a foray into false positive results in mineral dissolution and precipitation studies abstract this study presents and discusses several case studies of false positive results in mineral rock dissolution and precipitation studies and the lessons that can be learned from them

a model for fibrous illite nucleation and growth in sandstones we have developed a model for the formation of fibrous illite in sandstones where kaolinite is a primary reactant and potassium is derived from in situ k feldspar grain dissolution or imported into the model reference frame illite fiber nucleation and growth are modeled using arrhenius expressions that consider saturation state in addition to temperature and time nucleation occurs on pore walls and muscovite and detrital illite may be defined as energetically favorable substrates the model is integrated with other touchstone models to account for the influence of other diagenetic processes on surface area and reactant volumes and to provide input for permeability simulations we evaluated the illite model performance on two data sets 1 jurassic quartzose samples from offshore mid norway with maximum temperatures ranging from 108 to 173c 226 to 343f and 2 miocene lithic samples from offshore southeast asia that have maximum temperatures ranging from 157 to 182c 315 to 360f the model matches measured abundances of illite kaolinite and k feldspar in both data sets using identical kinetic parameters predicted k ar ages are consistent with available data given uncertainties associated with detrital contaminants although no illite particle size data are available from the analyzed samples modeled crystallite thicknesses from the norway data set are comparable to published measurements of 0 004 to 0 012 m from north sea samples with similar temperature histories

hydrogeochemical characterization of oilfield waters from southeast maracaibo basin venezuela diagenetic effects on chemical and isotopic composition free water and water in emulsion with oil from hydrocarbon wells onshore offshore lake maracaibo were extracted and analyzed the use of classical and new chemical diagrams evidenced that na hco3 and na cl are the main chemical facies whereas na so4 facies was rare and revealed only in faulted areas in which shallow groundwater was involved in deep circulation cretaceous ca cl waters were not revealed but their imprint on some samples was traced and evidenced by langelier ludwig brine differentiation and caexcess vs nadeficit diagrams these square plots allowing a discrimination between the water contributions from the different source rocks should be more exploited as hydrogeochemical tools in hydrocarbon exploration original data on monocarboxylic anions showed acetate concentration higher than ten times that of formate at sampling conditions but speciation under deep conditions indicated a reversed activity dominance calculation of saturation indexes and kinetic modeling showed an active precipitation of chalcedony kaolinite and clays especially where the temperature was lower t 100 c formation waters and lake water showed quite similar chloride concentration and hydrogen isotope composition however thermodynamic calculations at the oilfield p t conditions revealed chlorite and illite produced during previous diagenetic stages of misoa sandstones recrystallization to smectite and vermiculite in present day stage the diagenetic water produced in these reactions could have enriched the 18o isotopic composition of approximately 2 this isotope diagenetic effect was more evident in na hco3 waters which also showed a decrease in chloride at constant boron concentration as a consequence of clay dewatering in this regard a new boron geothermometer useful for formation waters is also proposed t c 5 1364 ln 1 b mg 1 5 728 0 180 0 164 0 012

evidence for deep anaerobic biodegradation associated with rapid sedimentation and burial in the beaufort mackenzie basin canada abstract formation waters of the 14 km thick late cretaceous cenozoic beaufort mackenzie basin were examined as part of a larger project to better understand the petroleum potential of the region where early exploration defined petroleum reserves of 744 109 bbls recoverable crude oil and 11 74 tcf gas historical water analyses 2583 samples from 250 wells drilled up to 5 km depth were compiled and culled to remove incomplete and poor quality samples the resultant database shows a broad range of salinity and water chemistry that has no systematic relationship with depth three main water types are defined paleo seawater and freshwaters related to a miocene age gravity driven flow system and low tds high alkalinity waters high alkalinity waters are isolated in overpressured fault blocks that were rapidly buried by post miocene iperk shale deposition the high alkalinities up to 9000 mg l are interpreted to be related to in situ co2 generation through anaerobic methanogenesis in response to freshwater invasion the dominant control on biogenic gas generation appears to be maximum burial temperature rather than the modern temperature distribution this is consistent with the paleopasteurization model that suggests once critical burial temperatures are reached sterilized rocks are inhibited from further biodegradation even when temperatures subsequently drop back into the habitable zone

mass transfer and problems of secondary porosity creation in deeply buried hydrocarbon reservoirs abstract secondary porosity created by the post depositional leaching of detrital grains and cements can be of great economic importance the term secondary porosity is only descriptive and does not give any information on whether the porosity of the rock has been enhanced in reality there are two end member volumetric outcomes to the leaching process firstly the mineral can be completely dissolved and its components transported out of the system in such an instance the porosity of the rock will be enhanced secondly it is possible that a mineral will decompose and by product minerals will be precipitated which will occupy approximately the same volume as the original mineral the latter case is more likely where the pore waters are close to equilibrium with the majority of the minerals of the rock this will occur at elevated temperatures and or where the pore waters have been in contact with the rock for a long time these conditions are commonly met during intermediate to deep burial many metastable minerals such as high temperature feldspars probably decompose in this way a consequence of the decomposition of such metastable minerals is that primary porosity will be swapped for secondary and that older and more deeply buried rocks will contain a higher proportion of secondary porosity the mass transport equation may be used to examine the physico chemical environment where enhanced porosities most likely occur during leaching by using a simple non equilibrium mass transport model based on the silica system it is possible to demonstrate the relative importance of flow rate reaction rates specific surfaces and temperature in controlling the areal extent of leaching wide leaching zones are related to high flow rates which in turn promote a high throughput of undersaturated fluid and dispersion leaching over a wide area is also favoured by low reaction rates which will occur at low temperatures although it is true to say that less of a given silicate mineral can be dissolved by an undersaturated pore water at low temperature than at high temperature it is probable that pore fluids from deeply buried sediments are more likely to be close to equilibrium with a given mineral than for instance meteoric water thus the most likely environment for the production of widespread enhanced porosity is in the shallow subsurface the lower flow rates and higher temperatures that are encountered during deep burial diagenesis will result in far narrower more intense leaching zones as a result providing sufficient reactive mineral is present any undersaturated pore water will approach equilibrium in a short distance in order to transport undersaturated pore waters appreciable distances in the deep subsurface it is necessary to minimize their interaction with the host rock this could occur if the pore fluid were to move through a fracture system where it would interact with a far smaller surface area of reactive mineral than if it moved through the pore network

fault reactivation and structural inversion backarc and intraplate compressive deformations example of the eastern sunda shelf indonesia the tectonic evolution of the eastern sunda platform in indonesia is revealed by the study of seismic profiles and comprises three general tectonic periods during the cenozoic extension with graben infill from the paleogene to the early miocene a tectonically quiet period with only broad subsidence and a middle miocene recent period of folding inversion and thrusting these structures reveal changes in the intraplate stress field which was controlled by the changes in the boundary conditions of the plates collisions on the northern and eastern sides and interplate coupling southward along the java subduction zone inversions often occurred without reactivation of pre existing normal fault planes at the upper levels of these faults the factors controlling these deformations are discussed with special interest placed on the conditions for the potential reactivation of cohesionless normal faults inversion and reactivations were mostly controlled by the orientation of the pre existing fault planes in relation with the regional stress field and by the tectonic stress regime in a strike slip regime effective principal stresses 1 and 3 horizontal with 1 2 3 the optimum conditions for reactivation in isotropic rocks are pre existing faults that strike close to the potential two vertical conjugate failure planes the same mechanical properties as for pre existing fault prevail in such conditions strike slip movements and reverse strike slip movements and associated en echelon folds were generally observed in a compressive regime 1 and 2 horizontal with 1 2 3 typical steeply dipping normal faults are difficult to reactivate by slip movements we suggest that reverse slip movements occurred in the deeper levels of faults and were accommodated by flexure above the pre existing fault planes and the inversion of grabens the strongly inverted grabens are those whose orientation is perpendicular to the regional compression nevertheless with strong regional shortening large uplifts and thrust faults were observed with reactivation in the form of reverse or reverse strike slip movements along pre existing steeply dipping normal fault planes

chemistry of oil field brines in relation to diagenesis of reservoirs 1 use of mineral stability fields to reconstruct in situ water composition example of the mahakam basin abstract during mineral diagenesis the chemical composition of pore water can be strongly influenced by mineral reactions especially under geological conditions where water rock ratios are low to model and eventually to predict diagenesis it is important to have an understanding of formation water composition however available water analyses in many basins are not complete enough for this purpose in most cases reliable values for ph dissolved silica and aluminium are lacking such data can be reconstructed using various relations developed herein these data are constrained by petrographic observations because of their well preserved or authigenic character some alumino silicates are considered as relatively stable and can be thought of as effectively saturated with respect to them the saturation state of the other minerals of the matrix can be discussed according to these constraints the sub arkosic series of the mahakam basin resevoirs mio pliocene are used as a reference example to perfect a method capable of reconstructing consistent compositions of diagenetic waters suitable for input into diagenetic modelling

recent developments in element concentration and isotope ratio analysis of individual fluid inclusions by laser ablation single and multiple collector icp ms abstract laser ablation inductively coupled plasma mass spectrometry la icp ms has become a most powerful technique for the elemental analysis of individual polyphase inclusions completely enclosed in minerals be they solid or a solid liquid gas mixture at the time of measurement simultaneous accurate quantification of major to ultra trace element concentrations from li to u by well controlled ablation of the entire fluid or melt inclusion content and successful use of largely matrix independent external calibration protocols are unique features of this method this contribution reviews fluid inclusion fundamentals relevant for their bulk analysis by la icp ms and discusses key aspects of the analytical protocol emphasis is on figures of merit precision accuracy obtained from the analysis of individual inclusions and fluid inclusion assemblages and procedures and technical developments to improving data quality are elaborated a new equation for the calculation of detection limits for la icp ms analysis is presented which closely follows iupac conventions applications are reviewed with emphasis on the use of synthetic fluid inclusions in constraining metal solubility and distribution between co existing phases new data for natural bismuth fluid inclusions document the seamless transition to melt inclusion analysis by la icp ms thus highlighting the fact that the procedures presented here are generally applicable to the analysis of inclusions in complex host minerals isotope ratio analysis of individual fluid inclusions by multicollector icp ms mc icp ms is a recent development that requires fast transient signals to be accurately recorded by instrumentation designed for high precision static measurements of long lasting stable ion beams we address the general principles based on pb isotopes and review a first application to the bingham canyon porphyry cu au mo deposit a pilot study using about 50 synthetic fluid inclusions containing srm 987 sr and variable nacl ca and rb concentrations demonstrates that accurate 87 sr 86 sr isotope ratios can be obtained on an individual rb poor fluid inclusion at absolute 2 precisions of 0 0003 to 0 002 a residual trend in 87 sr 86 sr as a function of the rb sr abundance ratio in the fluid inclusions suggests that interference correction of 87 rb on mass 87 assuming identical mass bias coefficients for the two elements may be inaccurate however the offset can be accurately corrected for by regressing the data to zero 87 rb the versatility and detection power of la icp ms makes this technique the method of choice for solute abundance and isotope ratio analysis of individual fluid inclusions significant future progress can be achieved by improvements in ion production transmission and data recording efficiency and by further improving control on inclusion ablation by pulsed laser beams data quantification strategies may also have to be further refined to keep pace with instrumental progress and innovation

geochronological evidence for continuous exhumation through the ductile brittle transition along a crustal scale low angle normal fault simplon fault zone central alps 1 major low angle normal faults juxtapose different structural levels of the crust that record both brittle and ductile deformation field relationships alone cannot establish whether these different responses to deformation represent 1 parts of a single process of exhumation along the detachment or 2 two separate events with the later more discrete brittle detachment exhuming a fossil ductile shear zone from depth these two general models are critically assessed for the low angle normal simplon fault zone sfz in the central alps the sfz shows a spatial transition from a broad ductile mylonitic shear zone to a discrete brittle detachment with identical kinematics the age of the ductile shear zone and ductile brittle transition is controversial we present a detailed geochronological study based on fission track 40ar 39ar and rb sr microsampling dating coupled with structural petrological and chemical analyses that provides tight constraints on sfz timing discontinuous mineral cooling ages over a broad range of temperatures across the fault zone argue for fault activity between 20 and 3 ma on the basis of synkinematic white mica in low temperature shear zones and necks of foliation boudinage the brittle ductile transition in the footwall could be dated at 14 5 10 ma overall the data presented here are consistent with a continuous transition from ductile shearing to a more localized zone of brittle deformation within the same geological framework over a period of 15 ma the sfz is therefore an example of a telescoped crustal section within a single major low angle fault involving a continuous period of exhumation rather than a two stage structure

regional trends in water chemistry smackover formation southwest arkansas geochemical and physical controls we present results on regional geochemical analyses of formation waters from jurassic carbonate strata of the upper smackover formation from the southwest arkansas shelf gulf coast basin these formation waters have been noted in earlier studies for their exceptional salinities and abnormally br rich composition our detailed and regionally extensive database shows that chemical heterogeneities occur on a relatively small scale and have spatial relations to faults which may act as pathways for fluid migration from deeper and hotter portions of the stratigraphic section one of the most pronounced heterogeneities in water chemistry is an h 2 s rich anomaly in the center of the shelf enrichments in h 2 s are accompanied by large increases in alkali elements li k and b and b which could be produced by higher temperature diagenesis of clay minerals and feldspars in deeper seated siliciclastic strata bromide enrichments in all smackover formation waters are apparent from relations between br and cl contents as well as br and divalent cation carpenter function values as suggested by earlier studies of these waters evaporatively concentrated seawater expelled from associated thick monomineralic evaporite sections may be an important source of br cl and na whereas ca mg and sr concentrations have been modified by carbonate mineral recrystallization and dolomitization mineral equilibria exert significant controls on concentrations of ca mg sr and so 4 in these brines br enrichment likely was produced by halite recrystallization in fluids already enriched in br from seawater evaporation elevated cl br ratios occur in regional association with lower salinity water near t e south arkansas fault system consistent with halite dissolution during meteoric water recharge thus despite relatively uniform reservoir lithology and simple structural setting processes on small spatial scales have resulted in significant heterogeneity in water chemistry within a small area of the gulf coast sedimentary basin although movement of fluids along these fault systems is not demonstrated in our study the regional trends in geochemistry are strongly suggestive of their important role in controlling chemical heterogeneities in smackover formation waters

sandstone diagenesis in the pattani basin gulf of thailand history of water rock interaction and comparison with the gulf of mexico abstract in the pattani basin a failed rift basin extensive water rock interaction has occurred between subquartzose alluvial sandstones of miocene age and their pore fluids diagenetic rates and pathways have been strongly influenced by high geothermal gradients high co 2 fugacities and low pore water salinities depositional pore water was fresh to brackish depending on the depositional environment of the sediments chloride concentrations in modern formation water are believed primarily to reflect the proportions of river and sea water in the depositional environment however the concentration of other important solutes and the isotopic composition of the formation waters can not be explained by roportional mixing of these two end member waters dissolution of detrital plagioclase an 3 and k feldspar are reactions of major significance that are reflected chemically in the na cl and k cl ratios of the formation water despite the high temperature of the sandstones 120 200 c diagenetic albite does not occur geochemical calculations indicate the formation water is undersaturated with respect to both orthoclase and albite this style of feldspar diagenesis differs significantly from that of sandstones of similar composition in other basins and has probably influenced other aspects of silicate diagenesis important authigenic minerals are 1 locally abundant calcite cement 13 c 12 8 18 o 17 3 pdb an early diagenetic phase that formed at about 60 c 2 pore filling kaolinite 18 o 9 9 d 83 5smow that was closely associated with feldspar dissolution and formed over a range of temperatures and 3 fibrous pore lining and pore bridging illite 18 o 9 8 d 86 7 smow the last significant cement formed at temperatures of 120 to 150 c potassium argon dates on illite indicate that sandstone diagenesis took place during a period of rapid sedimentation in the first two thirds of the burial history comparison of pattani basin diagenesis with diagenesis of sandstones of similar age in other sedimentary basins demonstrates that chemical diagenesis relative to mechanical compaction has been especially rapid in the pattani basin this reflects the effect of high temperatures on reaction rates the net effect is a high average rate of porosity loss with burial 11 km

carbonate equilibria and ph buffering by organic acids response to changes in pco2 abstract organic acid anions such as acetate a natural product of the thermal breakdown of kerogen may buffer the ph of natural formation waters the extent of ph buffering by acetate should determine whether an increase in the partial pressure of carbon dioxide p co 2 will lead to calcite dissolution porosity enhancement or calcite precipitation porosity destruction successful prediction of porosity therefore requires a quantitative understanding of this process this paper presents computer simulations of the response of the acetic acid carbonate system to changes in p co 2 for typical reservoir fluid compositions and temperatures the effects of variations in temperature ionic strength initial ca 2 concentration and the magnitude of the change in p co 2 were investigated natural formation waters from cenozoic reservoirs in the gulf coast basin and california show maximum organic acid concentrations at 100 20 c at 100 c the simulations show that for concentrations of acetic acid up to 0 06 m 3600 mg l 1 increases in the p co 2 of initially calcite saturated solutions will promote calcite undersaturation at higher temperatures equivalent buffering responses occur at lower acetic acid concentrations but acetic acid concentrations in natural formation waters also decrease rapidly with increasing temperature since present day concentrations of acetic acid in formation waters from cenozoic reservoirs rarely exceed 0 06 m increases in p co 2 will generally promote calcite dissolution by these waters in the gulf coast basin variation in organic acid concentrations as a function of reservoir age suggests that reductions in dissolved organic acid concentrations over geologic time have been relatively small it can be concluded therefore that at any point in the histories of these reservoirs an increase in p co 2 would have promoted calcite dissolution despite the effects of ph buffering by acetate since increases in p co 2 will in most cenozoic basins promote calcite dissolution knowledge of the location and timing of co 2 production in the subsurface can more easily be applied to the prediction of calcite cementation and dissolution

th pb ion probe dating of zoned hydrothermal monazite and its implications for repeated shear zone activity an example from the central alps switzerland th pb age dating of zoned hydrothermal monazite from alpine type fissures clefts is a powerful tool for constraining polyphase deformation at temperatures below 350 c and presents an alternative to k ar and 40ar 39ar dating techniques for dating brittle tectonics this study considers the relationship between cleft orientations in ductile shear zones and cleft mineral crystallization during subsequent brittle overprinting in the grimsel area located in the aar massif of the central alps horizontal clefts formed during a primary thrust dominated deformation while younger and vertically oriented clefts developed during secondary strike slip movements the change is due to a switch in orientation between the principal stress axes 2 and 3 the transition is associated with monazite crystallization and chloritization of biotite at around 11 5 ma quartz fluid inclusion data allow a link between deformation stages and temperatures to be established and indicate that primary monazite crystallization occurred in both cleft systems at 300 350 c while cleft monazite crystallization ceases at 11 ma in inactive shear zones monazite growth and or dissolution reprecipitation continues under brittle deformation conditions in vertical clefts during later deformation until 7 ma this younger shear zone activity occurs in association with dextral strike slip movement of the rhone simplon fault system with the exception of varying th u values correlated with the degree of oxidation there is only limited compositional variation in the studied cleft monazites

appraisal of the use of experimental and analogue studies in the assessment of the role of organic acid anions in diagenesis abstract to fully understand and predict the processes which act to produce the diagenetic phenomena of porosity enhancement or reduction observed within sandstone reservoir rocks it is necessary to understand the role of formation waters in mineral dissolution and precipitation reactions of particular interest are the mechanisms of feldspar and clay mineral dissolution precipitation which involve the transport of al and si in solution it has been suggested that organic acid anions known to be present within formation waters may enhance aluminosilicate mineral solubility by complexing with al and si reducing the volumes of water required for transport to take place however the possible significance of organic acid anion complexing still cannot be fully assessed because of limited experimental evidence and poor knowledge of the stability of dissolved organic species under appropriate pressure temperature conditions a series of experiments was undertaken to investigate the possible role of acetate oxalate and citrate representing potential complexing agents present within formation waters in diagenetic processes involving feldspar and quartz dissolution and clay precipitation and to determine solute yields from organic rich mudrocks at 150 c 50 mpa albite solubility is enhanced to the greatest extent in the presence of citrate whereas acetate enhances the rate of albite dissolution however in experiments with pb rich alkali feldspar the rapid decay of acetate reduces its complexing potential and hence its availability for the transport of metals artificial maturation of organic rich mudrocks under conditions appropriate for diagenesis yields a mixture of organic acid anions including acetate oxalate and citrate but there is no evidence for solute yields significantly in excess of those predicted for mineralogical buffers of fluid composition in the absence of organic species domestic waste disposal sites are considered as potential analogues to diagenetic systems the associated chemical processes show close parallels between the chemical maturation of landfill leachates and the development of sediment pore waters

physical and chemical controls on the composition of waters in sedimentary basins abstract the composition of subsurface waters is determined not only by diagenetic reactions but also by physical processes of fluid convection and hydrodynamic dispersion the south west louisiana gulf coast provides an instructive field example of the net effects of diagenetic reaction and solute transport on pore water compositions in a regional siliciclastic sequence most formation waters here have compositions totally unlike the compositions of the connate meteoric and marine fluids that were buried with their host sediments at the time of deposition dissolved chloride has been generated by the subsurface dissolution of salt domes and has been pervasively transported by fluid flow throughout most of the upper 3 km of the sedimentary section the simultaneous systematic variation in dissolved na k mg ca and alkalinity with chloride in these waters supports the hypothesis that metastable thermodynamic buffering by silicate carbonate mineral assemblages is a first order control on fluid compositions the chemical potential of chloride or alternatively total anionic charge appears to be a master variable which ranks in importance with such other variables as pressure and temperature in driving diagenesis in this region this variable is controlled largely by physical processes of advection and dispersion in the upper 3 km of the section and by dehydration reactions in deeper mudstone dominated sediments where the composition of the fluid is largely rock buffered the ultimate origin of the fluid and its pathway of chemical evolution may be obscured at least in terms of major solute composition non buffered components such as cl and br or isotopic compositions are more likely to retain information on original end member fluid compositions and reaction pathways

dating brittle tectonic movements with cleft monazite fluid rock interaction and formation of ree minerals 1 two millimeter sized hydrothermal monazites from an open fissure cleft that developed late during a dextral transpressional deformation event in the aar massif switzerland have been investigated using electron microprobe and ion probe the monazites are characterized by high th u ratios typical of other hydrothermal monazites deformation events in the area have been subdivided into three phases d1 main thrusting including formation of a new schistosity d2 dextral transpression and d3 local crenulation including development of a new schistosity the two younger deformational structures are related to a subvertically oriented intermediate stress axis which is characteristic for strike slip deformation the inferred stress environment is consistent with observed kinematics and the opening of such clefts therefore the investigated monazite bearing cleft formed at the end of d2 and or d3 and during dextral movements along nnw dipping planes interaction of cleft filling hydrothermal fluid with wall rock results in rare earth element ree mineral formation and alteration of the wall rock the main newly formed ree minerals are y si y nb ti minerals and monazite despite these mineralogical changes the bulk chemistry of the system remains constant and thus these mineralogical changes require redistribution of elements via a fluid over short distances centimeter low grade alteration enables local redistribution of ree related to the stability of the accessory phases this allows high precision isotope dating of cleft monazite 232th 208pb ages are not affected by excess pb and yield growth domain ages between 8 03 0 22 and 6 25 0 60 ma monazite crystallization in brittle structures is coeval or younger than 8 ma zircon fission track data and hence occurred below 280 c

analysis of a rock slope with internal dilation a class of rock slope failures exists in which the mode of failure requires the existence or creation of internal shears to accommodate large internal slide mass distortion these internal displacements are required to allow motion along the basal slip surface this paper demonstrates that the more traditional limit equilibrium methods of analysis are often conservative when used to assess the stability of slopes with this failure mechanism as a result back analysis may overestimate the available shear resistance a method of analysis capable of handling this failure mechanism was proposed by s k sarma a case history from the revelstoke hydroelectric project british columbia is used to demonstrate that these internal shears were required for movement to occur and that passive anchors inside the sliding rock mass can be used to improve the overall slope stability no attempt is made to evaluate the actual factor of safety of the three dimensional slide mass key words rock slope foliation shear s

correlation of carbon dioxide abundance with temperature in clastic hydrocarbon reservoirs relationship to inorganic chemical equilibrium the mol co2 of gas reservoired in sandstones tends to increase with increasing temperature along both the us gulf coast and the norwegian continental shelf each of these provinces displays identical linear correlations between temperature and the log of the partial pressure of co2 i e pco2 in the reservoir fluid that is the product of the mol co2 the reservoir pressure this correlation is the same both in gulf coast reservoirs younger than 17 ma and in norwegian reservoirs of jurassic to triassic age 144 245 ma indicating that it is not dependent on reaction kinetics norwegian shelf data indicate that the same correlation also applies for gas condensate and oil reservoirs we suggest that the log pco2versus temperature correlation is the result of inorganic chemical equilibria between feldspar clay and carbonate minerals a trend of decreasing ph with increasing temperature is fixed by equilibria between aluminosilicate minerals feldspar kaolinite at lower temperatures followed by various reactions between kaolinite illite chlorite and feldspar as temperatures exceed 110 140 c responding to the ph set by the aluminosilicate system co2 activity in the formation water and thereby co2 partial pressure in any gas phase present is controlled by equilibrium with carbonate minerals this model predicts that co2 released by organic maturation will in general result in precipitation of carbonate minerals below 100 120 c and that there should be widespread carbonate dissolution at higher temperatures

stable isotope and ar ar evidence of prolonged multiscale fluid flow during exhumation of orogenic crust example from the mont blanc and aar massifs nw alps the spatial and temporal scales and the geometry of fluid pathways in a collisional orogen are investigated using stable isotope analysis o c and h and 40ar 39ar dating of vein minerals formed at circa 11 16 ma in the mont blanc and the aar external crystalline massifs in both massifs 40ar 39ar dating of veins adularia provides evidence for progressive crystallization from 16 to 9 ma and mainly at 11 12 ma following veins opening during shear zone activity the fluid flow duration thus ranges from 4 to 5 ma in the two massifs the 18o values of vein quartz and calcite are similar to those of undeformed crystalline and sedimentary host rocks suggesting rock buffering while carbon isotope ratios of vein calcites fall into three compositional groups a type veins have 13c values that are buffered by the helvetic metasediments which suggests that these veins formed in a closed system from a locally derived co2 rich fluid the fluid in equilibrium with c type veins has depleted 13c values similar to mantle co2 while the intermediate 13c values of b type veins suggest mixing between the a type and c type fluids these results are in agreement with crustal to lithosphere scale upward vertical fluid flow along vertical shear zones related to the strike slip system bounding the adriatic block since 16 20 ma connecting a deep seated fluid to some downward flow in the sedimentary cover of external crystalline massifs

formation water geochemistry of the sverdrup basin implications for hydrocarbon development in the high arctic abstract the sverdrup basin holds significant reserves of discovered and undiscovered petroleum that have not been produced to date given its remote arctic setting loss of perennial sea ice increases the prospects for producing reserves from this region potential renewed drilling and production requires knowledge of formation water geochemistry to aid both exploration as well as regulatory development historic data shows formation waters typically have a seawater origin although there is high variability in formation water geochemistry salt diapirs lead to localized brine generation with salinities over an order of magnitude higher than average for the basin evidence also shows fresh water influx occurred along the basin margins that initiated biogenic gas generation

an isochemical model for formation of authigenic kaolinite k feldspar and illite in sediments abstract although thermodynamic calculations indicate a narrow range of conditions for coprecipitation of kaolinite and k feldspar coexistence of authigenic kaolinite and k feldspar is common in jurassic sandstones of the north sea this paper shows that the coexistence of authigenic k feldspar and kaolinite may indicate that this narrow range of conditions did prevail and that the system behaved as a closed system these conditions are presence of muscovite in the sediment pore waters supersaturated with respect to quartz and temperatures lower than 50 c at higher temperatures the authigenic kaolinite and authigenic k feldspar may react and form authigenic illite muscovite and quartz cement the direction of the isochemical reaction is thermodynamically determined by the degree of supersaturation with respect to quartz in the formation water published data on the degree of supersaturation with respect to quartz in formation waters suggest that the illitization of kaolinite may take place at temperatures as low as 50 c much lower than previously believed

geochemical constraints from formation water analyses from the north sea and the gulf coast basins on quartz feldspar and illite precipitation in reservoir rocks

chemistry of oil field brines in relation to diagenesis of reservoirs 2 reconstruction of palaeo water composition for modelling illite diagenesis in the greater alwyn area north sea abstract input data for water composition are required in order to numerically simulate diagenetic episodes in rock dominated closed system modelling the composition of water initially present in the pores has at most only a minor influence on the stability of the system and is controlled by the mineral composition in open system modelling the composition of infiltrating water can play an important role brent water salinities of the greater alwyn area exhibit a regional trend from the north to the south the k cl ratio of these waters varies along two trends it is higher in the north than in the south alkalinity and published pco2 values are used to calculate ph under subsurface conditions very few silica measurements are available aluminium analyses of pore water are not available some reservoirs of the greater alwyn area experienced a major reduction in permeability due to illite and quartz precipitation coeval with kaolinite and k feldspar dissolution saturation of present day brent waters with respect to these minerals is examined the method proposed in part 1 is applied to reconstruct a range of possible compositions for ancient waters in brent reservoirs between several extreme conditions one of these extremes is provided by buffering water with a set of minerals chosen from petrographic considerations the other extremes correspond to reasonable states of over saturation with respect to particular minerals e g quartz and concern the values of dissolved si and al which are the least well characterised concentrations from the routine analyses of formation waters

accommodation of transpressional strain in the arabia eurasia collision zone new constraints from u th he thermochronology in the alborz mountains north iran the alborz range of n iran provides key information on the spatiotemporal evolution and characteristics of the arabia eurasia continental collision zone the southwestern alborz range constitutes a transpressional duplex which accommodates oblique shortening between central iran and the south caspian basin the duplex comprises nw striking frontal ramps that are kinematically linked to inherited e w striking right stepping lateral to obliquely oriented ramps new zircon and apatite u th he data provide a high resolution framework to unravel the evolution of collisional tectonics in this region our data record two pulses of fast cooling associated with sw directed thrusting across the frontal ramps at 18 14 and 9 5 7 5 ma resulting in the tectonic repetition of a fossil zircon partial retention zone and a cooling pattern with a half u shaped geometry uniform cooling ages of 7 6 ma along the southernmost e w striking oblique ramp and across its associated nw striking frontal ramps suggests that the ramp was reactivated as a master throughgoing n dipping thrust we interpret this major change in fault kinematics and deformation style to be related to a change in the shortening direction from ne to n nne the reduction in the obliquity of thrusting may indicate the termination of strike slip faulting and possibly thrusting across the iranian plateau which could have been triggered by an increase in elevation furthermore we suggest that 7 6 m y old s directed thrusting predated inception of the westward motion of the south caspian basin