n three end member packing states have been identified random loose packing rlp p 45 4 n 5 2 random close packing rcp p 36 3 n 7 0 and bridged random close packing bridged rcp p 39 5 n 5 2 unlike previously proposed models rampage can simulate the transition from rlp to any other point in the stability field the rlp state is fully consistent with wet packed porosities of synthetic sands with lognormal mass size distributions reported in the literature the much higher in situ porosity values reported for modern air packed sands are unlikely to be preserved at depth on geological time scales data on the relation between intergranular volume and burial depth indicate that the observed intergranular volume reduction in the upper 800 m of the sediment column corresponds to the evolution of rlp to rcp and is thus fully explained by non destructive grain rearrangement

modeling circulation and seasonal fluctuations in perennially ice covered and ice walled lake untersee antarctica lake untersee antarctica is a freshwater perennially ice covered lake bounded along its north by the anuchin glacier the massachusetts institute of technology general circulation model used on a representative wedge shaped lake and actual bathymetry for lake untersee produces estimates for circulation and long term temperature and mixing trends modeled circulation is dominated by an anticyclonic gyre in front of the glacier with slower 3 mm s 1 flow exhibited around the lake s perimeter allowing effective mixing throughout most of the lake with time scales of one month estimated velocities bound maximal glacial flour particle size at 10 m for effective transport throughout the lake consistent with the sediment s mostly fine composition observed in field studies and mixing time scales mean nonuniformities in measured o2 concentration likely require recent or ongoing sources areas in which large temperature gradients prevent exchange of fluid demonstrate minimal mixing such as the lake s upper water layers and the anoxic basin in the south and circulation is consistently slowed in the northern sheltered bay area mean flow velocities fluctuate by about one fifth of their magnitude between summer and winter and the lake s almost homothermal body temperature varies by about one tenth of a degree over the same period while calculated temperature profiles qualitatively agree with field data the model s long term equilibrium temperature differs substantially likely due to poor description of heat transfer with the glacier model robustness tests show results differ by 10 when either grid scale or water temperature are halved

seismic observation of narrow plumes in the oceanic upper mantle 1 we have found seismic evidence for the existence of narrow about 200 km diameter and hot plumes in the upper mantle above the 410 km discontinuity they are located mainly under ocean islands not however under the central parts of major continents these results are obtained from comparing globally the observed travel time differences between ps conversions and ss precursors from the discontinuity at 410 km depth using published data and new observations from ocean island stations comparison of ss precursors and ps conversions however leads also to indications of significantly fewer narrow plumes in the upper mantle transition zone between 410 and 660 km depth than in the upper mantle above 410 km depth

field study on currents in a shallow ice covered lake a field study on current structure and circulation characteristics in lake vendyurskoe a small shallow icecovered lake in karelia russia is presented the current velocity magnitudes were generally found to be small the most pronounced currents had an oscillating character with velocity amplitudes on the order of millimeters per second the oscillation period obtained from spectral density calculations corresponded to that of a barotropic uninodal seiche the seichelike nature of the current oscillations was supported by the results from analysis of icelevel fluctuations giving identical periods and a phase shift of one fourth the period between the two types of oscillations mean currents measured during the winter were on the order of millimeters per second because lake vendyurskoe does not have any significant river inflow or outflow during winter the most probable cause of these currents is horizontal temperature pressure gradients scaling analysis indicated that these currents are geostrophic this was supported by theoretical estimates based on observed horizontal temperature gradients being of the same order as the observed currents the mean current velocities increased considerably after spring convection from 1 to several millimeters per second less