weathering rates of historic sandstone structures in semiarid environments ebro basin ne spain abstract long term weathering rates of sandstones were obtained for well dated from 400 to 1100 years bp historic structures wall sections of the alberuela castle in a semiarid environment of the central ebro basin ne spain using an exclusive movable contour plotting frame this sandstone has a low durability controlled mainly by textural properties differences observed in weathering rates are clearly controlled by the age of building as well as the orientation of the studied sandstone wall sections weathering processes related to insolation control the asymmetric spatial pattern of weathering rates in surfaces exposed for since 1100 bp which are higher in southeast facing sandstone wall sections 8 4 mm 100 years than in southwestern 4 4 mm 100 years and northern faces 3 9 mm 100 years in addition exponential increase in weathering rates with time from 1 6 2 2 mm 100 years in surfaces exposed for since 800 400 bp to 8 4 mm 100 years in surfaces exposed for since 1100 bp in southeastern faces may be probably related to environmental changes during the last millennium

effects of grand cycle cessation on the diagenesis of upper cambrian carbonate deposits in the southern appalachians u s a abstract the vertical transition from the mainly subtidal alternating shale and carbonate units or grand cycles of the conasauga group middle to upper cambrian to the peritidal dolostone of the knox group upper cambrian to lower ordovician marks a major change in the early paleozoic passive margin sedimentation of the southern appalachians the grand cycles represent an interplay between intrashelf shale basin and carbonate platform deposition the end of grand cycle deposition occurred in response to carbonate platform progradation over the infilled intrashelf basin and is associated with a prominent change in diagenetic patterns observed in the uppermost conasauga group strata the maynardville formation the maynardville records upward shallowing from subtidal shale and limestone into peritidal dolostone this change from a subtidal into a peritidal depositional regime influenced the early diagenesis of the maynardville and in conjunction with the changes in the regional facies distribution at the end of grand cycle deposition extended its influence upon the burial diagenesis of these deposits the subtidal deposits of the maynardville contain a variety of calcite cements dolomite is not abundant in these deposits where it occurs mainly as a fine crystalline ferroan phase associated with argillaceous layers and pressure dissolution features the local source for this dolomite during burial was provided by expulsion of pore fluids from interbedded shale deposits diagenesis of clay minerals and pressure dissolution of carbonates ferruginous coatings on hardgrounds in the subtidal deposits provided an additional local source of iron for the formation of replacement saddle dolomite the subtidal deposits also contain saddle dolomite cement associated with mississippi valley type mvt minerals which indicate the involvement of externally derived fluids during late burial diagenesis of these strata the peritidal deposits on the other hand are extensively dolomitized fine crystalline dolomite most likely represents early diagenetic replacement of tidal flat carbonate sediment coarse crystalline replacement dolomite formed during burial by recrystallization of early dolomite and dolomitization by warm burial brines fenestrae desiccation cracks and evaporite dissolution voids are occluded with dolomite cement which is commonly complexly zoned zoned dolomite precipitated during shallow to intermediate burial and is postdated by saddle dolomite cement saddle dolomite in pore centers dissolutional voids and tectonic fractures formed during late burial from warm fluids associated with the migration of mvt brines the effects of changes in the depositional setting on diagenesis are reflected in similar early diagenesis between the subtidal maynardville and the underlying conasauga group carbonate deposits and between the peritidal deposits of the upper maynardville and the rest of the overlying knox group the depositional environments and early diagenesis also affected the burial diagenesis the changes in regional facies distribution caused by carbonate platform progradation at the end of grand cycle deposition resulted in a decrease in the proportion of siliciclastic components this decrease was accompanied by a decline in the amount of ferroan carbonate in the peritidal deposits the maynardville is conformably overlain by carbonate deposits of the knox group this allowed the maynardville knox succession to behave as a single hydrologic unit for the migration of burial fluids in response to the formation of the ordovician foreland sevier basin to the southeast during the conversion from a passive into a convergent continental margin the nolichucky shale separates the maynardville knox succession from the carbonate deposits of the underlying conasauga group which were in contrast affected by the migration of burial fluids from the cambrian intrashelf shale basin to the west

multiple fluid flow events and the formation of saddle dolomite case studies from the middle devonian of the western canada sedimentary basin abstract in western canada sedimentary basin wcsb large scale and focused fluid flow that caused hydrothermal dolomitization have been suggested with different timing and intensity in this study we conducted a petrographic and geochemical comparison between the middle devonian sulphur point and slave point carbonates from northwestern alberta the results demonstrate the presence of both an early fluid flow event associated with the late devonian to mississippian antler orogeny as well as a later event coincident with the late cretaceous to early tertiary laramide orogeny early fluid flow event is characterized by high salinity fluids and high temperatures and oxygen isotopic values of marine or slightly enriched values as demonstrated in saddle dolomite from the slave point formation in contrast later fluids that caused the precipitation of saddle dolomite in the sulphur point formation are characterized by having slightly saline values comparable homogenization temperatures but more enriched 18 o values and slightly depleted 13 c signatures geochemical data indicate that this later hydrothermal fluid was mixture of middle devonian brines and radiogenic basement fluids

sulfide formation in reservoir carbonates of the devonian nisku formation alberta canada an ion microprobe study the processes affecting sulfur during diagenesis in carbonates have been investigated by ion microprobe analysis of 34s values of pyrite marcasite and anhydrite from the devonian nisku formation in the western canada sedimentary basin pyrite and marcasite from three nisku wells have 34s ranging between 35 and 20 cdt most lie between 35 and 5 cdt the 34s values of sulfides are heterogeneous within individual thin section sized samples varying by as much as 25 the 34s values increase by up to 50 with increasing well depth in two different nisku wells and 34s also increases by up to 15 as grain size increases from 40 to over 100 m in individual samples the appearance of native sulfur is accompanied by an increase of up to 30 in pyrite 34s values bulk and ion probe analyses of anhydrite are relatively uniform 34s 22 to 30 cdt the predominately low 34s values of nisku sulfides indicate bacterial sulfate reduction textural relations indicate that fe sulfide formation in nisku carbonates may have occurred by two different mechanisms much of the sulfide has 34s values that suggest that it was associated with bacterial sulfate reduction although most fe sulfides did not form until after pervasive matrix dolomitization depths of 300 1000 m other sulfide may have formed later during deep 4 km burial via thermochemical sulfate reduction the range in 34s values in a single thin section and correlations between pyrite morphology and isotopic values suggest that sulfate reduction was a very localized process and that the sulfate reduction environment varied considerably on a small scale

diagenetic stabilization of the upper permian dalan formation persian gulf basin abstract the upper permian dalan formation is a supergiant gas reservoir in the persian gulf basin reservoir quality is controlled by the original facies type dolomitization and anhydrite cementation this study focuses on the texture petrographic analysis bulk stable isotope values and trace element composition in order to identify the original mineralogy of carbonate grains and cements and to assess the synsedimentary and diagenetic processes including the timing and setting of dolomitization the limestone data suggest that the initial precipitation of isopachous acicular to fibrous aragonite cements in the marine phreatic environment was followed by dissolution of aragonite grains and cements in the meteoric phreatic zone and neomorphism and low mg calcite cementation blocky and drusy crystals syntaxial overgrowths in the active freshwater phreatic zone the relatively pristine 18o and 13c values low mn and relatively high sr and na contents indicate that this meteoric diagenesis was relatively limited the remaining porous space was cemented by blocky calcite later during burial four types of dolomites are identified within a complex mixture of texturally and isotopically different dolomites dolomicrite dolomicrosparite dolosparite and saddle dolomite dolomicrite formed syndepositionally from the evaporated permian seawater or by reflux of dense evaporated brines during shallow burial the second major episode of dolomitization dolomicrosparite and dolosparite was driven by reflux of warmer and more saline fluids at greater depth which affected both previously non dolomitized and dolomitized sediments the final diagenetic stabilization during deep burial is associated with extensive pressure solution fracturing vein filling of calcite and dolomite cements and crystallization of saddle dolomite

fluid compartmentalization of devonian and mississippian dolostones western canada sedimentary basin petrologic and geochemical evidence from fracture mineralization1 integrated petrographic geochemical and fluid inclusion study of fracture mineralization and associated host rock in selected mississippian and devonian carbonates extending from southeastern alb

simulating the growth of tafoni throughout the world large caves in rocks tafoni are found which originate from salt weathering the mechanisms that control their development are poorly understood the growth of tafoni has been studied with a model that describes how a rock surface containing a small pit disintegrates by salt crystallization during wetting drying cycles in the model the rock is mapped on a grid the migration and crystallization of salts are simulated explicitly in the drying phase of a cycle at the end of each wetting drying cycle the amount of salt deposited at the grid nodes is evaluated and the shape of the rock surface is adjusted by removing nodes the length of the drying period in a single cycle proved to be the key parameter for short drying periods the amount of crystallized salt at the surface is proportional to the drying rate therefore for short drying periods most salts are deposited at the more exterior parts of the rock surface as a result most damage will develop at these parts of the surface which results in smooth surfaces due to the characteristics of the drying process for long drying periods most salts accumulate at regions with low evaporation rates which are the sheltered parts of the rock surface these parts are not exposed to the wind or the sun as a result the pit grows and a tafone develops copyright 2004 john wiley sons ltd

origin and development of tafoni in tunnel spring tuff crystal peak utah usa bedding parallel tafoni are well developed over much of the surface of the tunnel spring tuff oligocene exposed in 300 m high crystal peak an inselberg the tunnel spring tuff is a crudely stratified non welded rhyolite ash flow tuff with 30 per cent porosity clasts of palaeozoic dolomite limestone and quartzite make up 10 per cent of the tuff the tafoni are remarkable because of their size up to 20 m wide but rarely wider than 4 m shape of the openings spherical arch like or crescent shaped and abundance up to 50 per cent of an outcrop face they are actively forming today calcite is abundant 10 to 40 per cent by weight in tafoni as an efflorescence in spalling flakes of tuff on their roofs and walls halite and gypsum generally make up less than 0 01 per cent of the efflorescence the absence of corroded quartz and feldspar grains in spall fragments indicates that chemical weathering is unimportant in development of the tafoni calcite aragonite halite and gypsum dust from modern salt pans less than 20 km from crystal peak are potential sources of salt in the tuff but the prevailing winds are in the wrong direction for significant amounts of these evaporite minerals to reach the inselberg calcite is the only evaporite mineral present in the tafoni in more than trace amounts and this mineral is readily available within the tuff itself as a result of rock weathering we propose that meteoric water containing carbonic acid infiltrates the tuff dissolves carbonate clasts and migrates to the steep flanks 20 of the peak through abundant megapores and micropores there it evaporates and precipitates calcite crystallization pressure spalls off grains and sheets as the physical manifestation of salt weathering the quasi uniform spacing of tafoni suggests that a self organization process is active in the water flow copyright 2000 john wiley sons ltd

a morphological study of greenschist weathering on dated coastal structures south devon uk three dated structures up to 450 years in age display the effects of coastal weathering of the greenschist of which they are constructed a variety of weathering forms is present the various topographic surfaces of the structures create variation in weathering environments and consequent weathering processes and rates weathering is enhanced by direct exposure to salt bearing spray and by humid conditions and apparently limited by direct exposure to solar radiation the maximum rates of weathering on the three surfaces approximate to 0 6 mm a 1 over this period consistent with measured contemporary weathering rates for a natural surface formed by this rock type in a nearby coastal location 1997 by john wiley sons ltd

the magnesium isotope δ26mg signature of dolomites abstract dolomite precipitation models and kinetics are debated and complicated due to the complex and temporally fluctuating fluid chemistry and different diagenetic environments using well established isotope systems 18 o 13 c 87 sr 86 sr fluid inclusions and elemental data as well as a detailed sedimentological and petrographic data set we established the precipitation environment and subsequent diagenetic pathways of a series of proterozoic to pleistocene syn depositional marine evaporative sabkha dolomites syn depositional non marine evaporative lacustrine and palustrine dolomites altered marine mixing zone dolomites and late diagenetic hydrothermal dolomites these data form the prerequisite for a systematic investigation of dolomite magnesium isotope ratios 26 mg dol dolomite 26 mg ratios documented here range from 2 49 to 0 45 26 mg mean 1 75 1 08 n 42 the isotopically most depleted end member is represented by earliest diagenetic marine evaporative sabkha dolomites 2 11 0 54 2 n 14 in comparing ancient compositions to modern ones some of the variation is probably due to alteration altered marine 1 41 0 64 2 n 4 and earliest diagenetic lacustrine and palustrine dolomites 1 25 0 86 2 n 14 are less negative than sabkha dolomites but not distinct in composition various hydrothermal dolomites are characterized by a comparatively wide range of 26 mg ratios with values of 1 44 1 33 2 n 10 by using fluid inclusion data and clumped isotope thermometry 47 to represent temperature of precipitation for hydrothermal dolomites there is no correlation between fluid temperature 100 to 180 c and dolomite mg isotope signature r 2 0 14 nor is there a correlation between 26 mg dol and 18 o dol magnesium isotope values of different dolomite types are affected by a complex array of different mg sources and sinks dissolution precipitation and non equilibrium fractionation processes and overprinted during diagenetic resetting further progress on the use of 26 mg dol as a proxy will require new theoretical and experimental data for 26 mg fluid dol that includes dehydration effects of the free mg aquo ion versus fluid temperature in ancient diagenetic systems complex variables must be considered these include fluid chemistry and physical properties mg sources and sinks temporal changes during precipitation and post precipitation processes including open and closed system geochemical exchange with ambient fluids all of these factors complicate the application of 26 mg dol as proxy for their depositional or diagenetic environments nevertheless the data shown here also indicate that 26 mg dol can in principle be interpreted within a detailed framework of understanding

origin of high h2s concentrations in the upper permian changxing reservoirs of the northeast sichuan basin china abstract intense thermochemical sulfate reduction tsr and up to 18 h 2 s are found in the upper permian changxing formation p 3 ch in the northeast ne sichuan basin china despite that rare gypsum or anhydrite was found in this formation here we present new concentration data of carbonate associated sulfate cas from carbonate host rocks c o and sr isotope data for tsr related calcites and s isotope data for sulfur compounds obtained during this study these data along with spatial temporal changes in palaeogeopressure conditions hydraulic conductivity and the physical capacity indicate that the h 2 s was generated locally from tsr within the p 3 ch reservoirs we propose that the reactive sulfates were derived from cas released during dolomitization and recrystallization of earlier dolomite within the p 3 ch fm and from the cross formational migration of evaporative brines from the lower triassic feixianguan formation t 1 f to p 3 ch fm our calculation shows that the two sources could provide enough so 4 2 for the generation of h 2 s within the p 3 ch reservoirs early downward migration of sulfate rich evaporative brines from the t 1 f formation occurred in near surface and shallow burial diagenetic settings mainly 3 ch fm subsequently tsr calcites precipitated from the pore water have high sr concentrations up to 7767 ppm close to the t 1 f tsr calcites and 87 sr 86 sr ratios mainly from 0 7074 to 0 7078 which are significantly higher than those of late permian seawater but within the range of early triassic seawater

magnesium isotopes in high temperature saddle dolomite cements in the lower paleozoic of canada abstract mg isotopes are used to better understand the genesis of hydrothermal saddle dolomite cements in lower paleozoic successions in canada these cements occur in fault bounded dolostones that overlay lithologically diverse basement rocks ordovician dolomite lies over the precambrian craton whereas the silurian and devonian dolomites overlay a succession of tectonically accreted sedimentary volcanic and ultramafic units of cambrian to ordovician age lower silurian saddle dolomites have the most negative 26mgdsm3 values of our dataset 3 25 to 1 13 and plot in two distinct groups a strongly negative subset that characterizes higher temperature 175 c dolomites and a less negative subset for lower temperature 153 c dolomites upper ordovician saddle dolomites precipitated at significantly lower temperatures 102 c and their 26mgdsm3 values range from 1 26 to 0 71 lower devonian saddle dolomites formed at very high temperature 350 c and have 26mgdsm3 values ranging from 1 29 to 0 78 no experimental data on high temperature 100 350 c fluid dolomite mg isotope fractionation factors have been published and recent research suggests that no significant fractionation occurs between diagenetic fluids and dolomites at high temperatures in closed to semi closed diagenetic systems our results indicate that the isotopic signature of diagenetic fluid is the primary control for the 26mgdsm3 values in these high temperature dolomites

cavernous weathering in the capitol reef desert utah tafoni and honeycomb weathering are abundant in sandstones at capitol reef national park cavities are particularly common in talus blocks resting on alkali rich soil in vertical walls of desert washes and in sandstones containing calcareous cement chemical analyses x ray diffraction data and field observations suggest that salt weathering is the most important cause of disintegration but dissolution of calcite cement also occurs in calcareous sandstones cavernous weathering at capitol reef produces cavities similar in appearance to features found in arkose outcrops along the coast of northwest washington even though the environmental conditions at these two locations are very different

salt weathering influence of evaporation rate supersaturation and crystallization pattern micro and macroscale experiments which document the dynamics of salt damage to porous stone have yielded data which expose weaknesses in earlier interpretations previously unexplained differences are found in crystal morphology crystallization patterns kinetics and substrate damage when comparing the growth of mirabilite na2so4 10h2o and thenardite na2so4 versus halite nacl the crystallization pattern of sodium sulphate was strongly affected by relative humidity rh while a lesser rh effect was observed for sodium chloride macroscale experiments confirmed that mirabilite crystallizing at rh 50 per cent and thenardite crystallizing at rh 50 per cent tend to form subflorescence in highly localized areas under conditions of constant rh and temperature this crystallization pattern was more damaging than that of halite since halite tended to grow as efflorescence or by filling the smallest pores of the stone in a homogeneous fashion a result which contradicts wellman and wilson s theoretical model of salt damage low rh promoted rapid evaporation of saline solutions and higher supersaturation levels resulting in the greatest damage to the stone in the case of both sodium sulphate and sodium chloride crystallization at any particular crystallization condition sodium chloride tended to reach lower supersaturation levels resulting in the crystallization of isometric crystals and created negligible damage while sodium sulphate reached higher supersaturation ratios resulting in non equilibrium crystal shapes resulting in significant damage esem showed no damage from sodium sulphate due to hydration instead after water condensation on thenardite crystals rapid dissolution followed by precipitation of mirabilite took place resulting in stone damage by means of crystallization pressure generation it is concluded that salt damage due to crystallization pressure appears to be largely a function of solution supersaturation ratio and location of crystallization these key factors are related to solution properties and evaporation rates which are constrained by solution composition environmental conditions substrate properties and salt crystallization growth patterns when combined with a critical review of salt damage literature these experiments allow the development of a model which explains variations in damage related to combinations of different salts substrates and environmental conditions

late leaching under deep burial conditions a case study from the miocene zhujiang carbonate reservoir south china sea abstract the zhujiang carbonates are the hydrocarbon reservoir rock of the liuhua 11 1 field they comprise three porous zones of around 20 m intercalated with tight zones parallel to bedding thin section cathodoluminescence and stable isotope data were combined with well log information in order to investigate the origin and spatial distribution of the abundant porosity the vast majority of pores in the porous zones was generated or overprinted in deep burial settings by late leaching two tight zones however lack this porosity because of meteoric cementation and increased compaction and related calcite precipitation prior to late leaching we propose that migrating corrosive fluids were diverted along bedding parallel flow barriers of the tight zones and leached the intervals in between the porous zones thereby emphasizing the depositional pattern

weathering of coastal defensive structures in southwest england a 500 year stone durability trial historic structures can be viewed as exposure trials of the stone of which they are constructed as such they represent a geomorphological weathering experiment several structures of henrician sixteenth century and greater age on the coast of southwest england have been exposed to coastal salt weathering for 500 600 years long term weathering rates on five different rock groups are derived from careful study of weathering depths and forms there is significant variation in weathering rate between five major rock groups rank ordering of weathering rate values reveals a durability order of these rock groups which is confirmed by local juxtapositions controls on rock durability in the coastal weathering environment include both mechanical and mineralogical characteristics specific density and combined quartz and muscovite content are positively related to durability high feldspar and chlorite content are associated with low durability copyright 2000 john wiley sons ltd

rates of tafoni weathering on uplifted shore platforms in nojima zaki boso peninsula japan many tafoni occur on the faces of marine cliffs on three uplifted shore platforms with different altitudes and with known ages of emergence the mean value of the ten largest depths of tafoni d cm was obtained and the period of their formation t years was estimated the relationship between the two variables was found to be d 20 3 1 e 0 005t

salt as an agent in the development of cavernous weathering a well recognized spatial association exists between cavernous weathering and the presence of soluble salts disaggregation of rock in caverns is attributed to mechanical pressures exerted by crystallization and or hydration of salts however scanning electron micrographic analysis demonstrates that solutional etching is the major weathering process in caverns in near coastal locations in western australia and new south wales australia it is suggested here that the breakdown of the caverned sandstone is caused by sodium chloride which increases the rate of quartz dissolution

impact of diagenesis and low grade metamorphosis on isotope δ26mg δ13c δ18o and 87sr 86sr and elemental ca mg mn fe and sr signatures of triassic sabkha dolomites abstract dolomite is a common rock forming mineral in the geological record but its value as archive of ancient seawater 26 mg signatures and their variations in time is at present underexplored unknown factors include the sensitivity of 26 mg ratio to processes in the diagenetic and low grade metamorphic domain this paper documents and discusses the first detailed 26 mg data set from early diagenetic and burial dolomites samples come from the upper triassic hauptdolomit dolomia principale the dolomites italy and include coeval dolomicrites that underwent differential burial diagenesis and low grade metamorphosis in a temperature range between about 100 and more than 350 c magnesium isotope data are complemented by dolomite 13 c 18 o and 87 sr 86 sr isotope ratios as well as ca mg mn fe and sr elemental abundances and data on the dolomite crystal structure as indicated by dolomicrite 87 sr 86 sr ratios and petrographic evidence sabkha calcian d1 dolomites precipitated from evaporated seawater and stabilized at an early marine pore water diagenetic stage to d2 dolomites analysed here with increasing burial temperature dolomite 26 mg ratio scatter in the data set decreases with increasing mg ca ratio and degree of order specifically 26 mg ratio variability is reduced from 0 36 2 at burial temperatures beneath 100 c to about 0 14 2 at temperatures in excess of 350 c respectively with mean 26 mg values ranging constantly near 1 9 this suggests that at least for the rock buffered system investigated here dolomicrite 26 mg proxy data are conservative and preserve near pristine values even at elevated burial temperatures at present the main element of uncertainty is the mg isotope fractionation factor between evaporated seawater and dolomite a possible solution to this problem includes the compilation of data from modern sabkha environments including pore water and calcian dolomite 26 mg isotope signatures

the origin of honeycomb weathering honeycomb weathering occurs throughout the world but the origin remains a matter of controversy wind erosion exfoliation frost shattering and salt weathering have been proposed as explanations although few attempts have been made to substantiate these hypotheses with chemical or mineralogical studies chemical analyses and field observations indicate that honeycomb weathering in coastal exposures of arkosic sandstone near bellingham washington results from evaporation of salt water deposited by wave splash microscopic examination of weathered surfaces show that erosion results from disaggregation of mineral grains rather than from chemical decomposition thin walls separating adjacent cavities seem to be due to protective effects of organic coatings produced by microscopic algae inhabiting the rock surface cavity walls are not reinforced by precipitation of elements released by weathering as has often been suggested at other locations honeycomb weathering develops rapidly and can be observed on surfaces that were planar less than a century ago

cavernous weathering dynamical instability and self organization cavernous weathering may be conceptualized as a self reinforcing process characterized by positive feedback within the weathering system a morphometric study of caverns in the valley of fire state park nevada usa demonstrates the applicability of a dynamically unstable or conditionally unstable model of cavernous weathering systems outcrop surfaces displaying caverns tend to show increasing fragmentation of the surface in the early stages of cavernous weathering succeeded by convergent evolution of the surface in which caverns tend to grow and coalesce a paradoxical relationship exists between the weathering system output at the scale of individual forms and the outcrop scale caverns tend toward minimum interior surface area by developing a spheroidal form yet the outcrop surface tends toward maximum exposed surface area by increasing the degree of fragmentation of the surface copyright 2004 john wiley sons ltd

genesis of copper lead mineralization in the regionally zoned agnigundala sulfide belt cuddapah basin andhra pradesh india shallow marine sandstone shale carbonate sedimentary rocks of the paleoproterozoic northern cuddapah basin host copper nallakonda deposit copper lead dhukonda deposit and lead mineralization bandalamottu deposit which together constitute the agnigundala sulfide belt the cu sulfide mineralization in sandstone is both stratabound and disseminated and pb sulfide mineralization occurs as stratabound fracture filling veins and or replacement veins within dolomite systematic mineralogical and sulfur carbon and oxygen isotope studies of the three deposits indicate a common ore fluid that deposited copper at nallakonda copper lead at dhukonda and lead at bandalamottu under progressive cooling during migration through sediments the ore fluid was of low temperature 200 c and oxidized thermochemical reduction of basinal water sulfate produced sulfide for ore deposition it is envisaged that basal red bed and evaporite bearing rift related continental to shallow marine sediments might have acted as the source for the metals rift related faults developed during sedimentation in the basin might have punctured the ore fluid pool in the lower sedimentary succession and also acted as conduits for their upward migration the ore bearing horizons have participated in deformations during basin inversion without any recognizable remobilization

origin of dolomite in the third member of feixianguan formation lower triassic in the jiannan area sichuan basin china abstract the third member of feixianguan formation jiannan area consists of marine carbonates parts of which were dolomitized to form high quality reservoirs based on petrographical features three types of dolomite phases are distinguished including very finely finely crystalline dolomite type d1 medium crystalline dolomite type d2 and coarse crystalline saddle dolomite type d3 type d1 dolomite has characteristics of micritic to fine crystals 18 o values imply partly diagenetic stabilitization during burial type d2 dolomite has characteristics of mainly medium 100 250 m subhedral crystals with zoned to uniform dull red cl the estimates from 18 o values suggest precipitation at depths of 760 1100 m values of 13 c and 87 sr 86 sr of type d2 are similar to values of host limestone suggesting that dolomitization was mediated by t 1 f 3 seawater like fluids type d3 is strictly associated with type d2 high homogenization temperatures and limited distributions indicate that type d3 formed in a deep closed environment dolostone made of type d1 dolomite is volumetrically minor and is characterized by micropores and low permeability while type d3 as a void filling cement slightly reduces the reservoir porosity high quality reservoirs are only intimately associated with dolostone made of type d2 a burial compaction model is proposed for type d2 mg 2 was derived from seawater derived compaction fluid that was expelled from the trough strata the potential high quality reservoirs are likely to occur in opposing margin shoals

origin of vein hydrothermal carbonates in triassic limestones of the espadan ranges iberian chain e spain dolomitization and hg bearing veins in the lower triassic limestones of the espadan ranges iberian chain e of spain

hydrothermal dolomitization of the bekhme formation upper cretaceous zagros basin kurdistan region of iraq record of oil migration and degradation abstract the common presence of oil seepages in dolostones is widespread in cretaceous carbonate successions of the kurdistan region of iraq this integrated field petrographic chemical stable c o and sr isotopes and fluid inclusion study aims to link dolomitization to the origin and geochemical evolution of fluids and oil migration in the upper cretaceous bekhme carbonates flux of hot basinal hydrothermal brines which is suggested to have occurred during the zagros orogeny resulted in dolomitization and cementation of vugs and fractures by coarse crystalline saddle dolomite equant calcite and anhydrite the saddle dolomite and host dolostones have similar stable isotopic composition and formed prior to oil migration from hot 81 115 c basinal nacl mgcl2 h2o brines with salinities of 18 22 wt nacl eq the equant calcite cement which surrounds and hence postdates saddle dolomite has precipitated during oil migration from cooler 60 110 c nacl cacl2 h2o brines 14 18 wt nacl eq the yellowish fluorescence color of oil inclusions in the equant calcite indicates that the oil had api gravity of 15 25 composition which is lighter than present day oil in the reservoirs api of 10 17 this difference in oil composition is attributed to oil degradation by the flux of meteoric water which is evidenced by the low 13c values 8 5 to 3 9 vpdb as well as by nil salinity and low temperature in fluid inclusions of late columnar calcite cement this study demonstrates that linking fluid flux history and related diagenesis to the tectonic evolution of the basin provides important clues to the timing of oil migration degradation and reservoir evolution

thermochemical sulphate reduction can improve carbonate petroleum reservoir quality abstract interest in the creation of secondary pore spaces in petroleum reservoirs has increased because of a need to understand deeper and more complex reservoirs the creation of new secondary porosity that enhances overall reservoir quality in deeply buried carbonate reservoirs is controversial and some recent studies have concluded it is not an important phenomenon here we present petrography geochemistry fluid inclusion data and fluid rock interaction reaction modeling results from triassic feixianguan formation sichuan basin china core samples and explore the relative importance of secondary porosity due to thermochemical sulphate reduction tsr during deep burial diagenesis we find that new secondary pores result from the dissolution of anhydrite and possibly from dissolution of the matrix dolomite assuming porosity before tsr was 16 and the percentage of anhydrite was 6 modelling shows that due to tsr 1 6 additional porosity was created that led to permeability increasing from 110 md range 72 168 md within a 95 confidence interval to 264 md range 162 432 md within a 95 confidence interval secondary porosity results from the density differences between reactant anhydrite and product calcite the addition of new water during tsr and the generation of acidity during the reaction of new h2s with the siderite component in pre existing dolomite in the reservoir fluid pressure was high during tsr and approached lithostatic pressure in some samples this transient overpressure may have led to the maintenance of porosity due to the inhibition of compactional processes an additional 1 6 porosity is significant for reserve calculations especially considering that it occurs in conjunction with elevated permeability that results in faster flow rates to the production wells

strontium isotopic composition of devonian dolomites western canada sedimentary basin significance of sources of dolomitizing fluids abstract dolomites from the middle and upper devonian of five regions in the western canada sedimentary basin wwere studied middle devonian winnipegosis formation outcrop keg river formation reefs in the rainbow field and presqu ile barrier and the upper devonian miette buildup and the wabamun group of northern alberta data from the upper middle devonian swan hills formation in the rosevear field and nisku formation reefs were also included in these areas of western canada there are at least three dolomitizing events 1 early dolomites in winnipegosis reefs and the subsurface part of the presqu ile barrier have 87sr 86sr values near 0 7080 wabamun sabkha dolomites have 87sr 86sr values near 0 7083 and all fall on their respective parts of the seawater curve thus the sources of dolomitizing fluids were devonian seawaters 2 secondary matrix dolomites from the presqu ile barrier rainbow rosevear miette nisku and wabamun all have slightly higher 87sr 86sr values 0 7081 0 7094 than the correspnding devonian seawater dolomitizing fluids were probably modified from devonian seawater with small amounts of radiogenic 87sr being added during early compaction from adjacent or underlying clastics or older carbonates 3 late stage saddle dolomites from presqu ile rainbow rosevear miette nisku and wabamum are all highly radiogenic 0 7100 up to 0 7151 except for most pine point saddle dolomites indicating significant input of radiogenic 87sr with likely sources from the underlying cambrian and precambrian clastics and or the precambrian basement at pine point some presqu ile saddle dolomites have sr isotopic values similar to matrix dolomites suggesting a different fluid source possibly from subsurface brines that may have evolved from pressure solution of devonian strata alternatively formation waters derived from the deeper part of the basin may have been progressively diluted with low 87sr 86sr shallow formation waters as they moved updip along the presu ile barrier the 87sr 86sr values support and refine earlier conclusions based on textual evidence c and o isotopes geochemistry and fluid inclusions they considerably limit the sources of the dolomitizing fluids in open diagenetic systems however the timing of the widespread secondary matrix dolomites can only be broadly constrained to sometime during the late paleozoic the similarity of the o and sr isotopes in the replacement dolomites from widely separated locations geographically and stratigraphically tentatively suggests that these dolomites may have precipitated from similar formation waters on a basin wide scale the late stage saddle dolomites and probably the coarsely crystalline dolomites could only have formed near maximum burial conditions in the late cretaceous to early tertiary from warm hydrothermal brines that moved updip from more deeply buried portions of the basin later coarse crystalline calcites precipitated from highly saline brines and are generally more radiogenic than saddle dolomites they apparently formed from more radiogenic formation waters than have curretly been reported from the basin

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

metal accumulation during and after deposition of the kupferschiefer from the sangerhausen basin germany abstract a densely sampled profile 58 cm in thickness composed of 13 samples of the kupferschiefer and overlying zechstein carbonates from the sangerhausen basin germany has been analysed by various geochemical and microscopic methods in order to clarify the mechanism of base metal accumulation in this location the kupferschiefer is only slightly influenced by the hematite bearing oxidized fluids called rote faule the determination of facies dependent parameters along the profile indicates that kupferschiefer from the sangerhausen basin was largely deposited in a marine environment only at the beginning of kupferschiefer sedimentation did euxinic conditions prevail the bottom part of the profile is significantly enriched in trace elements such as cu ph zn as co ag and u the cu concentration amounts to 19 88 wt post depositional oxidation of the organic matter is observed only in the transition zone between the kupferschiefer and the zechstein conglomerate indicating the influence of ascending oxidizing brines microscopic analyses show that only fe sulfides form framboidal textures cu minerals are present along the total profile preferentially in fractures and as patchy structures composed of chalcocite chalcopyrite and bornite in the highly mineralized bottom section cu sulfides are associated with pyrobitumen sparry calcite and arsenopyrite results from maturation studies of organic matter suggest that the maximum temperature affecting the kupferschiefer was approximately 130 c a 3 step process of metal accumulation is proposed during deposition of the sediment framboidal pyrite and pyrite precursors were precipitated by bacterial so 4 2 reduction bsr during diagenesis the pyrite and pyrite precursors were largely replaced by mixed cu fe minerals and by chalcocite pr in the section with very high cu contents 8 reduced sulfur from fe sulfides was not sufficient for precipitation of cu and other trace metals from ascending solutions in this part of the profile thermochemical so 4 2 reduction tsr occurred after pyrite replacement as indicated by the presence of pyrobitumen and sparry calcite

origin of the nchanga copper cobalt deposits of the zambian copperbelt the zambian copperbelt forms the southeastern part of the 900 km long neoproterozoic lufilian arc and contains one of the world s largest accumulations of sediment hosted stratiform copper mineralization the nchanga deposit is one of the most significant ore systems in the zambian copperbelt and contains two major economic concentrations of copper and cobalt hosted within the lower roan group of the katangan supergroup a lower orebody copper only and upper orebody copper and cobalt occur towards the top of arkosic units and within the base of overlying shales the sulfide mineralogy includes pyrite bornite chalcopyrite and chalcocite although in the lower orebody sulfide phases are partially or completely replaced by malachite and copper oxides carrollite is the major cobalt bearing phase and is restricted to fault propagation fold zones within a feldspathic arenite hydrothermal alteration minerals include dolomite phlogophite sericite rutile quartz tourmaline and chlorite quartz veins from the mine sequence show halite saturated fluid inclusions ranging from 31 to 38 wt equivalent nacl with homogenisation temperatures thtot ranging between 140 and 180 c diagenetic pyrites in the lower orebody show distinct relatively low 34s ranging from 1 to 17 whereas arenite and shale hosted copper and cobalt sulfides reveal distinctly different 34s from 1 to 12 for the lower orebody and 5 to 18 for the upper orebody there is also a clear distinction between the 34s mean of 12 1 3 3 n 65 for the upper orebody compared with 5 2 3 6 n 23 for the lower orebody the 13c of dolomites from units above the upper orebody give 13c values of 1 4 to 2 5 consistent with marine carbon however dolomite from the shear zones and the alteration assemblages within the upper orebody show more negative 13c values 2 9 to 4 0 and 5 6 to 8 3 respectively similarly shear zone and upper orebody dolomites give a 18o of 11 7 to 16 9 compared to lower roan dolomites which show 18o of 22 4 to 23 0 two distinct structural regimes are recognized in the nchanga area a weakly deformed zone consisting of basement and overlying footwall siliciclastics and a moderate to tightly folded zone of meta sediments of the katangan succession the fold geometry of the lower roan package is controlled by internal thrust fault propagation folds which detach at the top of the lowermost arkose or within the base of the overlying stratigraphy and show vergence towards the ne faulting and folding are considered to be synchronous as folding predominantly occurred at the tips of propagating thrust faults with local thrust breakthrough the data from nchanga suggests a strong link between ore formation and the development of structures during basin inversion as part of the lufilian orogeny sulfides tend to be concentrated within arenites or coarser grained layers within shale units suggesting that host rock porosity and possibly permeability played a role in ore formation however sulfides are also commonly orientated along but not deformed by a tectonic fabric or hosted within small fractures that suggest a significant role for deformation in the development of the mineralization the ore mineralogy hydrothermal alteration and stable isotope data lend support to models consistent with the thermochemical reduction of a sulfate and metal enriched hydrothermal fluid at the site of mineralization there is no evidence at nchanga for a contribution of bacteriogenic sulfide produced during sedimentation or early diagenesis to the ores

multiple episodes of dolomitization and dolomite recrystallization during shallow burial in upper jurassic shelf carbonates eastern swabian alb southern germany abstract the upper jurassic of the eastern swabian alb is composed of oolitic platform sands with associated microbe siliceous sponge mounds at the platform margins they are surrounded by argillaceous or calcareous mudstones and marl limestone alternations deposited in adjacent marl basins partial to complete dolomitization is predominantly confined to the mound facies six types of dolomite as well as one type of ankerite document a complex diagenetic history during shallow burial with multiple episodes of dolomite formation and recrystallization the earliest massive matrix dolomitization is ca rich has slightly depleted oxygen isotope values relative to late jurassic seawater and carbon isotopic values in equilibrium with late jurassic seawater this initial massive matrix dolomitization occurred during latest jurassic to earliest cretaceous and is related to pressure dissolution during very shallow burial at temperatures of at least 50 c hydrologic conditions and mass balance calculations indicate that burial compaction provided sufficient fluids for dolomitization mg is derived from negligibly modified seawater that was expelled from the adjacent off reef strata into the mound facies position of the mounds along the platform margins controlled the distribution of the shallow burial dolomite covariant trends between textural modification increasing stoichiometry partial changes in trace element content mn fe sr and depletion in stable isotopes as well as distinctive cl pattern illustrate two recrystallization phases of the precursor matrix dolomite during further burial at elevated temperatures strong sr enrichment of the second phase of recrystallized dolomite is ascribed to sr rich meteoric waters descending from overlying aragonite bearing reef limestones or evaporite bearing peritidal carbonates late stage coarsely crystalline dolomite cements occur as vug and fracture fillings and formed during burial ankerite associated with sulphide and sulphate minerals and saddle dolomite are assumed to have formed from hydrothermal waters that moved to higher stratigraphic levels along fracture conduit systems that developed during late cretaceous to tertiary alpine orogenesis

diagenesis of an evaporite related carbonate reservoir in deeply buried cambrian strata tarim basin northwest china many hydrocarbon shows have been reported from potential evaporite related carbonate reservoirs in the lower to middle cambrian units in the tarim basin petrography facies and geochemical analyses from outcrop and core samples were integrated to document the effect of diagenetic evolution on these evaporite related carbonate reservoirs a simplified depositional model has been developed that reveals a restricted carbonate platform dominated by an evaporatic inner platform lagoon with shoal and reef facies developed around the platform margins grainstones and packstones were deposited at the platform margin and on inner platform shoals whereas carbonate mudstones and wackestones were deposited in the lagoon associated with evaporites early dolomitization and dissolution occurred related to the reflux of evaporated seawater breccia associated fracture porosity caused by a short period of meteoric water induced dissolution is the predominant type of effective porosity in the anhydrite bearing dolomudstone reservoir thermochemical sulfate reduction tsr occurred in the deep subsurface diagenetic environment characterized by high temperature 100 c 160 c and high salinity 16 26 wt aqueous fluid inclusions in tsr calcite that has relatively negative 13c values as low as 12 vienna peedee belemnite and elevated 87sr 86sr values the porous dolomites contain solution enlarged pores caused by dissolution of anhydrite and carbonate as a secondary consequence of tsr this study shows that deeply buried evaporite related ancient carbonate rocks may be effective reservoirs because of a combination of fracture porosity and tsr induced secondary porosity evaporitic cambrian carbonates buried to over 5000 m 16 500 ft can thus be considered as targets for future exploration

mid cretaceous hydrothermal vents and authigenic carbonates in a transform margin basque cantabrian basin western pyrenees a multidisciplinary study a comprehensive study of authigenic carbonates and associated fauna in late albian organic rich deep water deposits the black flysch group reveals that carbonate precipitation was a by product of the anaerobic oxidation of hydrocarbon rich hydrothermal fluids the authigenic carbonates are exposed along the kardala and alkolea sea cliffs in the western pyrenees the two vent carbonates occur 1 km apart adjacent to the synsedimentary right reverse mutriku fault but in contrasting structural domains the kardala carbonates occur on a structural ridge hangingwall and the alkolea carbonates are positioned at the base of an erosional scarp folded downward footwall the similarity in pattern of the carbonate phases and complex paragenetic events for both vent precipitates implies that hydrothermal fluid generation processes and pore water evolution during early and late diagenesis were similar nevertheless a comparison of the geochemistry fossil fauna and morphology of carbonate structures of both precipitates suggests that the vented hydrocarbon type flow intensity and temperature of hydrothermal fluids were different at the kardala vent intense focused flow of hot up to 109 c oil rich fluids were generated allowing the development of a relatively abundant chemosynthesis based fauna in contrast at the alkolea vent diffuse flows of warm thermogenic methane rich fluids were expelled to the sea floor and no chemosynthetic fauna developed these differences are related to the contrasting structural setting of each locality similar d 13 corg values for both pyrobitumen fills and host unit organic matter suggest that the hydrocarbon source was the black flysch group contact alteration of these organic rich sediments by syndepositional hydrothermal fluids generated hydrothermal petroleum oil and gas hydrocarbons which probably migrated updip to the sea floor by contemporary compression tectonics

fluid inclusions in pre ore minerals from the carbonate hosted mercury deposits in the espadan ranges eastern spain abstract the fluids associated with pre ore minerals in the carbonate hosted mercury veins from the espadan ranges eastern spain have been studied using microthermometry cryo sem eds raman spectroscopy and crush leach analyses in order to determine their origin and the processes responsible for mineralization three primary fluid inclusion types have been found in quartz and dolomite 1 type s composed of liquid vapour mainly co 2 halite 2 type s v composed of liquid high density co 2 double bubble halite and 3 type v composed of high density co 2 n 2 mixtures type s fluid inclusions are preferentially trapped in betxi type veins characterized by the presence of hydrothermal dolomite whereas type v fluid inclusions are trapped almost exclusively in eslida type veins which were without dolomite from these data two different fluids can be inferred to occur during the period of formation of the veins 1 a complex polysaline brine of nacl kcl cacl 2 mgcl 2 so 4 2 h 2 o composition with a salinity of up to 33 eq wt nacl and a 2 co 2 n 2 rich fluid the brine halogen composition points to its secondary origin caused by the dissolution of underlying marine evaporites it is thought that these deposits were formed during jurassic or cretaceous rifting periods as a consequence of fluid mixing between a high salinity secondary brine and connate waters the generation of a co 2 n 2 rich phase caused the pressure increase of the fluids that favoured the hydraulic fracturing of the enclosing rock dolostone the origin of this volatile rich phase probably occurred due to the oxidation and thermochemical breakdown of the organic matter contained in the host rock this event is recorded by the coetaneous trapping of both aqueous and carbonic fluids in quartz crystals the formation conditions for the hypogene sulphide paragenesis are obscure but the coincidence between the preferential trapping of type v fluid inclusions the absence of hydrothermal dolomite and the major abundance of mercury in eslida type veins is thought to be linked to a mercury fractionation event to the vapour phase during the release of the volatiles

the influence of thermochemical sulphate reduction on hydrocarbon composition in nisku reservoirs brazeau river area alberta canada abstract the upper devonian nisku formation reservoirs of the brazeau river area of west central alberta canada produce oil and sweet and sour gas condensate generally oil pools are located updip in the study area and sour 6 31 h2s gas condensate downdip h2s in the study area is formed by thermochemical sulphate reduction tsr all liquid hydrocarbons probably have one source with the duvernay formation being the most likely candidate assessing the relative maturity of the oils and condensates is difficult because of the wide variation in thermal maturity and the effects of tsr but the ratio of pristane n heptadecane does appear to decrease with increasing maturity for this sample set with increasing tsr the following changes were noted decrease in the saturate aromatic hydrocarbon ratio increase in the relative abundance of organo sulphur compounds e g benzothiophenes 34s values of liquid hydrocarbon samples approached the values for anhydrite of the nisku formation in the study area and an increase in 13c of the saturate fraction h2s concentrations in hydrodynamically isolated pools provide a good estimate of the extent of tsr in these reservoirs however other pools have anomalously high h2s concentrations for their depth suggesting that the h2s was generated at greater depths and migrated updip into these pools

epigenetic dolomitization of the přaidoli formation upper silurian the barrandian basin czech republic implications for burial history of lower paleozoic strata stratabound epigenetic dolomite occurs in carbonate facies of the barrandian basin silurian and devonian czech republic the most intense dolomitization is developed in bioclastic calcarenites within the transition between micritic limestone and shaledominated p idoli and lochkov formations deposited on a carbonate slope medium crystalline 100 400 m inclusion rich xenotopic matrix dolomite 18o 4 64 to 3 40 pdb 13c 1 05 to 1 85 pdb which selectively replaced most of the bioclastic precursor is volumetrically the most important dolomite type coarse crystalline saddle dolomite 18o 8 04 to 5 14 pdb 18c 0 49 to 1 49 pdb which precipitated in fractures and vugs within the matrix dolomite represents a later diagenetic dolomitization event in some vugs saddle dolomite coprecipitated with petroleum inclusion rich authigenic quartz crystals and minor sulfides which in turn were post dated by semisolid asphaltic bitumen the interpretation of the dolomitization remains equivocal massive xenotopic dolomite although generally characteristic of a deeper burial setting may have been formed by a recrystallization of an earlier possibly shallow burial dolomite deeper burial recrystallization by reactive basinal pore fluids that presumably migrated through the more permeable upper portion of the p idoli sequence appears as a viable explanation for this dolomitization overprint saddle dolomite cement of the matrix dolomite is interpreted as the last dolomitization event that occurred during deep burial at the depth of the oil window zone the presence of saddle dolomite the fluid inclusion composition of associated quartz crystals and vitrinite paleogeothermometry of adjacent sediments imply diagenetic burial temperatures as high as 160 c although high geothermal gradients in the past or the involvement of hydrothermally influenced basinal fluids can account for these elevated temperatures burial heating beneath approximately 3 km thick sedimentary overburden of presumably post givetian strata no longer preserved in the basin appears to be the most likely interpretation this interpretaion may imply that the magnitude of post variscan erosion in the barrandian area was substantially greater than previously thought

tectonic deformation around the eastern himalayan syntaxis constraints from the cretaceous palaeomagnetic data of the shan thai block summary lower to middle cretaceous red sandstones were sampled at four localities in the lanpin simao fold belt of the shan thai block to describe its regional deformational features most of the samples revealed a characteristic remanent magnetization with unblocking temperatures around 680 c primary natures of magnetization are ascertained through positive fold test a tilt corrected formation mean direction for the jingdong 24 5 n 100 8 e locality which is located at a distance of 25 km from the ailaoshan red river fault revealed northerly declination with steep inclination dec inc 8 3 48 8 95 7 7 n 13 however mean directions obtained from the zhengyuan 24 0 n 101 1 e west zhengyuan 24 0 n 101 1 e and south mengla 21 4 n 101 6 e localities indicate an easterly deflection in declination such as dec inc 61 8 46 1 95 8 1 n 7 dec inc 324 2 49 4 95 6 4 n 4 and dec inc 51 2 46 4 95 5 6 n 13 respectively the palaeomagnetic directions obtained from these four localities are incorporated into a palaeomagnetic database for the shan thai block when combined with geological geochronological and gps data the processes of deformation in the shan thai block is described as follows subsequent to its rigid block clockwise rotation of about 20 in the early stage of india asia collision the shan thai block experienced a coherent but southward displacement along the red river fault prior to 32 ma this block was then subjected to a north south compressive stresses during the 32 27 ma period which played a key role in shaping the structure of chongshan lancang chiang mai belt following this some local clockwise rotational motion has occurred during the pliocene quaternary time in central part of the shan thai block as a result of internal block movements along the reactivated network of faults

tafoni on coastal slopes south devon u k cavernous weathering tafoni development occurs on coastal slopes in greenschist bedrock at elevations up to 40 m above sea level the freshly weathered surfaces of the cavern interiors are irregular in morphology discordant to major rock structure formed by substantially weakened rock and associated with granular weathering debris the weathering debris contains soluble elements in proportions similar to those present in seawater and the penetration of elements associated with sea salts into the weathering surface to estimated depths of at least 0 1 0 2 m is indicated by the presence of chlorine scanning electron microscopy and microprobe analyses suggest that rock breakdown occurs principally through limited chemical weathering at grain boundaries the mechanism for the emplacement of marine salts within sheltered rock surfaces in the tafoni is postulated to be a combination of dry deposition under turbulent atmospheric conditions and wetting by coastal fog

chemical and carbon isotopic fractionations of gaseous hydrocarbons during abiogenic oxidation abstract the variations of chemical and carbon isotopic compositions in gaseous hydrocarbons upon abiogenic oxidation by sulfates hematite and both in laboratory studies were clearly demonstrated through a unique two step experimental approach in the first step a large number of small gold capsules containing gaseous hydrocarbons were prepared by kerogen pyrolysis experiments which were conducted under the same condition 450 c and 50 mpa for 72 h using the same kerogen the analytical results demonstrated that the chemical and carbon isotopic compositions of gases were almost identical among the capsules in the second step each of these gaseous hydrocarbon bearing capsules along with mineral oxidant hematite or magnesium sulfate heptahydrate or both and deionized water 15 of the amount of the oxidant was placed into a large gold capsule after welding sealing the small gaseous hydrocarbon bearing capsule was forced to leak while the large capsule remained undamaged by compressing the large capsule from the outside at the position where this small capsule was located then these large capsules containing gas and mineral reactants were heated isothermally at 350 c and 50 mpa for 72 144 216 and 288 h respectively the results of the oxidation experiments with increasing heating time can be outlined as follows 1 the amount of methane remained almost unchanged in the experiments using hematite and the mixed oxidants hematite mgso4 while it increased substantially in the experiment using mgso4 after 72 h indicating methane was one of the final products of c2 oxidation 2 the amounts of c2 c5 hydrocarbons decreased consistently and more rapidly in the experiment using mgso4 than that using hematite and the mixed oxidants 3 the oxidation rates of gas hydrocarbons increased with increasing carbon number of hydrocarbons 4 the oxidation rates of i butane and i pentane were substantially higher than those of the corresponding n butane and n pentane 5 the amount of h2s increased substantially in the experiment using mgso4 whereas it was below the detection level in the experiments using hematite and the mixed oxidants 6 the 13c values of c1 c5 hydrocarbons became less negative and the isotopic fractionation extent increased with increasing carbon number of hydrocarbons and oxidation extent 7 carbon isotopic fractionation factor k12 k13 decreased with increasing oxidation rate of gas hydrocarbons and 8 the rate of thermochemical sulfate reduction tsr was strongly dependent upon the presence and concentrations of h2s

saddle dolomite as a by product of chemical compaction and thermochemical sulfate reduction petrographic stable and radiogenic isotope trace element and fluid inclusion data from the devonian nisku reef trend in the subsurface of alberta canada suggest that saddle dolomite was a by product of chemical compaction and thermochemical sulfate reduction in subsiding dolomitized rocks furthermore thermochemical sulfate reduction may be representative of other processes that increase the amount of saddle dolomite via an increase in carbonate alkalinity the data also confirm that saddle dolomite is most likely to form at elevated temperatures from hypersaline brines

structural features and genesis of microscopic pores in lacustrine shale in an oil window a case study of the dongying depression the paleogene shale of the dongying depression a continental basin in eastern china is taken as the study subject to examine the microscopic features of lacustrine shale reservoirs in the oil window this study shows that shale pores in this evolutionary stage are present at the micrometer to nanometer scale but fractures commonly have extension distances at the millimeter scale pores and fractures can be divided into three types namely primary pores secondary pores and cracks primary pores commonly have good connectivity at shallow burial depth with the increase of burial depth primary porosity is reduced because of compaction and cementation secondary pores are important in shale including dissolved pores inside grains and at grain edge and dissolution pores inside the hybrid of organic matter om and clay minerals and evaporite minerals including carbonates or sulfates types of cracks were observed bedding fissures dissolution fractures and structural fractures the development of bedding fissures is related to the deposition of shale laminae the formation of dissolution fractures is related to acidic fluids such as organic acids and hydrogen sulfide whereas the formation of structural fractures is jointly controlled by fault development fluid overpressure and lithofacies the pores and fractures in the oil window of lacustrine shale can store and channel oil and gas the hybrid om clay carbonate sulfate and the pores inside are important through the oil window moreover the development of the pores depends not only on hydrocarbon generation but also on the interaction of hydrocarbons and organic acid dissolution this finding has important significance in the accumulation of oil and gas in continental shales

differences in the depositional environment of basal zechstein in southwest poland implication for base metal mineralization abstract base metal enrichment in the permian basal zechstein sediments occurs in two major areas in southwest poland the north sudetic syncline and the fore sudetic monocline two profiles of kupferschiefer and adjacent limestones from the konrad mine and from the rudna mine were studied by petrographic and geochemical methods in order to recognize similarities and differences in the comparative depositional environment of both areas in the north sudetic syncline basal zechstein was deposited in a shallow marine environment under slightly oxidizing sedimentary conditions it is composed of 1 2m thick mergelkalk which might be the equivalent of werrakalk ca1 in the fore sudetic monocline the basal zechstein is a black shale the so called kupferschiefer t1 with a thickness of 0 2 0 5 m microscopical investigations have shown that a substantial contribution of higher continental plant material is observed only in the north sudetic syncline because of its location near the zechstein sea shore pyrite is present in small amounts only in the fore sudetic monocline apart from the former shoreline of the zechstein sea the organic matter is preferentially composed of degraded algae due to its deposition under deeper anoxic bottom waters abundant pyrite is also present differences in the chemical composition of the respective kerogens are reflected in the results obtained from rock eval analyses hydrogen indices hi are significantly higher in the t1 section of the profile from the rudna mine in comparison to the basal zechstein of the konrad mine profile a depletion of hydrogen is only observed in the highly mineralized section and in rote faule of the konrad mine this indicates that hydrogen equivalents from organic matter participated in metal sulfide precipitation by thermochemical sulfate reduction tsr this is further confirmed by the occurrence of pyrobitumen saddle dolomite and calcite spars in contrast in the mineralized section of the rudna mine profile a depletion of hydrogen is not observed pyrobitumen is missing and saddle dolomite and calcite spars are very rare instead abundant pyrite originating from bacterial sulfate reduction bsr could have been the source of reduced sulfur species available for base metal precipitation after deposition of the sediment pyrite replacement the results allow one to conclude that in both areas base metals were precipitated from ascending solutions due to the interaction with the reducing zechstein barrier several observations argue for a post depositional mineralization process at a temperature of less than 130 c in both areas

hydrothermal dolomite a product of poor definition and imagination abstract the latest dolomite bandwagon is the hydrothermal dolomite model in its present form this bandwagon is doomed or at least very much overstated for at least two reasons 1 there are several definitions of hydrothermal and hardly any author specifies which one s he is using 2 very few of the dolomites hitherto called hydrothermal have been demonstrated to be hydrothermal according to any definition except the worst as presently applied the term hydrothermal dolomite is confusing and or meaningless we suggest to use white s geol soc amer bull 68 1957 1637 time honored definition of hydrothermal as aqueous solutions that are warm or hot relative to its surrounding environment with no genetic implications regarding the fluid source hence a dolomite should be called hydrothermal only if it can be demonstrated to have formed at a higher than ambient temperature regardless of fluid source or drive furthermore this definition does not carry a lower or upper temperature limit even a dolomite formed at 40 c could be hydrothermal by extension dolomites formed at temperatures lower than ambient are not hydrothermal even if they formed at a rather high temperature for example groundwater may penetrate a rock sequence through a highly permeable pathway such that it is heated to 150 c at a depth where the surrounding rock has a temperature of 250 c we suggest to call dolomite formed from this water hydrofrigid dolomite formed in or near thermal equilibrium with the surrounding rocks may be called geothermal furthermore not all saddle dolomite formation requires advection fluid flow to transport mg saddle dolomite can be formed in at least three ways i e from advection local redistribution of older dolomite during stylolitization and as a by product of thermochemical sulfate reduction in a closed or semi closed system only the first and the last of these three possibilities have a chance of being hydrothermal almost all dolomites and dolostones in the western canada sedimentary basin have recently been re interpreted as hydrothermal applying the rationale outlined above reveals however that this basin contains very little hydrothermal dolomite rather most dolomites in this basin and almost all dolostones south of the peace river arch are geothermal and or the proof of a hydrothermal origin has not been made this has important implications beyond the various case studies at hand as attempts to tie dolomitization to orogenic events become moot at least in the southern part of the basin

evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the puguang gas field sichuan basin china the puguang gas field is the largest gas field found in marine carbonates in china marine carbonate reservoirs in this field were buried to a depth of about 7000 m 22 966 ft and experienced maximum temperature up to 220c before uplift to the present day depth of 5000 5500 m 16 404 18 045 ft with present day thermal maturity between 2 0 and 3 0 equivalent vitrinite reflectance ro sulfur rich pyrobitumens with reflectance up to 3 5 are widespread in the reservoirs and resulted from thermal cracking of crude oils most likely generated from upper permian source rocks and thermochemical sulfate reduction tsr natural gases in the puguang gas field have wide variations in nonhydrocarbon gas contents with h2s contents between 5 1 and 58 3 and co2 contents between 7 9 and 18 0 the hydrocarbon gases originated mainly from thermal cracking of accumulated oil but were altered by tsr thermochemical sulfate reduction in the study area exerted different effects on the isotope compositions of different hydrocarbon gas components at different tsr stages the differential increase of 13c values for different gas components reflects transformation from a heavy hydrocarbon gas dominated tsr stage to a methane dominated tsr stage this caused a decrease of 13cmethane13cethane values and a corresponding conversion from reversed to normal isotope distributions thermochemical sulfate reduction in the study area appears to have been limited by sulfate concentrations in the reservoirs a successive three stage tsr series namely liquid hydrocarbon involved tsr heavy hydrocarbon gas dominated tsr and methane dominated tsr occurred in reservoirs with sufficient sulfate concentration methane can be the dominant organic reactant for tsr but only at elevated temperature and after most c2 hydrocarbons are exhausted

effects of progressive burial on matrix porosity and permeability of dolostones in the foreland basin of the alpine orogen switzerland abstract the changes in rock matrix porosity and permeability that carbonate reservoirs undergo with increasing burial depth are poorly understood this lack of understanding raises the risks involved in exploring and engineering deep reservoirs for geo energy applications to provide more insight into compaction processes the present study examines the effects of progressive burial on two dolomitized mudstone units belonging to the middle triassic muschelkalk within the swiss molasse basin situated in the foreland of the alpine orogen based on investigations of wireline logs and drill cores retrieved from up to 5000 m depth we report the burial modification of crystal textures pore sizes pore geometries and their impact on matrix porosity and permeability within the first 1500 m below surface porosity is found to drop from 40 2 to 18 1 vol and permeability drops from 105 15 to 1 md at depths 3000 m porosity and permeability maintain nearly constant values of 6 2 vol and 1900 m mechanical compaction is inactive and pressure solution at crystal contacts and along stylolites both diagenetic and tectonic without any associated cementation accounts for porosity loss at depths 3000 m collapse of pores by pressure solution is compounded by pore clogging by hydrothermal dolomite introduced by external fluids throughout the entire depth range stylolitization incrementally thins the formations however the dissolved material is not locally reprecipitated

a new technique for non destructive field measurement of rock surface strength an application of the equotip hardness tester to weathering studies tafone like depressions have developed on the aoshima sandstone blocks used for a masonry bridge pier in the coastal spray zone a thin layer of partial granular disintegration was found on the surface in depressions to evaluate quantitatively the strength of the thin weathered layer the hardness was measured at the surface of the sandstone blocks using both an equotip hardness tester and an l type schmidt hammer comparison of the two testing results indicates that the equotip hardness value is more sensitive in evaluating the strength of a thin layer of weathered surface rock than the schmidt hardness value by applying two methods i e both the repeated impact method and the single impact method the equotip tester can evaluate the strengths of fresh internal and weathered surficial portions of rocks having a thin weathering layer comparison of the two strengths enables evaluation of strength reduction due to weathering copyright 2007 john wiley sons ltd

rates of development of tafoni in the moenkopi and kaibab formations in meteor crater and on the colorado plateau northeastern arizona tafoni are pits formed by non uniform weathering in otherwise uniform rock two equations have been proposed for the rate of development of tafoni both based on 2000 year old outcrops from the coast of japan we have taken tafoni measurements from the meteor crater arizona and vicinity that extend the equations back at least 50 000 years as reported in earlier studies we found pit depth to be the best tafone parameter to measure the size of the pit decreases significantly with increasing inclination of the rock surface however the size of the pit can vary greatly for other reasons in some cases the measurements are statistically significantly different between two stations taken from contiguous areas of similar inclination and aspect in an apparently homogeneous bed it is clear however that over tens of thousands of years tafoni enlarge significantly our data are generally log normal and all are markedly heteroscedastic the 1991 equation proposed by matsukura and matsuoka does not fit our data the 1996 equation proposed by sunamura provides a better fit we propose a sigmoidal equation d b1 e b2 b3 t where d is the depth t is the age and b1 b2 and b3 vary with lithology this new equation fits our data far better than the earlier published equations copyright 2002 john wiley sons ltd

primary and diagenetic isotopic signals in fossils and hemipelagic carbonates the lower jurassic of northern spain stable isotope and trace element analyses of 230 jurassic pliensbachian toarcian samples from northern spain have been performed to test the use of geochemical variations in fossils belemnites and brachiopods and whole rock hemipelagic carbonates as palaeoceanographic indicators although the succession analysed reinosa area westernmost basque cantabrian basin has been subject to severe thermal alteration during burial diagenesis the samples appear to be well preserved the degree of diagenetic alteration of the samples has been assessed through the application of integrated petrographic chemical and cathodoluminescence analyses it is demonstrated that brachiopods and whole rock carbonates although widely used for palaeoceanic studies do not retain their primary marine geochemical composition after burial diagenesis in contrast there is strong evidence that belemnite rostra preserve original isotopic values despite pervasive diagenesis of the host rock well preserved belemnite shells non luminescent to slightly luminescent typically show stable isotope values of 4ae3 to 0ae7 d 13 c 0ae7 to 3ae2 d 18 o and trace element contents of 950 l gg 1 sr and sr mn ratios 80 this study suggests that the degree to which diagenesis has affected the preservation of an original isotopic composition may differ for different low mg calcite fossil shells and hemipelagic bulk carbonates behaviour that should be considered when marine isotopic signatures from other ancient carbonate rocks are investigated multiple non luminescent contemporaneous belemnite samples passed the petrographic and geochemical tests to be considered as palaeoceanic recorders yet their d 13 c and d 18 o values exhibited moderate scatter such variability is likely to be related to the palaeoecological behaviour of belemnites and or highfrequency secular variations in sea water chemistry superimposed on the longterm isotopic trend a pronounced positive carbon isotope excursion up to 4ae3 is documented in the early toarcian serpentinus biozone which correlates with the toarcian d 13 c maximum reported in other european and tethyan regions

gas souring by thermochemical sulfate reduction at 140 c reply natural gas in the permian triassic khuff formation of abu dhabi contains variable amounts of h2s gas souring occurred through thermochemical sulfate reduction of anhydrite by hydrocarbon gases sour gas is observed only in reservoirs hotter than a critical reaction temperature 140 c petrographic examination of core from a wide depth range showed that the anhydrite reactant has been replaced by calcite reaction product only in samples deeper than 4300 m gas composition data show that only reservoirs deeper than 4300 m contain large quantities of h2s i e 10 at present day geothermal gradients 4300 m is equivalent to 140 c fluid inclusion analysis of calcite reaction product has shown that calcite growth only became significan at temperatures greater than 140 c thus three independent indicators all show that 140 c is the critical temperature above which gas souring by thermochemical sulfate reduction begins the previously suggested lower temperature thresholds for other sour gas provinces 80 130 c derive from gas composition data that may not allow adequately either for the reservoir temperature history or for the migration of gas generated at higher temperatures into present traps conversely published proposals for higher threshold temperature 180 200 c derive from short duration experimental data that are not easily extrapolated to geologically realistic temperatures and time scales therefore the temperature of 140 c derived from our study of the khuff formation may be th best estimate of temperature required for in situ thermochemical sulfate reduction to produce the high h2s concentrations encountered in deep carbonate gas reservoirs

a physical model for the rate of coastal tafoni development to explain the temporal change in growth rates of tafoni on homogeneous insoluble rocks modeling is carried out based on a formulated relationship for weathering induced reduction of rock strength the model is expressed by z z c 1 sqrt t ast t where z is the depth of tafoni z c is the ultimate depth t is the time and t is the time lag application of the model to the pacific coast of japan reveals that 1 the model accurately describes tafoni development 2 average growth rates presented by most previous studies are underestimated and 3 an exponential decrease in growth rates stems from strength reduction by inherent rock weathering

a preliminary scanning electron microscope study of honeycomb weathering of sandstone in a coastal environment honeycomb weathering has been observed in a carboniferous sandstone at a coastal location near ballycastle on the north coast of northern ireland specimens of this sandstone have been analysed by x ray diffraction scanning electron microscopy and energy dispersive spectrometry results reveal that calcium sulphate gypsum is the only salt present and is found only at and immediately below the rock surface sem observations suggest that crystallization of salts in pores could easily dislodge quartz grains to promote granular disintegration whilst etching of quartz grain surfaces attests to chemical weathering activity within the rock however the reason for the development of the honeycomb pattern is not known

biogenic origin of coastal honeycomb weathering honeycomb weathering occurs in two environments in late cretaceous and eocene sandstone outcrops along the coastlines of south west oregon and north west washington usa and south west british columbia canada at these sites honeycomb weathering is found on subhorizontal rock surfaces in the intertidal zone and on steep faces in the salt spray zone above the mean high tide level in both environments cavity development is initiated by salt weathering in the intertidal zone cavity shapes and sizes are primarily controlled by wetting drying cycles and the rate of development greatly diminishes when cavities reach a critical size where the amount of seawater left by receding tides is so great that evaporation no longer produces saturated solutions encrustations of algae or barnacles may also inhibit cavity enlargement in the supratidal spray zone honeycomb weathering results from a dynamic balance between the corrosive action of salt and the protective effects of endolithic microbes subtle environmental shifts may cause honeycomb cavity patterns to continue to develop to become stable or to coalesce to produce a barren surface cavity patterns produced by complex interactions between inorganic processes and biologic activity provide a geological model of self organization surface hardening is not a factor in honeycomb formation at these study sites salt weathering in coastal environments is an intermittently active process that requires particular wind and tidal conditions to provide a supply of salt water and temperature and humidity conditions that cause evaporation under these conditions salt residues may be detectable in honeycomb weathered rock but absent at other times honeycomb weathering can form in only a few decades but erosion rates are retarded in areas of the rock that contain cavity patterns relative to adjacent non honeycombed surfaces copyright 2010 john wiley sons ltd

diagenesis and paleofluid flow in the devonian southesk cairn carbonate complex in alberta canada abstract one of the largest geological features in the deep part of the alberta basin in western canada is the middle to upper devonian southesk cairn carbonate complex sccc the sccc lies at depths of about 5000 m in the vicinity of the disturbed belt of the rocky mountains and is rising to depths of 2000 m in a northeasterly direction over a distance of 150 km the complex contains several natural gas pools which contain up to about 30 h2s the major focus of this study is to construct a detailed diagenetic history and concurrent pore fluid flow over time the southesk cairn complex reveals a complicated diagenetic history that consists of five phases of pore water evolution 1 synsedimentary to shallow burial 2 intermediate burial 3 deep burial 4 maximum burial and 5 present depth each of these phases influenced the character and the quality of the reservoir rocks phase 4 is of special interest because the strontium isotope data of late diagenetic calcite cements suggest the influence of basin external fluids the 87sr 86sr isotope ratios of these calcite cements are high up to 0 7320 directly adjacent to the disturbed belt and they decrease with increasing distance towards the foreland basin 0 7100 the radiogenic strontium signal can be traced for about 100 km into the foreland basin circumstantial evidence suggests that the radiogenic strontium was carried by fluids that were expelled during the laramide orogeny from the rocky mountain main ranges and or the underlying basement

rates of weathering and temporal changes in strength of bedrock of marine terraces in boso peninsula japan the weathering rates of bedrock of the dated erosional marine terraces were examined for two definitions 1 the rates at which the thicknesses of weathered zones with different strengths increase with weathering time and 2 the rates at which the strength of weathered materials at an arbitrary depth decrease with time the bedrock of terraces in the study area consists mainly of tertiary sandstone and mudstone the weathering profiles were observed from drill cores on three levels of terrace surfaces i e 2850 290 and 70 years b p in the emergence age of terraces which is regarded as the weathering time of bedrock the degree of weathering grade was described by the residual strength ratio which is defined as the ratio of the needle penetration hardness of weathered part to that of fresh part of bedrock the weathering rates of the first definition do not decrease monotonically but decelerate logarithmically with time zones with lower weathering grades have faster weathering rates than those with higher weathering grades with respect to the weathering rates of the second definition mudstone starts to weather earlier than sandstone but is exceeded by the latter after a certain time has elapsed because mudstone has a higher slaking susceptibility

diagenesis and its impact on a microbially derived carbonate reservoir from the middle triassic leikoupo formation sichuan basin china the uppermost middle triassic leikoupo formation in the western sichuan basin of china has recently been shown to host as much as 5 3 tcf 1 5 1012 m3 of natural gas resources the reservoir rocks composed mainly of microbially derived dolomudstone e g thrombolites and stromatolites are characterized by low porosity 8 and permeability 0 001 to 10 md the limestone is commonly tight and not of reservoir quality because of abundant meteoric calcite cementation whereas the dolostone has various types of pores dominated by solution enlarged pores and vugs microbial framework pores and micropores breccias are well developed in places probably because of dissolution of underlying evaporites e g anhydrite by an influx of low salinity fluids e g freshwater and seawater during an early burial stage early dolomitization created micropores in the dolomudstone and subsequent diagenetic events were dominated by calcite dolomite quartz cementation pyrite replacement compaction fracturing and development of stylolites localized hydrothermal activity has been evidenced by high homogenization temperatures 160 c 200 c obtained from fluid inclusions in fracture filling cements bacterial sulfate reduction probably resulted in h2s generation pyrite precipitation and solution enlarged pore and vug formation whereas part of the current h2s in these reservoirs may have been sourced from thermochemical sulfate reduction or an underlying formation e g the feixiangguan formation development of microfractures and associated micropores was probably the final diagenetic event which improved pore interconnectivity this study confirms the effect of diagenesis on the development of a microbial dolomudstone reservoir which may be applicable to other similar microbial carbonate reservoirs elsewhere for example middle triassic sections of the tethys region and offshore brazil

origin of dolostone reservoir rocks smackover formation oxfordian northeastern gulf coast u s a formation of regionally extensive dolostone reservoir rocks in the smackover can be understood despite the possible effects of recrystallization geochemical and petrographic data suggest that dolomitization took place in 1 seawater seepage 2 reflux 3 near surface mixed water 4 shallow burial mixed water and 5 deeper burial environments which overlapped in time and space to form a platform scale dolostone body composed of a complex mixture of dolomites seawater seepage and reflux dolomitization occurred in the near surface penecontemporaneously with deposition of the smackover and overlying haynesville formations dolomitization by seawater seepage occurred within an oolite grainstone sill which separated an intraplatform salt basin from the open sea seawater flowed landward through the sill in response to evaporitic drawdown of brines in the isolated intraplatform basin isolation of the salt basin occurred during the oxfordian when the shoreline retreated from the conecuh embayment dolomite located at the top of the smackover enriched in 18 o suggests additional dolomitization by reflux of hypersaline brines reflux occurred as buckner coastal sabkhas prograded over smackover oolite grainstone shoreface deposits vugs lined with shallow burial calcite and dolomite cements indicate flushing of the smackover grainstone aquifer with fresh water freshwater intrusion probably occurred following sea level lowstands during the late jurassic and early cretaceous leaching in the proximal portion of the freshwater aquifer produced excellent limestone reservoir rocks in the updip smackover dolomitization in the contemporaneous downdip mixed connate freshwater zone formed dolostone reservoir rocks with depleted isotopic compositions consistent with a shallow burial mixed water origin

generation of isotopically and compositionally distinct water during thermochemical sulfate reduction tsr in carbonate reservoirs triassic feixianguan formation sichuan basin china abstract thermochemical sulfate reduction tsr the reaction of petroleum with anhydrite in reservoirs resulting in the growth of calcite and the accumulation of h2s has been documented in the feixianguan formation dolomite reservoir in the sichuan basin china fluid inclusion salinity and homogenization temperature data have shown that tsr results in a decrease in salinity from a pre tsr value of 25 wt down to 5 wt as a result of water created as a byproduct of progressive tsr we have studied the isotopic character of the water that resulted from tsr in the feixianguan formation by analyzing the oxygen isotopes of tsr calcite and determining the oxygen isotopes of the water in equilibrium with the tsr calcite at the temperatures determined by aqueous fluid inclusion analysis we have compared these tsr waters to water that would have been in equilibrium with the bulk rock also at the temperatures determined by aqueous fluid inclusion analysis we have found that the tsr waters are relatively depleted in oxygen isotopes by up to 8 compared to what would be expected at equilibrium between the bulk rock and water since this type of water was specifically derived from anhydrite the generation of relatively large volumes of low salinity low 18o water associated with advanced tsr in the feixianguan formation has also been reported in the permian khuff formation in abu dhabi and from sour devonian fields in the western canada basin this suggests that tsr derived water may be a common phenomenon the effects of which on mesogenetic secondary porosity and reservoir quality have previously been underappreciated

the geochemistry of carbonate diagenesis the past present and future stable carbon and oxygen isotopes 18o and 13c values and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years as valuable as these insights have been many problems regarding the interpretation of geochemical signals within mature rocks remain for example while the 18o values of carbonate rocks are dependent both upon the temperature and the 18o value of the fluid and additional information including trace element composition aids in interpreting such signals direct evidence of either the temperature or the composition of the fluids is required such information can be obtained by analysing the 18o value of any fluid inclusions or by measuring the temperature using a method such as the clumped isotope technique such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well known tendency for 18o values of carbonate rocks to decrease with increasing age unlike the 18o 13c values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time however many studies have not sufficiently emphasized the effects of diagenesis and other post depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored finally many additional proxies are poised to contribute to the interpretation of carbonate diagenesis although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods care should be taken to understand both old and new proxies before applying them to the ancient record

thermochemical sulphate reduction in the upper devonian cairn formation of the fairholme carbonate complex south west alberta canadian rockies evidence from fluid inclusions and isotopic data the fairholme carbonate complex is part of the extensively dolomitized upper devonian carbonate reefs in west central alberta the studied formations contain moulds up to 10 cm in diameter which are filled partially with saddle dolomite quartz and calcite cements these cements precipitated from a mixture of brines that acquired high salinity by dissolution of halite and brines derived from evaporated sea water the fluids were warm homogenization temperature of primary fluid inclusions of 76 to 200 c and saline 20 to 25 wt nacl equivalent and testify to thermochemical sulphate reduction processes the latter is deduced from s in solid inclusions co2 and h2s in volatile rich aqueous inclusions and depleted d 13 c values down to 26 vienna pee dee belemnite high 87 sr 86 sr values 0ae7094 to 0ae7110 of the cements also indicate interaction of the fluids with siliciclastic sequences the thermochemical sulphate reduction related cements probably formed during early laramide burial another younger calcite phase characterized by depleted d 18 o values 23ae9 to 13ae9 vienna pee dee belemnite low na 27 to 37 p p m and sr 39 to 150 p p m concentrations and non saline 0 wt nacl equivalent fluid inclusions is attributed to post laramide meteoric water

evidence of honeycomb weathering on mars features similar to honeycomb weathering are observed in some martian rocks at the pathfinder landing site honeycomb weathering on earth develops in a range of environments including extremely arid and cold areas in antarctica and on many types of rocks e g basalts and andesites subjected to physical and or chemical weathering salt weathering in combination with frost shattering and chemical weathering provides a viable explanation for the development of honeycomb features on mars under current environmental conditions data supporting this theory are presented and discussed these findings are important in understanding the present climate geology and in particular the geomorphology of mars they also suggest new paths for research and experimentation in future missions to mars

constraining temperatures of formation and composition of dolomitizing fluids in the upper devonian nisku formation alberta canada with clumped isotopes abstract three dolomite phases in the burial dolomitization system of the nisku formation in central alberta canada were measured for carbonate clumped isotopes independent temperature constraints using fluid inclusions and traditional oxygen isotope geothermometry allow us to test the application of this method using a new calibration curve extrapolated to higher temperatures these three phases of dolomite correspond to three diagenetic stages in the burial history the oldest phase is replacive gray matrix dolomite formed during relatively shallow burial depths at average clumped isotope temperatures 47t of 80 7 c previous temperature estimates from standard oxygen isotope geothermometry ranged from 50 to 80 c a calculated 18o h2o of 3 1 4 vsmow points to precipitation from devonian seawater modified by evaporation and or water rock interaction the first phase of saddle dolomite cement formed by pressure solution during intermediate burial depths yielded average 47t of 96 4 c previously calculated at 80 c and 18o h2o of 4 1 vsmow a second phase of saddle dolomite formed in association with thermochemical sulphate reduction during deep burial records average 47t of 151 5 c comparable to fluid inclusion temperatures ranging from 125 to 145 c the average 18o h2o of the fluid that formed this saddle dolomite phase is calculated as 8 1 vsmow suggesting much more advanced water rock interaction 47t are consistent with previous temperature estimates from a combination of geological data conventional o isotope and fluid inclusion thermometry another important finding is that the 47 signatures are preserved even after reaching deeper burial and or during uplift and cooling which points to a closure temperature in dolomites in excess of 180 c thus dolomite appears to be a better phase for clumped isotope analysis than calcite for rocks that have been exposed to higher temperatures upon further burial this study demonstrates the power of the clumped isotope thermometer to reconstruct complex diagenetic histories in burial diagenetic environments where multiple generations of diagenetic cements can be analyzed individually

dolomites in the tazekka pb zn district eastern morocco polyphase origin from hydrothermal fluids many different types of water and processes have been proposed for the formation of dolomites the three phases of hydrothermal dolomites in the middle atlas causse were investigated to elucidate their formation processes the first two of these are associated with sphalerite and galena in stratiform and open space filling deposits these formed early in the history of the deposition of the pb zn mineralization and commonly reveal a paragenetic overlap a later phase post dating pb zn mineralization is reflected in saddle dolomite all three phases show a decrease in 18o and 13c values passing from sterile unmineralized to mineralized rocks and isotopic signatures are independent of the carrier facies however early formed dolomites can be separated into two distinct groups on the basis of 18o values type 1 dolomites host stratiform ore deposits whereas type 2 dolomites host an open space filling ore body later saddle dolomites are more depleted in 18o than either of these the early hydrothermal and saddle dolomites precipitated from similar fluids during three distinct events but formed by two mechanisms replacement hydrothermal dolomite and cement precipitation saddle dolomite they show different isotopic signatures and apparently formed at different temperatures field data petrographic and stable isotope results suggest a continuum of replacement during the carixian for the early hydrothermal dolomite 1 and during the toarcian for early hydrothermal dolomite 2 followed by a cement precipitation phase for saddle dolomite

carbon and oxygen isotope study of hydrothermal carbonates in the zinc lead deposits of the san vicente district central peru a quantitative modeling on mixing processes and co2 degassing abstract carbon and oxygen isotope studies of the host and gangue carbonates of mississippi valley type zinc lead deposits in the san vicente district hosted in the upper triassic to lower jurassic dolostones of the pucara basin central peru were used to constrain maiels of the ore formation a mixing model between an incoming hot saline slightly acidic radiogenic pb sr fluid and the native formation water explains the overall isotopic variation 13 c 11 5 to 2 5 relative to pdb and 18 o 18 0 to 24 3 relative to smow of the carbonate generations the dolomites formed during the main ore stage show a narrower range 13 c 0 1 to 1 7 and 18 o 18 7 to 23 4 which is explained by exchange between the mineralizing fluids and the host carbonates combined with changes in temperature and pressure this model of fluid rock interaction explains the pervasive alteration of the host dolomite i and precipitation of sphalerite i the open space filling hydrothermal white sparry dolomite and the coexisting sphalerite ii formed by prolonged fluid host dolomite interaction and limited co 2 degassing late void filling dolomite iii or calcite and the associated sphalerite iii formed as the consequence of co 2 degassing and concomitant ph increase of a slightly acidic ore fluid widespread brecciation is associated to co 2 outgassing consequently pressure variability plays a major role in the ore precipitation during the late hydrothermal events in san vicente the presence of native sulfur associated with extremely carbon light calcites replacing evaporitic sulfates e g 13 c 11 5 altered native organic matter and heavier hydrothermal bitumen from 27 0 to 23 0 13 c points to thermochemical reduction of sulfate and or thiosulfate the 13 c and 18 o values of the altered host dolostone and hydrothermal carbonates and the carbon isotope composition of the associated organic matter show a strong regional homogeneity these results coupled with the strong mineralogical and petrographic similarities of the different mvt occurrences perhaps reflects the fact that the mineralizing processes were similar in the whole san vicente belt suggesting the existence of a common regional mineralizing hydrothermal system with interconnected plumbing

application of an s shaped curve model to the temporal development of tafoni of salt weathering origin to describe temporal change in tafone development an s shaped curve equation is proposed z zc 1 n 1 exp t n exp 1 1 n t where z is observed tafone depth zc is ultimate tafone depth t is time and n and are constants the applicability of this model is examined using tafone data selected from seven sites which are categorized into three different salt weathering environments a spray splash dominant occasionally wave affected supra tidal zone aerosol affected coastal regions and inland desert areas the results indicate that the equation can well describe tafone development in each of these environments an investigation based on the values of n and determined through a best fit of the equation to the data suggests that n characterizes site specific environmental conditions and reflects the magnitude of factors controlling the recession mechanism of tafone surfaces it is found that 1 the maximum rate of tafone growth dramatically decreases from supra tidal through coastal to desert environments and 2 the growing mode of tafoni is different depending on the environmental settings the erosional force to facilitate the development of tafoni at supra tidal sites is estimated to be about 400 times greater than that in the general coastal area copyright 2011 john wiley sons ltd

diagenesis of the devonian southesk cairn carbonate complex alberta canada marine cementation burial dolomitization thermochemical sulfate reduction anhydritization and squeegee fluid flow abstract one of the largest geological features in the deep part of the alberta basin in western canada is the middle to upper devonian southesk cairn carbonate complex sccc the sccc is located at depths of about 5000 m adjacent to the disturbed belt of the rocky mountains rising to depths of 3000 m in a northeastward direction over a distance of 150 km the sccc produces oil and gas with up to about 30 h2s from several stratigraphic levels the diagenesis of the sccc was investigated by means of petrography and geochemistry stable isotope compositions were determined for fibrous calcite cements matrix dolomite saddle dolomite and sparry calcite fibrous calcites have stable isotope values of 18o 7 5 to 4 5 pdb and 13c 2 0 1 0 pdb matrix dolomite samples have stable isotope values of 18o 8 1 to 4 9 pdb and 13c 0 to 1 0 pdb saddle dolomite samples range from 18o 9 8 to 7 1 pdb and 13c 1 0 to 0 pdb sparry calcite samples have 18o 13 7 to 7 2 pdb and 13c 24 0 to 0 pdb the sr isotope values of all analyzed samples span a broad range from 0 7080 to 0 7320 showing considerable scatter but also one distinctive pattern i e values higher than 0 7220 occur only at depths in excess of about 4000 m whereas lower values scatter throughout the entire depth range furthermore the sr isotope values of sparry calcites decrease northeastward similarly the fluid inclusion homogenization temperatures of these calcites display a general decrease updip whereas the salinities show no general spatial trends the uncorrected homogenization temperatures range from about 85 to 180 c and the nacl equivalent salinities range from about 17 to 24 weight the d values of brines obtained from seven wells range from 79 to 20 smow and their 18o values range from 1 5 to 8 7 smow the sulfur isotope values of the dissolved sulfate ranges from 34s 10 1 to 24 5 cdt the sr isotope values of the brine samples range from 0 7109 to 0 7255 elemental sulfur isotope values cluster tightly around 34s 18 0 to 18 4 cdt the values of secondary anhydrites range from 34s 18 4 to 26 7 cdt values for solid bitumen vary from 34s 19 5 to 20 2 cdt integration of all data reveals a complex diagenetic evolution from the middle devonian to the recent the paragenetic sequence consists of 24 distinct diagenetic phases which have been grouped into 5 diagenetic stages stage i density driven convection in near surface settings likely superimposed on and or aided by tidal pumping facilitating marine cementation stage ii cellular convection facilitating pervasive matrix dolomitization aided by compaction stage iii hydrocarbon migration stage iv thermochemical sulfate reduction forming sour gas and regional squeegee flow forming sparry calcite cements and localized flow through faults stage v fluid flow driven by erosional rebound in the overlying mesozoic strata and remnant squeegee flow this study yielded several important results 1 the prolific repertoire of diagenetic textures found in the southesk cairn carbonate complex serves as a means of comparison for petroliferous carbonate complexes elsewhere especially in sour gas provinces close to orogenic fronts 2 the diagenetic evolution contains long periods of relative tranquility that were punctuated by several episodes of intense diagenetic activity 3 both thermochemical sulfate reduction and squeegee type fluid flow are recorded in late diagenetic sparry calcite cements 4 the effect of the various burial diagenetic episodes on the hydrocarbon reservoir properties throughout the entire reef complex was highly variable specifically matrix dolomitization thermochemical sulfate reduction and the emplacement of secondary and tertiary anhydrites affected the hydrocarbon reservoir properties throughout the entire reef complex on the other hand squeegee fluid flow was insignificant in terms of affecting the reservoir properties

thermochemical sulfate reduction and fluid evolution of the lower triassic feixianguan formation sour gas reservoirs northeast sichuan basin china the dolomite hosted lower triassic feixianguan formation from the northeast sichuan basin china is an economically important reservoir that contains sour natural gas these reservoirs were initially filled with oil later replaced by gas during burial to 7000 m 22 965 ft followed by uplift to about 4000 m 13 123 ft we have studied the souring process thermochemical sulfate reduction tsr and diagenetic evolution of the feixianguan formation using detailed petrology fluid inclusion studies and stable isotope data from carbonate minerals pre tsr diagenesis included in time order the eodiagenetic main stage of dolomitization by a reflux mechanism fracture related calcite cementation barite quartz celestite and fluorite mineralization and a dolomite recrystallization stage thermochemical sulfate reduction resulted in anhydrite replacement by calcite petroleum destruction formation of sulfur rich pyrobitumen and elemental sulfur and generation of large volumes of and diagenesis during tsr can be subdivided into oil stage tsr and gas stage tsr with oil stage tsr defined by the presence of primary oil and bitumen inclusions in the tsr calcite based on aqueous inclusion homogenization temperatures oil stage tsr commenced at a temperature of 116 c with a mode between 130 c and 140 c gas stage tsr started at a temperature of 135 c and continued to maximum burial temperatures of about 220 c trace amounts of pyrite barite quartz and celestite grew during tsr post tsr diagenesis was dominated by fracture related calcite precipitation as well as celestite and anhydrite crystallization formation water salinity increased from depositional values 3 5 wt up to 24 wt during pre tsr dolomite recrystallization probably because of an influx of evaporite associated water from the overlying jialingjiang formation although pre tsr barite quartz celestite and fluorite mineralization was associated with a transient decrease in water salinity during tsr formation water salinity decreased from 26 wt to as low as 4 wt as a result of water being produced during tsr reactions

burial diagenesis of middle ordovician carbonate buildups alabama usa documentation of the dominance of shallow burial conditions abstract middle ordovician carbonate buildups at two localities red mountain expressway rme tidwell hollow twh record evidence of burial diagenesis burial diagenetic phases include from oldest to youngest 1 blocky calcite 2 planar dolomite associated with stylolites 3 saddle dolomite in primary voids and late veins and 4 blocky calcite in late veins all blocky calcite cements post date ordovician meteoric cement based on cross cutting and superposition relationships and blocky calcite is the most common burial diagenetic phase present in the chickamauga buildups blocky calcite not in late veins formed from post ordovician meteoric water based on the multiple lines of evidence which include 1 variable 18o values 5 2 to 11 4 some of which are significantly more negative than values from ordovician meteoric cement 2 non radiogenic 87 sr 86 sr values 0 70847 to 0 70869 and 3 low fe and mn values near or below detection limit 400 ppm planar dolomite associated with stylolites has variable 18o values 1 4 to 6 1 e is ca rich non stoichiometric and has relatively low mn and fe values this phase is interpreted as forming during the mississippian or earliest pennsylvanian at a relatively shallow burial depth rare saddle dolomite has 880 5 9 to 7 2 0 this phase post dates planar dolomite and appears to record migration of basinal brines into the platform succession during the alleghenian orogeny late veins filled with saddle dolomite and blocky calcite have variable 18o compositions like blocky calcite 4 8 to 12 8 0 and likely formed under elevated temperature conditions if precipitation from 18o enriched burial brines is assumed

origins of honeycomb weathering the role of salts and wind honeycomb weathering is a common surface phenomenon affecting a variety of rocks in a range of environments it develops on building stones and it shapes ocean cliffs rocks in hot deserts and arctic landscapes honeycomb weathering may also help alter rocks on other planets such as mars although first noted in the nineteenth century its origins are still not well understood and a dearth of laboratory experiments testing the many theories proposed for its development has added to the ambiguity incipient honeycomb weathering in a homogeneous limestone has been experimentally reproduced by wind exposure and salt crystallization our experiments show that heterogeneous wind flow over a stone surface is important in the development of this weathering pattern wind promotes evaporative salt growth between grains on a stone surface resulting in the development of small randomly distributed cavities a reduction in air pressure within the cavities results in increased wind speed and rapid evaporation a high evaporation rate and evaporative cooling of the saline solution in the cavity leads to more rapid and greater granular disintegration than in the surrounding areas it seems that this local supersaturation and subsequent buildup of salt crystallization pressure ultimately result in the formation of honeycomb features for the first time these experimental results demonstrate the close relationship between salts wind and honeycomb weathering they also offer new ways to understand the genesis of this striking and sometimes harmful weathering pattern