structurally controlled hydrothermal dolomites in albian carbonates of the ason valley basque cantabrian basin northern spain abstract shallow marine carbonate sedimentation dominated during the albian in the western part of the basque cantabrian basin in northern spain forming the large ramales platform this platform originated on a less subsiding tectonic block facing deeper and more subsiding areas to the south and east which were created by tectonic activity in the basin fracture related hydrothermal dolomites hosted in these albian carbonates are well exposed in the ason valley area mapping in the studied area revealed several dolomite bodies related to main faults that cut the stratification almost at right angles the bodies show a vertical development along fault strike up to 900 m thick from which kilometre scale branches expand following the stratification dolomitization is pervasive and independent of the limestone facies main dolomite facies are fine replacive sucrosic and saddle petrography c o and sr isotopes and fluid inclusion analysis support a polyphase hydrothermal dolomitization at fluid temperatures between 75 c and 240 c and highly variable salinity of up to 22 wt nacl fine dolomite replaced limestone first and then sucrosic and saddle dolomites replaced part of the first dolomite and cemented newly created fracture porosity together with different calcite cements zebra dolomites and hydroclastic breccias are products of this later stage burial analysis of the host rock supports maximum burial temperatures of 80 c and intense tectonic activity from the albian to turonian with a latest albian peak subsidence albian stretching of the crust and subsequent ascent of the isotherms in the area is suggested to have produced sufficient heat to the dolomitizing fluids the structural analysis indicates a strong transtensional tectonic activity in the studied area during albian to turonian time with the creation of an overstep between w e trending and n s trending faults fluids moved from subsiding areas to fractured uplifted parts of the ramales platform enhanced by diapiric activity

fluid flow and late diagenesis of fault infill carbonates in the aptian dolostones at jabel semmama kasserine area western central tunisia abstract in western central tunisia kasserine area dolomitization of jabel semmama aptian limestones of the serj formation fm led to the dolostones of the orbata fm these dolostones are cross cut by several generations of faulting and fracturing the current study focused on e w trending faults and fractures system incorporating carbonate infills and freshwater flows which have stimulated continental diagenetic processes the faulting fracturing and rock fluid interactions yielded nine fault infill diagenetic and tectonic sedimentary phases the purpose of studying the tectonic stresses and the nine phases was to elucidate their late impact on the aptian dolostone reservoir quality this enquiry was based on lithological tectonic microfacies and oxygen carbon o c isotopes investigations analyses of stable o c isotopes are performed on crushed and dedolomitized dolostones as well as on calcite cement laminae and spherulites c1 c2 the crushed and dedolomitized dolostones 18o and 13c data averages 4 0 v pdb 1 75 v pdb remain close to the moderately depleted results of the aptian replacive dolostone host rocks averages 3 6 v pdb 2 0 v pdb inversely calcite cement laminae and spherulites c1 c2 yielded strikingly depleted oxygen values 18o averages 7 77 v pdb 8 0 v pdb pointing out to freshwater controlled diagenesis the discrepancy between the two records 0 23 v pdb is ascribed to cooling during the precipitation of calcitic cement spherulites the divergence between the two depleted 13c values of the two cements c1 c2 averages 6 19 v pdb and 4 0 v pdb is attributed to climatic control wet dry alternations that controlled the micritization phenomenon the latter orchestered the release of ca12co3 being more mobile than ca13co3 this process is in agreement with the strong variation of fluid flow supplies vadose phreatic transcribed in the lithology as cement phases lamellar spherulites gelatinous vadose and granular the study series record two major climatic changes a wet period encompassing phases p1 p4 and a dry period extending on phases p5 p9 two short cool and wet episodes accompany this period the nine tectonic sedimentary and diagenetic phases occurred into two periods through the telogenetic stage uplift they started at the miocene pliocene during the atlasic foldings along e w faults and fractures phases p1 p6 and continued throughout the quaternary period villafranchian along a reactivation of the same direction phases p7 p9 we believe that the nine phases arose after the oae s organic matter maturity and thus had nearly no impact on any early oil migration while the earlier diagenesis phases remain the most useful for the reservoir quality

tectonic hydrothermal brecciation associated with calcite precipitation and permeability destruction in mississippian carbonate reservoirs montana and wyoming the mississippian madison formation contains abundant fracture zones and breccias that are hydrothermal in origin based on their morphology distribution and geochemical signature the hydrothermal activity is related to crustal shortening during the laramide orogeny brecciation is accompanied by dedolomitization late stage calcite precipitation and porosity occlusion especially in outcrop dolomites the tectonic hydrothermal late stage calcite reduces permeability in outcrops and potentially high quality subsurface reservoir rocks of the subsurface madison formation bighorn basin the reduction of permeability and porosity is increased along the margins of the bighorn basin but not predictable at outcrop scale the destruction of porosity and permeability by hydrothermal activity in the madison formation is unique in comparison to studies that document enhanced porosity and permeability and invoke hydrothermal dolomitization models hydrothermal breccias from the owl creek thrust sheet are classified into four categories based on fracture density calcite volume and clast orientation shattered breccias dominate the leading edge of the tip of the owl creek thrust sheet in the eastern owl creek mountains where tectonic deformation is greatest whereas fracture mosaic and chaotic breccias occur throughout the bighorn basin the breccias are healed by calcite cements with 18o values ranging between 26 5 and 15 1 peedee belemnite pdb indicating that the cements were derived from isotopically depleted fluids with elevated temperatures in the chaotic and mosaic breccia types large rotated and angular clasts of the host rock float in the matrix of coarse and nonzoned late stage calcite this appearance combined with similar 18o values across even large calcite veins indicates that the calcite precipitated rapidly after brecciation values for 13c 5 12 pdb from the frontal part of the owl creek thrust sheet indicate equilibrium between methane and co2 bearing fluids at about 180c fluid inclusions from the eastern basin margin show that these cements are in equilibrium with fluids having minimum temperatures between 120 and 140c and formed from relatively low salinity fluids less than 5 wt nacl strontium isotope ratios of these hydrothermal fluids are more radiogenic than proposed values for mississippian seawater suggesting that the fluids mixed with felsic rich basement before migrating vertically into the madison formation we envisage that the tectonic hydrothermal late stage calcite cemented breccias and fractures originated from undersaturated meteoric groundwaters that migrated into the burial environment while dissolving and incorporating ca2 and and radiogenic sr from the dissolution of the surrounding carbonates and the felsic basement respectively in the burial environment these fluids were heated and mixed with hypersaline brines from deeply buried parts of the basement expulsion of these fluids along basement rooted thrust faults into the overlying strata including the madison formation occurred most likely during shortening episodes of the laramide orogeny by earthquake induced rupturing of the host rock the fluids were injected forcefully and in an explosive manner into the madison formation causing brecciation and fracturing of the host rock whereas the subsequent and sudden decrease in the partial pressure of co2 caused the rapid precipitation of calcite cements the explosive nature of hydrothermal fluid migration ultimately produces heterogeneities in reservoir quality carbonates in general flow units in the madison formation are related to sequence boundaries which create vertical subdivisions in the porous dolomite the late stage calcite cement surrounds hydrothermal breccia clasts and invades the dolomite reducing porosity and permeability of the reservoir quality rock as a consequence horizontal flow barriers and compartments are established that are locally unpredictable in their location and extent and regionally predictable along the margins of the bighorn basin

partial dolomitization of a pennsylvanian limestone buildup by hydrothermal fluids and its effect on reservoir quality and performance dolomite was studied in the reinecke field in west texas to determine its origin reservoir characteristics and effect on oil production the reinecke reservoir includes approximately 25 dolomite in an upper pennsylvanian to lowest permian limestone buildup dolomites are fine to coarsely crystalline with undulose extinction petrography indicates that dolomite formed diagenetically late dolomites overgrow stylolites indicating precipitation during deep burial low 18o values are consistent with primary fluid inclusions that indicate dolomite precipitation or recrystallization at 92 to 118 c these temperatures are much higher than those in the existing reservoir or what should have been encountered during regional burial two types of dolomite are observed one type is stratiform replaced micrite rich facies and is interpreted as early dolomite that recrystallized in hydrothermal brines the second type crosscuts stratigraphy and depositional facies and has vertically continuous morphologies that trend northwest southeast the latter dolomite is interpreted as forming in hydrothermal brines derived from latest permian aged highly evaporated seawater these brines descended deep into the basin because of their greater density and then ascended because of heating and thermal convection reinecke dolomites have lower porosity but higher permeability than the surrounding limestones as a result dolomites provide raceways for fluids moving through the reservoir these dolomites helped an artificially enhanced bottom water drive to recover approximately 50 of the original oil in place the dolomite has complicated a crestal co2 flood these hydrothermal reinecke dolomites have lower porosity but much higher permeability than most nearby permian dolomites formed in evaporated seawater during shallow burial arthur h saller is a stratigrapher with chevron energy technology company in houston texas he has worked in the petroleum industry for 27 yr helping with exploration and production projects in west texas canada angola and southeast asia he received his b s degree from the university of kansas m s degree from stanford university and ph d from louisiana state university 1984 john a tony d dickson received his ph d from queen mary college london university in 1967 he has had academic positions in cardiff nottingham and cambridge universities he has worked on smackover andrews horseshoe atoll shu iaba tengiz and other oil reservoirs he retired from academia in 2007 and is now consulting and conducting courses in carbonate diagenesis

early cleavage development in the late ordovician of northeast victoria australia abstract a pervasive cleavage s that subparallels bedding in the upper ordovician rocks of northeast victoria can be related to an incremental strain distribution in a shear environment during the establishment of the first set of regional tectonic folds and faults the cleavage is a strong preferred orientation of phyllosilicates which overprints sedimentary features is contemporaneous with early formed extension fractures and postdates the formation of clastic dykes the cleavage is attributed to tectonic and metamorphic processes rather than a static mimetic recrystallization of clay minerals during burial metamorphism there is little evidence to suggest that this is an early deformation originating during imbricate stacking of an ordovician accretionary prism

saddle dolomite and calcite cements as records of fluid flow during basin evolution paleogene carbonates united arab emirates abstract field observations indicate that tectonic compression anticline formation and concomitant uplift events of marine paleogene carbonates in eastern united arab emirates which are related to the zagros orogeny have induced brecciation karstification and carbonate cementation in vugs and along faults and fractures structural analysis stable isotopes and fluid inclusion microthermometry are used to constrain the origin and geochemical evolution of the fluids fluid flow was related to two tectonic deformation phases initially the flux of moderately 87 sr rich basinal nacl mgcl 2 h 2 o brines along reactivated deep seated strike slip faults have resulted in the precipitation of saddle dolomite in fractures and vugs and in dolomitization of host eocene limestones 18 o v pdb 15 8 to 6 2 homogenization temperatures of 80 115 c and salinity of 18 25 wt eq nacl subsequently compression and uplift of the anticline was associated with incursion of meteoric waters and mixing with the basinal brines which resulted in the precipitation of blocky calcite cement 18 o v pdb 22 to 12 homogenization temperatures of 60 90 c and salinity of 4 5 9 wt eq nacl saddle dolomite and surrounding blocky calcite have precipitated along the pre and syn folding e w fracture system and its conjugate fracture sets the stable isotopes coupled with fluid inclusion micro thermometry homogenization temperatures of 50 c and salinity of 87 sr rich cool meteoric waters this carbonate cementation history explains the presence of two correlation trends between the 18 o v pdb and 13 c v pdb values i a negative temperature dependent oxygen isotope fractionation trend related to burial diagenesis and to the flux of basinal brines and ii positive brine meteoric mixing trend this integrated study approach allows better understanding of changes in fluid composition and circulation pattern during evolution of foreland basins

paleozoic stromatactis and zebra carbonate mud mounds global abundance and paleogeographic distribution carbonate mud mounds with zebra and stromatactis structures are present in every paleozoic system and series but are more common in devonian and carboniferous deposits reaching their acme in mississippian system lower carboniferous rocks global distributions illustrate that mud mounds spanned the planet ranging from tropical to polar circles such a wide latitudinal span signifies that they not only grew in and occupied warm depositional environments but also in settings where oceanic waters were cold and seasonally light limited moreover their proliferation during the devonian and carboniferous was at a time when planet wide climatic ice house conditions are thought to have prevailed mud mounds therefore may also be products of cool and cold water carbonate sedimentation

the effect of hydrothermal fluids on ordovician carbonate rocks southern ordos basin china abstract hydrothermal processes have an important influence on the spatial distribution and formation of carbonate reservoirs and play a critical role in certain deep reservoirs where hydrothermal fluid rock interaction may result in permeability creation due to carbonate dissolution or permeability destruction due to the deposition of cementing minerals this study analyzed certain hydrothermal indicator minerals quartz celestite multi stage calcareous fillings in carbonate rocks from the ordovician majiagou formation southern ordos basin china the petrological and geochemical characteristics of these indicator minerals imply that the majiagou formation was affected by two metasomatic events related to the hercynian indosinian and yanshanian orogenies and linked to magmatic hydrothermal fluids derived from basaltic melts most of the ce and eu values obtained from matrix dolomite samples were lower than one but u th and v c ratios showed positive anomalies the 13c and 18o values of the matrix dolomite were concentrated between 12 8 and 6 0 whereas 13c showed a bipolar distribution at the same depth elemental geochemistry results showed that the ordovician carbonate rocks were subjected to freshwater leaching followed by evaporation and widespread hydrothermal dissolution successively as a result of the multi episodic hydrothermal activity that affected the southern ordos basin and the path of migration channel a parental magmatic hydrothermal fluid fluid 1 that had been generated during the underplating of the southern north china craton by mesozoic alkaline basalt melts separated into two types of fluids with different chemical properties an acidic hydrocarbon bearing fluid fluid 2a mixed with cambrian oil and gas and a weakly to moderately alkaline brine fluid 2b not in contact with hydrocarbons both fluids 2a and 2b then interacted with the ordovician carbonate rocks with fluid 2a having undergone hydrothermal sulfate reduction with so42 in the layers in which gypsum was developed and convection of fluid 2a having resulted in brine formation the reaction products of h2s and co2 together with organic acid co dissolved carbonate minerals in the ordovician strata played a major role in the formation of a high porosity reservoir conversely owing to the progressive reduction of temperature and solubility during fluid ascent fluid 2b precipitated quartz calcite fillings in the residual pore spaces resulting in the localized destruction of the original reservoir permeability in summary this study analyzed the petrological and geochemical characteristics of 60 carbonate rock drill core samples established the likely formation mechanism of fluids 2a and 2b and determined the effects of these hydrothermal fluids on the ordovician majiagou formation carbonate reservoir

in situ geochemistry of lower paleozoic dolomites in the northwestern tarim basin implications for the nature origin and evolution of diagenetic fluids lower paleozoic sedimentary rocks in the northwestern tarim basin were strongly altered by complicated geofluids which resulted in the occurrence of various diagenetic minerals e g dolomite here in situmajor trace and rare earth element geochemistry of lower ordovician diagenetic dolomite grains as well as petrography were performed to unravel the geochemical features the nature and origin of the diagenetic fluids the results indicate that different geochemical information can be detected within a single sample even within a single dolomite grain five generations of diagenetic dolomite have been identified based on geochemical signatures resulting from four distinct types of diagenetic fluids 1 hree enrichment paas normalized low rree no eu anomaly low mn ba moderate fe and high sr contents are probably due to early burial dolomitizing fluids 2 mree enrichment high rree high mn fe and low sr content are likely to be associated with devonian deep circulating crustal hydrothermal fluids 3 flat or lree enrichment pattern with obviously positive eu anomaly is inferred to be linked to permian magmatic hydrothermal fluids and 4 flat ree pattern moderate rree no eu anomaly low mn ba moderate fe and high sr contents are probably due to late burial dolomitizing fluids the significances of in situmethod demonstrated in this study compared with the whole rock analysis include not only contamination free analysis but also unraveling the internal geochemical variation within a single sample or a mineral grain thus for the geochemical study of complicated diagenetic process in situmethod should be preferentially considered

nature and distribution of diagenetic phases and petrophysical properties of carbonates the mississippian madison formation bighorn basin wyoming usa abstract the purpose of this study is to characterize factors controlling the nature and distribution of the diagenetic phases that gave rise to carbonate reservoirs to do so a pluridisciplinary approach was carried out integrating sedimentologic and diagenetic studies on the madison formation lower carboniferous 357 340 my which is a carbonate reservoir in subsurface the madison formation outcropping in the bighorn basin wyoming usa is a 340 m thick carbonate series composed of four to seven 3rd order depositional sequences s1 to s7 depending of the palaeogeographic location the first three sequences s1 to s3 were deposited under arid and warm conditions during tournaisian times which favoured high accumulation of carbonates leading to a morphological change from a quite flat ramp s1 to s2 to a wide platform s3 it also probably favoured the early calcite cementation isopachous and syntaxial calcite cements of the subtidal deposits and the early dolomitization d1 of the supratidal to intertidal sabkha ones in addition the very flat profile occurring during s3 was also responsible for the postponed dolomitization d2 of s1 s2 due to reflux of brines at various degrees depending on the palaeogeographic location the deposition of s4 to s7 under humid conditions during visean times were associated with 1 a decrease of the carbonate accumulation and of the dolomitization 2 dissolution processes at micro pores network and macro scales karst collapse breccia and calcite cementation c1 the spatial distribution of all these first diagenetic phases acts as a controlling factor on the distribution of the later burial diagenetic phases thus the mesogenetic calcite cements are mostly observed in the secondary porosity created by dissolution karsts collapse breccias or dolomitization the burial diagenesis of the madison formation was characterized by 1 a dolomitization phase d3 in proximal parts of the platform leading to an increase of porosity and permeability and 2 calcite cementations c2 c3 in the distal parts of the platform leading to a decrease of these properties the madison formation provides a good spatial representation of sedimentary and diagenetic heterogeneities that may occur in the carbonate reservoirs due to various palaeogeographic locations climates burial history and structures

structurally controlled hydrothermal alteration of carbonate reservoirs introduction langhorne taury smith heads the reservoir characterization group at the new york state museum he holds a b s degree from temple university a ph d from virginia tech and did postdoctoral work at the university of miami he also worked for chevron as a development geologist and current research interests are focused on carbonate reservoir characterization and hydrothermal alteration of carbonate reservoirs graham davies received his b sc honors degree and ph d from the university of western australia his doctoral thesis was on modern carbonates in shark bay western australia published in aapg memoir 13 after a postdoctoral fellowship with james lee wilson at rice university he joined the geological survey of canada gsc in calgary after 7 years with the gsc graham co founded and became principal owner of agat applied geoscience and technology consultants laboratories since 1983 he has operated through gdgc ltd he has published about 70 papers on the geology of australia and canada and has authored or coauthored more than 600 consulting reports graham received the cspg douglas medal in 2002 for his work on arctic paleozoic carbonates and evaporites and on the triassic and other aspects of western canadian geology his principal current interest is in hydrothermal dolomites this issue of the aapg bulletin is dedicated to structurally controlled hydrothermal alteration of carbonate rocks and reservoirs this type of alteration covers the spectrum of carbonate hosted sulfide ore deposits to hydrothermal dolomite to leached limestones and in many cases has a major impact on reservoir type geometry quality and distribution hydrothermal dolomite htd reservoirs have a long history of exploration and development in north america extending from the earliest discoveries in the ordovician lima indiana and other trends of the northeastern united states hurley and budros 1990 wickstrom et al 1992 and continuing today with the trenton black river play

the effect of non parallel thrust fault interaction on fold patterns the geometry of two subsurface non parallel thrust faults are systematically altered within mechanical models to determine the range of resulting fold map patterns and the effect of fault interaction on these fold patterns and ultimately to infer fault geometry from complex fold shapes multiple faults can create both complex fold patterns with more than one fold axis and single folds without evidence of a second fault fold length may not correlate to fault length when remote contraction is oblique to the faults thus contraction direction should be determined before fault length is inferred the influence of fault interaction is studied by comparing the fold pattern created by interacting faults to the fold pattern created from the superposition of folds produced by equivalent but isolated faults under small strains larger faults tend to have greater interaction but even interacting faults may not produce fold patterns notably distinct from the superposed fold patterns this suggests that fault interaction need not necessarily be considered when inferring fault geometry from fold shape in the field however under non coaxial strain conditions fold patterns may differ from those presented here

controls on reflux dolomitisation of epeiric scale ramps insights from reactive transport simulations of the mississippian madison formation montana and wyoming abstract prediction of the geometry and petrophysical properties of dolomite geobodies depends on understanding both the hydrological system supplying reactive fluids and the chemistry of these fluids however patterns are complicated by the non linear response of the diagenetic system to depositional texture which controls both fluid flux via permeability architecture and reaction rate via effective surface area this study explores interactions between extrinsic controls spatial distribution of brine composition and temperature and intrinsic controls permeability and reactivity using local and regional scale reactive transport models of sequential episodes of brine reflux that resulted in partial dolomitisation of the mississippian madison ramp inter well scale models show preferential early dolomitisation of fine grained more reactive beds pervasive dolomitisation can occur most readily beneath the brine pool where flow is perpendicular to bedding and is most rapid at high brine fluxes down dip of the brine pool bedding parallel flow is focused in relatively permeable coarse grained beds providing reactants for strongly preferential alteration of intervening more reactive fine grained beds in contrast thicker sequences of fine grained beds dolomitise more slowly limited by the rate of supply of magnesium regional scale models with injection of brines of increasing salinity towards the ramp interior reproduce the observed pattern of dolomitisation however more realistic simulations in which reflux is driven by lateral density contrasts generate flow rates orders of magnitude too low for significant dolomitisation simulations suggest pervasive dolomitisation of epeiric scale ramps by a platform wide reflux circulation as often invoked is not feasible rather dolomitisation of such extensive systems critically requires local scale flow systems such as may result from topographically controlled variations in restriction of platform top seawater circulation

geochemistry of the mississippian delle phosphatic event eastern great basin u s a abstract the middle mississippian delle phosphatic event marks a significant change in depositional conditions in the devonian mississippian antler foreland basin of north america stratigraphic and petrographic studies show that deposition of delle event rocks ended a long period of open marine carbonate sedimentation when facies shifted to phosphorites cherts fine grained siliciclastics and lime mudstones from highly productive waters that may have been episodically anoxic this drastic change in lithology is reflected in the chemistry of the two suites of rocks 18o values of the delle event lime mudstones and phosphatized wackestones packstones affected by the delle event are 3 5o oo lighter than underlying and overlying open marine wackestones packstones sr concentrations of delle event lime mudstones and open marine wackestone packstone facies are low suggesting that both of these lithologies have undergone similar postdiagenetic alteration the uniform sr concentrations and variable 18o data suggest that either the younger delle event rocks were exposed to meteoric water during diagenesis or that the relatively light 18o values represent a primary brackish water depositional signal either interpretation is consistent with sedimentological evidence of the delle event occurring in a predominantly shallow water setting 13c values of delle event lime mudstones are also anomalously light indicating a high input of organic carbon during their formation mn concentrations of the delle event lime mudstones and stratigraphic equivalents are highly variable suggesting fluctuating oxygen concentrations during their deposition and diagenesis rare earth element data from phosphorites and cherts of the delle event indicate oxic to suboxic depositional conditions

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

diagenesis of continental carbonate country rocks underlying surficial travertine spring deposits abstract diagenesis of miocene pliocene continental rocks in extensional teruel basin related to deposition of a travertine at surface has been studied most of the diagenetic processes described here are in relation to the travertine deposition so they are not widespread along the basin due to their high induration degree these rocks may be misinterpreted with travertine tufa facies so there is a need of clear criteria for their recognition as they can supply important hydrological and paleoenvironmental information the diagenetic processes that deeply modified the substrate include dissolution recrystallization and cementation and lead to a high induration of the miocene pliocene rocks these processes were driven by groundwater flow leading to travertine deposition at surface and appeared to be closely related to the fluvial incision of alfambra river and to extensional fracturation during middle pleistocene times probably under humid conditions the cementation sequences and the organization of fractures and vadose micrite point to alternating vadose phreatic conditions and syndiagenetic movements of the faults isotopic signal show lighter 18 o than that of the unaltered miocene pliocene rocks and similar 13 c suggesting lighter oxygen signal in waters during interglacial pleistocene periods and a common origin for carbon from marine mesozoic rocks from the substrate of the basin all of that suggest the strong control of tectonics and climate at different timescales in diagenesis of the rocks serving as substrate for surficial travertine deposition

melt depletion and subsequent metasomatism in the shallow mantle beneath koolau volcano oahu hawaii xenoliths from pali oahu hawaii include samples of the mantle lithosphere underlying koolau shield volcano most such xenoliths are spinel peridotites the remainder being plagioclase spinel peridotites and garnet free pyroxenites clinopyroxene separates from pali peridotite xenoliths have relatively depleted 87sr 86sr 0 70309 0 70346 and 143nd 144nd 0 512967 0 513206 the mantle lithosphere beneath the koolau volcano has a range of present day epsilonnd 6 9 11 on a 143nd 144nd versus 147sm 144nd diagram they define a 61 20 ma errorchron within error of the estimated age of 80 85 ma of the pacific lithosphere at oahu and an intercept of epsilonnd 6 8 we interpret the pali spinel peridotites as samples of the pacific lithosphere residual to melt extraction of the pacific crust these rocks were not metasomatized by melts having isotopic composition similar to the shield building koolau volcano but they could have been metasomatized by melts related to the honolulu volcanic series plagioclase mineral separates from two peridotites are in 87sr 86sr isotopic disequilibrium with coexisting clinopyroxenes the plagioclases are light rare earth element depleted and have enriched 87sr 86sr ratios 0 70436 and 0 70443 the delta18o of olivine 5 09 5 12 and clinopyroxene 5 32 5 33 from spinel peridotites are typical for oceanic upper mantle rocks in contrast delta18o of olivine pyroxene and plagioclase in plagioclase bearing peridotites are enriched by 0 5 relative to the spinel peridotites e g delta18o ratios of olivines from plagioclase peridotites are 5 4 5 5 it is here interpreted that the plagioclases represent phases that formed by reaction with or precipitation from a melt that intruded the pacific lithospheric mantle the pacific lithospheric isotopic composition is reflected in the clinopyroxene separates the high delta18o and 87sr 86sr of the plagioclase resemble koolau lavas and suggest that the melt passing through the lithosphere was plume related the elevated delta18o of the plagioclase indicates that the melt from which it fractionated had a component of recycled crustal material not acquired from local crustal contamination it is also estimated here that the duration of the porous melt flow of the koolau like melt though the mantle lithosphere was short lived 10 2 10 4 years

petrologic and stable isotopic studies of a fossil hydrothermal system in ultramafic environment chenaillet ophicalcites western alps france processes of carbonate cementation abstract the late jurassic chenaillet ophiolitic complex western alps represents parts of an oceanic core complex of the liguria piemonte domain a model for the origin and evolution of the chenaillet ophicalcites based on textural and isotopic characterization is presented the chenaillet ophicalcites correspond to brecciated serpentinized peridotites that record seafloor shallow serpentinization at a minimum temperatures of 150 c followed by authigenic carbonation carbonation starts with a network of micrometric to millimetric pre or syn clast formation calcite veins accompanied by a pervasive carbonation of residual olivine and serpentine inside the serpentinite mesh core a matrix of small calcite 13 c vpdb values that range between 5 and 0 4 the lower values were obtained for calcite within the serpentinite clasts the 18 o vsmow values have a range between 11 and 16 in carbonated clasts the 18 o values in the matrix are fairly homogeneous with an average at 12 and the late calcite veins have values between 12 5 and 15 5 these values suggest a relatively high temperature of formation for all the carbonates carbonates within clast are mainly characterized by a formation temperature in the range of 110 c to 180 c assuming a 18 o value of seawater of 0 the matrix forms at a temperature of ca 165 c late veins are characterized by a formation temperature ranging between 120and 155 c we propose a model where serpentinization is followed by discrete carbonation then brecciation and cementation as a consequence of continuous hydrothermal fluid circulation in the serpentinite basement this is comparable to observations made in the stockwork of present day long lived oceanic hydrothermal systems

exploring the geological features and processes that control the shape and internal fabrics of late diagenetic dolomite bodies lower khuff equivalent central oman mountains abstract the lower permian carbonate rock succession of the oman mountains represents a stratigraphical surface analogue for middle eastern lower khuff hydrocarbon reservoirs the characteristics of the carbonate rocks are highly variable both laterally and vertically due to the superposition of several episodes of dolomitization a stratabound early dolomitization event affected the well bedded limestones during triassic times subsequently a later burial dolomitization led to the emplacement of several hundreds of meter wide and tens of meter thick dolomite bodies the goal of our study is to understand the distribution and genetic controls on the formation of late diagenetic dolomite bodies field mapping in wordian to capitanian beds was conducted in the jebal akhdar tectonic window in the central oman mountains to determine the shape internal fabrics and associated structures of late diagenetic dolomite bodies the longest extension of late diagenetic dolomite bodies was found to follow predominantly the strike direction of fracture planes bed to bed contacts the precambrian permian unconformity and the bottom rim of early diagenetic dolomite two groups of diagenetic fabrics were observed in association to fractures and bedding planes first vug related fabrics comprise vuggy pores and zebra dolomite second breccia related fabrics consist of either small scale crackle mosaic breccia fabrics and centimeter sized vugs followed by bedding parallel vugs lined up petrographic analysis revealed three types of diagenetic phases referred to as d1 d2 and d3 the matrix replacive phase d1 represents the main diagenetic phase formed during burial coarse crystalline dolomite rhombs phase d2 and saddle dolomite phase d3 were found along stylolites in vugs and in fractures based on the presence of saddle dolomite and zebra dolomite textures late diagenetic dolomite bodies are interpreted to have formed by the circulation of warm dolomitizing fluids of at least 60 c

carbonate porosity creation by mesogenetic dissolution reality or illusion many authors have proposed that significant volumes of porosity are created by deep burial dissolution in carbonate reservoirs we argue however that this model is unsupported by empirical data and violates important chemical constraints on mass transport because of the ubiquitous presence and rapid kinetics of dissolution of carbonate minerals the mesogenetic pore waters in sedimentary basins can be expected to be always saturated and buffered by carbonates providing little opportunity for the preservation of significantly undersaturated water chemistry during upward flow even if the initial generation of such undersaturated pore water could occur a review of the literature where this model has been advanced reveals a consistent lack of quantitative treatment in consequence the presumption of mesogenetic dissolution producing a net increase in secondary porosity should not be used in the prediction of carbonate reservoir quality

ages and inferred causes of late pleistocene glaciations on mauna kea hawai i glacial landforms on mauna kea hawai i show that the summit area of the volcano was covered intermittently by ice caps during the late pleistocene cosmogenic 36 cl dating of terminal moraines and other glacial landforms indicates that the last two ice caps called older makanaka and younger makanaka retreated from their maximum positions approximately 23 ka and 13 ka respectively the margins and equilibrium line altitudes of these ice caps on the remote tropical pacific island were nearly identical which would seem to imply the same mechanism for ice growth but modelling of glacier mass balance combined with palaeotemperature proxy data from the subtropical north pacific suggests that the causes of the two glacial expansions may have been different older makanaka air atop mauna kea was likely wetter than today and cold whereas younger makanaka times were slightly warmer but significantly wetter than the previous glaciation the modelled increase in precipitation rates atop mauna kea during the late pleistocene is consistent with that near sea level inferred from pollen data which suggests that the additional precipitation was due to more frequent and or intense tropical storms associated with eastward moving cold fronts these conditions were similar to modern la nina weak enso conditions but persisted for millennia rather than years increased precipitation rates and the resulting steeper temperature lapse rates created glacial conditions atop mauna kea in the absence of sufficient cooling at sea level suggesting that if similar correlations existed elsewhere in the tropics the precipitation dependent lapse rates could reconcile the apparent difference between glacial time cooling of the tropics at low and high altitudes copyright 2008 john wiley sons ltd

paleogeographic and paleotectonic setting of laramide sedimentary basins in the central rocky mountain region in the rocky mountain region between central montana and central new mexico sedimentologically isolated nonmarine basins were produced by basement deformation during the laramide orogeny within the area formerly occupied by a broad late cretaceous foreland basin in which laterally continuous marine facies had accumulated previously laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere laramide basins include perimeter basins along the cratonic periphery of the arcuate laramide province axial basins along a north south intramontane trend and ponded basins located farther west closer to the overthrust belt twelve specific stratigraphic and sedimentologic criteria for the onset duration and termination of laramide deformation allow the chronology of basin development to be inferred independently for each basin maastrichtian initiation of laramide deformation was approximately synchronous throughout the laramide province but termination of laramide deformation was systematically diachronous from north to south between early and late eocene time widespread eocene erosion surfaces truncate syntectonic laramide sequences and are overlain by largely volcanic and volcaniclastic post laramide strata of eocene age in the north and oligocene age in the south fluvial depositional systems draining toward the great plains were dominant in perimeter and axial basins but ponded basins were occupied at times by large lakes that served as regional sediment traps paleocene drainages from ponded basins also led eastward toward the continental interior but partly interconnected lakes that developed within several ponded basins by mid eocene time were either closed hydrologically or else they drained westward into the tyee paleoriver of the pacific northwest

fracture controlled paleohydrogeology in a basement cored fault related fold sheep mountain anticline wyoming united states new geochemical and microstructural data constrain the origins and pathways of paleofluids during the growth of sheep mountain anticline wyoming united states oxygen carbon and strontium isotope studies were performed on prefolding and fold related calcite veins and their sedimentary host rocks and combined to fluid inclusion microthermometry results we show that most of the cements precipitated from paleogene meteoric fluid stable isotopes and fluid inclusion homogenization temperatures further indicate that most veins were mineralized from upward moving fluids after these fluids were heated at depth t 110 c this implies that fluids migrated along the basement thrust underlying the fold and or at the base of the cover above the fault tip the fluids circulated rapidly in the diffuse synfolding and early folding fracture network the zone of preferential migration of the warm fluids is currently located in the backlimb of the fold which supports some of the previously published structural interpretation of the subsurface this study also highlights the potential of combined fracture analysis and geochemical analyses of paleofluid flows in fractures to constrain both the deformation history and the fluid flow during basement involved shortening in laramide style forelands

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

formation of saddle dolomites in upper cambrian carbonates western tarim basin northwest china implications for fault related fluid flow abstract the saddle dolomites occur more intensely in cores closely to fault than that in cores far away from the fault in upper cambrian carbonate of western tarim basin suggesting that formation of the saddle dolomites is likely related to fault controlled fluid flow they partially fill in fractures and vugs of replacement dolomite the saddle dolomites exhibit complex internal textures commonly consisting of core and cortex in comparison with the matrix dolomites the saddle dolomites show lower sr content and 87 sr 86 sr ratios higher fe and mn content and more negative 18 o values combined with high th 100 130 c of primary fluid inclusions it is suggested that the saddle dolomites precipitated from hydrothermal fluid derived from the deep evaporite bearing middle cambrian strata and the magnesium source may be due to dissolution of host dolomite during hydrothermal fluid migration fault activity resulted in petrographic and geochemical difference of the core and cortex of the saddle dolomites the cores precipitated from the formation water mixed by deep brines at the early stage of fault activity and the cortexes precipitated from the deep fluid with higher temperatures through the middle cambrian later in summary the formation of the saddle dolomites implies a hydrothermal fluid event related to fault activity which also resulted in high porosity in upper cambrian carbonate in western tarim basin

structural and microstructural evolution of the rattlesnake mountain anticline wyoming usa new insights into the sevier and laramide orogenic stress build up in the bighorn basin the rocky mountains in western us provide among the best examples of thick skinned tectonics following a period of thin skinned tectonics related to the sevier orogeny the compressional reactivation of basement faults gave birth to the so called laramide uplifts arches the bighorn basin located in wyoming is therefore a key place to study the transition from thin to thick skinned tectonics in orogenic forelands especially in terms of microstructural and stress strain evolution our study focuses on a classic laramide structure the rattlesnake mountain anticline rma wyoming usa a basement cored anticline located in the western part of the bighorn basin stress and strain evolution analysis in folded sedimentary layers and underlying faulted basement rocks were performed on the basis of combined analyses of fractures fault slip data and calcite twinning paleopiezometry most of the fractures are related to three main tectonic events the sevier thin skinned contraction the laramide thick skinned contraction and the basin and range extension serial balanced cross sections of rma and displacement profiles suggest that all thrust faults were coeval evidencing strain distribution in the basement during faulting the comparison of rma with another structure located in the eastern edge of the bighorn basin i e the sheep mountain anticline sma allows to propose a conceptual model for the geometric and kinematic evolution of laramide related basement cored anticlines finally the stress evolution is reconstructed at both the fold scale and the basin scale we show that the evolution of stress trends and magnitudes was quite similar in both structures rma and sma during laramide times thick skinned tectonics in spite of different stress regimes during sevier thin skinned tectonics and post laramide times stress trends and fracture patterns were different in these two structures these results suggest that the distance to the orogenic front influenced the fracture patterns but not the foreland stress magnitudes which were likely controlled by the structural style

dedolomitization as an analogue process for assessing the long term behaviour of a co2 deep geological storage the alicun de las torres thermal system betic cordillera spain abstract carbon dioxide deep geological storage especially in deep saline aquifers is one of the preferred technological options to mitigate the effects of greenhouse gases emissions thus in the last decade studies characterising the behaviour of potential co 2 deep geological storage sites along with thorough safety assessments have been considered essential in order to minimise the risks associated with these sites the study of natural analogues represents the best source of reliable information about the expected hydrogeochemical processes involved in the co 2 storage in such deep saline aquifers in this work a comprehensive study of the hydrogeochemical features and processes taking place at the natural analogue of the alicun de las torres thermal system betic cordillera has been conducted thus the main water co 2 rock interaction processes occurring at the thermal system have been identified quantified and modelled and a principle conclusion is that the hydrogeochemical evolution of the thermal system is controlled by a global dedolomitization process triggered by gypsum dissolution this geochemical process generates a different geochemical environment to that which would result from the exclusive dissolution of carbonates from the deep aquifer which is generally considered as the direct result of co 2 injection in a deep carbonate aquifer therefore discounting of the dedolomitization process in any co 2 deep geological storage may lead to erroneous conclusions this process will also influence the porosity evolution of the co 2 storage formation which is a very relevant parameter when evaluating a reservoir for co 2 storage the geothermometric calculation performed in this work leads to estimate that the thermal water reservoir is located between 650 and 800 m depth which is very close to the minimum required to inject co 2 in a deep geological storage it is clear that the proper characterisation of the features and hydrogeochemical processes taking place at a natural system analogous to a man made deep geological storage will provide useful conceptual semi quantitative and even quantitative information about the processes and consequences that may occur at the artificial storage system

constraining the fluid history of a co2 h2s reservoir insights from stable isotopes ree and fluid inclusion microthermometry reservoirs that host co2 h2s bearing gases provide a key insight into crustal redox reactions such as thermochemical sulphate reduction tsr despite this there remains a poor understanding of the extent duration and the factors limiting this process on a reservoir scale here we show how a combination of petrography fluid inclusion rare earth element ree and carbon 13c oxygen 18o and sulphur 34s stable isotope data can disentangle the fluid history of the world s largest co2 accumulation the labarge field in wyoming usa the carbonate hosted labarge field was charged with oil around 80 ma ago which together with nodular anhydrite represent the reactants for tsr the nodules exhibit two distinct trends of evolution in 13c with both 34s and 18o that may be coupled to two different processes the first trend was interpreted to reflect the coupled dissolution of anhydrite and reduction to elemental sulphur and the oxidation of organic compounds and associated precipitation of calcite during tsr in contrast the second trend was interpreted to be the result of the hydrothermal co2 influx after the cessation of tsr in addition mass balance calculations were performed to estimate an approximate tsr reaction duration of 80 ka and to identify the availability of organic compounds as the limiting factor of the tsr process such an approach provides a tool for the prediction of tsr occurrence elsewhere and advancing our understanding of crustal fluid interactions

ore forming fluids in archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec canada archean gold bearing quartz veins at the sigma mine occur in deformed metavolcanics intruded by two generations of preore porphyries the veins were emplaced during a late compressional tectonic event and they consist primarily of quartz tourmaline carbonates pyrite and scheelite vein minerals have been deformed and are also cut by abundant healed fractures outlined by fluid inclusions in the veins gold typically occurs in late fractures and in recrystallized quartz microthermometric sem eda leachate analysis and gas chromatographic techniques were used to study fluid inclusions in quartz most fluid inclusions at sigma occur in late fractures and are secondary the fluid inclusions are commonly found in fractures parallel to or less commonly within gold bearing fractures and are genetically related to the gold three types of fluid inclusions are identified 1 high salinity na ca cl aqueous inclusions 25 34 wt salt with small amounts of co 2 some containing halite calcite and nahcolite as daughter minerals 2 co 2 rich inclusions containing small amounts of h 2 o and ch 4 and 3 low salinity h 2 o co 2 inclusions 10 equiv wt nacl with co 2 contents which are variable but commonly in the range of 15 to 30 mole percent co 2 leachate analyses show that in addition to na and ca the aqueous fluids contain minor amounts of k fe mg and li homogenization temperatures of h 2 o co 2 inclusions with constant phase ratios range from 285 degrees to 395 degrees c those of aqueous inclusions range from 60 degrees to 295 degrees c the aqueous and co 2 rich inclusions are interpreted to have trapped two coexisting fluids resulting from the unmixing of a homogeneous h 2 o co 2 parent fluid itself occasionally trapped in late fractures the ore bearing fluid is interpreted to be a low salinity 10 equiv wt nacl fluid containing 15 to 30 mole percent co 2 at minimum temperatures in the range of 285 degrees to 395 degrees c the relatively low homogenization temperatures of aqueous inclusions are interpreted to be the result of co 2 effervescence from a co 2 saturated aqueous fluid due to pressure fluctuations at the time of entrapment it is also proposed that the onset of gold deposition is related to the onset of fluid unmixing

fluid rock interactions associated with regional tectonics and basin evolution an integrated approach consisting of fracture analysis petrography carbon oxygen and strontium isotope analyses as well as fluid inclusion micro thermometry led to a better understanding of the evolution of fluid rock interactions and diagenesis of the upper permian to upper triassic carbonates of the united arab emirates the deposited carbonates were first marked by extensive early dolomitization during progressive burial the carbonates were affected by dolomite recrystallization as well as precipitation of vug and fracture filling dolomite quartz and calcite cements after considerable burial during the middle cretaceous sub vertical north south oriented fractures f1 were cemented by dolomite derived from mesosaline to hypersaline fluids upon the late cretaceous maximum burial and ophiolite obduction sub vertical east west fractures f2 were cemented by dolomite dc2 and saddle dolomite ds derived from hot highly saline fluids then minor quartz cement has precipitated in fractures from hydrothermal brines fluid inclusion analyses of the various diagenetic phases imply the involvement of increasingly hot 200 degrees c saline brines 20 to 23 nacl eq through one dimensional burial history numerical modelling the maximum temperatures reached by the studied rocks are estimated to be in the range of 160 to 200 degrees c tectonically driven flux of hot fluids and associated diagenetic products are interpreted to have initiated during the late cretaceous maximum burial and lasted until the oligocene miocene compressional tectonics and related uplift the circulation of such hydrothermal brines led to partial dissolution of dolomites dc2 and ds and to precipitation of hydrothermal calcite c1 in new mainly oriented north south f3 and pre existing reactivated fractures the integration of the obtained data confirms that the diagenetic evolution was controlled primarily by the interplay of the burial thermal evolution of the basin and the regional tectonic history hence this contribution highlights the impacts of regional tectonics and basin history on diagenetic processes which may subsequently affect reservoir properties

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

alteration of early formed dolomite during shallow to deep burial mississippian mission canyon formation central to southwestern montana dolomite in the mississippian mission canyon formation osagean to middle meramecian of central and southwestern montana precipitated during multiple episodes of dolomitization and or recrystallization the earliest generation of dolomite most likely precipitated from evaporated sea water in near surface diagenetic environments when the mission canyon platform intermittently aggraded to sea level this dolomite has relatively positive delta sup 18 o values minus 1 5 to 7 5 and typically is nonstoichiometric and trace element enriched a second generation of dolomite has very negative delta sup 18 o values minus 11 2 to minus 1 5 and always is stoichiometric and trace element depleted dolomite with delta sup 18 o values minus 6 8 is in the upper 100 180 m of the mission canyon formation beneath a regional post mission canyon unconformity and karst surface that extends across the entire study area dolomite depleted in sup 18 o may have 1 precipitated initially in a regional mixing zone during post mission canyon subaerial exposure and karstification 2 precipitated at elevated temperatures during later diagenesis or 3 recrystallized from an earlier generation of dolomite during later shallow to deep burial diagenesis very negative delta sup 18 o values and low trace element contents for the second generation of dolomite suggest that diagenetic solutions were dominated by meteoric water whereas delta sup 13 c values indicate that diageneticmore fluids were buffered by osagean carbonate rocks 54 refs 10 figs less

strain response and re equilibration of ch4 rich synthetic aqueous fluid inclusions in calcite during pressure drops abstract aqueous fluids in sedimentary basins often contain dissolved methane particularly in petroleum environments pvtx pressure volume temperature composition reconstructions performed using fluid inclusion data are largely based on the assumption that inclusions do not change from the time of trapping until the present many authors however consider that fluid inclusions can re equilibrate particularly in fragile minerals like calcite in order to understand this re equilibration phenomenon in the metamorphic domain previous experiments have been performed under high pt conditions but few have been performed at low to medium pt conditions such as those associated with sedimentary burial diagenesis and no previous studies have examined ch 4 bearing aqueous inclusions in calcite an experimental study of the preservation modification of ch 4 rich synthetic fluid inclusions in calcite during isothermal decompression was conducted an autoclave was used for accurate ptx control allowing equilibrium between liquid and vapour in the ch 4 h 2 o system ptx conditions were maintained at four stages of decreasing pressure with each stage held for 7 days to simulate an isothermal pressure drop in order of decreasing pressure the pressure temperature conditions monitored were 276 10 bar at 180 7 c 176 10 bar at 180 7 c 76 10 bar at 180 7 c and 10 3 bar at 180 15 c at the end of the experiment the calcite was recovered and analyzed by microthermometry and raman microspectroscopy for ptx reconstruction a careful procedure was adopted to limit re equilibration of inclusions during analytical procedures four types of inclusion shapes and four types of strain patterns were differentiated classification of the petrographic strain patterns was carried out these strain patterns were associated with inclusion stretching and or leakage regarding ch 4 t h and p h compared to experimental conditions factors controlling the preservation or acquisition of strain patterns included the initial shape and size of the inclusion and the pressure differential p between the confining pressure p cf and the internal pressure p i within the inclusion most fluid inclusions seemed to be trapped during the first 7 days of the experiment although few 4 of these preserved the initial pt conditions of 276 10 bar whereas 8 preserved the second and third run of pt conditions overall the majority of inclusions 88 did not reflect accurately the ptx trapping conditions a petrographic guide to the inclusions is presented here that allows strain identification for pvt reconstructions re equilibration patterns and evidence for preferential methane leakage from aqueous inclusions in calcite are important findings revealed by this study and may be useful for the reconstruction of post trapping events in investigations of natural samples and in other experiments using synthetic inclusions in calcite

porosity permeability relationships in interlayered limestone dolostone reservoirs porosity and permeability data from five carbonate platform successions of different settings ages and burial depths are examined to identify overall similarities and differences in the reservoir quality of interlayered limestones and dolostones each succession consists mainly of limestone and dolostone with subordinate proportions of intermediate partly dolomitized compositions in the three deeply buried platforms the key features are that limestones have much lower average porosity than associated dolostones and that limestones and dolostones show little difference in average permeability for given porosity in contrast the shallowly buried platforms show little difference in average porosity between limestones and dolostones and also display higher average permeability for given porosity in dolostones than limestones these data suggest the following general guidelines for depositional and diagenetic controls on reservoir architecture in carbonates consisting of interlayered limestone and dolostone 1 reservoir compartmentalization by the formation of tight limestone barriers is largely a burial diagenetic process involving calcite cementation locally produced by chemical compaction 2 both the pattern of early dolomitization and the distribution of clay minerals because they influence the localization of chemical compaction are key factors that determine the distribution of tight limestone barriers separating flow units thus the pattern of eventual burial compartmentalization follows a template that is hardwired into the stratigraphic architecture by depositional mineralogy and early diagenesis 3 after burial 2 3 km 1 2 1 8 mi dolostones should not be expected to have higher permeability for given porosity than associated limestones these rules assume dolomitization to occur early relative to chemical compaction which will commonly be the case because of combined hydrologic and mass balance constraints dolomitization and concentration of clay appear linked to cycle and sequence architecture in the present examples and thus may have a useful degree of predictability at least in terms of statistical parameters such as net gross and probability of flow unit thickness distribution

calcite vein system and its importance in tracing paleowater flow and hydrocarbon migration in the ordovician carbonates of the tazhong area tarim basin china it is difficult to determine flow pathways of paleofluids in sedimentary basins petrography fluid inclusion and isotopic compositions from different phases of calcite veins in the ordovician in the tazhong area were studied our results reveal a complex diagenesis process with two importantly crossformational fluid movements coupled with three phases of calcite veins c1 c2 and c3 precipitation and thermochemical sulfate reduction by hydrocarbons fluids that precipitated c2 and c3 calcites were saline and enriched in 18o and thus may have originated from underlying evaporates whereas fluid that precipitated c3 had an additional contribution from 87sr rich ba rich and zn rich precambrian basinal clastic units fluid inclusion homogenization temperatures salinities and 18owater values of c2 veins show decreasing trends with increasing distances from strike slip faults this may indicate that c2 fluid might migrate into ordovician carbonate reservoirs through strike slip faults within 6 km during late hercynian orogeny late permian the similarly decreasing trends are observed in c3 veins indicating that the eastern part of no 1 fault seemed to be the main fluid flow pathway for c3 calcite precipitation during himalayan orogeny hydrocarbon may have migrated along the similar pathways as supported by 1 abundant primary hydrocarbon bearing fluid inclusions in the c2 and c3 calcites and 2 well documented hydrocarbon migration and mixing this is an ideal case showing how spatial variation in geochemical data from fracture fillings can be used to trace paleowater and likely hydrocarbon flows in sedimentary basins it thus may be useful in predicting fluid flow and distribution of hydrocarbons in the tarim basin and other basins

transvaporite model of ore genesis and an exploration strategy for new giant ore deposits abstract in the current work we stand for the revival of the transvaporite ore genesis model which was introduced by hungarian academician e szadeczky kardoss more than half a century ago but has now fallen out of favor and superseded by the orthomagmatic model our arguments in support of the transvaporite model are based on the contemporary overpressure theory developed for mesozoic cenozoic petroleum basins we propose a new variant of the transvaporization model whereby the formation of giant ore deposits occurs in the areas where former petroleum rich basins are intersected by large igneous provinces lips at such sites structural magmatic activity leads to hydrocarbon deposits being destroyed and many new ore deposits being created beneath a new regional volcanic fluid seal built by rapidly accumulating up to 3 4 km in 1 ma lava flows in such conditions a zone of significant overpressure that is 1 4 1 8 times higher than hydrostatic pressure develops subsequently under the influence of compressive mechanism of mass transfer gas saturated brines of sedimentary rocks migrate toward intruding melts and then mix with them as a result unusual rocks transvaporites are formed and giant ore deposits created here we review six supergiant ore deposits of various types and ages with different associations of metals and host rocks including the norilsk district in russia cu ni co pt pd el teniente los bronces district in chile cu mo almaden deposit in spain hg ermakovskoe deposit in russia be pebble copper deposit in southern alaska cu mo au and olympic dam deposit in southern australia u cu au ree noting that all listed metals in each and every deposit do reach giant reserves in all these deposits transvaporization appears to play the key role in ore genesis given the 1 short duration of the main ore genesis stage a significant practical implication of the transvaporite model is that it should modify future strategies for ore deposits exploration instead of pursuing just any deposit exploration efforts should focus specifically on giant ore deposits concentrating search efforts at the sites where former petroleum basins have been intersected by lips basin analysis of enriched oil and gas provinces is already being conducted in detail by seismic transects involving two dimensional cross sections and areal surveys followed by three dimensional mapping of productive reservoirs the same strategy should be applied to ore bearing regions but at better precision and accuracy given the smaller velocity contrast in seismic wave propagation in ore rocks compared with oil and gas bearing sedimentary rocks important advances in the ore geology of lips such as geological mapping and gravity and magnetic surveys have already produced significant new data these studies can be divided into two types which first involve a general description of the lip and are then followed by an investigation of major intrusions likely to be directly linked to ore genesis a combination of these investigations into hydrocarbon basins and lips will lay the foundation for transvaporite exploration which will be the key criterion for the successful search for new giant ore deposits

the role of fractures in the structural interpretation of sheep mountain anticline wyoming the development of fractures in the sedimentary layers of sheep mountain anticline a laramide asymmetric fault cored fold of the bighorn basin is documented and interpreted to constrain the kinematic evolution of the fold the fracture pattern is interpreted to identify five main fracture sets a fracture set striking 110 set i oblique to the future fold trend is interpreted as a regional fracture set that predates laramide compression a joint set set ii striking 045 and present in the hinge and backlimb is related to the ne oriented laramide compression just prior to and during initial anticline growth joints striking 135 set iii parallel to the fold trend are found within the hinge and are interpreted to have developed in response to the bending of layers the two youngest fracture sets are attributed to a late stage of fold growth a joint set set iv in the backlimb striking parallel to the set i fractures but vertical and a fracture set in the forelimb consisting of set i fractures reactivated as reverse faults the relative chronology mode of formation opening vs shearing and structural locations of these fractures suggest the following kinematics of folding at sheep mountain anticline little or no lateral fold propagation no hinge migration and limb rotation with only minor limb flexure and stretching