thin section and s e m textural criteria for the recognition of cement dissolution porosity in sandstones cement porosity relationships are described from the lower triassic sherwood sandstone group and the middle jurassic ravenscar group in the united kingdom calcite cemented sandstones display a variety of replacement textures with preferential replacement of grains and of overgrowth faces with high free surface energy dolomite and siderite cemented sandstones display similar textures but replacement is less specific and euhedral overgrowth surfaces are commonly embayed by carbonates examination of the more porous sandstones with the scanning electron microscope reveals a range of pitting and embayment textures in authigenic overgrowths and in detrital grains these range from small v shaped notches and pits through regular and irregular shaped embayments into large depressions these textures appear to be morphologically similar to the quartz surfaces seen in thin sections of carbonate cemented sandstones and are interpreted to have been formed by the dissolution of pore filling and grain replacive authigenic carbonates this is confirmed by examination of experimentally exhumed overgrowth surfaces from carbonate cemented sandstones these textures indicate that part of the intergranular porosity in these sediments is secondary in origin and has been generated by the dissolution of carbonate cements the identification of such textures may lead to a more confident interpretation of the nature of intergranular porosity in the subsurface

petrofacies of eocene sand injectites of the tumey giant injection complex california usa abstract the forearc succession of great valley group in central california provides some of the best examples of giant sand injection complexes in the world and is therefore considered valuable analogues for injectite systems in the subsurface several sand injection complexes are well described in the outcrop and subsurface however the petrographic characteristics of injectites are still poorly documented in this paper we present the results of an integrated study of field observations quantitative and qualitative sandstone petrography provenance and petrofacies analysis of the tumey giant injection complex tgic in order to understand its lithostratigraphy and petrological evolution and its impacts on reservoir petrofacies characteristics and fluid migration the tgic intrudes into a 450 m thick deep water succession of slope mudrocks and sandy channel fills of the kreyenhagen shale eocene forming an interconnected network of sandstone sills dykes and injection breccias the complex generated a horizontal and vertical plumbing system for fluid migration connecting isolated sandy channel fills among low permeability mudrocks the primary detrital composition diagenetic products microtextures and provenance signatures allowed for the definition and discrimination of depositional and intrusive petrofacies and their genetic relations petrofacies associations confirm that the gypsum cemented feldspathic litharenites from the kreyenhagen shale channel fills are the only source for the injection complex eodiagenetic compaction and extensive gypsum cementation reduced the primary porosity of the complex while telodiagenetic dissolution of autigenic constituents formed pervasive secondary porosity the underlying calcite cemented arkosic sandstones of the lodo and domengine formations acted as barriers for fluid flow aiding lateral fluid migration and overpressure buildup within the overlying kreyenhagen channel fills intense grain microfracturing occurred during sand remobilization and injection along with erosion of the host mudrocks this study has significance for the generation of reliable stratigraphic and petrological models for sand injection complex genesis and evolution that consequently can help the understanding and exploration of injectite complexes elsewhere

quantification and visualization of finite strain in 3d viscous numerical models of folding and overthrusting abstract finite strain analysis and three dimensional 3d numerical modeling are important methods to understand the deformation history of rocks here we analyze finite strain in 3d numerical simulations of power law viscous folding and overthrusting simulations with different and laterally varying detachment geometries cause a lateral transition from folding for thicker detachments to overthrusting we compute the 3d finite strain tensor the principal strain values and their orientations we compute the nadai strain e s and the lode s ratio representing the strain symmetry constriction or flattening we apply hsu diagrams to visualize strain distribution in e s space in combination with color coding indicating the corresponding position of strain data in the 3d model we analyze the orientation and spatial variation of finite strain with polar and rose diagrams and quantify the spatial and temporal evolution of finite strain with strain profiles our results show that i the magnitude of e s generally increases from folding to overthrusting ii an initial flat ramp geometry of the detachment generates a distinctive pattern of e s iii lateral variations of e s and can be used to identify lateral variations in sub surface structures and v internal strike slip shearing is generated due to the folding overthrusting transition

the fluid regime during fracturing of the embla field central trough north sea abstract the deep embla field located in the central trough north sea represents the first oil reservoir in paleozoic rocks to be developed on the norwegian shelf the reservoir consists of braided fluvial and alluvial fan sandstones interlayered with floodplain lacustrine mudstones and volcanics and intrusives extensive fracturing of the reservoir has led to the formation of numerous mineralised veins three main types of mineralised fractures can be identified 1 early diagenetic veins and veins which occur in and around mafic igneous rocks 2 carbonate dominated veins 3 barite clay minerals and bitumen veins this article presents fluid inclusion evidence from fracture filling minerals suggesting that the fractures were important fluid conduits of both petroleum and brine migration the petroleum inclusion data are consistent with an undersaturated oil composition a range in salinity between 23 and 12 wt nacl eq is found for the aqueous inclusions the variation in salinity indicates mixing between a high saline fluid related to evaporites and a fluid with lower salinity microthermometry data suggest that the fracturing events occurred at deep basinal levels at temperatures in excess of 100 c

the benfontein kimberlite sills magmatic reactions and high intrusion temperatures the kimberlite sills at benfontein south africa were intruded as xenolith poor phenocryst rich carbonated peridotite magmas the dominant pre intrusion phenocryst phases are olivine fo sub 86 92 mg ilmenite ss 13 0 18 0 wt percent mgo 0 5 3 0 wt percent cr 2 o 3 mg al chromite and ree perovskite post intrusion magmatic reaction mantling has encapsulated all pre existing groundmass oxides in mg al titanomagnetite the benfontein sills appear to have been emplaced at much higher temperatures than are probably typical for most other kimberlites in a high heat flow environment comparable to that recorded by the lesotho geotherm under open system vapor composition conditions an adiabatic diapiric upwelling of peridotitic magma may be capable of producing the high temperatures that apparently prevailed during the intrusion the temperature of reaction for coupled fe based and c based fo 2 buffers in the upper mantle for example emog mh hematite mol percent isopleths increases with decreasing pressure and at near surface pressures approximates the temperature of intrusion for the phase compositions observed at benfontein modified journal abstract

multiple generations of high salinity formation water in the triassic sherwood sandstone wytch farm oilfield onshore uk abstract the origin and heterogeneity of oilfield formation water in the lower triassic sherwood sandstone at wytch farm in the wessex basin uk have been investigated using production data detailed water geochemistry and o s and h stable isotope data the formation waters are highly saline nacl type brines with tds values of up to 230 000 mg l there is a general decrease in salinity from the flanks of the field to the crest with cl decreasing from about 136 000 to 109 000 mg l the cl br ratio of the water shows that salinity was largely derived from the dissolution of upper triassic continental evaporites found off structure to the west and north of the field the water in the field had a meteoric source although variation in 2h values suggests that there may be meteoric waters of different ages in the oilfield reflecting recharge under different palaeoclimatic conditions at the crest of the field aqueous so 4 2 resulted from dissolution of anhydrite in the reservoir in contrast in other parts of the field there is an indication that some of the dissolved so 4 2 was derived from oxidation of pyrite at some point on the recharge path of meteoric water to the field there were two meteoric influx events bringing different cl concentrations and different 2h values the first was probably before the eocene oil influx and could have occurred in the lower cretaceous or early tertiary the second meteoric influx event probably occurred after or during oil migration into the wytch farm structure since the second meteoric water is found at the flanks of the field adjacent to the regions where salt is found in the stratigraphy the preservation of heterogeneities in oilfield formation water compositions suggests that there has been little aqueous fluid movement or diffusive flux for over 40 million years mass flux has been restricted by density stratification within the aquifer and the very low effective permeability for the aqueous phase in the oil filled reservoir

determination of fabric and strain ellipsoids from measured sectional ellipses implementation and applications abstract geologists examine fabrics to constrain models of formation or of deformation of rocks and it is often convenient to summarise the results by a fabric ellipsoid as fabric data are commonly collected on planar sections through the rock estimating a fabric ellipsoid from sectional ellipses often with arbitrary orientations is an important task an algebraic method to calculate such an ellipsoid presented in an earlier paper has been implemented with the program ellipsoid it is used here on examples that illustrate questions and issues that arise when collecting selecting and processing sectional fabric data and when assessing the results the quality of fit of the ellipsoid to the data is assessed in all cases examples include a case in which the average sizes of markers on individual sections can be used in the determination of the ellipsoid and other cases in which such sizes are not useful a case in which sectional ellipses are not obtained from closed markers and a case in which data scatter and insufficient coverage of section orientations lead to a hyperboloid instead of an ellipsoid

texture analysis of volcanic rock samples quantitative study of crystals and vesicles shape preferred orientation from x ray microtomography data abstract in the texture analysis of volcanic rocks the preferred orientation of the constituents can provide useful information for the interpretation of the processes involved in the rock formation we present here a new data analysis technique based on x ray microtomography measurements and on shape preferred orientation analysis to obtain the orientation distribution functions of the constituents of volcanic rocks this procedure proved to be very suitable for volcanic samples where diffraction based techniques developed for crystallographic preferred orientation studies are of limited utilization in addition to the fact that they cannot provide any information about vesicles or bubbles moreover the analysis performed directly in three dimensions 3d overcomes the problems that usually occur when employing stereological methods for the analysis of the images obtained via microscopy based techniques in this study two scoriae from stromboli and etna and a tube pumice from campi flegrei were measured via x ray microtomography and then the resulting volumes were analyzed following the proposed procedure results highlight little preferred orientation for the vesicles in the two scoria samples whereas the pumice shows a marked preferred orientation crystals also divided by mineral species were taken into account as well and in the two scoria samples there is no crystal preferred orientation in contrast with the pumice where crystal preferred orientation features are very similar to the ones found for the vesicles overall we found strong differences in preferred orientation weak for vesicles in scoriae showing an axial symmetry with the axis parallel to the elongation axis of the sample and a stronger and more complex orientation texture in the pumice sample for both crystals and vesicles the promising results obtained suggest that this procedure is potentially very useful for the analysis of preferred orientation in volcanic rocks and geomaterials in general

the santa rita nickel sulfide deposit in the fazenda mirabela intrusion bahia brazil geology sulfide geochemistry and genesis the santa rita ore zone within the fazenda mirabela intrusion is an unusual example of high tonnage magmatic ni cu pge sulfide mineralization developed as a stratiform layer within the main cumulus sequence of a layered intrusion the intrusion as a whole is about 4 2 5 km in outcrop dimensions and at least 3 km in original maximum stratigraphic thickness with an original boat shaped morphology the ore zone comprises a 50 to 200 m thick interval containing up to 5 wt percent disseminated sulfides within a sequence of mostly unaltered harzburgites and orthopyroxenites this orthopyroxene dominated sequence lies between the dominantly olivine rich cumulates that comprise the lower half of the intrusion and the gabbroic cumulates that make up the upper half sulfide tenor variations and silicate host rock compositions have been derived from a very large database of continuous assays of more than 500 diamond drill cores consistent patterns of sulfide tenor gradually decreasing ni from over 20 to about 10 wt percent near constant cu and gradually decreasing pges with cyclically varying pt pd ratios are found within the santa rita ore zone across the entire intrusion and display relatively little lateral variability the position of the sulfide mineralization envelope within the harzburgite orthopyroxenite package migrates to higher stratigraphic levels from the northern to the southern margin of the intrusion maximum grades and thicknesses correlate with the maximum thickness of orthopyroxene bearing cumulates asymmetrically disposed toward the southern margin of the intrusion nickel tenors of the sulfides range from 15 to 25 wt percent exceptionally high for rocks with relatively low values of mgo mgo feo and are closer to the typical values observed in komatiite hosted deposits although cu tenors typically 5 8 wt are much higher than komatiite values sulfide pge tenors range from less than 100 ppb to greater than 10 ppm of both pt and pd with maximum values in s poor rocks at the base of the sulfide bearing interval pges become significantly more depleted toward the top of the santa rita zone we attribute these features along with the localization of the sulfide bearing interval at the contact between ultramafic and mafic cumulates to mixing of an initially s undersaturated moderately ni enriched resident magma with a relatively low ni pge depleted significantly lower temperature replenishing magma charged with suspended sulfide liquid droplets

synchrotron x ray computed microtomography studies on vesiculated basaltic rocks synchrotron x ray computed microtomography cmt was applied to five basalt samples from different locations in order to determine the microgeometrical structures and physical properties of vesicles two vesiculated basaltic lavas and one calcite filled vesiculated basaltic lava were collected from the lesotho highland south africa and two basaltic scoria tephra were taken from the lungkuan volcano group in northeast china they were examined with cmt on the x27c beam line of the brookhaven national synchrotron light source the voxel size varied from 10 to 20 m depending on the chosen field of view of the tomographic instrument a fast filtered back transform ffbt algorithm was employed in the tomographic reconstruction a 3d image geometry analysis package was used to measure the vesicularity specific surface area and interconnectivity of the vesicles the results of the analyses showed that the bubbles have roughly spheroidal forms of different sizes vesicularity ranged from 45 for lavas to 80 for scoria at least 90 of the vesicles are interconnected specific surface area was determined both by counting voxel faces and by use of two point correlation functions

sayh al uhaymir 094 a new martian meteorite from the oman desert sayhal uhaymir sau 094 is a 223 3 g partially crusted strongly to very strongly shocked melanocratic olivine porphyric rock of the shergottite group showing a microgabbroic texture the rock consists of pyroxene 52 0 58 2 vol dominantly prismatic pigeonite en60 68fs20 27wo7 9 associated with minor augite en46 49fs15 16wo28 31 brown shock oxidized olivine fo65 69 22 1 31 completely isotropic interstitial plagioclase glass maskelynite an50 64or0 3 0 9 8 6 13 0 chromite and titanian magnesian chromite 0 9 1 0 traces of ilmenite ilm80 86 pyrrhotite fe92 100 0 1 0 2 merrillite 0 1 and pockets 4 8 6 7 consisting of green basaltic to basaltic andesitic shock glass that is partially devitrified into a brown to black product along boundaries with the primary minerals the average maximum dimensions of minerals are olivine 1 5 mm pyroxene 0 3 mm and maskelynite 0 3 mm primary melt inclusions in olivine and chromite are common and account for 0 1 0 6 of the rock x ray tomography revealed that the specimen contains 0 4 vol of shock melt associated vesicles up to 3 mm in size which show a preferred orientation fluidization of the maskelynite melting and recrystallization of pyroxene olivine and pyrrhotite indicate shock stage s6 minor terrestrial weathering resulted in calcite veining and minor oxidation of sulfides the meteorite is interpreted as paired with sau 005 008 051 the modal composition is similar to dar al gani 476 489 670 735 876 with the exception that neither mesostasis nor titanomagnetite nor apatite are present and that all phases show little zonation the restricted mineral composition predominance of chromite among the oxides and abundance of olivine indicate affinities to the lherzolitic shergottites

three dimensional observation and morphological analysis of organic nanoglobules in a carbonaceous chondrite using x ray micro tomography abstract organic nanoglobules are submicrometer spherical often hollow organic grains ubiquitously distributed throughout primitive solar materials such as carbonaceous chondrites until now organic nanoglobules have been examined by tem only after sectioning by ultramicrotomy so it has not been possible to determine whether fluids or mineral grains occur in the hollow cores h 2 o rich fluids might be present in hollows of the nanoglobules if they originate from dust particles composed of organic materials and ice prior to or in an early stage of the solar system formation or fluids incorporated into nanoglobules during aqueous alteration on the asteroidal parent body in order to determine whether or not any fluids or mineral grains are present in the nanoglobules a carbonaceous chondrite sample tagish lake c2 meteorite was observed non destructively using synchrotron radiation based x ray ct computed tomography and then microtomed sections were observed using a transmission electron microscope tem we observed three dimensional shapes of thirty eight organic nanoglobules in the meteorite sample their size and shape distributions are consistent with a hypothesis that nanoglobules originate from icy dust particles nanoglobule candidates observed in ct images were confirmed by the tem images however the presence or absence of fluid could not be judged because ct images of nanoglobules are affected by x ray refraction simulation of ct images by considering x ray refraction shows that the presence or absence of water in nanoglobules cannot be distinguished with ct images alone however the outer shapes of nanoglobules can be determined quantitatively and nanoglobules containing silicate cores can be easily identified the thirty eight nanoglobules we examined did not have silicate cores

applications of x ray computed tomography in the geosciences x ray computed tomography ct is a technique that allows non destructive imaging and quantification of internal features of objects it was originally developed as a medical imaging technique but it is now also becoming widely used for the study of materials in engineering and the geosciences x ray ct reveals differences in density and atomic composition and can therefore be used for the study of porosity the relative distribution of contrasting solid phases and the penetration of injected solutions as a non destructive technique it is ideally suited for monitoring of processes such as the movement of solutions and the behaviour of materials under compression because large numbers of parallel two dimensional cross sections can be obtained three dimensional representations of selected features can be created in this book various applications of x ray ct in the geosciences are illustrated by papers covering a wide range of disciplines including petrology soil science petroleum geology geomechanics and sedimentology

the role of detrital composition and climate on the diagenetic evolution of continental molasses evidence from the cambro ordovician guaritas sequence southern brazil abstract the cambro ordovician molassic guaritas sequence camaqua basin southern brazil comprises alluvial fan and braided fluvial sandstones and conglomerates with intercalated aeolian and lacustrine deltaic deposits and andesitic lava flows the sediments display a complex detrital composition derived from plutonic gneissic acidic volcanic and low grade metamorphic source rocks this detrital assemblage was strongly modified by semi arid continental near surface diagenesis early cementation by hematite smectite quartz and calcite and the relatively limited burial prevented strong compaction and preserved some primary macroporosity in most of the sandstones whereas the absence of early cements and or abundance of ductile grains promoted substantial porosity destruction by compaction and the inhibition of further diagenetic modifications the diagenetic dissolution and replacement of volcanic rock fragments and detrital feldspars by clays and albite changed the original framework composition as well as the tectonic provenance classification of the sandstones detailed quantitative petrographic study allowed the reconstruction of the original detrital compositions and the distinction of six different pathways of diagenetic evolution of the sandstones attesting to the efficiency of this method for diagenetic modelling and provenance analyses of ancient sandstones

plagioclase chain networks in slowly cooled basaltic magma plagioclase crystals in the slowly cooled interior of the thick holyoke flood basalt flow of connecticut linked to form monomineralic chains at an early crystallization stage partial melting experiments reveal that when the quartz tholeiite was only 25 crystallized the chains had already linked to form a continuous 3 d network at such an early stage of crystallization the network was weak highly permeable and easily deformed consequently the mush of plagioclase chains and interstitial pyroxene crystals underwent compaction in the lower third of the flow with the expelled liquid rising to the center of the flow where it crystallized to form coarse grained sheets of fractionated basalt plagioclase chains are most easily seen in the basalt after it has been partly melted and the late crystallizing minerals converted to glass the chains are several crystals wide the crystals which are approximately 0 5 mm long are attached together randomly normal zoning patterns indicate crystals had a brief period of growth before linking together the chains branch every few millimeters to form the 3 d network which was mapped using serial polished sections and x ray ct scans the chain frequency measured along vertical and horizontal traverses decreases toward the center of the flow in the compaction zone the frequency in the vertical direction is greater than in the horizontal making the reasonable assumption that these frequencies were initially the same the difference is used to calculate the degree of compaction the resulting pattern through the flow matches almost exactly the pattern indicated by variations in the incompatible elements plagioclase chains are also found in some coarser grained plutonic rocks if they are common their fabric may provide a new direct means of measuring the degree of compaction in crystal mushes

topology of syntectonic melt flow networks in the deep crust inferences from three dimensional images of leucosome geometry in migmatites we have determined the three dimensional form of leucosome in two migmatites produced by syntectonic anatexis of different protoliths 1 stromatic migmatite derived from pelite which comprises sheets of leucosome quartzofeldspathic layers with grt with walls of melanosome bt rich selvages in mesosome schistose layers of bt pl qtz or grt or crd and 2 a migmatitic garnet amphibolite derived from basalt which is composed of spindle shaped leucosomes qtz pl spatially associated with peritectic grt in melanosome hbl qtz or cpx three dimensional images were generated from two dimensional representations of spatial data obtained by two methods 1 serial grinding and 2 high resolution x ray computed tomography hr x ray ct projections of three dimensional images of stromatic migmatite derived using data from either method show the planar nature of leucosome throughout the sample melt transport through this rock when it was partially molten could be modeled as flow in parallel conduits in the image derived from hr x ray ct data garnet in leucosome is only rarely in contact with melanosome which suggests these garnet grains were suspended in melt during flow projections of three dimensional images of the migmatitic garnet amphibolite do not reveal the full extent of leucosome connectivity due to the irregular geometry of leucosome connectivity in this sample can be shown however by virtual slicing of the three dimensional images perpendicular to the plane of the two dimensional representations approximately parallel to the lineation defined by the leucosome and by using three dimensional projections of a single leucosome connectivity tree constructed by projecting leucosome patches from slice to slice and noting the overlap based on leucosome geometry and volume we estimate effective porosity for flow in this rock to have been 20 vol at stagnation leucosome in the migmatitic garnet amphibolite occurs in strain shadows around garnet which are inferred to have been obstacles to flow along linear paths blocking of inferred flow channels by garnet contributes to the high degree of path length tortuosity in this sample tau 2 6 which is expressed visually by the complex form of the leucosome in three dimensions cross sectional areas for individual inferred melt flow paths are highly variable over 2 3 orders of magnitude and minimum channel radius is changeable by approximately 1 order of magnitude meaning there was large variability along the channels and implying strong local flow divergences based on these data unusually straight and uniform channels would have dominated the mesoscopic melt flux through this rock when it was partially molten

quantitative 3d petrography using x ray tomography application to bishop tuff pumice clasts abstract textures are traditionally studied using the petrographic microscope which limits observations to 2d sections of 3d objects given the difficulty in retrieving information on shapes sizes and spatial distribution of objects in 3d from random sections a method that can yield observations in 3d is highly desirable x ray tomography yields a 3d map of the linear x ray attenuation coefficient which is a function of the atomic number and density of the material and the x ray energy used based on the compositional contrasts between different phases various phases can be identified in a rock pumice clasts characterized by isolated crystals surrounded by a low density matrix are ideal for the application of this technique we present our first results obtained using x ray tomography for pumice clasts from the bishop tuff samples studied are from units f7 and f8 and are representative of the variability in density and crystal contents observed in the early erupted bishop tuff data were collected on the geosoilenvironcars beamline at the advanced photon source argonne national laboratory using a 22 kev monochromatic x ray beam image analysis was performed using the software blob3d brightness contrasts in the resulting images allow the separation of magnetite sanidine quartz including minor plagioclase glass and vesicles crystals smaller than 5 voxels in diameter cannot be quantified properly mostly due to noise in the images this constrains the spatial resolution of the resulting images such that there is a trade off between sample size and spatial resolution we employed two runs with samples of varying sizes 5 and 10 mm to obtain adequate resolution voxel sizes ca 8 and 17 m we have previously studied quartz size distributions in bishop pumice clasts using a crushing sieving and winnowing procedure the agreement between our new and previous results is excellent quartz size distributions obtained using the tomography data are concave upward on population density vs size plots and resemble the size distributions for all quartz crystals including both whole crystals and fragments these size distributions were probably generated by fragmentation through which large crystals were converted in a larger number of small crystals petrographic observations suggest fragmentation occurred mostly under magmatic conditions sanidine size distributions are similar to those observed for quartz magnetite size distributions are typically linear indicating a simple magmatic history in contrast with the tektosilicates

platinum ore in three dimensions insights from high resolution x ray computed tomography platinum group elements referred to as pge comprising pt pd rh ru ir and os are strategic metals with a wide variety of industrial applications most of the world s pge production is mined from large mafic ultramafic intrusions such as the bushveld complex in south africa which currently provides 75 of the world s pt production the pge mineralization is found within distinctive layers tens to hundred of centimeters thick but extending laterally for many tens of kilometers where the pge occur at low parts per million levels as platinum group minerals pgm and in solid solution within disseminated base metal sulfides there is still heated debate at the most fundamental level about the mode of formation of this class of deposit genetic models range from primary magmatic sulfide collection to concentration by migrating halogen rich fluids a crucial line of evidence is the spatial relationship between the pgm which are the most important pge bearing phases and the base metal sulfide aggregates or blebs so far all observations have been carried out using two dimensional mineralogical studies where textural relationships with other minerals are ambiguous and with statistical limitations owing to sampling of trace phases intersecting random surfaces we present the first detailed three dimensional in situ analysis of the pgm at the sample scale using high resolution x ray computed tomography coupled with conventional microscopic and mineralogical study we find a striking and highly consistent relationship of pgm grains with the edges of complex shaped magmatic sulfide blebs and the intersection of these blebs with chromite silicate grain boundaries these new three dimensional observations strongly support an orthomagmatic model coupled with nucleation and growth of pgm at the margins of sulfide liquid droplets

a review of onshore uk salt deposits and their potential for underground gas storage the uk faces a major change in the nature of its gas supply as north sea production declines and the country becomes increasingly reliant upon gas imports as a result the uk government recognizes that significant investment in gas supply infrastructure is required to maintain security of supply and manage the gas market part of that infrastructure will be additional underground gas storage capacity in specially designed and engineered salt caverns this paper summarizes the distribution and nature of halite rock salt deposits in england and northern ireland and reviews the details of existing and planned storage sites in salt caverns there is considerable potential for further salt cavern development however not all of the uk salt fields are suitable with the halite beds being too shallow thin or impure

structural deformation of the leoville chondrite foliations defined by alignment of elongated chondrules have been noted previously in chondrites but none displays this effect so well as leoville cv3 the shapes of leoville chondrules were produced by deformation in situ as indicated by inclusions and clasts with similar shapes and preferred orientations to those of chondrules similarities in the aspect ratios of apparent strain ellipses measured for chondrules alone 1 9 and 2 0 by several methods and for the whole meteorite 2 0 indicate either that leoville deformed homogeneously or that it deformed as a framework of touching chondrules this amount of strain corresponds to approximately 33 uniaxial shortening assuming constant volume because the strain ellipse was measured in only one orientation this strain value is a minimum estimate for the meteorite lack of correlation between foliation and either shock or thermal effects argues that impact or metamorphism are unlikely to have produced this deformation compaction due to overburden from progressive accretion on the chondrite parent body is suggested to have been its cause estimates of maximum deviatoric stresses in the interiors of asteroid sized bodies and constraints on maximum temperatures for cv3 chondrites are consistent with diffusional flow as the deformation mechanism for olivine in these chondrules diffusional flow is also suggested by the scarcity of observed lattice dislocations deformation of leoville olivines by this mechanism at geologically reasonable strain rates appears to require higher temperatures than those believed to have been experienced by this meteorite 600 c however differences in olivine grain size the presence of water or a more complex deformation history might explain this discrepancy

quantifying the three dimensional shapes of spheroidal objects in rocks imaged by tomography abstract the shapes of objects in rocks are sometimes measured through tomographic sections either physical sections or virtual x ray computed tomography approximation of these objects as ellipsoids is often sufficient in these analyses here we present an easily implemented algorithm to best fit an ellipsoid to a set of n points n 9 distributed on the surface of a spheroidal object observed in tomographic sections the method is demonstrated on x ray computed tomography data of 90 chondrules in a cr2 chondrite by averaging the ellipsoids for all the objects measured in a rock one can then readily describe its fabric by an average ellipsoid and if that average ellipsoid is interpreted as a strain ellipsoid and assuming all objects have undergone the same strain each object can be retro deformed and the inferred initial population of objects studied

average properties of ellipsoidal fabrics implications for two and three dimensional methods of strain analysis abstract many rocks contain ellipsoidal objects such as pebbles or reduction zones which display a variety of shapes and orientations in deformed rocks such objects may be used for strain analysis by using the concept of an average ellipsoid here called the fabric ellipsoid two fabric ellipsoids are defined which are the results of two different algebraic averaging processes during deformation of ellipsoidal distributions the fabric ellipsoids change as if they were themselves material ellipsoids and are therefore of fundamental importance in strain analysis in most studies to date such 3 d fabric ellipsoids have been obtained from 2 d average ellipses determined on section planes cut through the rock sample previous work has assumed that the average ellipses will approximate to section through a single fabric ellipsoid i show here that this is not the case as sectioning introduces a systematic bias into the section ellipse data this bias is distinct from the statistical errors due to finite sample size and measurement errors discussed in other work and must be considered in any method of strain analysis using section planes

meteorite 3 d synchrotron microtomography methods and applications methods of synchrotron x ray computed microtomography xrcmt are described which allow nondestructive high spatial and contrast resolution imaging of the density structures of meteorites and their components in three dimensions images of bulk chondrites to one cubic centimeter in size reveal compound chondrules chondrule matrix volumetric ratios metal and sulfide distribution petrofabrics and 3 d chondrule and calcium aluminum inclusion cai sizes and shapes images of separated chondrules and cais reveal void spaces mineral intergrowth textures and the true locations of crystal rims and cores at resolutions to 8 cubic micron volume element images of achondrites reveal mineral fabrics and crystal zoning lunar glass spherules can be searched for phenocrysts bearing deeply sourced melt inclusions a companion dvd and url contain images for classroom and research use numerical techniques for quantification of x ray computed microtomography xrcmt data and its potential applications are discussed three dimensional x ray images of meteorites provide a way to discover components of interest and to precisely slice samples to expose these components with minimal damage and loss of material three dimensional studies of petrographic features size shape texture and modal abundance of chondrites and their components as well as other meteorites have definite advantages over standard 2 d studies using randomly sliced thin sections

three dimensional grain fabric measurements using high resolution x ray computed tomography abstract high resolution x ray computed tomography hrxct provides detailed imagery of the interiors of rocks up to hand sample size non destructively and in three dimensions new tools described here allow these data to be used for analysis of grain fabrics two separate sets of measurement techniques have been developed the first concentrates on quantifying individual crystals phenocrysts porphyroblasts or other discrete objects or void spaces within a sample quantifiable properties include location size shape orientation and contact relationships with adjacent objects the second set of techniques performs a more general fabric analysis on any distinguishable component in a sample a fabric tensor can be computed based on a number of metrics including the star volume distribution svd star length distribution sld and mean intercept length mil methods the fabric tensors provide principal component directions and magnitudes which in turn provide a measure of degree of anisotropy and shape indices because the svd and sld measure only the material of interest whereas the mil is also influenced by spatial distribution results can be divergent for sparse phases also introduced is a three dimensional rose diagram that can be viewed interactively and inspected to reveal further details about non orthogonal directional components these techniques are demonstrated through analysis of a garnet kyanite schist from mica dam british columbia

diagenesis of carbonate cements in permo triassic sandstones in the wessex and east yorkshire lincolnshire basins uk a stable isotope study

three dimensional investigation of volcanic textures by x ray microtomography and implications for conduit processes 1 synchrotron x ray computed microtomography ct was applied for the first time to clasts of pumice and scoria generated by active explosive volcanoes characterized by a range of eruptive styles mild strombolian to plinian and magmatic composition basaltic to trachytic the obtained two dimensional 2d tomographic images corresponding to sample views at different rotation angles were processed to reconstruct three dimensional 3d volumes and then used to make 3d measurements of vesicularity vesicle number density volume and connectivity for quantitative characterization of the investigated tephras the results indicate a positive correlation between vesicle number density and eruption intensity that is used to investigate modes of magma degassing in explosive eruptions in addition the vesicle geometry affecting the connected gas flow pathways in pyroclasts from strombolian eruptions is determined and related to the known permeabilities implications on the dynamics of explosive eruptions is discussed and ultimately used to quantitatively discriminate between different eruptive styles

quantitative analysis of the joint variability of rock texture and composition in thin section the generalized griffiths descriptor and its application to sediment generation from granitoids abstract the general protocol for quantitative petrographic description of clastic sediments envisioned by j c griffiths approximately 60 years ago is built on specification of the following five statistically independent fundamental properties of aggregates modal composition spatial arrangement size distributions shape distributions and orientation distributions of the elements each element is represented by a grain falling into one of an exhaustive set of petrographic classes we introduce the so called generalized griffiths descriptor ggd which extends this protocol to igneous and metamorphic rocks statistical independence of the fundamental properties permits rigorous testing of petrogenetic hypotheses as well as simulation of rocks based on any conceivable combination of them the benefits of a unifying scheme for description of crystalline parent rocks and their weathering products are illustrated by exploratory analysis of a petrographic data set derived from five granodiorites from across north america comprising the parent rocks and three size fractions coarse medium and fine of their detritus compositional data analysis is used to estimate sediment generation and evolution sge trajectories for each of the plutons sampled the bulk mineral composition and the mineral composition of the rock fragment assemblage display a common trend of relative depletion of quartz and k feldspar with decreasing grain size compensated for by relative enrichment of biotite and accessory minerals the evolution of crystal interface composition suggests that the balance between chemical and mechanical weathering may be reflected in relative stabilities of crystal interfaces we propose a computationally efficient stereological inversion procedure for estimating the parameters of crystal size probability distributions in rocks based on the assumption of lognormality the lognormal distribution is an acceptable default model for crystal size in granitoids at the level of individual thin sections about 87 passes the tests for lognormality but less so at the level of entire plutons only 62 success ratio the current implementation of the ggd is not yet complete because it deals explicitly with the first three fundamental properties only as a consequence of adopting the simplifying assumption that all elements which make up a rock are spherical shape and orientation distributions may be included in future implementations of the ggd dedicated data sets comprising sediments generated from a single parent rock under different conditions will be needed to advance sge studies by shedding more light on the balance between chemical and mechanical weathering

an introduction to the application of x ray microtomography to the three dimensional study of igneous rocks imaging rocks in three dimensions through x ray microtomography enables routine visualization of structures in samples which can be spatially resolved down to the sub micron scale although x ray tomography has been applied in biomedical research and clinical settings for decades it has only recently been applied to studies of rocks and few geoscientists realize its value and potential this contribution provides an introduction to the principles and techniques of x ray microtomography to the study of igneous rock textures as well as reviewing the current state of the art we hope that this short review will encourage more geoscientists to apply x ray microtomography in their research and that this will lead to new insights into the processes that occur in magmatic as well as other geological systems

diagenetic controls on the reservoir quality of the tight gas collyhurst sandstone formation lower permian east irish sea basin united kingdom abstract the lower permian collyhurst sandstone formation is an important gas reservoir both offshore and onshore in the east irish sea basin of north western uk and is considered to be a tight gas reservoir diagenetic processes have significantly influenced the reservoir quality of the sandstone this study employs petrographic mineralogical and stable isotopic analyses on two different onshore core successions to investigate the composition and texture the diagenetic history and the impact of lithology and diagenesis on reservoir quality the collyhurst sandstone formation consists of fine to very coarse grained poorly to well sorted sub arkosic to lithic arenite sands the reservoir properties of the sandstone are relatively poor with average porosity and permeability of 6 6 and 0 387 md respectively on the basis of petrographic analyses five sandstone lithofacies were identified fine grained conglomeratic carbonate cemented quartz cemented and matrix free contrasting characteristics between the reservoir units suggest the development of divergent diagenetic pathways features observed during early diagenesis include compaction growth of pore filling early carbonate cements calcite i dolomite and siderite i and vermicular kaolinite late diagenetic features include further carbonate cementations ferroan dolomite ankerite calcite ii and siderite ii iii clay mineral cementation illite and chlorite with both grain coating and pore filling textures also common in late diagenesis are syntaxial quartz overgrowths albite barite and anhydrite are minor late diagenetic cement phases compaction processes rather than cementation accounts for the major destruction of the initial sandstone porosity but cementation still accounts for up to 32 of porosity loss carbonate cements mostly fill intergranular pores thus block pore throat and prevent migration of fluid clay minerals bridge and occlude pores resulting in further destruction of the reservoir quality the study provides insights into how lithofacies primary composition texture and sorting and diagenesis can influence reservoir qualities of tight gas sandstones

diagenesis and reservoir quality of the sherwood sandstone triassic corrib field slyne basin west of ireland abstract the triassic sherwood sandstone in the corrib field slyne basin west of ireland on the european continental margin is a dry gas reservoir with a mercia mudstone top seal petrographic analysis combined with x ray diffraction stable isotope fluid inclusion and core analysis have been used to assess timing of growth and origin of mineral cements whether sandstone diagenesis involved mass flux the controls on reservoir quality and how reservoir quality is likely to vary in more deeply buried sections depositional and early diagenetic characteristics of the sherwood in corrib are typical of a semi arid fluvial environment containing groundwater of meteoric origin early diagenesis included the development of copious dolomite cement in the form of dolocrete as well as the formation of abundant clay while less volumetrically important grain coating haematite and k feldspar cement also grew burial diagenesis witnessed the initial growth of minor chlorite and albite quartz overgrowths and ankerite followed and are the most important burial cements growing over a temperature interval between 100 and 1165 c albite commenced growth at about 90 c quartz cement at 100 c and ankerite at 110 c these cements reached the zenith of their development at 105 110 c for albite 125 130 c for quartz and 135 145 c for ankerite siderite and anhydrite are relatively minor late stage cements the formation water has been consistently highly saline during burial and in terms of mineral cement derived oxygen stable isotope values is likely to be a diagenetically evolved version of the initial depositional water suggesting long term stagnation of the pore fluids the diagenetic reactions that formed burial cements all seem to be essentially isochemical including the ankerite that has carbon isotope characteristics of the cannibalisation of dolocrete reservoir quality is mainly controlled by early diagenetic dolomite and clay although grain size is also important because only sandstones with 200 m grain size have 50 md permeability both shallower and deeper sections than corrib may have similar porosity and permeability since temperature dependent diagenetic controls on reservoir quality e g quartz and illite cement are peripheral

classification and genesis of calcrete and gypsite lithofacies in paleodrainage systems of inland australia and their relationship to carnotite mineralization calcrete and gypsite lithofacies are chemical sedimentary rocks irre spective of age and or stratigraphic position the depositional environments of these deposits indicate that since early tertiary silting up of inland drainage developed internal salt lake systems with gypsite as the dominant evaporitic lithotype elongate bodies of phreatic calcrete formed in paleochannels almost all primary calcretes and gypsites have been modified by phreatic and vadose diagenetic processes because the diagenetic processes are complex stratigraphic analysis is difficult carnotite mineralization is probably not contemporaneous with calcretization assuming that waters from fertile uraniferous sources are flowing through the sediments several controls will govern carnotite mineralization ponding by physical or chemical barriers solution chemistry p co2 co 3 hco 3 v u and k concentrations eh ph and postdepositional physical or chemical changes for significant carnotite bodies to remain primary deposition must have been followed by a freezing mechanism to preserve the deposit diagenetic conditions which may cause remobilization vary between deposits therefore it may be unwise to use a single deposit as the basis for a widely applicable exploration model modified journal abstract