diagenesis and reservoir quality of the aldebaran sandstone denison trough east central queensland australia the lower permian aldebaran sandstone is the principal hydrocarbon reservoir in the denison trough bowen basin east central queensland australia it accumulated in a wide range of fluvio deltaic and nearshore marine environments detailed petrological study of the unit by thin section x ray diffraction scanning electron microscopy electron microprobe and isotopic analysis reveals a complex diagenetic history which can be directly related to depositional environment initial composition and burial temperature history early diagenetic effects included the precipitation of pyrite siderite and illite smectite rims o smow 8 9 to 11 3 deep burial effects included physico chemical compaction and the formation of quartz overgrowths siderite c pdb 34 0 to 11 5 o 0 7 to 22 7 illite illite smectite and ankerite c 9 3 to 4 9 o 7 6 to 14 4 involved fluids were in part connate meteoric water derived from compaction of the underlying freshwater reids dome beds important post maximum burial effects controlled by deep meteoric influx from the surface were ankerite and labile grain dissolution and formation of kaolinite o 7 8 to 8 9 d 115 to 99 calcite c 9 5 to 0 9 o 9 0 to 20 0 and dawsonite c 4 0 to 2 3 o 9 8 to 19 8 the formation of dawsonite reflecting eventual stagnation of the aquifer entrapment of contained hydrocarbons was a relatively recent event which may be continuing today reservoir quality varies from marginal to good in the west to poor in the east with predictable trends being directly linked to depositional environment and diagenesis

carbon dioxide reaction processes in a model brine aquifer at 200 c and 200 bars implications for geologic sequestration of carbon the reactive behavior of supercritical co2 under conditions relevant to geologic storage and sequestration of c is largely unknown experiments were conducted in a flexible cell hydrothermal apparatus to determine the extent of fluid rock reactions in addition to carbonate mineral precipitation that may occur in a brine aquifer aquitard system that simulates a saline aquifer storage scenario the system was held at 200 c and 200 bars for 59 days 1413 h to approach steady state then injected with co2 and allowed to react for another 80 days 1924 h in addition to magnesite precipitation silicate minerals quartz plagioclase microcline and biotite in the aquifer and the aquitard display textures etch pits mineralization indicating significant reaction changes in elemental abundances in the brine following addition of co2 include ph decrease and enrichment in cl partly due to supercritical co2 desiccation of the brine geologic sequestration systems have potential for geochemical reactions that extend beyond simple aqueous dissolution of co2 and precipitation of carbonate these reactions may produce geochemical and geotechnical consequences for sequestration and provide important characteristics for monitoring and evaluation of stored co2 a n understanding of multi phase equilibrium relationships between supercritical co2 and aquifer brine systems also raises new questions for a variety of geologic systems multi phase fluid equilibria may for example account for the large amounts and heterogeneous distributions of calcite cement in a wide variety of geologic systems particularly in sedimentary basin sandstones published by elsevier science ltd

from passive continental margin to mountain belt insights from analytical and numerical models and application to taiwan abstract the physics of deformation at plate boundaries is insufficiently understood taiwan is particularly interesting in this context since geological and geophysical arguments indicate that passive margin sediments were exhumed into an active mountain belt over a relatively short period of time classically taiwan has been described as a critical wedge orogeny in which deformation is entirely restricted to the crust more recent data however indicate that deeper parts of the lithosphere also participate in deformation in order to study the physical feasibility of various endmember hypothesis we have performed two dimensional 2 d thermo mechanical numerical experiments in which both mantle lithospheric and surface processes are taken into account and in which crustal deformation evolves as a function of mantle flow our setup consists of ocean ocean subduction followed by passive continental margin deformation results indicate that in most cases the subducting slab becomes close to vertical upon arrival of continental material to the trench whereas crustal rheology deformation and surface processes do not modify mantle flow significantly they are important for crustal scale processes relatively little crustal exhumation occurs if the crust is homogeneous and erosion rates are small significant exhumation in a dome like structure is obtained if a weak lower crust and large erosion rates are present the presence of an oceanic arc in the overriding oceanic lithosphere modifies but does not control crustal deformation synthetic metamorphic facies maps and thermochronological data computed from the numerical models are in reasonable agreement with observations in southern taiwan if exhumation occurs in a dome like structure further physical insight in the doming mechanisms is obtained by a combination of analytical techniques and crustal scale numerical models this analysis indicates that the dome like exhumation mechanism can be attributed to a compressional mechanical instability facilitated by both a weak lower crust and large rates of erosion both conditions are likely to be satisfied in taiwan

numerical models of salt diapir formation by down building the role of sedimentation rate viscosity contrast initial amplitude and wavelength formation of salt diapirs has been described to be due to upbuilding i e rayleigh taylor like instability of salt diapirs piercing through a denser sedimentary overburden or syndepositional down building process i e the top of the salt diapir remains at the surface all the time here we systematically analyse this second end member mechanism by numerical modelling four parameters are varied sedimentation rate nu sed salt viscosity eta salt amplitude delta of the initial perturbation of the sedimentation layer and thewavenumber k of this perturbation the shape of the resulting salt diapirs strongly depends on these parameters small diapirs with subvertical side walls are found for small values of nu sed and eta salt or large values of delta whereas taller diapirs with pronounced narrow stems build for larges values of nu sed and eta salt or small values of delta two domains are identified in the four parameter space which separates successful down building models from non successful models by applying a simple channel flow law the domain boundary can be described by the non dimensional law nu sedcrit c 1 1 2 delta 0 rho sed k 2 k 2 c2 where rho sed is the sediment density scaled by the density contrast delta rho between sediment and salt the wavelength is scaled by the salt layer thickness h salt and velocity is scaled by eta salt h salt 2 delta rho g where eta salt is the salt viscosity and g is the gravitational acceleration from the numerical models the constants c 1 and c 2 are determined as 0 0283 and 0 1171 respectively

petrographic and stable isotopic evidences of co2 induced alterations in sandstones in the lishui sag east china sea basin china abstract dawsonite bearing sandstones in the lishui sag east china sea basin china have been investigated as a natural occurrence for co2 induced diagenesis using petrography x ray diffraction and stable isotopes the sandstones are predominantly litharenite with the order of cements growth comprising clay coatings and pyrite siderite pyrite quartz overgrowths ankerite kaolinite and microcrystalline quartz dawsonite and late calcite dawsonite which has recently been considered as trapping mineral after massive co2 charging constituted up to 7 of the whole rock volume occurring as pore filling clusters or replacement of detrital grains stable isotopic data suggest that the co2 in equilibrium with dawsonite and calcite display a mantle magmatic origin and is in close proximity to those of the co2 gas now present in the reservoir indicating the same origin for both the elevated co2 concentrations induced by a mass influx of magmatic co2 into host rock was favourable for dawsonite cementation plagioclase was then considered as the precursor of dawsonite k feldspar could also serve as the aluminum source of dawsonite with an external supply of na in a geochemically open system

dynamics and implications of slab detachment due to ridge trench collision 1 the approach of a buoyant spreading ridge to a subduction zone may lead to detachment of a subducted slab previous work has called upon the detachment process as an explanation for observed ridge abandonment and slab window related magmatism e g in baja california but such a scenario has not been tested using fully dynamic numerical models we use dynamic two dimensional models including a non newtonian rheology to study the approach of a trench parallel spreading ridge to a subduction zone we find that before the ridge approaches within 100 km of the trench detachment of the subducted slab occurs due to the combined effects of increased buoyancy and reduced strength of the lithosphere with ridge proximity in models exploring effects of subducted slab length distance of the ridge from the trench shear zone strength and lithospheric yield strength we find the following 1 a decrease in subduction velocity as the ridge approaches the trench 2 a shrinking surface plate that maintains a uniform plate like subduction velocity 3 ridge abandonment distances 100 275 km from the trench and 4 slab gap distances 195 285 km from the trench these results are consistent with observations in baja california where detachment of the cocos slab may explain abandonment of observed segments of the east pacific rise 50 200 km outboard of the trench and the presence of nonarc related magmatism located 100 250 km inboard of the trench ridge subduction does not occur within any of these two dimensional cases of ridge trench collision indicating that 3 d geometric effects may be responsible for ridge subduction

testing the tracer ratio method for modeling active compositional fields in mantle convection simulations 1 tracer methods are attractive for modeling compositional fields because they offer the potential of zero numerical diffusion composition is typically taken to be proportional to the absolute local concentration of tracers but an increasingly popular method is to have dense and regular tracers with composition being equal to the local fraction of dense tracers this paper tests this ratio method using established benchmarks and by comparing the performance of the two tracer methods and grid based methods for simulating the long term evolution of a convecting mantle with a thick dense stable layer for this scenario the ratio method is found to have several advantages giving sharp stable long term layering with no tracer settling minimal statistical noise and low entrainment even with only 5 tracers per cell the method is equally applicable to finite volume and finite element treatments of the underlying flow entrainment in grid based advection methods is heavily dependent on resolution and numerical details and is reduced 1 order of magnitude by the filter proposed by a lenardic numerical determination of physically correct entrainment rates remains a challenging problem comparing tracer and grid based methods the spatial pattern of the thermal and chemical fields appear to be converging on the finest grids however the estimated entrainment differs significantly

numerical modeling study of mineralization induced by methane cold seep at the sea bottom abstract in order to investigate the response of authigenic minerals to gas hydrate geo systems the biogeochemical processes and its induced mineralization were predicted by employing the comprehensive reactive transport modeling approach based on the available data extracted from the northern continental slope area of the south china sea a 1 d vertical column model was developed three cases with different upward methane flux rates and three cases with different mineral compositions i e a total of six cases were designed to investigate the effects of variations in the depth of sulfate methane transition zone smtz and in the mineral composition on the formation of authigenic minerals the simulation results indicate that the smtz depth influenced by both the upward methane flux rate and the initial composition played an important role in the formation of authigenic minerals the aom reaction is intensive at the interface and the precipitation amount of calcite is large which is mainly controlled by aom when the methane leakage rate is 20 times higher than the base case aragonite starts to precipitate during the simulation oligoclase k feldspar smectite na smectite ca chlorite dissolved our study specific to this area as a starting point may provide a quantitative approach for investigating carbonate and pyrite formation in hydrate bearing sediments accounting for methane oxidation and sulfate reduction the method presented here and the model built in this study can be used for other sites with similar conditions in addition this study may serve as an indication for the potential natural gas hydrate reservoir in depth and is also significant for marine carbon and sulfur cycle

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

the influence of second scale convection on the thickness of continental lithosphere and crust abstract the effect of sublithospheric convection on the thickness and structure of the continental lithosphere is studied in numerical models assuming different rheologies newtonian non newtonian and temperature and pressure dependent heat fluxes and heating modes bottom versus internal heating the stagnant regions near the top of the models are identified with the lithosphere we distinguish a seismic and thermal lithosphere controlled by temperature an elastic lithosphere controlled by viscosity and a mechanical lithosphere controlled by strain rate strong lateral thickness undulations with a thick lithosphere above the downwelling regions and a thin lithosphere above the upwelling regions develop the thickness of the elastic lithosphere is about two thirds of the mechanical lithosphere while the ratio of the elastic to the thermal lithosphere varies between 0 4 and 0 8 the time dependence of some models with internal heating and a surface heat flow of 20 mw m 2 is characterized by long periods of the order of 1 ga close to steady state and short periods 100 ma of changing cell patterns and lithospheric thicknesses models showing thick lithospheric roots suggest that the mantle beneath some old shields may be associated with cold slowly downwelling convective flow rather than being firmly attached to the lithosphere the missing gravity and topography signals from such regions can be explained by elastic bending of the lithosphere and by dynamic adjustment of the moho observed lithospheric thickness variations in europe and their minimum to maximum spacing agree well with the models suggesting sublithospheric convection to be the cause compared to other observed features thickness undulations of the lithosphere seem to provide the strongest indications for sublithospherical convection relatively low viscosities in the lower crust may lead to a long term decoupling of surface topography from moho topography and mantle dynamics the moho depth is capable of adjusting to mantle stresses within relaxation times of the order of 100 ma while surface topography may flatten out such a mechanism may be important in regions with a deep moho such as fennoscandia or with a shallow moho such as the pannonian basin neither region showing a pronounced surface topography

evaluation of the impact of co2 co contaminant gas aqueous fluid and reservoir rock interactions on the geologic sequestration of co2 lowering the costs of front end processes e g separation in the geologic sequestration of co2 can dramatically lower the overall costs one possible approach is to sequester less pure co2 waste streams that are less expensive or require less energy to separate from flue gas or a coal gasification process etc the objective of this research is to evaluate the impacts of co2 itself as well as an impure co2 waste stream on geologic sequestration using reaction progress models reactive transport simulators and analogous reactive transport experiments run in a plug flow reactor specifically we are investigating the potential for co injecting the sox nox and h2s present in coal fired waste streams along with the co2 we present here the results of reactive transport simulations made to investigate the long term impact of dissolved co2 h2s and so2 on carbon sequestration in the frio fm tx the results suggest that addition of relatively large amounts of h2s to a co2 injection should not adversely impact injectivity or sequestration compared to the injection of co2 alone the co injection of so2 however could produce significantly different results if conditions are such that so2 can oxidize to sulfate in the long term the simulations suggest that carbonate mineral formation can sequester a significant fraction of the injected carbon through the formation of calcite magnesite and dawsonite

predictive modelling of structure evolution in sandbox experiments in this paper a computational approach is presented that is able to forward model complex structural evolution with multiple intersecting faults that exhibit large relative movement the approach adopts the lagrangian method complemented by robust and efficient automated adaptive meshing techniques a constitutive model based on critical state concepts and global energy dissipation regularized by inclusion of fracture energy in the equations governing state variable evolution the efficacy of the approach is benchmarked by forward simulation of two extensional analogue experiments that exhibit the development of a roll over anticline with a series of superimposed crestal collapse graben systems these sandbox experiments are excellent benchmarks for computational models as both intersecting localisations with different rates of slip and large relative movement on localisations must be represented while other complex phenomena associated with structural evolution over geological timeframes are not present

a comparison of numerical surface topography calculations in geodynamic modelling an evaluation of the sticky air method summary calculating surface topography in geodynamic models is a common numerical problem besides other approaches the so called sticky air approach has gained interest as a free surface proxy at the top boundary the often used free slip condition is thereby vertically extended by introducing a low density low viscosityfluid layer this allows the air crust interface to behave in a similar manner to a true free surface we present here a theoretical analysis that provides the physical conditions under which the sticky air approach is a valid approximation of a true free surface two cases are evaluated that characterize the evolution of topography on different timescales 1 isostatic relaxation of a cosine perturbation and 2 topography changes above a rising plume we quantitatively compare topographies calculated by six different numerical codes using finite difference and finite element techniques using three different topography calculation methods i direct calculation of topography from normal stress ii body fitting methods allowing for meshing the topography and iii lagrangian tracking of the topography on an eulerian grid it is found that the sticky air approach works well as long as the term st ch hst l 3 is sufficiently small where st and hst are the viscosity and thickness of the sticky air layer and ch and l are the characteristic viscosity and length scale of the model respectively spurious lateral fluctuations of topography as observed in some marker based sticky air approaches may effectively be damped by an anisotropic distribution of markers with a higher number of markers per element in the vertical than in the horizontal direction

factors controlling normal fault offset in an ideal brittle layer we study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two dimensional elastic plastic brittle layer floating on an inviscid fluid the yield stress of the layer is the sum of the layer cohesion and its frictional stress faults are initiated by a small plastic flaw in the layer we get finite fault offset when we make fault cohesion decrease with strain even in this highly idealized system we vary six physical parameters the initial cohesion of the layer the thickness of the layer the rate of cohesion reduction with plastic strain the friction coefficient the flaw size and the fault width we obtain two main types of faulting behavior 1 multiple major faults with small offset and 2 single major fault that can develop very large offset we show that only two parameters control these different types of faulting patterns 1 the brittle layer thickness for a given cohesion and 2 the rate of cohesion reduction with strain for a large brittle layer thickness 22 km with 44 mpa of cohesion extension always leads to multiple faults distributed over the width of the layer for a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault when the offset is very large the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes

free surface formulation of mantle convection i basic theory and application to plumes in mantle convection models the top surface is traditionally approximated as a free slip boundary and the dynamic topography is obtained by assuming that the normal stress on the free slip boundary is compensated instantaneously through surface deformation it has already been shown that this approximation is valid for long wavelength topography based on both viscous and viscoelastic models with a free surface we have found that the characteristic time for topographic growth is comparable to the timescales of mantle convection 10 6 year for short and intermediate wavelengths 10 3 km or less and or a high effective lithospheric viscosity 10 24 pa s this suggests that the topography is history dependent under these conditions and that a free surface formulation is required to study the topography at these wavelengths we have developed a new eulerian finite element technique to model a free surface since the technique is based upon an undeformable eulerian grid this enables us to study long term free surface dynamics in the presence of evolving buoyancy we have compared numerical with analytic solutions of viscous relaxation for fixed buoyancy problems as long as the magnitude of topography is much smaller than the wavelength we find that the finite element method is very accurate with relative errors of less than 1 per cent this numerical technique can be applied to a variety of geophysical problems with free surfaces in applying this technique to dynamic models of mantle plumes we find that surface relaxation retards the topography at intermediate and short wavelengths and produces a smoother topography compared with topography from free slip calculations this reduced topography has a significant influence on the geoid at the corresponding wavelengths moreover free surface models by allowing vertical motion on the free surface yield a hotter lithosphere over ascending plumes than models with free slip boundaries

an experimental study of co2 oil brine rock interaction under in situ reservoir conditions to understand the mineralogical and chemical changes in oil bearing reservoirs e g depleted oil reservoirs during massive co2 injection we have carried out a core flooding experimental study of co2 oil brine rock interactions under a simulated reservoir condition of 100 c and 24 mpa the experimental condition is based on field data from a co2 eor project in the southern songliao basin this oil bearing co2 flooding experiment used the same experimental setup reservoir conditions and workflow as the oil free experiment reported by yu et al 2012 the sandstone core samples used in the experiment have similar mineralogical compositions as that used in the previous experiment compared with the oil free experiment the presence of oil appears to substantially reduce the reaction degree between the co2 fluid and some sensitive minerals the dissolution rates of the k feldspar and carbonate minerals for the oil bearing experiment are 1 5 and 1 4 of that for the oil free experiments respectively for the silicate minerals represented by the k feldspar the presence of oil mainly delays the dissolution during the experiment and reduces the equilibrium dissolution rate for the carbonate minerals the presence of oil appears to primarily affect the dissolution at the beginning of the experiments and reduce the maximum dissolution rate attained the core permeabilities for the oil free and oil bearing cases are both reduced after experiments the reduction in permeability is probably due to the precipitation of fine siliceous mineral and clay particles released by the dissolution of the carbonate cement which may clog some pore throats the results provide some new insights on the fluid rock interaction during co2 injection in depleted oil reservoirs or during co2 eor

on topography and geoid from 2 d stagnant lid convection calculations 1 temperature dependent convection in the stagnant lid regime with a frank kamenetskii and arrhenius viscosity formulation are compared when properly scaled the nusselt number and root mean square velocity of the fluid for the two viscosity formulations are similar but the surface stresses and hence predicted dynamic topography and geoid differ significantly the arrhenius viscosity formulation results are insensitive to a viscosity cutoff at the high viscosity limit as long as the cutoff is 104 times the basal viscosity the condition number for stagnant lid convection matrices using a penalty formulation is smaller than the penalty number times viscosity contrast value a reasonable estimate of the condition number explaining why large viscosity contrast convection problems with the penalty method agree with other formulations and analytic solutions the difference in magnitude between the maximum and minimum dynamic topography could be used to constrain the magnitude of the viscosity contrast across the stagnant lid in planetary bodies

computational approaches to studying non linear dynamics of the crust and mantle we outline a mathematical formulation for mantle convection which can deal with the viscoelastic plastic rheology of the cool parts of the lithosphere this formulation is then analyzed to expose the numerical challenges inherent in the equations and a suitable solution strategy is outlined with this strategy in place we discuss a parallel implementation explaining how we maintain computational efficiency in a tiered and modular software environment we show two examples from geodynamics which demonstrate the capability of the numerical method and its implementation c 2007 elsevier b v all rights reserved

a benchmark comparison for mantle convection codes summary we have carried out a comparison study for a set of benchmark problems which are relevant for convection in the earth s mantle the cases comprise steady isoviscous convection variable viscosity convection and time dependent convection with internal heating we compare nusselt numbers velocity temperature heat flow topography and geoid data among the applied codes are finite difference finite element and spectral methods in a synthesis we give best estimates of the true solutions and ranges of uncertainty we recommend these data for the validation of convection codes in the future

using natural co2 reservoir to constrain geochemical models for co2 geological sequestration abstract numerical modeling of geochemical transport processes is necessary to investigate long term co 2 storage in deep saline formations because aluminosilicate mineral alteration is very slow under ambient deep formation conditions and is not amenable to experimental study geochemical transport modeling can solve many problems and answer questions related to co 2 geological sequestration the numerical modeling provides valuable insights regarding the physical and chemical consequences of co 2 injection in the subsurface environment however the reliability and applicability of the models need to be tested and validated if they are applied for co 2 geological sequestration issues on model validations are important if co 2 injection technologies are to be implemented safely efficiently and predictably validation of geochemical transport models could be different from conventional model validation methods for groundwater flow and solute transport for the short term behaviors the models can be validated using laboratory and field experiments for the long term mineral alteration and co 2 sequestration the natural analogue using high pressure co 2 reservoirs could be a best way to validate the model in this paper a natural co 2 reservoir in southern songliao basin of china which is past accumulations of co 2 in geological formation associated with magmatic or volcanic activity was selected although the length of co 2 exposure and hence the rates of reaction for the natural system is not known in detail we have shown that it is indeed possible to use observation data of mineral alteration in the natural co 2 reservoir to constrain thermodynamic and kinetic data of minerals used in the model and to confine conditions of temperature pressure salinity and primary mineral composition

the role of boundary conditions in numerical models of subduction zone dynamics abstract numerical studies of subduction zone dynamics on a regional scale usually use a limited spatial extent for their models and therefore need to define boundary conditions on all model edges these boundary conditions imply a choice for the mechanical and thermal state of the surrounding regions which may influence the evolution of the model system we investigate the role of the surface and lateral boundary conditions for simple mechanical subduction models using a standard finite element method we find that subduction is only possible if the slab can decouple from the surface this decoupling can be achieved by a true free surface a free slip surface with a weak crust for the subducting plate or a free slip surface with a layer of low viscosity low density material sticky air between the model surface and the crust models of slab dynamics that employ a free slip surface reproduce trench migration slab sinking velocities and slab geometry of models with a free surface as long as they use either a weak crust which can be viscous viscoelastic and or brittle or a sticky air layer the vertical topography will however not be reproduced for free slip models without a sticky air layer for ocean ocean convergent models we find that the application of inflow boundary conditions at the edges of the subducting or overriding lithosphere controls trench motion and the geometry of the subducting slab inflow on the overriding side causes trench retreat and a slab resting on the lower mantle whereas inflow restricted to the subducting side can cause trench advance and a slab which folds on the lower mantle

petrology and isotopic geochemistry of dawsonite bearing sandstones in hailaer basin northeastern china abstract the co2 gas reservoir sandstones in the hailaer basin contain abundant dawsonite and provide an ideal laboratory to study whether any genetic relationship exists between dawsonite and the modern gas phase of co2 the origins of dawsonite and co2 in these sandstones were studied by petrographic and isotopic analysis according to the paragenetic sequence of the sandstones dawsonite grew later than co2 charging at 110 85 ma the dawsonite 18o value is 7 4 smow and the calculated 18o values of the water present during dawsonite growth are from 11 4 to 9 2 smow this combined with the nahco3 dominated water linked to dawsonite growth suggests meteoric water being responsible for dawsonite growth the 13c values of gas phase co2 and the ratios of 3he 4he of the associated he suggest a mantle magmatic origin of co2 rich natural gas in hailaer basin dawsonite 13c values are 5 3 to 1 5 average 3 4 and the calculated 13c values of co2 gas in isotopic equilibrium with dawsonite are 11 4 to 7 3 these c isotopic values are ambiguous for the dawsonite c source from the geological context the timing of events together with formation water conditions for dawsonite growth dawsonite possibly grew in meteoric derived water atmospherically derived co2 maybe or at least the dominant c source for dawsonite it seems that there are few relationships between dawsonite and the modern gas phase of co2 in the hailaer basin

continental scale magmatic carbon dioxide seepage recorded by dawsonite in the bowen gunnedah sydney basin system eastern australia abstract dawsonite naalco3 oh 2 is widespread as a cement replacement and cavity filling in permo triassic sedimentary rocks of the bowen gunnedah sydney basin system eastern australia the origin of dawsonite in these rocks was studied by petrographic and stable isotope analysis dawsonite d13c pdb values range from 4 0 to 4 1 and are remarkably consistent throughout the bowen gunnedah sydney basin system these values indicate either a marine carbonate or magmatic source for carbon in the dawsonite a magmatic carbon source is considered more likely on the basis that 1 evidence of and the cause for widespread marine carbonate dissolution in the sedimentary successions are not apparen 2 dawsonite is widespread in both marine and nonmarine facies 3 the region has been the site of major igneous activity 4 other dawsonite deposits of similar carbon isotopic composition are linked to igneous activity and 5 magmatic co2 accumulations are known in parts of the bowen gunnedah sydney basin system the timing of igneous activity in the bowen basin constrains the timing of dawsonite formation in the bowen gunnedah sydney basin system to the tertiary consistent with textural relationships which indicate that dawsonite formed late during the burial history of the permo triassic sequences the distribution and interpreted origin of dawsonite implies magmatic co2 seepage in the bowen gunnedah sydney basin system on a continental scale

effects of elasticity on the rayleigh taylor instability implications for large scale geodynamics summary although parts of the lithosphere may be expected to behave elastically over certain timescales this effect is commonly ignored in models of large scale mantle dynamics recently it has been demonstrated that elasticity and in particular viscoelasticity may have a significant effect on the buckling instability and on the creation of lithospheric scale shearzones it is however less clear whether elasticity also has an effect on mantle convection and density driven lithospheric instabilities the focus of this work is therefore to study the effects of elasticity on the twolayer rayleigh taylor rt instability consisting of a maxwell viscoelastic layer overlying a viscous layer of lower density we analyse this problem by performing systematic numerical simulations that are compared with newly derived analytical solutions it is demonstrated that elasticity can be important for certain parameter combinations it leads to a speedup of the rt instability the cause for this speedup is that the rt instability is only sensitive to the viscous fraction of deformation in the viscoelastic layer elasticity reduces the viscous fraction of deformation at timescales shorter than the maxwell relaxation time t m t m g where is the viscosity and g the elastic shear module for plate tectonics on earth the parameters are such that the effect of elasticity on instability growth is negligible for most boundary conditions whereas elasticity does not or only slightly change the timescales for lithospheric detachment of the upper mantle it does significantly alter the response and stress build up in the overlying crust numerical simulations illustrate this effect for lithospheric detachment and show that peak stresses in a viscoelastic crust are smaller than stresses that develop in a viscous crust moreover if the timescale for delamination of the mantle lithosphere is equal or smaller than the maxwell relaxation time of the crust the topography of the crust is increased compared to viscous models

styles of crustal deformation in compressional orogens caused by subduction of the underlying lithosphere abstract crustal scale deformation is calculated for models in which the driving mechanism corresponds to the asymmetric detachment and underthrusting of the underlying mantle lithosphere the plane strain finite element model results provide indications of the styles of deformation to be expected in small compressional orogens in particular these styles occur where shortening of the mantle lithosphere is achieved by the nearly rigid convergence between lithospheric mantles and the subduction of one mantle beneath the other the crust is modelled using coulomb plastic frictional and thermally activated power law viscous rheologies and the effects of compositional layering and variable geothermal gradients are included results are presented for a range of models in which the strength of the coupling between the model crust and its basal boundary surface denudation partial and total subduction of the crust and compositional layering are examined the results show the development of inclined step up shear zones which are a consequence of conjugate thrusting in regions of coulomb controlled rheology these zones link to sub horizontal shear zones which occur where composition and temperature render the crust viscously weak the model strain fields are interpreted in terms of deformation on discrete planes and the seismic reflectivity fabric that may be associated with this discrete deformation finally we ask whether similar seismic reflectivity fabric can be recognized in observations from small compressional orogens

diapirism and topography summary a new numerical technique using markers and a deformable lagrangian mesh is used to study the deformation of the surface above a rising diapir this method allows modelling of a free surface boundary in a self consistent way the method makes it possible to investigate many different problems with non regular geometry the codes were verified through comparison with analytical solutions for the initial stages and with other numerical codes for mature stages our simulations led to the following conclusions 1 the growth rate of the diapir is more strongly influenced by the viscosity contrast than the wavelengths this is due to the strong growth rate difference during the initial stage 2 the maximum elevation above the diapir linearly increases with increasing wavelength and is approximately the same for different viscosity contrasts 3 the elevation will be considerably higher for a low viscosity diapir at a given depth than for an isoviscous diapir comparisons for different thicknesses of the buoyant layer showed that the highest topography is produced when the two layers have equal thickness we showed the dependence of the topographic behaviour on the parameter r the ratio of the density difference between two layers to the density of the upper layer it was found that topography behaves linearly up to r 0 15 it indicates that a posteriori estimation of topography for traditional calculations using the free slip boundary condition works well within this limit

genesis of authigene carbonate minerals in the upper cretaceous reservoir honggang anticline songliao basin a natural analog for mineral trapping of natural co2 storage abstract dawsonite naalco 3 oh 2 as a trace mineral of co 2 is abundant in the upper cretaceous reservoir of the honggang anticline southern songliao basin petrologic and isotopic geochemical characteristics have been investigated by a polarizing microscope a scanning electron microscope sem x ray diffraction xrd cathode luminescence cl fluid inclusions and carbon oxygen isotopic analysis the diagenetic sequence indicated that dawsonite late generation microcrystalline quartz late generation kaolinite late generation calcite and ankerite was formed after co 2 flooding meanwhile as a function of analysis on carbon oxygen isotope the mantle magmatic co 2 provided a carbon source for the precipitation of dawsonite late generation calcite and ankerite the three kinds of carbonate minerals mentioned earlier grew in a progressively evolved condition using a basic thermodynamic fraction equation the total quantity of natural co 2 captured by dawsonite bearing sandstones per cubic meter was calculated to be 99 51 kg

gas water rock interactions in frio formation following co2 injection implications for the storage of greenhouse gases in sedimentary basins to investigate the potential for the geologic storage of co2 in saline sedimentary aquifers 1600 t of co2 were injected at 1500 m depth into a 24 m thick sandstone section of the frio formation a regional brine and oil reservoir in the u s gulf coast fluid samples obtained from the injection and observation wells before co2 injection showed a na cacl type brine with 93 000 mg l total dissolved solids tds at near saturation with ch4 at reservoir conditions following co2 breakthrough samples showed sharp drops in ph 6 5 5 7 pronounced increases in alkalinity 100 3000 mg l as hco3 and fe 30 1100 mg l and significant shifts in the isotopic compositions of h2o dissolved inorganic carbon dic and ch4 geochemical modeling indicates that brine ph would have dropped lower but for the buffering by dissolution of carbonate and iron oxyhydroxides this rapid dissolution of carbonate and other minerals could ultimately create pathways in the rock seals or well cements for co2 and brine leakage dissolution of minerals especially iron oxyhydroxides could mobilize toxic trace metals and where residual oil or suitable organics are present the injected co2 could also mobilize toxic organic compounds environmental impacts could be major if large brine volumes with mobilized toxic metals and organics migrated into potable groundwater the d 18 o values for brine and co2 samples indicate that supercritical co2 comprises 50 of pore fluid volume 6 mo after the end of injection postinjection sampling coupled with geochemical modeling indicates that the brine gradually will return to its preinjection composition

structural evolution of a three dimensional finite width crustal wedge abstract we present the results of three dimensional numerical experiments designed to study the response of a layer of crustal material subjected to convergence through an imposed basal velocity discontinuity and to surface processes erosion sedimentation we focus on the three dimensional response of the system arising from the finite width of the imposed velocity discontinuity in particular we describe the complex structures that develop around the wedge and their interactions with the loading unloading produced by the surface processes we show that the pro and retro shear zones that develop in a doubly vergent two dimensional wedge curve around the end of the velocity line discontinuity to merge into the strike slip structure that naturally develop i e as a consequence of the imposed boundary conditions along the edge of the wedge along the retro shear zone the stress orientation rotates along a vertical axis which implies that the retro shear zone is a pure thrust along all of its curved length whereas along the pro shear zone stresses rotate along a horizontal axis which in turn implies that the pro shear zone progressively evolves towards an oblique thrust in its curved section furthermore the outward motion i e perpendicular to the direction of imposed shortening along the curved section of the retro shear zone is accommodated by oblique extension along a secondary kinked structure antithetic to the retro shear zone we also show the complex evolution of the wedge when ductile flow and ductile strain softening is activated by increasing the imposed basal temperature in this situation the wedge is broader as structures develop at finite distances on either side of the line discontinuity and its dynamics resembles more that of a vise like orogen at the surface a flat plateau forms that accumulates sediment from the surrounding actively deforming mountain ranges until a channel breaks through one of the sides and flushes the inward draining basin of its sedimentary content the complex numerical models provide a detailed insight into the geometry and complex three dimensional structures within and at the margins of an orogen and how they can be affected by erosional processes at the surface

anelasticity explains topography associated with basin and range normal faulting topography associated with normal faulting in the basin and range western united states is usually modeled as a flexure of a broken elastic plate however modeled effective rigidities are usually 100 times lower than the rigidity deduced from upper crustal thickness this discrepancy may be related to a significant anelastic deformation which we explore through numerical modeling because experimental rock rheology evidences a pressure dependent yield stress beyond the elastic limit in crustal rocks we made a finite element model that accounts for such a crustal rheology and also for the frictional behavior of an embedded high angle fault resulting topography after extension is similar to that obtained from a thin elastic plate model however the corresponding strain pattern differs first the footwall rotates in a rigid fashion over a width of 15 km that matches the typical size of uplifted basin and range blocks second hanging wall subsidence results from a significant horizontal extension and rotation of the crust we suggest that plastic deformation in the deep part of the footwall could trigger the development of a new high angle fault with a fault spacing that matches basin and range structure

laboratory experiment and numerical simulation on authigenic mineral formation induced by seabed methane seeps abstract in natural gas hydrate marine settings cold seepage of methane fluid is a widely observed phenomenon where authigenic minerals serve as an indication of potential gas hydrate bearing reservoirs at depth in this study based on the data from the site hd196 near dongsha island northern continental slope of south china sea laboratory experiments and numerical simulation studies were conducted to investigate the biogeochemical processes of authigenic mineral formation induced by methane seepage the bioreactor experimental results show that in response to methane flux ph increased to 8 5 after 20 days of reaction and eh declined rapidly first and then remained unchanged at about 100 mv the decrease in so42 hs and hco3 concentrations indicated the occurrence of anaerobic oxidation of methane coupled with sulfate reduction aom sr the depletion of fe2 implied the formation of iron bearing minerals e g iron sulfides carbonate minerals were also identified in the experimental phase of this study most iron sulfides existed as massive pieces and in some cases as spherical or rod shape pieces the calcium carbonates were observed as blocky pieces numerical simulations were also performed to reproduce the biogeochemical reactions that occurred in the reactor experiments based on experimental data kinetic parameters associated with the observed reactions were calibrated the model simulated results are general consistent with those obtained by the experiments conducted in this study the combination of simulation and experimental studies provided a powerful tool to investigate the biogeochemical processes in the methane leakage environment at different temporal and spatial scales this study gave a new perspective to understand the formation of cold seep authigenic minerals in marine sediments and was significant for future investigations on the effects of hydrate decomposition

mineralogical and geochemical consequences of the long term presence of co2 in natural reservoirs an example from the springerville st johns field arizona and new mexico u s a abstract the springerville st johns co2 field in eastern arizona and western new mexico is one of more than a dozen gas fields developed within the colorado plateau and southern rocky mountain region extensive travertine caco3 deposits record a long history of co2 migration and leakage to the atmosphere the oldest travertine deposits may have formed during the initial filling of the co2 reservoir when groundwaters exsolved co2 upon reaching the surface the youngest travertine deposits are associated with springs on the floor of the little colorado river valley but travertine deposition appears to be insignificant today older deposits occur up to 325 m above the valley floor geologic relationships suggest travertine deposition began in the late pleistocene after volcanic activity ended at 0 3 ma most of the caco3 could have been derived from dissolution of the underlying limestones and dolomites interactions between the reservoir fluids and rocks were observed in core samples from one of the intervals that produced dry gas these reactions resulted in the dissolution of carbonate cements and detrital feldspars and the formation of dawsonite and kaolinite geochemical simulations suggest that the dawsonite could have been deposited when the co2 fugacity reached 20 bars and that the kaolinite formed as the co2 fugacity decreased corrosion of drill pipe by acidic waters and a pronounced hco3 anomaly above the co2 reservoir provide evidence of a continuing flux of co2 from depth co2 storage occurs primarily as dissolved carbonate species and as gas accumulations only a small percentage of the co2 was sequestered in secondary minerals

computer simulation of co2 trapped through mineral precipitation in the rose run sandstone ohio abstract equilibrium path of reaction and kinetic modeling of co2 brine mineral reactions in the rose run sandstone one of ohio s deep saline aquifers was conducted in order to investigate the factors that are likely to influence the capacity of this formation to trap injected co2 as solid carbonate mineral phases equilibrium modeling was applied to investigate the impact of temperature pressure mineralogy brine composition and co2 fugacity on mineral dissolution and precipitation the amount of co2 sequestered and the form of sequestration path of reaction and kinetic modeling were used to evaluate intermediate products and reaction progress as a function of time as well as to investigate the impact of brine to rock ratio the results of equilibrium modeling demonstrate that dissolution of albite k feldspar and glauconite and the precipitation of dawsonite and siderite are potentially very important for mineral trapping of co2 according to the path of reaction and kinetic modelling the stability of carbonate rocks is controlled by the brine to rock ratio the ph of the system the fugacity of co2 and the kinetic rate of dissolution in kinetic modeling with a brine to rock ratio of 1 25 reactive surface area of 10 cm2 g 10 mpa f co 2 and 12 porosity 1 5 2 g of co2 per kg of reacted rock significant quantities 10 40 g of carbonate minerals were precipitated from both the sandstone and mixed rock assemblages the rose run sandstone has the potential to store co2 over millennia as a negatively buoyant aqueous solution and ultimately as immobile carbonate minerals

numerical simulation of rayleigh taylor instability for single and multiple salt diapirs abstract the diapiric ascent of light rocks through a denser oberburden is simulated using a new numerical method based in part on finite differences rocks are modelled as viscous newtonian liquids and the internal interfaces are located with the help of passive markers these markers are relocated using a new algorithm equivalent to a cellular automaton rule this allows the markers to be redistributed very rapidly this new method appears to be well adapted to quickly resolving geological problems simulations of the growth of a single dome are performed under various viscosity ratios for the salt and the overburden the effect of asymmetrical initial deformations of the salt layer is to produce diapiric structures that slowly return to symmetry as the diapir matures and rises in an isotropic medium simulations involving continuous sedimentation of the overburden are also performed an initial geometry on a basin margin in which multiple diapirs eventually develop is studied diapirs grow relatively rapidly near the centre while on the margins the salt layer flows upward following the basement simulations with salt that is more viscous than sediments show the development of previously unnoticed asymmetrical structures

effects of strongly temperature dependent viscosity on time dependent three dimensional models of mantle convection numerical simulations of thermal convection in a wide 8 8 1 cartesian box heated from below with temperature dependent viscosity contrasts of 1000 and rayleigh number 105 show that boundary conditions and aspect ratio have an enormous effect on the preferred flow pattern with rigid upper and lower boundaries spoke pattern flow with small diameter 1 5 cells is obtained consistent with laboratory experiments and previous numerical results however with the arguably more realistic stress free boundaries the flow chooses the largest possible wavelength forming a single square cell of aspect ratio 8 with one huge cylindrical downwelling surrounded by upwelling sheets the addition of stress dependence to the rheology weakens the stiff upper boundary layer resulting in smaller cells though still with upwelling sheets and downwelling plumes

polydiapirs multiwavelength gravity structures polydiapirs domes in domes result from the evolution of a multiwavelength rayleigh taylor gravitational instability the present work investigates the initiation and development of polydiapirs by means of two dimensional finite difference models of gravitationally unstable triple layered sequences of newtonian fluids the values of viscosities thicknesses and densities were systematically varied for selected examples in triple layered sequences only two types of density stratification are likely to result in multiwavelength instabilities that will eventually develop into polydiapiric structures the limiting factors and evolution of such structures were studied for both these types of density stratifications finite difference calculations on models with physical parameters appropriate to evaporitic sequences and covered by a viscous clastic overburden showed that a very small downward density decrease in the evaporitic source may cause an internal overturn that will compete with the main overturn between source and overburden this may lead to the development of sequential or simultaneous polydiapirs the mature polydiapirs in the models exhibit spines curtain folds pendant repetitions of the source layers inside the large diapirs and anomalous interdiapiric distances all these features are known in natural salt diapirs suggesting that polydiapirs may be far more common in nature than hitherto recognized

an arbitrary lagrangian eulerian formulation for creeping flows and its application in tectonic models summary this paper presents and analyses numerical techniques developed to investigate viscoplastic stokes flows within a model of lithospheric deformation in particular the techniques are related to a subduction model of compressional orogens the driving mechanism in the model corresponds to the near rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust the numerical techniques use the arbitrary eulerian lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving eulerian finite element grid that conforms to the material domain a regridding algorithm allows the associated lagrangian motion and fields to be followed and in addition coupled back to the eulerian calculation of the flow mass flux boundary conditions are used so that the effects of erosion and deposition by surface processes and mass loss by subduction can be included in the model calculation the evolving model crustal layer is flexurally compensated using a general elastic beam formulation the applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed simple flows a linear viscous subduction model a whirl flow and a quasi convection model are used to show that the mass conservation regridding and surface tracking errors are small the broader applicability of the modelling techniques is reviewed

modeling the behavior of the continental mantle lithosphere during plate convergence in studying orogenic processes the mechanism of viscous rayleigh taylor type removal of gravitationally unstable lithosphere is often invoked to explain the behavior of the mantle lithosphere using numerical models we consider this mechanism and explore alternate styles of deep lithospheric deformation during tectonic convergence the numerical experiments incorporate a mix of viscous and plastic rheologies to model the mechanical evolution of the lithosphere asthenosphere system our results suggest that there are a number of deformational modes of the model mantle lithosphere 1 a dripping or rayleigh taylor type instability 2 an asymmetric underthrusting or subduction 3 symmetric ablative plate consumption 4 slab breakoff the failure and detachment of the strong lithosphere and 5 mixed modes with combinations of these processes the development of the modes is controlled by the rate of convergence associated with the background tectonic regime the density field and the rheology of the mantle lithosphere it is important to determine whether these modes occur in the earth beneath collisional orogens