geologic structural and fluid inclusion studies of el bronce epithermal vein system petorca central chile the el bronce epithermal vein system located in the western foothills of the andean cordillera of central chile contains 25 metric tons of gold 105 metric tons of silver and 16 000 metric tons of copper the veins are hosted by volcanic rocks of the cerro morado early cretaceous and las chilcas early to late cretaceous formations which consist chiefly of breccias tuffs and lavas of andesitic composition the subcircular morro hediondo caldera with a diameter of 14 to 16 km and of late cretaceous age is located immediately north of the district dacitic tuffs and andesitic flows and breccias associated with the caldera are assigned to the lo valle formation on the basis of k ar ages of 83 to 80 ma two groups of early to late cretaceous intrusive rocks occupying north trending belts are recognized in the area the older a quartz monzodiorite body west of petorca intruded the cerro morado formation the younger comprises dioritic to granodioritic stocks dikes and sills and includes the petorca porphyry 86 or 3 ma and the dioritic tonalitic ring dike the latter defines the margin of the morro hediondo caldera 80 79 ma large zones of hydrothermal silicification and argillic alteration are associated with the two groups of intrusive rocks the morro hediondo caldera is the most prominent geologic feature in the area faults and fractures related to the caldera provided structural controls for the mineralization several northeast to northwest trending faults are either radial or concentric with respect to the caldera the most outstanding structures are the quebrada de castro and el bronee structural systems the el bronce structural system containing most of the mineralization in the district consists of extensional faults dikes and veins exposed within a 3 km wide and a 17 km long northeast trending zone delimited by the north northwest south southeast quebrada de castro and petorca e1 durazno dextral strike slip faults the relative movement of this fault pair was responsible for the structural pattern at el bronce which involved development of a first order dilational jog within this jog two major groups of veins are recognized the el bronce guanaco la olla san lorenzo group located north of the el bronce creek fault and the pedro de valdivia el espino norte group located south of this structure both groups of veins are vertical to subvertical and strike northwest to northeast detailed studies of the orebodies have shown that each ore shoot is composed of several lenses which contain as many as four ore types and a dike 1 hydrothermal breccia ore cemented 2 massive ore 3 stockwork zone and 4 disseminated zone orebodies may also contain barren andesite dikes the contacts between the four ore types are generally abrupt but are locally gradational the andesite dikes generally display sharp locally sheared contacts ore minerals are dominated by coarse crystalline pyrite sphalerite chalcopyrite galena tetrahedrite tennantite and minor bornite gangue minerals consist of quartz carbonates barite and chlorite five paragenetic stages are recognized 1 quartz pyrite gold 2 quartz pyrite sphalerite chalcopyrite gold 3 tetrahedrite tennantite galena silver 4 barite and 5 carbonates chlorite sphalerite hydrothermal alteration with the formation of sericite kaolinite chlorite and carbonates mainly affected the host rocks of the stockwork and disseminated ore types carbonates are the principal alteration minerals in the andesite dikes fluid inclusions associated temporally with the precious metal mineralization show homogenization temperatures that range from 235 degrees to 344 degrees c and salinities from 4 to 10 wt percent nacl equiv gold mineralization underlies a shallow zone of boiling of the hydrothermal fluids the fluid inclusion data suggest that the precious metal zone was generated 400 to 1 200 m beneath the paleosurface temperature and salinity tend to decrease in the shallow levels of the veins the calculated vertical variations in enthalpy of the hydrothermal fluids suggest that a fluid mixing mechanism was responsible for ore deposition hot relatively saline metal rich fluids ascended and mixed with cooler meteoric fluids to cause precipitation of the iron copper and zinc sulfides and subsequently the gold preliminary sulfur isotope studies delta 34 s 0 5 2 3ppm suggest a magmatic source for the sulfur in sulfides at el bronce

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

diagenesis of the hild brent sandstones northern north sea isotopic evidence for the prevailing influence of deep basinal water abstract the deeply buried 3 8 4 3 km brent reservoir sandstones of the hild field northern north sea have undergone intense diagenetic alteration fluid inclusion o c h sr isotope and k ar age data combined with petrography and burial history constrain the conditions and timing of the main diagenetic transformations as well as the nature and origin of diagenetic fluids non ferroan calcite cement formed at shallow depth the reconstructed diagenesis of the hild brent reservoirs displays notable differences with diagenetic models developed in many fields of the northern north sea in particular diagenesis in hild is characterized by the prevailing influence of deep basinal water a feature related to the immediate proximity of the viking graben this study further illustrates that diagenetic processes related to compaction derived fluids commonly reported in sediments located within rift basins should also be expected to occur in neighboring sub basins along rift margins

the temperature dependence of hydrogen isotope fractionation between clay minerals and water evidence from a geopressured system abstract d values of water samples from two gulf coast geopressured fields vary linearly with temperature while the d values from the coexisting clays remain constant and are independent of temperature fluid how rates on the order of millimeters per year characteristic of gulf coast geopressured systems provide adequate residence time for hydrogen isotope equilibrium to be achieved between the extant porewaters and clay minerals since equilibrium is achieved these d values are used to calculate the temperature dependence of the hydrogen isotope fractionation factor between illite smectite and water clay waterh between 0 and 150 c 1000 in clay water h 45 3 10 3 t 94 7 this equation predicts values for clay waterh at 96 c which are equal to those predicted by a similar equation derived by yeh 1980 above and below 96 c this new equation predicts values which diverge from those predicted by the equation of yeh 1980 with a difference of 9 at 150 c and 25 at 0 c the possibility is raised that the very slow upward component of flow characteristic of gulf coast geopressured sediments combined with the slow downward subsidence of the clay rich sediments can result in a system which is rock dominated with respect to hydrogen

small intermediate fault segments can either aid or hinder rupture propagation at stepovers 1 large scale geometrical complexities along faults are known to be likely endpoints for coseismic rupture as suggested by analysis of historic ruptures and corroborated by models of rupture on bent or discontinuous faults however natural faults also include smaller scale complexities we use the 3d finite element method to model dynamic ruptures on strike slip fault stepovers with a smaller intermediate fault between the main strands we find that such small faults can have a controlling effect on rupture behavior and ground motion intensity and distribution in particular the intermediate fault can either aid or prevent rupture propagation across the stepover depending on its length and basal depth the results have important implications for hazard assessment of faults with large and small scale geometrical complexities and also suggest that more site specific modeling studies may be necessary to develop realistic rupture scenarios for individual complex fault systems

an integrated thermal and isotopic study of the diagenesis of the brent group alwyn south u k north sea abstract the petrography fluid inclusion thermometry and isotope geochemistry of diagenetic cements are used to reconstruct the pore fluid history of the middle jurassic brent group reservoir sandstones in the alwyn south area of the u k north sea the study focuses on a relatively limited area of three adjacent reservoir compartments at successively higher structural levels the cement assemblage of kaolinite quartz and illite has resulted in severe deterioration of otherwise good reservoir quality early precipitation of vermiform and late blocky kaolinite was succeeded by a period of relatively intense illite precipitation temperature estimates for kaolinite precipitation of 80 c and 18o of 15 3 suggest co existing fluids of 18o 3 quartz cementation overlapped both kaolinite and illite formation fluid inclusion data indicate that quartz cementation took place at temperatures of 109 7 c pore fluid salinities were 4 wt nacl with a h2o o isotopic composition of 1 o 0 5 smow the fluids which precipitated coexisting illite were compositionally homogeneous with equilibrium 18o water compositions of 0 5 smow illite sd values range from 33 to 50 smow these fluid inclusion and isotopic data suggest that both quartz and illite were precipitated from pore waters with a uniform marine signature this is consistent with the predominantly marine to paralic depositional context of the brent group in alywn south illite precipitation was followed by hydrocarbon emplacement between the middle eocene and lower oligocene

pressure temperature time composition p t t x constraints of multiple petroleum charges in the hild field norwegian north sea abstract the brent reservoirs of the hild field situated at 3 8 4 3 km depth in the northern north sea show evidence of multiple episodes of diagenetic events fracturing and petroleum charging data from fluid inclusions and drill stem test samples characterise the petroleum filling history of the hild field two main types of petroleum inclusions occur type 1 inclusions which occur in two of the fault compartments of the field show a heavier composition than the present reservoir fluids from a comparison of fluid inclusion analyses and burial history onset of reservoir filling is most likely represented by type 1 inclusions and took place at 125 130 c in well 30 7 8r this temperature corresponds to a burial depth of 3 5 3 7 km which occurred 35 32 ma ago type 2 inclusions occur in all wells and are compositionally similar to the reservoir fluids suggesting recent trapping considerable differences in homogenisation temperature and phase behaviour of type 2 inclusions between the three wells of the hild field indicate the compartmentalised nature of the reservoir pvt modelling of fluid properties and trapping conditions was based on microthermometry gc data for type 1 inclusions and present day fluid compositional data for type 2 inclusions the results show generally good agreement between estimated pressure for type 2 inclusions and present day reservoir conditions and an increase in overpressure between trapping of types 1 and 2 inclusions we suggest that quartz cementation may continue after onset of reservoir filling and may exert major control on the pressure evolution in deep reservoirs

diagenetic effects on porosity permeability relationships in red beds of the lower triassic bunter sandstone formation in the north german basin abstract carbonate and anhydrite cement clay clasts and inter granular clay are the main components that reduce reservoir quality in the studied bunter sandstone formation the impacts of these parameters on porosity and permeability are determined by combining petrographic mineral quantification with conventional core analysis of samples from the danish part of the north german basin the depositional environments are considered because they largely control the distribution of cements clays and grain sizes the lateral variability of depositional environments is defined by the position in the basin and the proximity to the source areas the stratigraphic distribution of depositional environments is related both to local topography and to climate because high aridity promoted aeolian deposition the bunter sandstone formation has high porosity and permeability in most of the sandstone intervals in the northern north german basin the reservoir quality is good as long as the cements and clays are present as confined bodies that leave the remaining pore spaces available for flow in contrast inter granular clay and pervasive cementation hinder virtually all flow through the sandstone the ephemeral fluvial deposits have an average porosity and permeability of 20 3 and 810 md respectively and the values are 24 6 and 807 md for the aeolian sandstones excluding the unconsolidated aeolian sands which presumably have higher porosity and permeability the aeolian sandstones of the volpriehausen member have very good reservoir quality since they have a thickness of about 25 m are laterally continuous are largely clay free and the cement occurs in small amounts the sandstones of the solling member consist mainly of ephemeral fluvial deposits which generally have good reservoir quality however some intervals have high contents of inter granular clays or pervasive carbonate anhydrite or halite cement and these components reduce the permeability significantly the lateral distribution of the ephemeral fluvial sandstones is variable and therefore difficult to predict when planning a geothermal exploration well thus the volpriehausen member is the preferred target

meteoric water diagenesis in late cretaceous canyon fill turbidite reservoirs from the espirito santo basin eastern brazil abstract parameters controlling the diagenetic evolution of passive margin marine turbidites which are important targets of hydrocarbon exploration are poorly constrained in the literature this study aims to unravel the conditions of diagenesis and its impact on the reservoir quality evolution from late cretaceous canyon filling turbiditic sandstones of the onshore portion of espirito santo basin eastern brazil kaolinization 18 o 13 3 to 15 2 d 96 6 to 79 6 and dissolution of framework silicate grains is attributed to meteoric water incursion during eodiagenesis in response to a considerable fall in relative sea level eogenetic alterations also include cementation by siderite average 18 o 7 2 13 c 9 3 and pyrite progressive sediment burial present depths 1530 2027 m resulted in the formation of poikilotopic calcite ferroan dolomite ankerite average 18 o 7 9 13 c 2 9 minor amounts of quartz overgrowths and in partial dickitization of kaolinite isotopic values of calcite and dolomite ankerite follow two trends of co variance of 13 c with decreasing 18 o and increasing temperature from a composition closer to marine 0 one trend goes towards positive 13 c values up to 22 4 for calcite 18 6 for dolomite ankerite indicating increasing input of carbonate from methanogenic fermentation the other trend develops towards negative 13 c values down to 17 2 for calcite 15 for dolomite ankerite suggesting increasing contribution from thermal decarboxylation with increasing temperature and depth despite the presence of various cement types mechanical compaction was more important than cementation in reducing depositional porosity in the onshore urucutuca sandstones

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

sources of ca2 in the major carbonate cements in eocene sandstones and conglomerates evidence from sr isotopes sr ca ratios and rare earth elements abstract different sources of calcium have important effects on the formation and distribution of carbonate cements on the basis of identifying all possible sources of calcium comprehensive quantitative analyses of sr c and o isotopic values and trace elements including rare earth elements were conducted to determine the ca2 sources and their changes in the same type of carbonate cements at different vertical positions in the same reservoirs ferroan calcite and ankerite were the major types of carbonate cements ca2 in ferroan carbonate cements in the sandstones near the mudstone sandstone m s interfaces was derived primarily from diagenetic fluids mixed with ca2 originating from the transformation of smectite to illite and from original sedimentary water in the adjacent mudstones however ca2 in the ferroan carbonate cements in the sandstones away from the m s interfaces originated from the transformation of smectite to illite the dissolution of carbonate minerals and original sedimentary water in source rocks the gradual change of ca source of the same type of ferroan carbonate cements indicates the diversity of migration direction of diagenetic paleo fluid in the same period during the transformation of smectite to illite concentration gradients were created from mudstones to sandstones ca2 mg2 fe2 and other cations in the mudstones diffused to the adjacent sandstones and provided sources of ferroan carbonate cements at the edge of the sandstones pore water with ca2 mg2 fe2 co32 and other cations in the source rocks was collectively charged into sandstones during oil emplacement and promoted the precipitation of ferroan carbonate cements in residual pore water due to oil emplacement and material supplied by the adjacent mudstones the content of ferroan carbonate cements in the sandstones was lower and the reservoir porosity was higher at a greater distance from the m s interfaces however in thick bedded sandstones due to lack of material supply from the adjacent mudstones ca2 mg2 fe2 and co32 in the ferroan carbonate cements were mainly derived from source rocks as a result no significant changes were observed in the content of ferroan carbonate cements and reservoir porosity in the vertical direction

rupture termination and jump on parallel offset faults proper evaluation of seismic hazard depends on accurate estimates of potential earthquake size in areas with complex multisegment fault systems such an estimate in turn depends on an ability to predict the circumstances under which rupture may jump between fault segments many observational and numerical studies have analyzed the phenomenon of jumping rupture but none has focused on how the process of rupture termination on the primary nucleating fault segment affects the ability of rupture to jump to a secondary fault segment in the current study i model the dynamics of a simple 2d strike slip fault system with two parallel segments arranged with either a compressional or extensional stepover i vary the suddenness with which the initial shear stress tapers to zero on the primary section if the initial shear stress goes to zero over a very small 100 m distance rupture readily jumps both compressional and extensional stepovers of 1 km if the initial shear stress tapers to zero over 1 km rupture can jump the compressional stepover but not the extensional stepover if the initial shear stress tapers to zero over 2 5 km rupture cannot jump either the compressional or the extensional stepover the results illustrate the importance of the slip gradient and the resultant static stress field and the acceleration of the rupture front and the resultant generation of stopping phases in determining the probability of jumping rupture

composition porosity and reservoir potential of the middle jurassic kashafrud formation northeast iran abstract in the kopet dagh basin of iran deep sea sandstones and shales of the middle jurassic kashafrud formation are disconformably overlain by hydrocarbon bearing carbonates of upper jurassic and cretaceous age to explore the reservoir potential of the sandstones we studied their burial history using more than 500 thin sections supplemented by heavy mineral analysis microprobe analysis porosity and permeability determination and vitrinite reflectance the sandstones are arkosic and lithic arenites rich in sedimentary and volcanic rock fragments quartz overgrowths and pore filling carbonate cements calcite dolomite siderite and ankerite occluded most of the porosity during early to deep burial assisted by early compaction that improved packing and fractured quartz grains iron oxides are prominent as alteration products of framework grains probably reflecting source area weathering prior to deposition and locally as pore fills minor cements include pore filling clays pyrite authigenic albite and k feldspar and barite existing porosity is secondary resulting largely from dissolution of feldspars micas and rock fragments with some fracture porosity porosity and permeability of six samples averages 3 2 and 0 0023 md respectively and 150 thin section point counts averaged 2 7 porosity reflectance of vitrinite in eight sandstone samples yielded values of 0 64 0 83 in the early mature to mature stage of hydrocarbon generation within the oil window kashafrud formation petrographic trends were compared with trends from first cycle basins elsewhere in the world inferred burial conditions accord with the maturation data suggesting only a moderate thermal regime during burial some fractures iron oxide cements and dissolution may reflect cenozoic tectonism and uplift that created the kopet dagh mountains the low porosity and permeability levels of kashafrud formation sandstones suggest only a modest reservoir potential for such tight sandstones fractures may enhance the reservoir potential

note on the maximum upward migration of pore water in response to sediment compaction abstract a simplified model of compaction shows the maximum vertical upward pore water migration that can be expected as a result of sediment compaction this occurs when overpressuring lateral escape or fault related flow are absent in such a model the water that is expelled from a given sedimentary layer must be accommodated in the overlying layer s both sediment and water move downward with respect to the water sediment interface water however migrates downward at a slower pace than the sediment and hence appears to be expelled upward relative to a given sedimentary horizon burial history diagrams provide a convenient means of representing the time depth evolution of 1 sedimentary layers and 2 selected levels of water contained in the sediment above basement two major points are revealed by this approach firstly water generally does not escape a basin undergoing deposition however water can escape a basin through the water sediment interface when a thick layer of mud is blanketed by a thin sandy unit secondly a significant portion of a given sedimentary basin as modelled is saturated with water that was incorporated into it with its oldest sediments

co2 sequestration and extreme mg depletion in serpentinized peridotite clasts from the devonian solund basin sw norway the conglomerates of the solund devonian basin of sw norway contain numerous locally up to 20 vol peridotitic clasts with concentric mm to 10 cm thick zones of varying red to black color the peridotite clasts show a clear alteration related textural evolution the least altered rocks are partly serpentinized peridotites showing a typical mesh texture with veins of serpentine magnesite and ni rich magnetite surrounding olivine fo91 relicts and its mg depleted clay like alteration product deweylite assemblage in the more advanced ophicarbonate stage the mesh cells contain calcite silica and are surrounded by talc in the final stage quartz calcite and hematite dominate the mineralogy and occur together with minor amounts of chromite talc cr chlorite and cr hydroandradite in tandem with this textural evolution is a decrease in mgo from 40 to 2 5 wt and a cao increase from 1 to 35 wt all peridotite clasts are characterized by high cr and ni concentrations the chemistry and the textural evolution show that the clasts formed by an extreme mg mobilization from the peridotite with development of secondary porosity and subsequent precipitation of calcite mgo removed from the clasts after burial is in part consumed by replacement reactions in the sediment matrix around the clasts where mg free minerals e g almandine are replaced by mg bearing minerals e g talc calculated apparent 87 sr 86 sr ratios of the clasts at 385 ma 0 7124 0 7139 corresponding to the inferred age of sediment deposition and incipient clast alteration indicate interaction with diagenetic basinal fluids we explain the reaction history as a three stage process involving a partial serpentinization of olivine in an oceanic environment b breakdown of olivine relicts to the deweylite assemblage resulting in mobilization of mgo under near surface conditions in a tropical devonian climate and c further mg mobilization and replacement of the deweylite assemblage by calcite and quartz after diagenesis sedimentary basins with abundant weathered peridotite represent potential sites for a permanent co2 storage by formation of calcite in a low temperature environment 2010 elsevier ltd all rights reserved

unraveling climatic changes from intraprofile variation in oxygen and hydrogen isotopic composition of goethite and kaolinite in laterites an integrated study from yaou french guiana an integrated study of o and h isotopes in the lateritic profile of yaou french guiana was undertaken to investigate the usefulness of stable isotopes as tracers of climatic changes in continental environments the studied profile is composed of a 27 m thick saprolite mostly developed in the past under wet and dry tropical climate in association with a duricrust overlain by a 3 m thick yellow latosol formed more recently under present equatorial hot and humid climate 18o d values determined for weathering goethite pseudomorphs after pyrite and kaolinite microcrystalline clay groundmass throughout the 30 m deep profile reflect formation temperatures consistent with present 25 c and realistic past climatic temperatures 20 c 30 c indicating that weathering minerals formed in isotopic equilibrium with their genetic environment and were not subjected to significant isotope exchange after formation a distinct shift downward 2 for 18o 15 for d from low to high 18o d values occurs around 20 m depth in the saprolite it is interpreted as recording the change from the past tropical to the present equatorial climate goethite and kaolinite in the 5 10 m thick saprolite interval immediately above the active basement weathering front are in isotopic equilibrium with modern water and must have formed under present equatorial humid conditions in contrast goethite and kaolinite found higher up in the saprolite and in the duricrust formed in the past under tropical wet and dry climate from waters distinctly depleted in 18o and d relative to modern water the marked depletion of paleo meteoric water at yaou most likely reflects a more contrasted or monsoonal character of the ancient tropical climate the present study shows that ancient weathering minerals in lateritic profiles preserve their 18o d values and carry a time signal the time signal is best expressed in minerals formed rapidly at the weathering front and not subjected to post formational remobilization such as the goethite pseudomorphs after pyrite occurring at yaou groundmass kaolinite is more susceptible to partial remobilization through successive dissolution precipitation reactions which may obliterate the paleoclimatic signal unraveling the climatic record carried by weathering minerals in old soil systems is greatly enhanced by studying both oxygen and hydrogen isotopic compositions

model for the evolution of oxygen isotope ratios in the pore fluids of mudrocks during burial abstract the determination of oxygen isotopic ratios in authigenic silicates and pore fluids provides vital information on the nature of water rock interactions in the subsurface and the origin of porosity occluding phases in reservoir rocks the 18 o values of pore fluids generally increase during burial through water rock interaction the transformation of detrital smectite to illite is thought to have a major influence on the isotopic evolution of pore fluids but previous attempts to model this effect have failed to accurately reproduce the observed water rock isotopic trends with depth a revised model has been developed which incorporates an empirical rate equation for the illitization reaction the input variables include the initial mineral and pore water isotopic compositions time porosity depth functions and geothermal histories and the assumption is made that the isotopic equilibration of the solid phases with pore waters only takes place during reaction the model is tested against the most complete data set available from the texas gulf coast usa and produces a good fit with the available pore fluid and mineral data the sensitivity of the model is tested for variations in the following parameters the illite water fractionation factor the fractionation of oxygen isotopes between the smectitic and illitic components of the detrital illite smectite the oxygen isotope fractionation between pore waters and the interlayer water within the smectite and the porosity depth function the results of the model are shown to be strongly dependent on the porosity depth function the best fit between the model and real data is provided by the dickinson equation for undercompacted shale modified to have a constant porosity below 2600 m to simulate the effects of overpressuring the model is also sensitive to the degree of oxygen isotopic fractionation between the illite and smectite within the sediment at the time of burial but not to the extent of fractionation between interlayer water within smectite and the adjacent pore waters the uncertainty within the illite water fractionation factor has only a minor effect on the results of the model although only incorporating a single mechanism for water rock interaction the model provides a tool for testing the interpretations of subsurface pore fluid evolution trends during sediment diagenesis

conditions and timing of quartz cementation in brent reservoirs hild field north sea constraints from fluid inclusions and sims oxygen isotope microanalysis abstract the conditions and timing of quartz cementation in the brent sandstones in the hild field norwegian north sea were constrained on the basis of oxygen isotope microanalysis and fluid inclusion microthermometry combined with optical and cathodoluminescence cl microscopy studies samples of the tarbert and the ness formations were investigated in three different wells presently lying at 3 8 4 3 km subsea and at 140 155 c quartz cement is present in all wells abundance varies from 3 to 26 and tends to increase with depth of burial two distinct separate episodes of quartz cementation referred to as q1 and q2 are readily distinguished by cl microscopy the two episodes are also recognized in the bimodal distribution of fluid inclusion temperatures and oxygen isotope microanalyses the data indicate that q1 quartz formed in the early tertiary 65 35 ma at 95 110 c and 2 3 3 0 km burial depth and that q2 mainly precipitated in the oligocene miocene 40 10 ma at 125 135 c and 3 1 3 7 km q1 quartz precipitation follows the onset of overpressuring in the reservoir while precipitation of q2 seems to slightly precede and accompany the main phase of oil filling 35 10 ma secondary ion mass spectrometry sims oxygen isotope microanalyses yield 18o values ranging from 17 to 25 averaging 23 2 for q1 and 19 5 1 5 for q2 calculated 18o values of quartz forming waters are similar for q1 and q2 ranging from 1 to 3 and are similar to the 18o value 2 of present day formation water in the reservoir quartz cementation in the brent reservoir of the hild field occurred during deep burial at high temperature from evolved 18o rich basinal water most likely derived from the viking graben two episodes of enhanced quartz precipitation are recorded around 100 c and 130 c no evidence was found for any significant contribution of 18o depleted jurassic meteoric water and or low temperature precipitation of quartz cement

the anatomy of a carboniferous epithermal ore shoot at pajingo queensland setting zoning alteration and fluid conditions the paleozoic scott lode gold silver epithermal deposit situated at pajingo northeast queensland is hosted by feldspathic volcanolithic sandstones quartzose sandstones ignimbrites andesitic voltanits with interbedded block and ash deposits lapilli tuffs and andesitic to dioritic intrusive rocks the scott lode deposit contains 1 23 million metric tons of 9 9 g t gold and 38 9 g t silver there are six principal types of alteration assemblages which are zoned with respect to the main vein structure propylitic potassic adularia intermediate argillic silicic kaolinitic and ferroan carbonate overprinting textural relationships indicate that the propylitic type is earliest progressively followed by potassic adularia then intermediate argillic and finally silicic alteration auriferous veins are largely coeval with the silicic and intermediate argillic alteration types kaolinitc and ferroan carbonate occur as late stage alteration products postdating the precious metal mineralization principal ore and sulfide minerals include pyrite sphalerite galena chalcopyrite hessitc tennantite tetrahedrite argentitc petzite electrum and native gold gangue minerals in the veins are quartz illitc adularia calcite and locally kaolinitc the main stage quartz is zoned with respect to mineralogy mineralization styles metal distribution patterns and vein textres three major zones can be recognized zone i an upper precious metal moss chalcedonic crustiform colloform quartz zone zone ii intermediate precious base metal crystalline quartz coarse grained sulfide dominated crustiform colloform quartz zone and zone iiideep sparse base metal weakly banded crystalline comb quartz zone the ore interval occurs in zones i and ii with the transition to zone iii effectively defining the limit of mineralization economic mineralization at the scott lode deposit was laid down from dilute fluids 0 7 2 5 wt nac1 equiv with a very low concentration of co2 homogenization temperatures of 170 o to 315oc exhibit vertical variation with higher temperatures at greater depth the presence of coexisting liquid and gas rich inclusions in mineralized quartz together with adularia vein brecciation and pronounced crustiform colloform banding indicates hydrothermal boiling as the principal precipitation mechanism for gold and silver deposition thermodynamic modeling of adiabatic boiling of a fluid with an initial concentration of 2 to 5 ppb gold at 300oc indicates that such a fluid can deposit ores containing 10 ppm gold and 260 ppm silver at the temperature intervals recorded by the fluid inclusion work

mixing zone origin of 13c depleted calcite cement oseberg formation sandstones middle jurassic veslefrikk field norway abstract carbonate cement is one of the more volumetrically important diagenetic components of sandstones the sandstones of the fan delta facies of the oseberg formation brent group in veslefrikk field contain abundant carbonate cemented zones commonly comprised of 30 40 carbonate by volume the cement is predominantly poikilotopic ferroan calcite with minor amounts of ankerite and siderite petrographie evidence for high porosities at the time of calcite precipitation indicate that cementation occurred under shallow burial conditions probably less than 500 m crosscutting textural relations demonstrate that calcite cementation was predated and postdated by tectonic stresses that produced both grain fractures and through going fractures respectively in the non calcite cemented parts of the formation major compaction pressure solution feldspar dissolution and cementation by quartz and clays postdated calcite cementation the oxygen isotopic composition of the calcite averages 8 5 0 pdb and shows large variations within individual calcite cemented zones up to 10 0 in 18 o the depleted and variable oxygen isotopic composition in conjunction with evidence for shallow precipitation indicate calcite growth occurred in a mixing zone environment variably influenced by meteoric water isotopic variation within many individual cemented zones is symmetric about the center of the zones indicating that growth of the cemented zones was from the center outwards in a concretionary fashion the high fe and mn content of the calcite avg 1 3 mol fe avg 0 4 mol mn and its association with disseminated pyrite indicate that porewater was reducing during precipitation carbon isotopic data avg 13 c 15 7 0 pdb range 3 to 3 1 0 show that major amounts of organic carbon at least partly derived by sulfate reduction of biogenic methane were incorporated in the calcite other documented occurrences of early 13 c depleted carbonate cemented sandstones are similar to the carbonate cement in veslefrikk field in their association with methane oxidation and their low temperature formation however the cement at veslefrikk differs in its mineralogical and morphological occurrence and in the fact that it formed from a porewater strongly influenced by mixing with meteoric water key geologic factors that determined the traits of calcite cement in veslefrikk field are the combination of nearshore marine deposition of permeable sands close proximity of organic rich marine muds and variations in relative sea level which provided opportunities for invasion by meteoric water these geologic factors had a similar impact on other brent group deltaic sandstones and are believed to explain some of the differences between the geochemistry of carbonate cements in deltaic and shelf sandstones of the north sea and elsewhere

diagenetic pore waters stratified for at least 35 million years magnus oil field north sea isotopically distinct pore waters remained stratified in the upper jurassic magnus sandstone reservoir for the magnus oil field in the northern north sea throughout diagenesis a petrographic and isotopic study of the reservoir sandstones in three wells indicates that a similar suite of diagenetic quartz overgrowths kaolinite magnesian siderite ankerite and illite developed across the field however the oxygen isotope ratios delta 18 o recorded in these diagenetic minerals from the crest of the field are consistently more negative by about 3 o oo than those from the downdip flank we propose that this consistent difference in delta 18 o values records the persistence of isotopically different and stratified pore waters in the magnus sandstones the sandstones w re deposited as marine turbidites but were subaerially exposed during periods of structural uplift and tilting during the early and middle cretaceous meteoric water entered the structural crest resulting in pore water stratification deposition of thick upper cretaceous mudstones and marls sealed the magnus structure during subsequent rapid burial diagenetic minerals in the crest of the oil field grew in a pore water with significantly more meteorically derived water than those downdip assuming that each diagenetic phase developed at a similar temperature in each well the pore waters remained stratified for at least 35 m y from middle cretaceous approximately 90 ma subaerial erosion of the sandstone until reservoir charging at 55 ma the sandstones and mudstones of the kimmeridge clay formation behaved essentially as a closed system during diagenesis in contrast to the open system concept envisaged in most other diagenetic reconstructions

isotopic geochemistry of burial metamorphosed volcanogenic sediments great valley sequence northern california isotopic and mineralogic data from an 8500 m thick section of the great valley sequence northern california indicate that changes in the 18o values of authigenic minerals resulted from the conversion of smectite to a 10 a clay mineral as temperature increased with burial in the jurassic cretaceous outer arc basin the clay mineral assemblage in mudstone is characterized by a proportional increase of the 10 a clay mineral with increasing stratigraphic depth and by a depletion in the 18o value of the mixed layer smectite 10 a clay mineral with descending stratigraphic position from 21 9 to 15 5 smow modeling of the oxygen isotopic data from authigenic phases based on equilibrium fractionation during clay mineral diagenesis indicates that 18o values of calcite in mudstones and of calcite cements in sandstone precipitated along a temperature gradient of about 25 c km during maximum burial to about 6 7 km d values of the mixed layer smectite 10 a clay mineral range between 69 to 44 smow using temperatures calculated from the oxygen isotopic data the deuterium and oxygen isotopic data indicate that the smectite underwent late stage dehydration and probably buffered the composition of formation waters from sea water values to isotopic compositions of 18o 8 smow and d 25 smow the 13c values of calcite from mudstone and sandstone imply that crystallization of authigenic calcite was linked to organic diagenesis during which dissolved hcot 3 was continuously enriched in 13c as temperature increased with burial at the base of the sequence and immediately overlying the ophiolitic basement rocks several hundred meters of strata were altered by more oxygen depleted 18o 4 to 5 hydrothermal fluids emanating from the ophiolitic rocks probably at maximum burial depth

mapping diagenetic fluid flow within a reservoir kar dating in the alwyn area uk north sea abstract the alwyn area demonstrates that variation in the timing of diagenesis on the field scale depends on the structure and the proximity to the sources of diagenetic fluids variation in reservoir quality within the structure is the result of facies control on fluid migration illite in alwyn south formed in the middle palaeocene to middle oligocene between 60 and 38 ma at this time the brent group was at depths of between 2400 and 3000 m and reservoir temperatures ranged from 110 to 120 c extensive k ar dating shows that illitization pre oil emplacement proceeded from the deepest reservoir panels along and up structure fluid migration may have occurred along the deep seated alwyn south ninian fault system where the reservoir is lithologically homogeneous illite precipitation proceeded systematically base upwards at rates of 5 5 m ma 1 the duration of illite formation at any one level was between 2 and 8 ma in contrast in heterolithic reservoirs fluid migration occurred initially along high permeability fairways and consequently illite ages do not vary systematically with depth

illitization and paleothermal regimes in the middle ordovician st peter sandstone central michigan basin k ar oxygen isotope and fluid inclusion data hydrocarbon reservoirs occur in the middle ordovician st peter sandstone in the central michigan basin at depths of 1 5 3 5 km and are diagenetically altered latest diagenetic cements include saddle dolomite pervasive microcrystalline illite and chlorite and quartz a k ar and 18o 16o study of the fine grained authigenic illite in 25 samples from 16 wells covering a large area within the basin yields k ar ages ranging from 367 to 322 ma and dgr18o values between 12 7 and 16 9 pmil smow the dgr18o values of diagenetic quartz overgrowths range from 15 2 to 18 9 pmil fluid inclusion temperatures in the quartz cement range from 70 to 170 c reflecting multiple generations of diagenetic quartz and or precipitation over m st of the diagenetic history reequilibrated fluid inclusions in the saddle dolomite cement yield temperatures ranging from 90 to 150 c a regionally significant episode of illitization occurred during the late devonian mississippian temperatures of illite formation are indirectly estimated to be in the range of 125 170 c and most paleodepths of illitization are between 2 8 and 3 2 km these results imply that 1 illite formed from 18o rich fluids and 2 elevated geothermal gradients i e greater than 34 c km existed in the michigan basin in the late paleozoic the k ar ages and the dgr18o values are not correlated to present depths of the samples or paleodepths of illitization illites with young ages and low dgr18o values tend to be geographically distributed along the north south branch of the buried precambrian rift the dgr18o values of the diagenetic quartz follow a similar trend the spread of illite k ar ages and dgr18o values and their geographic distribution are best explained as reflecting abnormally high thermal regimes in the part of the basin located above the presumably highly fractured basement along the rift

continental shelf morphology and stratigraphy offshore san onofre california the interplay between rates of eustatic change and sediment supply abstract new high resolution chirp seismic data acquired offshore san onofre southern california reveal that shelf sediment distribution and thickness are primarily controlled by eustatic sea level rise and sediment supply throughout the majority of the study region a prominent abrasion platform and associated shoreline cutoff are observed in the subsurface from 72 to 53 m below present sea level these erosional features appear to have formed between melt water pulse 1a and melt water pulse 1b when the rate of sea level rise was lower there are three distinct sedimentary units mapped above a regional angular unconformity interpreted to be the holocene transgressive surface in the seismic data unit i the deepest unit is interpreted as a lag deposit that infills a topographic low associated with an abrasion platform unit i thins seaward by downlap and pinches out landward against the shoreline cutoff unit ii is a mid shelf lag deposit formed from shallower eroded material and thins seaward by downlap and landward by onlap the youngest unit iii is interpreted to represent modern sediment deposition faults in the study area do not appear to offset the transgressive surface the newport inglewood rose canyon fault system is active in other regions to the south e g la jolla where it offsets the transgressive surface and creates seafloor relief several shoals observed along the transgressive surface could record minor deformation due to fault activity in the study area nevertheless our preferred interpretation is that the shoals are regions more resistant to erosion during marine transgression the cristianitos fault zone also causes a shoaling of the transgressive surface this may be from resistant antecedent topography due to an early phase of compression on the fault the cristianitos fault zone was previously defined as a down to the north normal fault but the folding and faulting architecture imaged in the chirp data are more consistent with a strike slip fault with a down to the northwest dip slip component a third area of shoaling is observed off of san mateo and san onofre creeks this shoaling has a constructional component and could be a relict delta or beach structure

application of sr isotopes in secondary silicate minerals to paleogroundwater hydrology an example from k metasomatized rocks in the western u s abstract strontium isotope analyses of secondary silicate minerals represent a largely unexploited method of investigating paleogroundwater hydrology guided by cathodoluminescent imaging we obtained sr isotopic data from physical microsamples of primary and secondary silicate minerals in a k metasomatized mid tertiary ash flow tuff at socorro new mexico the intent was to test the utility of silicate sr isotopic data in paleohydrologic studies and to assess models for the basin scale paleogroundwater flow responsible for the k metasomatism in the socorro area secondary adularia in the upper lemitar tuff from a location at the southwestern extent of k metasomatized rocks contain sr with significantly higher 87 sr 86 sr ratios 0 720 than igneous feldspars 0 709 at the approximate time of the late cenozoic metasomatism 6 ma estimates of saline brine 87 sr 86 sr ratios obtained from late cenozoic evaporites in the region are much lower 0 712 than the adularia values and suggest that such brines were not responsible for the k metasomatism in this area groundwaters that had passed through high 87 sr 86 sr precambrian rocks prior to entering the tuffs are more likely k metasomatizing fluids but isotopic data from altered upper lemitar tuff from throughout the socorro area are required to fully test this possibility

effect of meteoric water flow on calcite cementation in the middle jurassic oseberg formation well 30 3 2 veslefrikk field norwegian north sea abstract the middle jurassic delta front sandstones of the oseberg formation in well 30 3 2 in the veslefrikk oilfield contain abundant calcite cemented intervals with thicknesses of up to 2 15 m most of the calcite cement is characterized by 18 o pdb values in the range 5 to 10 and by low strontium concentrations mostly in the range 100 500 ppm the low strontium concentration in the studied calcite cement can probably best be explained by the precipitation of calcite during meteoric water flushing 18 o pdb values of 5 to 10 have also been considered characteristic of calcite cement precipitated from meteoric water and ubiquitous authigenic kaolinite and feldspar dissolution in the studied sandstones suggests that extensive meteoric water flushing of the oseberg formation has taken place during and soon after deposition a few of the studied calcite cemented intervals contain a calcite cement with a much higher strontium content 750 1430 ppm and more negative 18 o pdb values 11 1 to 14 3 these calcite cemented intervals are thought to have formed at greater depth after meteoric water flushing had terminated the calcite cement with the highest strontium concentration and the most negative 18 o values occurs within calcite cemented intervals with sharp boundaries the calcite cement with a low strontium concentration and intermediate 18 o values is found within calcite cemented intervals with gradational boundaries characterized by a transition zone of calcite cemented specks in uncemented sandstone this difference in cementation pattern can be explained by the effect of pore water flow rates on the nucleation and growth of calcite cement

the influence of clay mineralogy on formation damage in north sea reservoir sandstones a review with illustrative examples abstract this paper critically reviews the clay mineralogy of reservoir sandstones in the north sea as assessed from peer reviewed papers in the literature as well as from the authors personal experience in the particular context of formation damage the most common clay minerals in these sandstones are well crystallized kaolinite mainly occurring as pore filling vermiform and booklet like aggregates illitic clays usually in the form of pore filling networks of thin lath like filamentous or hairy particles but less frequently as platy aggregates and chlorite most commonly found as pore lining aggregates of interlocking well developed bladed crystals all these clay minerals are authigenic diagenetic in origin discrete smectite is rarely found in north sea reservoir sandstones even though associated pelitic rocks may contain an abundance of this type of clay mineral the crystallization of kaolinite in these sandstones has been attributed to a variety of stages in the paragenetic sequence particularly in relation to the authigenic formation of calcite and quartz however most evidence suggests that vermiform kaolinite is a product of early diagenesis at temperatures ranging from surface to 40 c either before or contemporaneously with carbonate cementation and before the formation of quartz overgrowths exceptions to this generalization may occur because of complex basin histories involving tectonic uplift and migration of fluids of varying chemistry with increasing depth of burial and at higher temperatures the kaolinite in these reservoir sandstones converts at least partially to dickite which occurs in more blocky aggregates consideration of recent theoretical studies of kaolinite indicate that at ph values 8 all face and edge surfaces will be uniformly negatively charged strongly suggesting that in these circumstances the kaolinite will become disaggregated to form stable dispersions capable of migration where there is sufficient force of hydrodynamic flow the illitic clay in north sea reservoir sandstones is usually described in terms of two discrete phases namely illite itself and mixed layer illite smectite i s evidence is presented to show that in all probability only one illitic phase exists in these sandstones and that the mixed layer phase in reality consists only of very thin illite 100 c sem observations showing that putative pore lining illite actually consists of dense masses of compacted illite fibres identical to those that fill pores and finally the similar chemical compositions of both pore lining and pore filling illite showing them both to be of a muscovitic or phengitic nature it is probable that the illitic clay in north sea reservoir sandstones is highly dispersible and is prone to redistribution particularly in the na saturated state and because of its very fine particulate form would therefore be readily mobilized by hydrodynamic forces finally several illustrative examples are presented to show that clay migration during fluid injection is a probable cause of formation damage in north sea reservoir sandstones sem observations following fluid treatments clearly show the breakup of kaolinite aggregates whereas the evidence for illite mobilization consists of the appearance of large holes in the network of pore filling illite laths in addition to this analytical evidence shows that both kaolinite and illite particles have exited core samples following fluid or gas flow thereby proving their mobility within the core

seismic constraints on the architecture of the newport inglewood rose canyon fault implications for the length and magnitude of future earthquake ruptures the newport inglewood rose canyon fault zone nirc is an active strike slip fault system within the pacific north american plate boundary in southern california located in close proximity to populated regions of san diego orange and los angeles counties prior to this study the nirc fault zone s continuity and geometry were not well constrained nested marine seismic reflection data with different vertical resolutions are employed to characterize the offshore fault architecture four main fault strands are identified offshore separated by three main stepovers along strike all of which are 2 kilometers or less in width empirical studies of historical ruptures worldwide show that earthquakes have ruptured through stepovers with this offset models of coulomb stress change along the fault zone are presented to examine the potential extent of future earthquake ruptures on the fault zone which appear to be dependent on the location of rupture initiation and fault geometry at the stepover these modeling results show that the southernmost stepover between the la jolla and torrey pines fault strands may act as an inhibitor to through going rupture due to the stepover width and change in fault geometry across the stepover however these results still suggest that rupture along the entire fault zone is possible

the patricia zn pb ag epithermal ore deposit an uncommon type of mineralization in northeastern chile the patricia ore deposit represents an unusual example of economic zn pb ag mineralization at the northernmost end of the late eocene oligocene metallogenic belt in chile it is hosted by volcano sedimentary units which are typically tuffaceous and andesitic breccias the ore body consists of a set of subvertical e w vein systems developed under a sinistral strike slip regime that included transtensive domains with generalized extensional structures where the ores were deposited the deposit is divided into two blocks by a set of nnw ese trending reverse faults which uplifted the eastern block and exhumed thicker and deeper parts of the deposit at least 200 m of volcano sedimentary pile hosting the mineralization has been eroded in this block by contrast the western block exposes a shallower part of the system where cherts amorphous silica and jasperoids occur three main stages of mineralization have been defined 1 pre ore stage is characterized by early quartz pyrite and arsenopyrite 2 base metal and silver stage characterized by sphalerite 6 to 15 mol fes galena chalcopyrite pyrrhotite and ag bearing minerals freibergite polybasite stephanite pyrargyrite freieslebenite and acanthite and 3 post ore stage characterized by late quartz kutnohorite and minor sulfides arsenopyrite sphalerite pyrite galena ag bearing minerals and pb sulfosalts whole ore geochemistry shows two groups of elements that are positively correlated 1 ag cd cu pb zn related to the base metal sulfides and 2 au as ge sb w related to arsenopyrite and pyrite hydrothermal alteration is pervasive in the outcropping mineralized areas including silicification and locally vuggy silica textures at depth chloritic and sericitic alteration is developed along vein selvages and is superimposed to the regional propylitic alteration fluid inclusions indicate that the base metal ores were deposited from 250 to 150 c moderate salinity fluids 1 9 wt nacl the pre ore stage is characterized by a saline fluid 6 22 wt nacl and between 210 and 250 c whereas the post ore stage has salinity of 4 8 wt and temperature from 175 to 215 c cooling was the mechanism of ore mineral precipitation in the patricia deposit although mixing of fluids could have occurred in the pre ore stage mineralogical geochemical and fluid inclusion evidence is consistent with an intermediate sulfidation is epithermal deposit type this study highlights the high potential for hidden economic mineralization at depth in the western block and for extension of the ore body both to the south and to deeper levels in the eastern block of the patricia ore deposit to a larger extent the implications of finding such polymetallic epithermal style of mineralization in the northern chile precordillera is relevant both to the regional metallogenic perspective and to the exploration potential of the region where the late eocene early oligocene metallogenic belt apparently disappears

a renewed look at calcite cement in marine deltaic sandstones the brent reservoir heather field northern north sea uk wireline and seismic acoustic impedance imaging show that the marine part of the clastic brent group reservoir in the heather field northern north sea contains much calcite cement in the flank parts of the structure the non marine ness formation and crest parts of the structure contain negligible calcite cement this localised calcite cement has led to relatively poor reservoir performance since first oil in 1978 although a new suite of wells has boosted production with plans to keep the field active until 2030 understanding the origin and distribution of calcite cement would help the development of more realistic reservoir models and boost production rates through optimum well location we have thus used a suite of techniques including standard point counting sem eds mineralogy bse microscopy fluid inclusion thermometry and stable isotope analysis to develop new and improved models of calcite distribution calcite seems to have attributes of both early and late diagenetic cement 30 to 40 intergranular volume in calcite cemented beds seems to support pre compactional growth but high temperature fluid inclusions and the presence of primary oil inclusions suggests late growth much calcite may have developed early but it seems to have recrystallized and possibly undergone redistribution at close to maximum burial or had a late growth event calcite cement probably originated as marine derived micrite bioclasts or early marine cement but adopted the isotopic characteristics of high temperature growth as it recrystallized quartz grains have corroded outlines in calcite cemented areas with one sample with 79 calcite cement displaying signs of nearly total replacement of quartz grains by calcite the flank localisation of calcite remains to be explained although it could be due to primary depositional factors early diagenetic loss of calcite from crestal regions or late diagenetic loss of calcite from crestal regions controversially the growth of calcite seems to be associated with quartz dissolution although the geochemical and petrophysical cause of this remains obscure diagenetic loss of quartz from sandstones cannot easily be explained by conventional modelling approaches and yet seems to be an important phenomenon in heather sandstones

portovelo a volcanic hosted epithermal vein system in ecuador south america the portovelo epithermal vein system in southwestern ecuador has produced more than 120 tons of gold and about 250 tons of silver the veins result from hydrothermal processes close to a miocene volcano which produced an andesitic to dacitic sequence followed by collapse and post collapse rhyolitic activity which generated most of the alteration and mineralization three main structural segments are defined by nw trending strike slip faults which show later stages of vertical movement these faults are responsible for development of an extensive n s dilatational jog within andesitic rocks which acted as the main host to ore deposition a large scale propylitic aureole surrounds a quartz chlorite sericite adularia core centered on the rhyolites within a system of collapse related ring structures a quartz chlorite sericite adularia calcite assemblage is the most common wall rock alteration close to the veins the size 4 x 15 km and vertical range 1400 m of the vein system is exceptional alteration textures and mineral assemblage including a quartz calcite gangue sulfides abundant sulfosalts and free gold electrum are quite typical of an adularia sericite epithermal deposit spatially the mineralization is arranged in three zones in addition three successive stages can be distinguished the bulk of economic mineralization was deposited during the second stage in association with a clear quartz and calcite gangue tm ice and tm clath data of fluid inclusions in the clear quartz indicate a high salinity similar to 10 5 eq wt nacl the homogenization temperatures of fluid inclusions in the gangue minerals and in the altered host rocks vary between 180 degrees and 310 degrees c quartz delta 18 o values from hydrothermally altered wall rocks reflect the original isotopic values of the latter these values show a narrower range in vein quartz delta o 18 between 7 7 parts per thousand and 11 5 parts per thousand smow in addition the delta o 18 values of the vein quartz increase systematically with decreasing homogenization temperature this suggests that quartz was in equilibrium with a large reservoir of water of constant o 18 o 16 composition at decreasing equilibrium temperatures the estimated isotopic composition of the fluids from which milky quartz and calcite of the main mineralization stage precipitated lies around 1 smow this value indicates a meteoric rather than a magmatic origin of the ambient fluid clear quartz of the second stage seems to have formed from a fluid with a delta 18 o of 3 parts per thousand smow this higher value can be due to a more intense water rock interaction or to mixing of meteoric with magmatic water

paleoseismicity of the north branch of the newport inglewood fault zone in huntington beach california from cone penetrometer test data abstract application of cone penetrometer testing cpt is a promising method for studying subsurface fault zones in stratified unconsolidated sediment where trenching is not feasible analysis of data from 72 cpts spaced 7 5 to 30 0 m apart and 9 borings indicates that the north branch fault the active strand of the newport inglewood fault zone nifz in huntington beach has generated at least three and most likely five recognizable surface ruptures in the past 11 7 0 7 ka additional smaller earthquakes similar to the m w 6 4 1933 long beach earthquake may also have occurred but would not be recognizable with this method the minimum right lateral holocene slip rate of the nifz in the study area is estimated to be 0 34 to 0 55 mm yr the actual slip rate may be significantly higher

sedimentary environment and diagenesis of the lower cretaceous chaswood formation southeastern canada the origin of kaolin rich mudstones abstract the lower cretaceous fluvial sandstone mudstone succession of the chaswood formation is the proximal equivalent of offshore deltaic rocks of the scotian basin that are reservoirs for producing gas fields this study interprets the mineralogical consequences of cretaceous weathering and early diagenesis in a 130 m core from the chaswood formation in order to better understand detrital and diagenetic minerals in equivalent rocks offshore mineralogy was determined by x ray diffraction electron microprobe analysis and scanning electron microscopy the rocks can be divided into five facies associations light gray mudstone dark gray mudstone silty mudstone and muddy sandstone sorted sandstone and conglomerate and paleosols facies transitions in coarser facies are related to deposition in and near fluvial channels in the mudstones they indicate an evolutionary progression from the dark gray mudstone facies association swamps and floodplain soils to mottled paleosols well drained oxisols and ultisols following syntectonic uplift facies transitions and regional distribution indicate that the light gray mudstone facies association formed from early diagenetic oxidation and alteration of the dark gray mudstone facies association probably by meteoric water principal minerals in mudstones are illite muscovite kaolinite vermiculite and quartz illite muscovite is of detrital origin but variations in abundance show that it has altered to kaolinite in the light gray mudstone facies association and in oxisols vermiculite developed from the weathering of biotite and is present in ultisols the earliest phase of sandstone cementation in reducing conditions in swamps and ponds produced siderite nodules and framboidal pyrite which were corroded and oxidized during subsequent development of paleosols kaolinite is an early cement coating quartz grains and as well crystallized pore filling booklets that was probably synchronous with the formation of the light gray mudstone facies association later illite and barite cement indicate a source of abundant k and ba from formation water this late diagenesis of sandstone took place when the chaswood formation was in continuity with the main scotian basin prior to oligocene uplift of the eastern scotian shelf findings of this study are applicable to other mid latitude cretaceous weathering and early diagenetic environments

biogenic gas in fluid inclusions from sandstones in contact with oil mature coals this study was initiated to investigate if coals on the norwegian offshore continental shelf nocs expel petroleum and in which form the results revealed that equally isotopically light methane c1 was released from fluid inclusions in sandstones and from adjacent coal 60 9 to 72 7 the analyzed samples were collected from cored northern north sea and mid norwegian shelf wells in the depth interval 3924 5095 m 12 874 16 716 ft the vitrinite reflectance ro values of the coals range between 0 53 and 1 12 with most values between 0 8 and 1 0 only minor c2 fractions were released showing much heavier gas isotope signatures similar to values seen in our in house nocs gas isotope database for comparable depths the similar light c1 isotope values released both from the coals and from the fluid inclusions in the adjacent sandstones suggest that the origin of the gas is the coal and that no isotope fractionation occurs during release of the gas in nature traditional isotope interpretation schemes suggest the c1 to have a biogenic origin whereas recently published data also show the possibility for an early mature thermogenic origin the isotope values represent averages of the total gas released from all the individual disintegrated fluid inclusions in each sample these did not form simultaneously but during multiple events potentially covering several million years the release of isotopically light c1 proves gas presence in the sandstone at the time of fluid inclusion formation we therefore speculate that significant volumes of isotopically light c1 have been expelled from the analyzed coals over time the expelled isotopically light c1 may mix with mature thermogenically produced gas and skew the overall methane isotope values of gas accumulations toward lighter values thus explaining the isotopically lighter than expected gas accumulations on the nocs e g troll frigg and draugen fields