bioturbation influence on reservoir quality a case study from the cretaceous ben nevis formation jeanne d arc basin offshore newfoundland canada the delineation ben nevis l 55 well located in the hebron ben nevis field offshore newfoundland targeted the cretaceous ben nevis formation in the petroleum rich jeanne d arc basin this case study focuses on the bioturbated net pay horizons and assesses the importance of animal sediment interactions in controlling the porosity and permeability of sandstone reservoir intervals our data reveal that bioturbation can either reduce permeability and porosity by as much as approximately 33 or enhance it by up to 600 dependent on burrow type and behavior of the trace making organism the net pay interval in the cored interval of ben nevis l 55 is characterized by ophiomorpha dominated ichnofabrics the action of bioturbators can be classified in terms of sediment mixing sediment cleaning sediment packing and pipe work building strategies bioturbation has the potential to 1 increase isotropy or uniformity of grain size by destroying sedimentary laminae through burrow homogenization or 2 decrease isotropy by selectively sorting grains into burrow lining and fill by grain size and through creation of open burrow systems filled with later sediments of differing character to the host sediment the petrophysical characteristics of the reservoir facies are highly dependent on trace fossil morphology presence or absence of burrow linings nature of burrow fills burrow size and bioturbation intensity mudstone rich facies and ichnofabrics containing mudstone filled and or lined burrows e g ophiomorpha and clusters of chondrites have the net effect of permeability reduction in contrast permeability enhancement is documented from muddy sandstone facies with clean sand filled burrows e g thalassinoides and clean sandstones with burrow mottled or diffuse to massive textures

data capture for multiscale modelling of the lourinha formation lusitanian basin portugal an outcrop analogue for the statfjord group norwegian north sea

the effect of carbonate cementation on permeability heterogeneity in fluvial aquifers an outcrop analog study abstract spatial variations in permeability have been widely recognized as a dominant control on solute transport and petroleum migration outcrop studies are one means of improving our understanding of the nature of permeability heterogeneity and the underlying geologic controls to date these studies have either neglected the importance of cementation or investigated only the modern cemented permeability without explicitly considering how cementation has affected permeability two outcrops of channel sand deposits in the plio pleistocene sierra ladrones formation new mexico are investigated to quantify 1 the effect of cementation on permeability at a given location and 2 the patterns of cementation and their impact on permeability heterogeneity we analyzed approximately 400 spatially located samples 10 cm 3 each from the two outcrops cementation is typically weak to moderate filling less than 60 of the original pore space permeability and cement content are bimodally distributed and negatively correlated permeability reduction is investigated by measuring modern permeability minus cement permeability minus cement porosity and volumetric cement content permeability is reduced locally by up to three orders of magnitude by combining models of permeability for uncemented sediments and cemented sediments we derive a model of cemented permeability k c that depends primarily on three factors uncemented permeability k u original porosity and cement fraction our results suggest that accounting for porosity reduction only may result in an overestimation of k c conversely accounting for porosity reduction increased specific surface area and increased tortuosity appears to result in an underestimation of k c accounting for porosity reduction and increased specific surface area provide the best estimate of k c cementation is highly variable spatially and is poorly correlated with lithofacies variogram analysis of cementation and permeability indicates that the spatial distribution of cementation is the dominant control on the permeability correlation structure the spatially variable cementation also results in an order of magnitude increase in the variance of log permeability applying stochastic models of effective flow and transport properties to the results of this study suggest that cementation has a relatively small effect on effective permeability but can potentially increase dispersivity by two orders of magnitude

how porosity and permeability vary spatially with grain size sorting cement volume and mineral dissolution in fluvial triassic sandstones the value of geostatistics and local regression although it is well known that sandstone porosity and permeability are controlled by a range of parameters such as grain size and sorting amount type and location of diagenetic cements extent and type of compaction and the generation of intergranular and intragranular secondary porosity it is less constrained how these controlling parameters link up in rock volumes within and between beds and how they spatially interact to determine porosity and permeability to address these unknowns this study examined triassic fluvial sandstone outcrops from the uk using field logging probe permeametry of 200 points and sampling at 100 points on a gridded rock surface these field observations were supplemented by laser particle size analysis thin section point count analysis of primary and diagenetic mineralogy quantitative xrd mineral analysis and sem edax analysis of all 100 samples these data were analyzed using global regression variography kriging conditional simulation and geographically weighted regression to examine the spatial relationships between porosity and permeability and their potential controls the results of bivariate analysis global regression of the entire outcrop dataset indicate only a weak correlation between both permeability porosity and their diagenetic and depositional controls and provide very limited information on the role of primary textural structures such as grain size and sorting subdividing the dataset further by bedding unit revealed details of more local controls on porosity and permeability an alternative geostatistical approach combined with a local modeling technique geographically weighted regression gwr subsequently was used to examine the spatial variability of porosity and permeability and their controls the use of gwr does not require prior knowledge of divisions between bedding units but the results from gwr broadly concur with results of regression analysis by bedding unit and provide much greater clarity of how porosity and permeability and their controls vary laterally and vertically the close relationship between depositional lithofacies in each bed diagenesis permeability and porosity demonstrates that each influences the other and in turn how understanding of reservoir properties is enhanced by integration of paleoenvironmental reconstruction stratigraphy mineralogy and geostatistics

salt weathering simulations under hot desert conditions agents of enlightenment or perpetuators of preconceptions because of access difficulties and inhospitable environmental conditions our understanding of rock weathering in hot deserts has been strongly influenced by laboratory simulations however a risk exists that results may come to owe more to experimental design than to actual environmental conditions experienced at the atmosphere rock surface interface this was especially true in early salt weathering simulations that seemingly sought to reinforce the orthodoxy of a physically dominated weathering environment driven by extremely high absolute temperatures and large diurnal temperature ranges however improvements in environmental sensors over the last 20 years have established the complexity of desert climates and the importance of microclimatic controls and allowed considerable refinement of experimental design this overview evaluates the implications of recent simulation studies and presents new information on possible fatigue effects of differential thermal expansion of salts and the salt weathering of test blocks under compressive loading in doing so it demonstrates how physical rock properties change during experimentation the importance of short term surface temperature fluctuations distinctions between the weathering of unconfined debris and larger rock surfaces the importance of moisture and salt applications that replicate their availability in deserts the importance of rock thermal properties in controlling their weathering response in environments dominated by radiative heating and cooling and the overall complexity of weathering regimes

simulation of potassium feldspar dissolution and illitization in the statfjord formation north sea prior work has suggested that potassium mobility is an important control on the formation of late stage illite in deep reservoirs of the north sea we developed a one dimensional finite difference reactive transport model to simulate the concurrent dissolution of k feldspar and illitization of kaolinite in statfjord formation reservoirs the kinetics of k feldspar dissolution is set to be faster than diffusion consistent with the observed behavior and the proposition that the reaction was transport limited k feldspar dissolution produced dissolved potassium and silica that diffused and then precipitated to form illite and quartz areas that had higher initial reactant k feldspar produced higher solute concentrations and formed chemical gradients having areas that had lower initial feldspar content in typical north sea cases this means mass transfer would be from sandstone to shale the diffusive transport moved significant amounts of potassium and silica over distances of several meters and created observable diagenetic alteration simulations of an oil filled reservoir showed that oil emplacement did not stop illitization but did limit potassium mobility and maximized illite formation within the sandstone preservation of preillitization permeability was favored in water filled reservoirs that had slower illite formation where diffusion could export potassium from the sandstone before precipitation similar patterns of transport and precipitation were predicted for silica from k feldspar dissolution the results of the model showed good agreement with field data from other north sea reservoirs

role of clastic sedimentology in the exploration and production of oil and gas in the north sea summary the nature and quality of clastic reservoir rocks is the result of a complex interplay of tectonic setting provenance depositional environment and diagenesis in particular it is now well established that the reconstruction of depositional environments in clastic successions provides the optimum framework for describing and predicting reservoir development and reservoir quality distribution on both regional exploration and field production scales this review in the context of the 21st anniversary of the british sedimentological research group bsrg expands the above mentioned theme in relation to the development of depositional models of each of the major oil and or gas bearing clastic reservoirs in the u k sector of the north sea which has been the subject of a surge in geological interest following the first gas discoveries approximately 21 years ago each major reservoir interval is used to illustrate specific themes with emphasis placed on how the application of depositional models has contributed to a better understanding of the exploration for and the production from the following reservoirs i permian aeolian and fluvial sands rotliegendes sandstones southern north sea ii jurassic fluvial deltaic and marginal marine sands lower jurassic statfjord formation and middle jurassic brent group northern north sea iii jurassic shallow marine and deeper marine mass flow sands and conglomerates upper jurassic fulmar piper type sands and brae magnus type deposits central and northern north sea and iv tertiary deep marine turbidite fan deposits palaeocene and eocene sands central and northern north sea

the gutenberg richter or characteristic earthquake distribution which is it abstract paleoearthquake and fault slip rate data are combined with the cit usgs catalog for the period 1944 to 1992 to examine the shape of the magnitude frequency distribution along the major strike slip faults of southern california the resulting distributions for the newport inglewood elsinore garlock and san andreas faults are in accord with the characteristic earthquake model of fault behavior the distribution observed along the san jacinto fault satisfies the gutenberg richter relationship if attention is limited to segments of the san jacinto that are marked by the rupture zones of large historical earthquakes or distinct steps in fault trace the observed distribution along each segment is consistent with the characteristic earthquake model the gutenberg richter distribution observed for the entirety of the san jacinto may reflect the sum of seismicity along a number of distinct fault segments each of which displays a characteristic earthquake distribution the limited period of instrumental recording is insufficient to disprove the hypothesis that all faults will display a gutenberg richter distribution when averaged over the course of a complete earthquake cycle but given that 1 the last 5 decades of seismicity are the best indicators of the expected level of small to moderate size earthquakes in the next 50 years and 2 it is generally about this period of time that is of interest in seismic hazard and engineering analysis the answer to the question posed in the title of the article at least when concerned with practical implementation of seismic hazard analysis at sites along these major faults appears to be the characteristic earthquake distribution

deposition and textural evolution of cool water limestones outcrop analog for reservoir potential in cross bedded calcitic reservoirs the eocene miocene te kuiti group is a succession of cool water carbonate and siliciclastic rocks on the north island of new zealand the limestones are subtidal bioclastic and largely composed of bryozoans echinoderms and benthic foraminifers these rocks can be divided into a horizontally bedded facies assemblage and a cross bedded facies assemblage the horizontally bedded facies assemblage represents deposition on a wave storm dominated shelf and is composed of grainstones and packstones containing textures resulting from biologic processes the cross bedded facies assemblage represents deposition on a current dominated shelf and consists of grainstones with textures formed chiefly by physical processes the te kuiti group can be used as an outcrop analog for cross bedded calcite dominated hydrocarbon reservoirs porosity permeability and reservoir potential of the horizontally bedded facies assemblage decreased rapidly during late shallow burial owing to mechanical compaction and to the fine grain size close packing and ineffective early cementation of the rock coarser grain size open framework and earlier cementation within the cross bedded facies assemblage resulted in significant heterogeneity and good reservoir potential before moderate burial tabular lens shaped and sigmoidal reservoir units are proposed primary porosity and early developed traps are essential to reservoirs formed of cool water limestones and those carbonates from calcite seas the influence of depositional setting extends into the eventual bedding style texture physical properties and reservoir potential of the rock

role of degassing of the noril sk nickel deposits in the permian triassic mass extinction event the largest mass extinction event in earth9s history marks the boundary between the permian and triassic periods at circa 252 ma and has been linked with the eruption of the basaltic siberian traps large igneous province slip one of the kill mechanisms that has been suggested is a biogenic methane burst triggered by the release of vast amounts of nickel into the atmosphere a proposed ni source lies within the huge noril sk nickel ore deposits which formed in magmatic conduits widely believed to have fed the eruption of the slip basalts however nickel is a nonvolatile element assumed to be largely sequestered at depth in dense sulfide liquids that formed the orebodies preventing its release into the atmosphere and oceans flotation of sulfide liquid droplets by surface attachment to gas bubbles has been suggested as a mechanism to overcome this problem and allow introduction of ni into the atmosphere during eruption of the slip lavas here we use 2d and 3d x ray imagery on noril sk nickel sulfide combined with simple thermodynamic models to show that the noril sk ores were degassing while they were forming consequent bubble riding by sulfide droplets followed by degassing of the shallow sulfide saturated and exceptionally volatile and cl rich slip lavas permitted a massive release of nickel rich volcanic gas and subsequent global dispersal of nickel released from this gas as aerosol particles

the petrography and geochemistry of komatiite flows from the abitibi greenstone belt and a model for their formation abstract the petrology and geochemistry of komatiites at four localities in the abitibi greenstone belt are presented the effects of alteration crystal fractionation and partial melting on their geochemistry are considered it is concluded that na k ca sr and rb have been disturbed during seafloor alteration but the other major elements and the trace elements ni cr co sc ree y zr and hf have not been mobilized variations in the geochemistry within the komatiite flows are the result of olivine and sulphide fractionation the komatiites consisting of branching olivines skeletal textured komatiites and coarse parallel plate olivines a 3 spinifex textured zones may represent crescumulates rather than frozen liquids mass balance calculations indicate that the flows were open systems and could represent lava tubes 26 34 partial melting of a primitive earth mantle 1 4 c 1 chondrite will produce melts similar to the komatiites at each deposit in terms of major elements ni cr co sc hree and y however on average the concentrations of highly incompatible elements lree zr and hf in the komatiites only are 60 70 of the modelled concentrations thus indicating that the komatiites were derived from a mantle depleted in highly incompatible elements

ephemeral fluvial deposits integrated outcrop and simulation studies reveal complexity ephemeral fluvial sandstones form the main reservoirs in many oil and gas fields production histories and development difficulties suggest these deposits are more complex than is commonly supposed in the petroleum industry there is inadequate published information to account for this observation relevant sedimentological literature on such deposits is qualitative and unsuitable for detailed reservoir characterization studies therefore integrated sedimentological petrophysical and fluid flow simulation studies have been conducted on outcrop analogs in the southwestern united states our key objectives were to define flow units and to find areas of sensitivity inherent both in the sediments and in the analytical and statistical procedures used to describe and model s ch systems we report results of integrated studies on the lower jurassic kayenta formation in southeastern utah the kayenta formation is a relatively proximal sand rich succession dominated by channel fill sandstones sedimentological analysis identified eight main genetic units in contrast reservoir simulations based on probe permeameter data collected at outcrop show that six of the main genetic units have uniform flow behavior intraformational mudclast conglomerates are the dominant permeability barriers both because of the clasts themselves and because they act as sites of preferential cementation stochastic permeability models conditioned to the outcrop data produce results little different from models based on averaged values for each facies the most sensitive feature of the mode s concerns the assumptions made about the mudclast conglomerates the model results show that only by integrating analytical approaches can we understand the full character of an outcrop analog in such a way that we can use it predictively

assimilation of sulfate and carbonaceous rocks experimental study thermodynamic modeling and application to the noril sk talnakh region russia most of the intrusions in the noril sk talnakh region siberia are hosted in thick sedimentary sequences including abundant evaporitic and terrigenous sedimentary rocks three mafic ultramafic intrusions in this region contain unusually thick massive sulfide deposits which represent one of the world s largest economic concentrations of ni cu and pge the interaction of siberian magmas with sulfate and organic matter rich sedimentary rocks has been proposed as a possible mechanism for the origin of these exceptional sulfide deposits but the interaction process and the reaction paths have never been fully investigated here we clarify by both experimental petrology and thermodynamic modeling how sulfate and organic matter assimilation occur in mafic ultramafic magmas affecting magma composition crystallization and sulfide saturation interaction experiments were conducted at conditions relevant to the emplacement of noril sk type intrusions 1200 c 80 mpa to simulate the assimilation of sulfate and or organic compounds by ultramafic magmas we used a picrite from noril sk1 intrusion and coal and anhydrite from the area as starting materials the experimental results show that the incorporation of anhydrite into the magma occurs by chemical dissolution in the melt which increases the magma s sulfur content but suppresses sulfide saturation and reduces olivine crystallization extreme assimilation leads to sulfate saturation in the magma and high dissolved sulfur contents of 0 9 0 1 wt s conversely coal assimilation promotes sulfide segregation and magma crystallization while decreasing the dissolved h2o content of the melt and increasing the amount of coexisting fluid phase we also employed gas melt thermodynamic calculations to quantify the effect of these assimilations on the redox conditions and the s content of the magma and investigate the role of temperature pressure and initial gas content of the magma in the assimilation process we quantify how sulfate assimilation strongly oxidizes the magma and increases its s content both effects are intensified by increasing pressure from 50 to 100 mpa in this study decreasing temperature from 1350 to 1200 c in this study and decreasing amounts of fluid phase initially coexisting with the magma from 2 to 0 wt the interaction with organic matter ch in this study induces a strong reduction of the magma even for extremely low degrees of assimilation few tenths of wt and the dehydration of the melt we therefore suggest that in the noril sk talnakh district 1 additional s was supplied to mantle derived magmas by the assimilation of evaporitic rocks and was transported during magma ascent in the form of dissolved oxidized s 2 a substantial reduction of the magma inducing sulfide segregation and important crystallization then occurred due to the interaction with carbonaceous sediments this mechanism can potentially produce massive sulfide deposits by important sulfate assimilation locally higher than 3 wt caso4 and minor organic matter assimilation few tenths of wt ch however if one of the two steps does not occur or the assimilation in 1 is not large enough disseminated sub economic or no sulfide deposits are produced we conclude that exceptional conditions favoring substantial assimilation of sediments are needed to generate exceptional ore deposits like those of the noril sk talnakh district

origin of the ug2 chromitite layer bushveld complex chromitite layers are common in large mafic layered intrusions a widely accepted hypothesis holds that the chromitites formed as a consequence of injection and mixing of a chemically relatively primitive magma into a chamber occupied by more evolved magma this forces supersaturation of the mixture in chromite which upon crystallization accumulates on the magma chamber floor to form a nearly monomineralic layer to evaluate this and other genetic hypotheses to explain the chromitite layers of the bushveld complex we have conducted a detailed study of the silicate rich layers immediately above and below the ug2 chromitite and another chromitite layer lower in the stratigraphic section at the top of the lower critical zone the ug2 chromitite is well known because it is enriched in the platinum group elements and extends for nearly the entire 400 km strike length of the eastern and western limbs of the bushveld complex where we have studied the sequence in the central sector of the eastern bushveld the ug2 chromitite is embedded in a massive 25 m thick plagioclase pyroxenite consisting of 60 70 vol granular cumulus orthopyroxene with interstitial plagioclase clinopyroxene and accessory phases throughout the entire pyroxenite layer orthopyroxene exhibits no stratigraphic variations in major or minor elements mg number1 4 79 3 81 1 however the 6 m of pyroxenite below the chromitite footwall pyroxenite is petrographically distinct from the 17 m of hanging wall pyroxenite among the differences are 1 phlogopite k feldspar and quartz are ubiquitous and locally abundant in the footwall pyroxenite but generally absent in the hanging wall pyroxenite and 2 plagioclase in the footwall pyroxenite is distinctly more sodic and potassic than that in the hanging wall pyroxenite an45 60 vs an70 75 the lower critical zone chromitite is also hosted by orthopyroxenite but in this case the rocks above and below the chromitite are texturally and compositionally identical for the ug2 we interpret the interstitial assemblage of the footwall pyroxenite to represent either interstitial melt that formed in situ by fractional crystallization or chemically evolved melt that infiltrated from below in either case the melt was trapped in the footwall pyroxenite because the overlying ug2 chromitite was less permeable if this interpretation is correct the footwall and hanging wall pyroxenites were essentially identical when they initially formed however all the models of chromitite formation that call on mixing of magmas of different compositions or on other processes that result in changes in the chemical or physical conditions attendant on the magma predict that the rocks immediately above and below the chromitite layers should be different this leads us to propose that the bushveld chromitites formed by injection of new batches of magma with a composition similar to the resident magma but carrying a suspended load of chromite crystals the model is supported by the common observation of phenocrysts including those of chromite in lavas and hypabyssal rocks and by chromite abundances in lavas and peridotite sills associated with the bushveld complex indicating that geologically reasonable amounts of magma can account for even the massive 70 cm thick ug2 chromitite the model requires some crystallization to have occurred in a deeper chamber for which there is ample geochemical evidence

facies discrimination in a mixed fluvio eolian setting using elemental whole rock geochemistry applications for reservoir characterization abstract fluvio eolian successions are generally characterized by a high degree of complexity and internal heterogeneity quantifying the rapid facies changes in time and space is a major challenge to characterization of hydrocarbon reservoirs we present a method for facies discrimination in fluvio eolian successions the method presented here is developed on the modern fluvio eolian sediments from the skeleton coast dune field namibia the examined sediments comprise eight different facies types eolian dune sands interdune fluvial channel sands intraerg mass flow deposits intraerg hyperconcentrated flow deposits fluvial channel sands fluvial mud lacustrine heterolithic sand and lacustrine heterolithic mud the contrasting provenance of the fluvial and eolian sediments results in a distinct source fingerprint which can be discriminated using elemental whole rock geochemistry a multivariate statistical technique performed on the geochemical data has enabled discrimination of seven of the eight facies types furthermore the facies discrimination method allowed a quantitative estimate of the degree of fluvial reworking of eolian sand the skeleton coast dune field was chosen as a test area for geochemical facies discrimination because of its remarkable difference between the immature fluvial source and the mature quartz rich eolian source we hope that the method presented here can be applied to subsurface data from geological settings with the same difference in fluvial and eolian sources it is furthermore hoped that the method can be used in more complex sedimentological settings

three dimensional reservoir characterization and flow simulation of heterolithic tidal sandstones tidal sandstone reservoirs contain significant intervals of hydrocarbon bearing heterolithic facies characterized by the presence of tide generated sedimentary structures such as flaser wavy and lenticular bedding millimeter to centimeter sand mud alternations we have characterized the reservoir properties sandstone connectivity effective permeability and displacement efficiency of these facies using three dimensional 3 d models reconstructed directly from large rock specimens the models are significantly larger than a core plug but smaller than a typical reservoir model grid block we find that the key control on reservoir quality is the connectivity and continuity of the sandstone and mudstone layers if the sandstone layers form a connected network they are likely to be productive even at low values of net to gross about 0 30 5 this may explain why the productivity of low net to gross heterolithic tidal sandstones is commonly underestimated or overlooked connectivity is the dominant control on the transition between productive pay and nonproductive nonpay heterolithic facies however connectivity is difficult to characterize because core plugs sampled from the subsurface are too small to capture connectivity whereas two dimensional outcrop measurements can significantly underestimate the true 3 d value our results suggest that core plug measurements of permeability and displacement efficiency are unlikely to yield representative values at the scale of a reservoir model grid block because the connectivity and continuity of sandstone and mudstone layers varies significantly with length scale

chromite in komatiites 3d morphologies with implications for crystallization mechanisms high resolution x ray computed tomography has been carried out on a suite of komatiite samples representing a range of volcanic facies chromite contents and degrees of alteration and metamorphism to reveal the wide range of sizes shapes and degrees of clustering that chromite grains display as a function of cooling history dendrites are spectacularly skeletal chromite grains formed during very rapid crystallization of supercooled melt in spinifex zones close to flow tops at slower cooling rates in the interiors of thick flows chromite forms predominantly euhedral grains large clusters up to a dozen of grains are characteristic of liquidus chromite whereas fine dustings of mostly individual 20 m grains form by in situ crystallization from trapped intercumulus liquid chromite in coarse grained olivine cumulates from komatiitic dunite bodies occurs in two forms as clusters or chains of euhedral crystals developing into chicken wire texture where chromite is present in supra cotectic proportions and as strongly dendritic semi poikilitic grains these dendritic grains are likely to have formed by rapid crescumulate growth from magma that was close to its liquidus temperature but supersaturated with chromite in some cases this process seems to have been favoured by nucleation of chromite on the margins of sulphide liquid blebs this texture is a good evidence for the predominantly cumulus origin of oikocrysts and in situ origin of heteradcumulate textures our 3d textural analysis confirms that the morphology of chromite crystals is a distinctive indicator of crystallization environment even in highly altered rocks

a large silent earthquake in the guerrero seismic gap mexico 1 geodetic measurements from a network of permanent gps stations along the pacific coast of mexico reveal a large silent earthquake along the segment of the cocos north american plate interface identified as the guerrero seismic gap the event began in october of 2001 and lasted for 6 7 months average slip of 10 cm produced measurable displacements over an area of 550 250 km2 the equivalent moment magnitude of the event was mw 7 5 recognition of this and previous slow event here indicate that the seismogenic portion of the plate interface is not loading steadily as hitherto believed but is rather partitioning the stress buildup with episodic as opposed to steady state or periodic slip downdip of the seismogenic zone this process increases the stress at the base of the seismogenic zone bringing it closer to failure these results call for a reassessment of the seismic potential of guerrero and other seismic gaps in mexico

uncertainty in well test and core permeability analysis a case study in fluvial channel reservoirs northern north sea norway reservoir permeability is one of the important parameters derived from well test analysis small scale permeability measurements in wells are commonly made using core plugs or more recently probe permeameter measurements upscaling of these measurements for comparisons with the permeability derived from drill stem tests dsts can be completed by statistical averaging methods dst permeability is commonly compared with one of the core plug averages arithmetic geometric or harmonic questions that commonly arise are which average does the dst derived permeability represent and over what region is this average valid another important question is how should the data sets be reconciled where there are discrepancies in practice the permeability derived from well tests is commonly assumed to be equivalent to the arithmetic in a layered reservoir or geometric in a randomly distributed permeability field average of the plug measures these averages are known to be members of a more general power average solution this pragmatic approach which may include an assumption on the near well geology is commonly flawed owing to several reasons that are expanded in this article the assessment of in situ reservoir permeability requires an understanding of both core plug and probe and well test measurements in terms of their volume scale of investigation measurement mechanism interpretation and integration this article presents a comparison of core and well test measurements in a north sea case study we undertook evaluation of three dsts and associated core plug and probe data sets from jurassic fluvial channel sandstones in a single field the well test permeabilities were generally found to differ from the core estimates begin page 1930 and no consistent explanation could be found for the group of wells however the probe permeameter data were able to further constrain the core estimates this study highlights the uncertainty in effective in situ reservoir permeability resulting from the interpretation of small core and reservoir well test scale permeability data the techniques used are traditional upscaling combined with the lorenz plot to identify the dominant flowing interval fluvial sandstones are very heterogeneous and this exercise is instructive in understanding the heterogeneity for the guidance of reservoir models in such a system

geostatistical analysis in weathering studies case study for stanton moor building sandstone geostatistical techniques for spatial prediction and spatial simulation have been used in an innovative application to the study of weathering of natural building stone the study investigates the differences in the spatial variation of permeability characteristics between fine and coarse grained stanton moor building sandstone non destructive permeability measurements using an unsteady state portable probe permeameter were made on three adjacent faces of two cubic blocks representative of fine to medium grained and medium to coarse grained stanton moor sandstone the findings provide greater understanding in the investigation of the durability characteristics of the stanton moor sandstone and show that the spatial distribution and variability of permeability is more important in predicting the overall strength and weathering properties than mean permeability and absolute minimum and maximum values the results suggest that both primary textural characteristics such as grain size and extent of cementation appear to be important factors in determining the overall durability properties of stanton moor sandstone as a building stone geostatistical analysis has been shown to be an important tool in the characterization of spatial variation for the investigation of weathering of building stones copyright 2006 john wiley sons ltd

contrasting copper and chromium metallogenic evolution of terranes in the palaeoproterozoic itabuna salvador curaca orogen sao francisco craton brazil new zircon shrimp and sm nd model ages and their significance for orogen parallel escape tectonics abstract the northern segment of the palaeoproterozoic itabuna salvador curaca collisional orogen in the sao francisco craton brazil comprises two high grade terranes separated by the 150 km long n s trending itiuba syenite the curaca terrane to the west contains copper sulphide deposits hosted by hypersthenite norite complexes e g caraiba body whereas the jacurici terrane to the east contains chromite deposits hosted by peridotite norite sills e g medrado body shrimp u pb dating of zircons combined with a significant major element compositional gap indicates that the caraiba 2580 10 ma and the medrado 2085 5 ma mafic ultramafic complexes are not related by fractional crystallisation as previously thought depleted mantle nd model ages for the regional gneisses are also distinct with the curaca terrane having younger ages 2 4 2 8 ga than the jacurici terrane 2 9 3 2 ga suggesting that they represent two independent fragments of continental crust accreted during the palaeoproterozoic orogeny emplacements of the itiuba syenite ca 2084 ma and of the medrado complex were apparently coeval probably controlled by orogen parallel block displacement or escape tectonics following continent continent collision

the role of viscous particle segregation in forming chromite layers from slumped crystal slurries insights from analogue experiments the question of how the mineral layering in layered intrusions forms has been extensively debated for many decades there are many types of layering and it is of course possible that a number of mechanisms are involved of particular interest is how chromite layers form because these may contain valuable metals such as platinum group elements in addition to cr one model for the formation of these layers is that they formed through slumping of semi consolidated cumulates from the margins of the intrusion into the magma chamber during this slumping the grains are sorted by density and or size differences this study examines the viability of this process using analogue modelling starting materials beads and glycerine were scaled to match the density and size of the minerals chromite orthopyroxene and plagioclase present in layered mafic ultramafic layered intrusions and to match the density and viscosity of the silicate magma a perspex flume tank divided by a removable partition at one end was fully filled with glycerine a homogenized mixture of the beads was placed in the smaller partition of the tank representing the margins of the magma chamber the tank was then inclined between 16 and 45 the partition was removed and the beads flowed into the main part of the box the experiments were recorded by video camera allowing us to follow the dynamics of the flow during each run segregation of the beads was observed in the final deposits the larger less dense beads representing plagioclase concentrated at the top of the flow with the intermediate sized and medium density beads representing orthopyroxene in the middle and the smaller denser beads representing chromite at the bottom thus mimicking natural examples in experiments where the angle of inclination was low long thin layers formed such as those found in the bushveld complex in experiments where the angle of inclination was high thick but short layers formed a dimensionless analysis allows better understanding of the dynamics of the flow at the macroscopic scale the flow regime is strongly influenced by the viscosity of the fluid and is considered macro viscous where the role of the interstitial liquid is non negligible

a slow thrust slip event following the two 1996 hyuganada earthquakes beneath the bungo channel southwest japan we report a slow thrust slip event that occurred beneath the bungo channel region southwestern japan on oct and dec 1996 two hyuganada earthquakes both mw 6 7 followed by afterslips occurred in addition a crustal movement characterized by an extremely slow rise was observed around the bungo channel about 200 km north from the epicenters and continued for about 300 days long assuming a slow slip on the plate boundary we estimate its duration and surface displacements from gps time series data by curve fitting and then determine the fault slip distribution we found that a slow slip without any earthquakes continued for nearly one year and released the seismic moment comparable to that of the hyuganada earthquakes occurrence of the slow thrust slip event suggests that this kind of event may be a characteristic mode of stress release at a transition region of interplate coupling

a comparison of plug derived and probe derived permeability in cross bedded sandstones of the virgelle member alberta canada the influence of flow directions on probe permeametry in this article we compare permeability measured on radially confined cylindrical plugs to permeabilities measured using a millimeter scale probe tip on unconfined end faces of the same plugs our investigation focuses on directional attributes of probe permeametry data as a means to understand the differences between plug scale and probe scale permeability magnitudes horizontal and vertical plug and probe flow tests were carried out on fine to medium grained three dimensional 3 d cross bedded sandstones collected from estuarine channel successions within the cretaceous age virgelle member at writing on stone provincial park southern alberta canada the sandstones are heterogeneous in that they contain thin discontinuous micaceous carbonaceous laminae and apparent bedding parallel grain scale fabric the resulting probe derived permeabilities 0 4 3 5 d are mostly higher than corresponding plug permeabilities 0 5 1 6 d and mean probe derived permeability anisotropy is significantly lower than that derived from plug scale measurements that is plug k v k h is less than probe derived k v k h the results can be understood as a consequence of the model flow geometry of probe flow tests goggin et al 1988 namely that flow is preferentially directed parallel to the sample surface radially outward from the probe tip on end faces of vertical plugs the bedding parallel fabric further enhances flow parallel to the sample surface and probe measurements yield reliable estimates of horizontal permeability having magnitudes comparable to the permeability of equivalent horizontal plugs for probe measurements on end faces of horizontal plugs flow paths appear to be variably confined parallel to bedding as a function of grain scale and laminae scale fabric systematically yielding an overestimate of horizontal permeability k prh k plh interpretation of probe data in this orientation however is very uncertain due to partitioning of flow paths parallel to and at angle to bedding significantly almost all probe permeameter core and outcrop studies are based on measurements of this type

permeability structure of a compound valley fill in the cretaceous fall river formation of south dakota a cross section through a compound valley fill sandstone in the lower cretaceous fall river formation is exposed in red canyon on the southwest flank of the black hills in south dakota a mapping of changes in stratal architecture facies and permeability along this cross section records rock heterogeneities that can influence subsurface fluid flow behavior the 3 5 km wide 30 m thick compound valley fill sandstone cuts into a succession of meters thick upward coarsening marine shore zone strata and muddy alluvial plain deposits the sandstone becomes less permeable on average both upward and laterally outward from its axis to its margin the sandstone contains four discrete valley fills each fill is floored by a major unconformity surface the second fill has a permeability mean and variance substantially greater than that of the first fill and it shows the most pronounced large scale vertical and lateral permeability trends the third and fourth fills are progressively narrower finer grained more heterolithic and on average less permeable this retrogradational pattern juxtaposes poor reservoir quality valley fills on older higher quality valley fill sandstones the permeability structure within individual valley fills reflects the geometry and stacking of internal sediment bodies and the distribution of lithofacies within these bodies deposits lower in valley fills are more permeable and typically lack lateral permeability trends whereas deposits higher in fills are less permeable and exhibit more pronounced lateral permeability trends over several tens to hundreds of meters permeability variations along internal sediment bodies are less pronounced than those across valley fills this study demonstrates the importance of defining rock property trends in a sedimentologic framework to characterize complex valley fill reservoir sandstones

the influence of parent material on topsoil geochemistry in eastern england the topsoil of around 10 000 km2 in eastern england has recently been sampled intensely at 4609 sites to characterize its geochemistry the parent materials which include both solid geology and quaternary sediments range in age from permian to holocene the distributions of the concentrations of major and trace elements have been characterized geostatistically and the role of parent material on their spatial structure anisotropy and their spatial relationships coregionalization have been investigated analysis of variance with the sites grouped by major parent material type showed that this classi cation accounted for 14 to 48 per cent of the variance for the various elements global variograms of 13 elements al as ca cr cu fe mg mo ni p pb ti and u have been computed and modelled eleven of the variograms seem to comprise two structures both of which we modelled with spherical functions one of short range 3 5 to 9 km and the other with a range of 15 to 23 km the models included a nugget variance which varied from 27 per cent for as fe and mg to 63 per cent for p of the total the long range structures are related to the separations of the major parent materials the variograms of several elements showed appreciable anisotropy most notably that of mg anisotropy is evident at short ranges of less than 5 km this accords with the geological structure of the beds which dip from west to east so that their outcrops are elongated from north to south a linear model of coregionalization tted to the data emphasized several important geochemical associations which we interpret elements commonly associated with clay minerals mg al and the clay size fraction ti are dominated by the long range structure of the coregionalization whilst several trace elements as cr ni and u are spatially correlated with fe over short distances through adsorption of the former on the surfaces of fe oxyhydroxides the topsoil around large urban areas is enriched in lead but it is not clear whether anthropogenic sources are responsible for this metal s anomalous spatial relationships with other elements crown copyright 2003 reproduced with the permission of her majesty s stationery of ce published by john wiley sons ltd

the origin of chromitites and related pge mineralization in the bushveld complex new mineralogical and petrological constraints this article reports a study of chromitites from the lg 1 to the ug 2 3 from the bushveld complex chromite from massive chromitite follows two compositional trends on the basis of cation ratios trend a decreasing mg mg fe with increasing cr cr al trend b decreasing mg mg fe with decreasing cr cr al the chromitites are divided into five stages on the basis of which trend they follow and the data of eales et al chemical geology 88 261 278 1990 on the behaviour of the mg fe ratio of the pyroxene and whole rock sr isotope composition of the environment in which they occur following eales et al chemical geology 88 261 278 1990 the different characteristics of the stages are attributed to the rate at which new magma entered the chamber and the effect of this on aal2o3 and in the case of stage 5 the appearance of cumulus plagioclase buffering the aal2o3 the similarity of pge profiles across the mg 3 and mg 4 chromitites that are separated laterally by up to 300 km and the variation in v in the ug 2 argue that the chromitites have largely developed in situ modelling using the programme melts shows that increase in pressure mixing of primitive and fractionated magma felsic contamination of primitive magma or addition of h2o do not promote crystallization of spinel before orthopyroxene in general they hinder it and that the cr2o3 content of the magma was of the order of 0 25 wt less than 20 of the chromite in the magma is removed before orthopyroxene joins chromite which implies a 13 km thickness of magma for the critical zone it is suggested that the large excess of magma has escaped up marginal structures such as the platreef the pge profile of chromitites depends on whether sulphide accumulated or not along with chromite modelling shows that contamination of critical zone magma with a felsic melt will induce sulphide immiscibility although not chromite precipitation the lg 1 to lg 4 chromitites developed without sulphide whilst those from the lg 5 upwards had associated liquid sulphide much of the sulphide originally in the lg 5 and above has been destroyed as a result of reaction with the chromite

swaley cross stratification produced by unidirectional flows bencliff grit upper jurassic dorset uk unusual bedforms occur in the bencliff grit at osmington mills dorset the geometry of cross strata in stacked predominantly fine grained sandstone units is dominated by low angle troughs with locally perfectly preserved antiforms which wedge out onto climbing low angle erosion surfaces many of the antiforms hummocks and troughs swales appear to have grown at similar rates of sediment fall out others show draped antiforms and thickening into swales in both cases laminae are concordant with the lower bounding surfaces of the cross sets demonstrating that vertical accretion was the dominant process this geometry together with the conspicuous absence of angle of repose cross strata give the bedforms a resemblance to amalgamated hummocky cross stratification or swaley cross stratification many workers consider amalgamated hummocky cross stratification and swaley cross stratification to be identified with storm sedimentation on the shelf to shoreface thereby bracketting the structures both in terms of process and environment however the structures in the bencliff grit suggest that such a precise interpretation of amalgamated hummocky cross stratification or swaley cross stratification is inadvisable the geometry of the cross stratification suggests deposition under a highly sediment charged but essentially unidirectional flow this study therefore supports recent views that hummocky and swaley styles of cross stratification are in reality a form of low angle trough cross stratification produced primarily by unidirectional flows clearly since undulatory forms of stratification are substrate and process dependent and not environmentally specific great care must be taken in the interpretation of palaeoenvironmental settings especially where exposures are limited as for instance in borehole cores it is unlikely that core data can be used to identify bedforms analogous to hummocky cross stratification or swaley cross stratification unless boreholes are highly deviated

patterns of seismic activity preceding large earthquakes we analyze the patterns of seismic activity which precede large events in a mechanical model of a fault the model generates a statistical distribution of events similar to that observed for a single fault with a scaling region consistent with the gutenberg richter law at small and moderate magnitudes and an excess of events at large magnitudes we find only slight variation in the scaling behavior during a loading cycle however we do observe systematic variations in space and time of the overall rate of activity in the model the activity accelerates dramatically preceding a large event and is usually a maximum hi the neighborhood of the future epicenter these results are compared to california seismicity data where we find that activity patterns vary regionally looking at patterns of activity in the san francisco bay area since 1948 we find an increase of activity on the calaveras fault near san jose beginning in the 1980s which if our model is relevant would forecast a large earthquake in that region the 1989 loma prieta earthquake occurred on the san andreas fault within 30 km of the section of the calaveras fault showing increased activity

detection and monitoring of ongoing aseismic slip in the tokai region central japan analysis of global positioning system data shows that the rate of crustal deformations in the tokai region of japan a seismic gap area changed over the past 18 months kalman filtering analysis shows aseismic slip on the plate boundary in the western tokai region centered on lake hamana adjacent to the anticipated tokai earthquake source area the cumulative moment magnitude reaches 6 7 in june 2002 with a relative slip increase northeast of lake haman from january 2002 an existence of aseismic slip in the western tokai supports the hypothesis of a silent event as the cause of uplifting several days before the 1944 tonankai earthquake

the development of fluvial stochastic modelling in the norwegian oil industry a historical review subsurface implementation and future directions abstract fluvial sandstones are an important reservoir type for the petroleum industry in the late 1970 s and early 1980 s large hydrocarbon discoveries in the norwegian north sea in fluvial strata prompted the need for generating geologically meaningful stochastic object based models of fluvial deposits the aim of this focus was to allow the geologist to provide the reservoir engineers with a more realistic representation of permeability contrasts within channelised fluvial deposits by being able to use appropriate measurements from outcrop analogues as direct input data into the modelling software this initiative resulted in the development of a suite of geologically driven stochastic modelling algorithms supported by an extensive fieldwork program aimed at collecting stratigraphic and quantitative data from ancient outcrop analogues to support enhanced reservoir characterisation and geological modelling today these reservoirs are still important hydrocarbon producing fields with accurate reservoir description and 3d modelling capabilities playing a vital role in targeting remaining oil especially now that many of the fields on the norwegian continental shelf are past peak production and are in a decline phase as both computing capabilities and quantitative outcrop analogue studies have increased the understanding of and the ability to model fluvial reservoirs so have stochastic modelling techniques continued to provide the most suitable and robust means of building geologically realistic 3d reservoir models that incorporate increased geological understanding and heterogeneity complexity in the recent past a multitude of data such as seismic and production data have been used to condition the stochastic algorithms this review paper aims to outline the role of stochastic algorithms in building geologically realistic 3d fluvial reservoir models and highlight the success of these developments with case studies from both producing fields and ancient outcrop analogue studies finally the paper will allude to possible improvements in stochastic fluvial modelling and future directions in the modelling of fluvial petroleum reservoirs these include the use of physical or process based models high resolution near wellbore models and multi point statistics that allow for more realistic representations of heterogeneities of fluvial deposits at a variety of scales and by a variety of methods

the chromite deposit of the ipueira medrado sill sao francisco craton bahia state brazil the ipueira medrado mafic ultramafic sill hosts the largest chromite deposit in brazil the sill is a 7 km long and 300 m thick elongated body consisting mainly of interlayered dunite and harzburgite but also containing minor mafic rocks the most distinctive feature of the ipueira medrado sill is a continuous 5 to 8 m thick massive chromitite layer that is currently being mined the anomalous thickness of this chromitite layer provides important constraints on current genetic models for the origin of massive chromitites the sill is divided into three zones marginal ultramafic and mafic the marginal zone 5 20 m thick consists of sheared gabbro and orthopyroxene rich harzburgite the ultramafic zone up to 250 m thick is subdivided into the lower ultramafic unit the main chromitite layer and the upper ultramafic unit the lower ultramafic unit consists of dunite with minor harzburgite and chain textured chromitite the main chromitite layer is composed of chain textured and massive chromitites and the upper ultramafic unit consists mainly of harzburgite but also minor chain textured chromitite and dunite pyroxene abundance increases toward the top in this zone and grades to an orthopyroxenite at the top the presence of magmatic intercumulus amphibole is significant up to 20 vol in this unit the mafic zone up to 40 m thick consists of leuconorites to melanorites that are partially metamorphosed under amphibolite facies conditions the variation of olivine and orthopyroxene compositions throughout the sill layers reveals two intervals of distinct magmatic evolution the interval located below the main chromitite layer is characterized by gradual upward evolution of mineral compositions toward more mg rich compositions fo93 and en94 the interval located above the main chromitite layer is characterized by rapid evolution toward more fe rich compositions fo85 and en84 with increasing stratigraphic height the interval below the main chromitite layer suggests crystallization in a dynamic magma chamber undergoing frequent replenishment with primitive magma the interval above the main chromitite layer suggests crystallization in a mainly closed fractionating magma chamber mass balance considerations suggest that an enormous amount of magma 10 000 m thick was associated with the formation of the main chromitite layer at the ipueira medrado sill such a volume of magma is obviously not represented in the stratigraphy of the ipueira medrado sill mineral chemistry data indicate that the composition of the resident magma was most primitive at the stratigraphic interval immediately below the main chromitite layer we suggest that the sill acted as a conduit through which a large volume of magma flowed under these conditions contamination of the parental magma with crustal material and fluids is a likely mechanism for explaining the origin of the chromitite abundant intercumulus amphibole in the upper ultramafic unit and high fe3 in chromites intercumulus orthopyroxene in main chromitite layer chromitites and sm nd and re os isotope data for silicate rocks and chromitites respectively indicate that the ipueira medrado sill formed from magmas contaminated by crustal material the host rocks of the ipueira medrado sill consist mainly of high grade gneiss and marbles that would have been appropriate source of fluids and sio2 and could have caused the increase in f o2

geostatistical models of secondary oil migration within heterogeneous carrier beds a theoretical example this paper critically evaluates the utility of two different geostatistical methods in tracing long distance oil migration through sedimentary basins geostatistical models of petroleum migration based on kriging and the conditional simulation method are assessed by comparing them to known oil migration rates and directions through a numerical carrier bed in this example the numerical carrier bed which serves as ground truth in the study incorporates a synthetic permeability field generated using the method of turning bands different representations of lateral permeability heterogeneity of the carrier bed are incorporated into a quasi three dimensional model of secondary oil migration the geometric configuration of the carrier bed is intended to represent migrat on conditions within the center of a saucer shaped intracratonic sag basin in all of the numerical experiments oil is sourced in the lowest 10 of a saucer shaped carrier bed and migrates 10 14 km outward in a radial fashion by buoyancy the effects of vertical permeability variations on secondary oil migration were not considered in the study two sets of numerical experiments are presented in this study in the first set computed oil migration patterns and rates through the numerical carrier bed are compared to kriged and optimal estimations based on a suite of 50 conditional simulations to determine the utility of each geostatistical approach the geostatistical models also are compared to a much simpler oil migration simulation using a homogeneous carrier bed an assumption typically made in cross sectional models of oil migration the permeability realizations generated by kriging and the conditional simulation method were constructed using 50 randomly placed sampling points a second set of numerical experiments was made to determine how data sample size 20 35 50 100 and 250 randomly selected points influences p edicted oil migration patterns using kriged permeability fields oil migration patterns through the synthetic carrier bed were quantitatively compared to geostatistical representations of oil transport by computing the degree of correlation r2 of oil lens thickness at 5 10 and 15 m y after the onset of migration quantitative results indicate that both kriging and the conditional simulation method produced realistic representations of spatial oil lens patterns and first arrival times both kriging and the conditional simulation method produced results far more realistic than the simulation with a homogeneous carrier bed after 15 m y the spatial correlations r2 between the known oil lens patterns and that of the kriged and ensemble average of the 50 conditional simulations were 0 67 and 0 74 respectively although the conditional simulation method produced the best results it did so at a significant computational expense computed oil migration patterns based on different kriged permeability maps that incorporated varying amounts of control data points did not improve signif cantly when more than 100 randomly placed wells were used the study provides compelling evidence that secondary oil migration is an inherently three dimensional process for well studied basins geostatistical representations of long range oil migration could be of value in ongoing exploration strategies end page 1679

reservoir characterization from downhole mineralogy abstract it is now a relatively established procedure with recent advances in logging technology to be able to generate a comprehensive continuous measurement of major element chemistry in the subsurface concurrent with these advances strategies have been developed which transform elemental data derived from nuclear logging measurements into a set of mineral modes that accurately represent the mineralogy of a rock resulting mineralogy logs are potentially valuable on their own especially for creating a geological model in the absence of or to enhance core measurements mineralogy logs can also be used for the determination of other petrophysically useful formation descriptors such that in wells where core recovery is poor they can help to extend evaluation across poorly defined zones a selection of applications are investigated these show that with good acquisition derived mineralogy may be used effectively for inter well correlation and for the determination of enhanced estimates of matrix density and porosity

probe permeametry is it worth the effort abstract an overview is given of the wide variety of current applications for probe permeameter technology and data in light of recent key advances in reservoir characterization the future trends in applications and tool development are discussed by examining the procedures and costs associated with data acquisition quality control and general information enhancement and by comparing these factors to more conventional methods for obtaining permeability it is concluded that probe permeametry is indeed worth the effort

komatiites and nickel sulfide ores of the black swan area yilgarn craton western australia 2 geology and genesis of the orebodies the black swan ultramafic succession hosts a number of magmatic fe ni cu pge sulfide ore shoots ranging from high grade massive ore to low grade disseminated sulfides of these the most economically significant is the silver swan massive sulfide orebody associated with the basal contact of the succession the deposit varies in thickness between 5 and 20 m reaches a n s strike length of 75 m extends for at least 1 2 km of vertical plunge and is open at depth overlying matrix net textured ore is rare inclusions of dacite are abundant within the lower 5 m of the massive sulfide they range from angular fragments through smooth sinuous and plumose morphologies to fine lace like intergrowths with the sulfide matrix and comprise variable proportions of cores of porphyritic dacite and carapaces with skeletal plagioclase phenocrysts dynamic crystallisation and kinetic melting textures in the carapaces indicate that the inclusions have been heated to various temperatures some well above their liquidus temperature the composition of the inclusions ranges from a perfect match with the immediate footwall dacites to mixtures of dacite with up to 30 komatiite the consistent thickness of the inclusion bearing basal layer within the massive sulphide is interpreted as the extent of 3 d physical connectivity between the inclusions and a partially molten underlying hybrid layer primary contacts between the silver swan massive sulfide orebody and overlying ultramafic rocks are marked by thin rinds containing coarse grained chevron textured chromites with skeletal textures compositions of these chromites match those from kambalda perseverance and other localities and are inconsistent with a metamorphic origin they are interpreted as markers of primary magmatic contacts the combination of this feature with the general paucity of matrix ore implies that the massive ore accumulated and solidified before the accumulation of the overlying thick sequence of olivine cumulates taken together with observations on the internal fractionation of platinum group elements within the massive ores these observations are consistent with a model where the massive ore were emplaced at the floor of a small partially drained lava tube the floor of the tube had been previously heated by passage of large volumes of lava such that it had reached its melting range the felsic inclusions within the ore are the result of buoyant ascent of partially molten substrate into the ore magma this constitutes strong evidence for the operation of thermo mechanical erosion during ore emplacement the disseminated cygnet and black swan orebodies show a number of distinctive features cygnet contains a assemblage of clasts and inclusions which are interpreted as the result of rip up transport and redeposition of sulfides from a pre existing massive sulfide orebody of which black duck may be a remnant the black swan orebody by contrast does not show xenolithic features but is characterised by an association of sulfide blebs with segregation vesicles and by unusually coarse grained olivine the black swan orebody is interpreted as the result of transport of sulfide droplets within a lava charged with a suspended load of coarse olivine crystals

minipermeameter based study of permeability trends in channel sand bodies 1 permeability is the single most important factor influencing fluid flow in reservoirs to map the distribution of this property in three dimensions we did outcrop studies with a minipermeameter ribbon like distributary channel sand bodies exposed along the yorkshire coast of england were selected as study objects results show a close connection between depositional facies and permeability in the ribbon channels as witnessed by the five facies related permeability classes semivariogram analyses and correlation of permeability profiles indicate that permeability measurements tend to be unrelated at distances exceeding 2 3 m thus the detailed permeability patterns seen in these distributary channel sand bodies in well bores may be significantly different from the perm ability distributions in the vicinity of the well the short permeability correlation length implies that at the macroscale or facies level flow units are expected to be strongly lenticular models of the spatial flow unit distribution in this sand body type should mimic such a pattern by using a scaling up procedure the complex macroscale pattern could be used to generate models of the gross permeability distribution in ribbon like channel sand bodies at less refined averaging scales in the active fills of ribbon like channel sand bodies the classical upward decrease in permeability was rarely seen the zones with highest permeability were mostly present in the central to uppermost parts of these sand bodies these trends along with the lensoid shape of the permeability class zones should be maintained in detailed modelings and simulations of this type of reservoir sand body clay content grain size and stratification type seem to be the factors most strongly influencing permeability in the investigated sand bodies

petrogenesis of the massive chromitite layer from the jacurici complex brazil evidence from inclusions in chromite the jacurici complex hosts the largest chromite deposit in brazil in an up to 8 m thick chromitite layer within a tectonically segmented 300 m thick intrusion the ore has been interpreted as the result of crustal contamination driven crystallization in a magma conduit this study addresses the stratigraphy mineralogical and textural relationships and mineral chemistry of the monte alegre sul segment focusing on chromite hosted inclusions from the main chromitite layer to understand the role of volatiles in the genesis of the massive chromitite silicate inclusions enstatite phlogopite magnesiohornblende diopside and olivine are commonly monomineralic and sub to euhedral and crystallized prior to or coeval with the chromite crystallization carbonate inclusions dolomite and magnesite are irregular or have negative crystal shapes suggesting entrapment as melt droplets sulfides pentlandite millerite heazlewoodite polydymite pyrite and chalcopyrite are often polymineralic irregular or hexagonal shaped indicating entrapment as sulfide melt and as monosulfide solid solution the inclusions indicate an h2o and s saturated resident magma with immiscible droplets of carbonate melt during chromite crystallization inclusion rich and inclusion free chromites that occur together have similar compositions and are considered to have formed from the same magma in response to variations in the degree of cr saturation hot primitive magma might have heated and mobilized co2 and probably water from devolatized and assimilated carbonate rich wall rocks increasing fo2 and triggering chromite crystallization we propose that the formation of the chromitite layer started as in situ crystallization with additional material added by slumping of locally remobilized chromite slurries facilitated by the presence of volatiles

mesoarchaean to palaeoproterozoic growth of the northern segment of the itabuna salvador curaca orogen sao francisco craton brazil the geology of the northern segment of the itabuna salvador curaca orogen sao francisco craton is reviewed and new u pb ages and nd isotope and major and trace element data are combined to improve understanding of its tectonic evolution the results indicate that oceanic crust and island arc sequences accreted at about 3 30 ga to form the mundo novo greenstone belt and between 2 15 and 2 12 ga to form the rio itapicuru and rio capim greenstone belts at about 3 08 2 98 ga mafic crust underwent partial melting to form the retirol ndia and jacurici tonalite trondhjemite granodiorite belts of the serrinha block

morphology and microstructure of chromite crystals in chromitites from the merensky reef bushveld complex south africa the merensky reef of the bushveld complex consists of two chromitite layers separated by coarse grained melanorite microstructural analysis of the chromitite layers using electron backscatter diffraction analysis ebsd high resolution x ray microtomography and crystal size distribution analyses distinguished two populations of chromite crystals fine grained idiomorphic and large silicate inclusion bearing crystals the lower chromitite layer contains both populations whereas the upper contains only fine idiomorphic grains most of the inclusion bearing chromites have characteristic amoeboidal shapes that have been previously explained as products of sintering of pre existing smaller idiomorphic crystals two possible mechanisms have been proposed for sintering of chromite crystals 1 amalgamation of a cluster of grains with the same original crystallographic orientation and 2 sintering of randomly orientated crystals followed by annealing into a single grain the ebsd data show no evidence for clusters of similarly oriented grains among the idiomorphic population nor for earlier presence of idiomorphic subgrains spatially related to inclusions and therefore are evidence against both of the proposed sintering mechanisms electron backscatter diffraction analysis maps show deformation related misorientations and curved subgrain boundaries within the large amoeboidal crystals and absence of such features in the fine grained population microstructures observed in the lower chromitite layer are interpreted as the result of deformation during compaction of the orthocumulate layers and constitute evidence for the formation of the amoeboid morphologies at an early stage of consolidation an alternative model is proposed whereby silicate inclusions are incorporated during maturation and recrystallisation of initially dendritic chromite crystals formed as a result of supercooling during emplacement of the lower chromite layer against cooler anorthosite during the magma influx that formed the merensky reef the upper chromite layer formed from a subsequent magma influx and hence lacked a mechanism to form dendritic chromite this accounts for the difference between the two layers

geological modeling for simulation studies reservoir heterogeneity is hierarchical in nature and different levels of reservoir heterogeneity need to be quantified by the reservoir geologist for numerical simulation studies at the largest scale field scale sand body continuity and interconnectedness are the most important parameters at the scale of a reservoir or a genetic sand body areal permeability trends presence and distribution of permeability baffles vertical profiles of permeability and presence of directional permeability within a sand body are the most critical parameters at a smaller scale the influence of cross bedding on displacement processes is shown to be significant and in fact should be assessed to obtain more meaningful results from larger scale simulation models the examples given show some of the effects of the different scales of heterogeneity on fluid flow patterns these examples indicate the degree of detail that must be considered at different scales if simulations are to give meaningful results

introduction to the special issue on aquifer sedimentology problems perspectives and modern approaches abstract progress towards a better understanding of groundwater circulation and transport processes in aquifers demands a multidisciplinary approach to a host of unresolved problems although much progress has been made within recent years in interpreting the dynamic character of groundwater systems many key issues remain to be addressed in particular several areas demand attention the role of sedimentological information heterogeneity in groundwater and transport models the scaling up of observations from outcrop scale to larger scales and the integration of geological and geophysical information of different quality into the description of an aquifer structure still nowadays many of the heterogeneities cannot be recognized directly because of the limitation of measurement techniques this is probably one of the reasons for the limited application of aquifer sedimentology and geophysics in practical cases in order to consolidate expand and make a larger number of people aware of the contribution of modern aquifer sedimentology including modelling and ultra high resolution geophysical methods several lines of intervention were identified 1 a better collaboration of the different disciplines on site specific applied problems 2 development of new modelling techniques combining data of different quality 3 development of optimizing tools position and number of wells additional geophysical methods baysian techniques 4 development of a common language among sedimentologists and hydrogeologists to overcome communication problems

the fluid dynamics of river dunes a review and some future research directions 1 dunes are present in nearly all fluvial channels and are vital in predicting flow resistance sediment transport and deposition within many rivers progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field laboratory and numerical investigations progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune additionally the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood with the feedback of dune three dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features this review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes five areas are then highlighted and discussed as a possible focus for future research 1 the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field 2 the influence of three dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress 3 flow field modification resulting from bed form superimposition and amalgamation 4 the scale and topology of dune related turbulence and its interactions with sediment transport and the flow surface and 5 the influence of suspended sediment on the dune flow field and dune morphology

laminaset geometries in fine grained shallow marine sequences core data from the rannoch formation north sea and outcrop data from the kennilworth member utah usa and the bencliff grit dorset uk sandbodies from storm dominated marine and marginal marine environments commonly contain intervals of laminated fine sandstones a characteristic of such lamination is the presence of low angle cross lamination in order to model correctly the effects of such lamination on a waterflood of an oil bearing shoreface sequence it was necessary to quantify the geometry of the laminaset elements this challenge has been greatly complicated by the lack of outcrop of the formation of interest the middle jurassic rannoch formation of the north sea only occurs in the subsurface where it is not possible in core to measure the aspect ratio of laminasets directly in this study the laminaset geometry data that can be obtained from core e g apparent set thicknesses were collected for the rannoch formation these data were compared with similar data from potential outcrop analogues in 1 the cretaceous kennilworth member of the blackhawk formation in utah usa and 2 the upper jurassic bencliff grit from the dorset coast uk a quantitative analysis of laminaset geometries has been used to compare subsurface core with potential outcrop analogues the rannoch formation core is characterized by numerous low angle truncations we have measured these features in two wells means of 7 2 and 12 1 mean apparent set thicknesses were 0 24 and 0 19 m in the outcrop sections studied truncation angles ranged from 9 6 to 13 4 and mean set thicknesses from 0 24 to 0 34 m mean bounding surface dips of 5 8 and 8 6 and mean laminaset lengths of 2 3 and 4 1 m were also measured directly in the field and by using photomosaics on the basis of this comparison the kennilworth member in utah was found to be the most suitable and the geometries i e aspect ratios measured there were used to generate an appropriate geometry of rannoch laminaset geometries for use in engineering studies laminaset length 2 0 m laminaset thickness 0 2 m

periodic slow earthquakes from the cascadia subduction zone continuous geodetic measurements from convergent margins have shown that deep transient creep events can release large amounts of strain energy without detectable seismic shaking and they are thus known as slow or silent earthquakes because subduction zones generate the largest earthquakes

the uitkomst intrusion and nkomati ni cu cr pge deposit south africa trace element geochemistry nd isotopes and high precision geochronology the uitkomst intrusion is a tubular mafic ultramafic layered body that hosts one of south africa s largest ni cu cr pge deposits nkomati the sulphide ore occurs in the form of massive lenses in the immediate quartzitic footwall and as disseminations within peridotite the chromite ore forms an up to 10 m thick layer in the lower portion of the intrusion uitkomst has generally been interpreted as a magma conduit possibly related to the bushveld event here we present a new high precision u pb zircon date of 2057 64 0 69 ma that overlaps with the age of the merensky reef of the bushveld complex and thus demonstrates a coeval relationship between the intrusions based on incompatible trace elements as well as o and nd isotope data end 4 5 to 6 2 we show that the uitkomst parent magmas were contaminated with up to 20 archean upper crust prior to emplacement and with up to 15 dolomitic country rock during emplacement ore formation at nkomati was critically aided by substantial devolatisation and removal of dolomitic floor rocks leading to hydrodynamic concentration of sulphide and chromite during slumping of crystal mushes into the trough like centre of the subsiding intrusion and its footwall

nature of the high speed rupture of the two dimensional burridge knopoff model of earthquakes the nature of the high speed rupture or the main shock of the burridge knopoff spring block model in two dimensions obeying the rate and state dependent friction law is studied by means of extensive computer simulations it is found that the rupture propagation in larger events is highly anisotropic and irregular in shape on longer length scales although the model is completely uniform and the emergent rupture propagation velocity is nearly constant everywhere at the rupture front the manner of the rupture propagation sometimes mimics the successive ruptures of neighbouring asperities observed in real large earthquakes large events tend to be unilateral with its epicentre lying at the rim of its rupture zone the epicentre site of a large event is also located next to the rim of the rupture zone of some past event event size distributions are computed and discussed in comparison with those of the corresponding one dimensional model the magnitude distribution exhibits a power law behaviour resembling the gutenberg richter law for smaller magnitudes which changes over to a more characteristic behaviour for larger magnitudes for very large events the rupture length distribution exhibits mutually different behaviours in one dimension and in two dimensions reflecting the difference in the underlying geometry this article is part of the theme issue statistical physics of fracture and earthquakes

seismicity simulation with a rate and state dependent friction law the dynamic motions and stabilities of a single degree of freedom elastic system controlled by different friction laws are compared the system consists of a sliding block connected to an elastic spring driven at a constant velocity the friction laws are a laboratory inferred friction law called the rate and state dependent friction law proposed by dieterich and ruina and a simple friction law described by dynamic and static frictions we further extend the solution to a one dimensional mass spring model which is an analog of a fault controlled by the rate and state dependent friction law this model predicts non uniform slip and stress drop along the rupture length of a heterogeneous fault this result is very different from some earlier modelings based on the simple friction law and a slip weakening friction law in those earlier modelings the stress and slip functions become smoother with time along the length of the fault rupture owing to the interactions between fault segments during slip because of this smoothing process the number of small events will decrease with time and the universilly observed stationary magnitude frequency relation cannot be explained the interaction between a fault segment and its neighboring segments can be measured when the post slip stress on this segment is compared with the stress on an identical segment represented by a block in this modeling without neighboring segments if the post slip stress of the former is much higher than that of the latter strong interaction exists if the two are close only weak interaction exists the interaction is determined by the relative motion between fault segments and the time duration of interaction our new modeling with the rate and state dependent friction law appears to show no such smoothing effect and provides a physical mechanism for the roughening process in the difference between the fault strength and stress that is necessary to explain the observed stationary magnitude frequency relation the noninstantaneous healing predicted by the rate and state dependent friction law may be repsonsible for the recurring nonuniform slip and stress drop and may be explained by the reduction of interaction among fault segments due to the low frictional strength during the fault stopping the very low friction during slip stopping allows much longer times than does the higher friction due to instantaneous healing for the fault segments to adjust their motions from an upper limit slip velocity to almost rest according to newton s second law a process with fixed masses and constant velocity changes involves low forces and weak interactions if it is accomplished in a long time period and vice versa our modeling also indicates that the existence of strong patches with higher effective stress on a fault is needed for the occurrence of major events the creeping section of a fault such as the one along the san andreas fault in central california on the other hand can be simulated with the rate and state dependent friction law by certain model parameters which however must not include strong patches in this case small earthquakes and aseismic creep relieve the accumulating strain without any large events

application of probe permeametry to the prediction of two phase flow performance in laminated sandstones lower brent group north sea abstract the rannoch formation lower brent group north sea is an important laminated oil bearing reservoir in the northern north sea recent work has shown that at the estimated field frontal advance rates for the rannoch formation capillary effects in laminated sandstones might significantly affect the two phase oil and water flow characteristics previous published reservoir simulation models of the rannoch have not accounted for this newly acquired probe permeameter measurements have been used to map the fine scale permeability structure and develop geologically reasonable effective properties permeability and capillary pressure data were combined for geologically meaningful groups of laminae to define dynamic pseudo properties absolute permeability relative permeability and capillary pressure by numerical simulation a geological model for packages of lamination was then used to combine these lamina groups at a scale of the grid block the flow performance of this geologically reasonable grid block containing the laminated structure is compared with that of simple models based on conventional procedure i e the arithmetic average in a horizontal direction or harmonic average vertical permeability and rock curves at this scale the formation is highly anisotropic using the more detailed model with quite different flow characteristics in the horizontal and vertical directions the simple homogeneous models show more isotropic recovery characteristics and give significantly higher recoveries for vertical displacements from these findings it is suggested that previous models of the rannoch formation should be modified to include the capillary effects associated with lamination the procedure outlined here is referred to as the geopseudo approach the approach is widely applicable to laminated reservoirs

time intervals between characteristic earthquakes and correlations with smaller events an analysis based on a mechanical model of a fault certain correlations between characteristic events and smaller events are studied in the context of a spatially homogeneous mechanical model for a fault the model satisfies a one dimensional quasi linear wave equation in which the key nonlinearity is associated with the slipstick friction force at the interface between tectonic plates we find that the model gives rise to slipping events of all sizes the smaller events are consistent with the gutenberg richter statistical law whereas the characteristic events have a separate statistical distribution the crossover from smaller events to characteristic events coincides with a correlation length which sets the scale for clustering of smaller events before the characteristic rupture numerical results and scaling arguments relate the features of the distribution of characteristic events to the corresponding features of the distribution of smaller events as well as the length of the fault the distribution of time intervals between characteristic events has the distinguishing features of asymmetry and a finite upper cutoff the mean t and standard deviation of the time interval distribution are found to be related to features of the magnitude versus frequency distribution of smaller events and the coefficient of variation t is independent of the driving rate because this analysis relates properties of characteristic earthquakes to statistical features of smaller more numerous events as well as geometric features of the fault it may ultimately be useful for long term earthquake prediction

thick chromitite of the jacurici complex ne craton sao francisco brazil cumulate chromite slurry in a conduit abstract the jacurici complex located in the ne part of the sao francisco craton hosts the largest chromite deposit in brazil the mineralized intrusion is considered to be a single n s elongated layered body disrupted into many segments by subsequent deformation the ore is hosted in a thick massive layer two segments ipueira and medrado have been previously studied we provide new geological information and chromite composition results from the monte alegre sul and varzea do macaco segments located farther north and integrate these with previous results the aim of this study is to determine and discuss the magma chamber process that could explain the formation of the thick chromitite layer all segments exhibit similar stratigraphic successions with an ultramafic zone 250 m thick hosting a 5 8 m thick main chromitite layer mcl and a mafic zone 40 m thick the chromite composition of the mcl mg numbers 0 48 0 72 and cr numbers 0 59 0 68 is similar to chromites from layered intrusions and other thick chromitites previous work concluded that the parental magma of the mineralized intrusion was very primitive based on olivine composition up to fo 93 and orthopyroxene composition up to en 94 from harzburgite samples and that it originated from an old subcontinental lithospheric mantle we estimate that the melt from which the massive chromitite layer crystallized was similar to a boninite or low siliceous high mg basalt with a higher fe mg ratio the petrologic evidence from the mafic ultramafic rocks suggests that a high volume of magma flowed through the sill which acted as a dynamic conduit crustal contamination has previously been considered as the trigger for the chromite crystallization based on isotope studies as the more radiogenic signatures correlate with an increase in the volumetric percentage of amphibole up to 20 the abundant inclusions of hydrous silicate phases in the chromites from the massive ore suggest that the magma was hydrated during chromite crystallization fluids may have played an important role in the chromite formation and or accumulation however the trigger for chromite crystallization remains debatable the anomalous thickness of the chromitite is a difficult feature to explain we suggest a combined model where chromite crystallized along the margins of the magma conduit producing a semi consolidated chromite slurry that slumped through the conduit forming a thick chromitite layer in the magma chamber where layered ultramafic rocks were previously formed subsequently the conduit was obstructed and the resident magma fractionated to produce a more evolved composition

a transient flow syringe air permeameter in response to the need to better describe spatial variations in permeability we designed and built a new portable field air permeameter for use on rocks in outcrop and core in this instrument a chamber containing a small volume of air in contact with the rockissuddenly increased involumecreating a vacuum causing air to flow from the rock into the chamber the instantaneous chamber volume and the air pressure within it were monitored we evaluated a theory to allow interpretation of these data for the rock permeability the theory required knowledge of some difficult to determine geometric constants this difficulty was circumvented in practice through a calibration procedure we described the theory for the design and construction of this instrument and reviewed some successful uses in field scale studies of rock permeability heterogeneity

fossilized worm burrows influence the resource quality of porous media burrow associated selective dolomitization in the yeoman formation limestone ordovician williston basin is characterized by distinct textural heterogeneity physical parameters such as permeability porosity tortuosity and dispersivity are therefore difficult to assess this study compares the relative dispersivities of three geologic media homogeneous sandstone fractured limestone and burrowed dolomitic limestone results show that the flow paths present in burrow associated dolomite are tortuous and that the interaction between the flow paths and the matrix is extensive such rocks act as dual permeability systems in the subsurface hydrocarbon production from such deposits will be strongly influenced by burrow related heterogeneity and its influence should be carefully considered before secondary recovery schemes are implemented

relating small scale sedimentary structures and permeability in a cross bedded aquifer summary the objective of this study is to investigate the relation between small scale sedimentary structures and permeability in the brussels sands formation an early middle eocene shallow marine sand deposit in central belgium that constitutes a major groundwater source in the region a field campaign was carried out consisting of field observations of the sedimentary structures and in situ measurements of air permeability the sedimentary structures were interpreted sketched digitally photographed and measured in a representative outcrop additionally a total of 2750 cm scale air permeability measurements were carried out in situ analysis of the spatial distribution of sedimentary structures and permeability shows that clay rich sedimentary features such as bottomsets and distinct mud drapes exhibit a different statistical and geostatistical permeability distribution compared to the other lithofacies in the cross bedded sands spatial analysis of the air permeability data shows that permeability anisotropy in the cross bedded lithofacies is dominated by the foreset lamination orientation these results show that small scale sedimentary heterogeneity strongly influences the local spatial distribution of the hydraulic properties and results in permeability heterogeneity and stratification that would produce anisotropy in upscaled permeability values

repeat times of large earthquakes implications for earthquake mechanics and long term prediction information on the time intervals between large earthquakes is now available for several fault segments along plate boundaries in japan alaska california cascadia and turkey when dates in a sequence are known historically as along much of the nankai trough they provide information on the natural intrinsic variability of the rupture process most sets of repeat times however are dominated by paleoseismic determinations of dates of older large earthquakes which contain measurement uncertainties in addition to intrinsic variability a bayesian technique along with prior information on measurement uncertainties is used to make maximum likelihood estimates of intrinsic repeat time and its normalized standard deviation the coefficient of variation cv it is these intrinsic parameters and their uncertainties that are most useful for understanding the mechanics of earthquakes and for prediction for timescales of a few decades our estimates of intrinsic cv are small 0 to 0 25 for several very active fault segments where deformation is relatively simple large events do not appear to be missing in historic and paleoseismic records and data are available at or near major asperities and away from the ends of rupture zones cv is larger for regions of multibranched faulting overlapping slip near the ends of rupture zones and for data from uplifted terraces at subduction zones a poisson process is an inferior characterization of all of the 11 segments we examined scenarios used by recent working groups that assume either poissonian behavior or renewal processes with cv of 0 5 0 2 for the most active fault segments in the san francisco bay area likely lead to incorrect 30 year probability estimates the hayward fault and perhaps the peninsular segment of the san andreas fault in the san francisco bay area appear to be advanced in their buildup of stress that will be released in future large earthquakes multibranched faulting may account for why the predicted tokai earthquake in japan has not occurred as of 2006 parkfield earthquakes from 1857 to 2004 were characterized by the largest uncertainty of the sequences we studied cv 0 37 which may account for the failure of past predictions the large cv for parkfield fits our hypothesis that relatively weak fault segments are characterized by more irregular earthquake recurrence paleoseismic data from coastal sites along the cascadia subduction zone are characterized by cvs of about 0 3

frequency moment distribution of earthquakes in the aleutian arc a test of the characteristic earthquake model the frequency moment distribution of earthquakes is derived for discrete regions of the entire aleutian arc both for segments which have ruptured in historic times and those regions which have been identified as seismic gaps this distribution is then compared to the moment of the largest event in the respective region or for gaps the largest event which is likely to occur based on a plate kinematic model of accumulated moment data for this study are based upon three data sources instrumental data are utilized for the 18 yr subsequent to the establishment of the world wide standardized seismograph network wwssn data for events of magnitude 7 and greater have been carefully collected and are presumed to be complete for the time period 1899 to present the last data set is comprised of the largest event in each region either an actual earthquake or the moment estimate based on current plate slip rates data were normalized to the recurrence time of the largest event in the respective zone in all cases the data seriously underestimate the largest moment event indicating that a characteristic earthquake model is appropriate for individual fault zones however if the data are combined for the entire arc the largest event is closely predicted this indicates that a b value model may be utilized only for regions in which there is a distribution in the size of earthquake producing faults

transport of metals and sulphur in magmas by flotation of sulphide melt on vapour bubbles emissions of sulphur 1 2 and transition metals 3 4 from magmas in the shallow crust have global impacts on human society by affecting climate after volcanic eruptions 5 6 and by forming giant copper and gold ore deposits which are massive anomalies of sulphur as well as of the ore metals themselves 3 4 7 8 these emissions vastly exceed the amounts that could have been derived from the associated magmas themselves 2 intrusion of denser hotter vapour and sulphide saturated magma beneath the cooler and more buoyant bodies that are finally erupted is the most probable source of the excess sulphur and metals in eruptive or hydrothermal systems 1 4 9 10 however the mechanism for the selective upward transfer of sulphur and metals remains poorly understood because their main carrier phase sulphide melt is dense and is generally assumed to settle to the bottoms of magma bodies here we show by experimental observation and by gas speciation and mass balance models that such transfers may be achieved by the flotation of compound drops 11 comprising droplets of sulphide melt attached to vapour bubbles by proposing and demonstrating the feasibility of a novel physical mechanism for upward mobility of sulphide liquids our work provides a mechanism for massive atmospheric releases of sulphur and contradicts widely held assumptions that dense sulphide liquids rich in sulphur copper and gold will remain sequestered in the crust

frictional weakening and slip complexity in earthquake faults previous work has shown that velocity weakening friction produces slip complexity in simple dynamical models of earthquake faults carlson and langer 1989 here i show that a different type of dynamical instability caused by slip weakening friction also produces slip complexity the deterministically chaotic slip complexity produced by slip weakening friction in a simple one dimensional model is studied and the scaling of the distribution of sizes of events with the parameters in the model examined in addition a possible physical origin of frictional weakening is examined through a very simplified mathematical representation of a physical process proposed by sibson 1973 whereby frictional heating causes an increase in pore fluid temperature and pressure thereby reducing the effective normal stress and friction the two different types of frictional weakening are derived from two opposing limits with slip weakening occurring when the dissipation of heating is slow compared to the rupture timescale as lachenbruch 1980 has shown while velocity weakening is shown to occur when the dissipation is fast compared to the rupture timescale since both end member cases of frictional weakening are seen to produce slip complexity slip complexity is argued to be a generic feature of frictional weakening and elastodynamics on a fault

hierarchical geocellular facies modelling of a turbidite reservoir analogue from the eocene of the ainsa basin ne spain geocellular models of outcrop analogues can provide useful insights into modelling strategies for the subsurface reservoirs a model of the outcropping ainsa slope turbidite system was built to test the effects of different scales of heterogeneity on production in deep water systems the modelling began with a structural reconstruction based primarily upon the outcrop data to remove the post depositional structural relief of the reservoir analogue the facies modelling workflow consisted of three nested stages each corresponding to a different scale a the first scale addressed the deterministic reconstruction of surfaces that bound sedimentary bodies b the second scale related to the modelling of the interfingering at the gradational boundaries of the sedimentary bodies c the third scale reproduced the internal heterogeneity within the sandstone dominated sedimentary bodies flow simulation was used as a dynamic test of the differences in recovery efficiency for the different scales at which the geological heterogeneity was modelled

pervasive assimilation of carbonate and silicate rocks in the hortavaer igneous complex north central norway the intrusive complex at hortav r represents a magma transfer zone in which multiple pulses of gabbroic and dioritic magmas evolved along fe and alkali enrichment trends extreme alkali enrichment resulted in nepheline normative and sparse nepheline bearing monzodioritic and monzonitic rocks more evolved monzonitic and syenitic rocks are silica saturated and in some cases quartz bearing previous and current research recognized an abundance of clinopyroxene and other ca rich phases such as scapolite grossular rich garnet and igneous textured calcite among the mafic and intermediate rocks even the most pyroxene rich samples contain low sc concentrations which suggests early intense fractionation of clinopyroxene these features and the alkali enrichment are consistent with assimilation of carbonate rich host rocks carbon isotope ratios of the igneous textured calcite indicate an origin of the carbon from host rocks rich in calcite consistent with assimilation however low end values 3 4 to 10 2 and moderate initial 87 sr 86 sr values 0 7052 to 0 7099 indicate the need for assimilation of quartzofeldspathic rocks as well models of combined assimilation and fractional crystallization indicate that assimilation of simple end members either carbonate or silicate cannot explain the entire data set instead variable proportions of carbonate and silicate materials were assimilated with the most pronounced assimilation effects in the mafic rocks the reasons for variable degrees of assimilation are as yet uncertain it is possible that assimilation of calc silicate rocks with variable carbonate silicate proportions resulted in the range of observed compositions however the importance of carbonate assimilation in mafic rocks compared to felsic ones suggests that assimilation of carbonates was predominant at high temperature and or mafic magma compositions and assimilation of silicates was predominant at lower temperature and or felsic magma compositions we suggest that the ability of the mafic magma to dissolve higher proportions of carbonate contaminants is the result of the magma s ability to form clinopyroxene as a product of assimilation in any case extensive carbonate assimilation was possible because co2 escaped from the system d 2004 elsevier b v all rights reserved

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

three dimensional modeling of a shoreface shelf parasequence reservoir analog part 2 geologic controls on fluid flow and hydrocarbon recovery wave dominated shoreface shelf parasequences are commonly modeled as simple layer cake reservoirs yet analysis of modern and ancient analogs demonstrates that these intervals contain a more complex physical stratigraphy we investigate the impact of depositional and diagenetic heterogeneity associated with gently dipping clinoform surfaces on fluid flow and recovery during water flooding using a three dimensional model reconstructed from a well exposed outcrop analog we demonstrate that the volume of oil in place is affected by variations in facies thickness associated with interfingering along clinoforms whereas waterflood sweep efficiency is affected by barriers to flow along clinoform surfaces such as calcite cemented layers mudstones and siltstones sweep efficiency is low when water flooding is down depositional dip because oil is bypassed at the toe of each clinothem as water flows preferentially through high quality sandstone facies in the upper part of the parasequence sweep efficiency is higher when water flooding is up depositional dip because the gravity driven downward flow of water sweeps poorer quality sandstone facies in the lower part of the parasequence in both cases injectors may offer limited pressure support to producers water flooding along depositional strike yields good pressure support but poor sweep because the gravity driven flow of water into the lower part of the parasequence is significantly reduced this yields highly variable fluid saturations but a uniform pressure gradient which is consistent with pressure and fluid saturation data from the mature rannoch formation reservoir brent field united kingdom north sea simple layer cake models fail to capture the range of flow behaviors described above and overpredict recovery by up to 20 as a result

application of reservoir geology to enhanced oil recovery from upper devonian nisku reefs alberta canada the upper devonian west pembina reef trend of west central alberta contains recoverable reserves of over 79 106 m3 500 million bbl of oil and 1 4 1010 m3 500 billion ft3 of gas within approximately 50 pinnacle reefs in the nisku formation although the oil is saturated with gas at original reservoir pressure primary depletion would soon lower the reservoir pressure below the bubble point decreasing recovery thus pressure maintenance is applied early in the producing life of the pools through waterflood or miscible flood schemes selection of the appropriate enhanced recovery scheme depends upon the internal flow unit geometry of the reefs the bigoray nisku c pool and the pembina nisku l pool for end members of the reservoir spectrum they can be used as flow unit models in the geological input for reservoir simulation studies the bigoray nisku c pool is dominantly limestone the primary textures well preserved in this reef provide the key to interpreting the relict textures in fully dolomitized reefs a well displayed ecological zonation of rugose corals passes vertically upward into laminar stromatoporoids which in turn pass into solenoporacean algae the coral and stromatoporoid zones each contain two generic flow unit types depending upon the presence or absence of dolomitization the incipient reef facies and the algal cap with associated tidal flat sediments represent two additional flow units due to the presence of horizontal permeability barriers associated with the limestone lithology the pool is developed with a waterflood displacement scheme ultimate recovery is estimated to be on the or er of 0 55 106 m3 3 5 million bbl or 46 of original oil in place ooip the pembina nisku l pool is a completely dolomitized reef representative of the majority of the reefs in the nisku fairway in contrast to the bigoray nisku c pool the complete dolomitization reduces the number of generic reservoir flow units observed in the l pool reef from six to three due to the excellent reservoir quality and absence of horizontal permeability barriers it is being exploited by a vertical miscible flood the nisku l pool is one of the largest pinnacle reefs discovered in the nisku reef fairway and contains an estimated 5 106 m3 31 million bbl ooip ultimate recovery is estimated to be approximately 4 1 106 m3 25 8 million bbl or 82 of ooip

probe permeametry an overview and bibliography applications of probe permeameters in both laboratory and outcrop studies of permeability heterogeneity are common we present an overview of the current status of probe permeameter data acquisition analysis and application the multidisciplinary nature of probe permeameter studies has led to their publication in diverse scientific journals we present a bibliography that includes literature covering both the technology and the application of probe permeametry

outcrop analog for sandy braided stream reservoirs permeability patterns in the triassic hawkesbury sandstone sydney basin australia permeability and porosity measurements on 1228 core plugs taken from the triassic hawkesbury sandstone in the sydney basin australia were analyzed by both sedimentological and geostatistical approaches to assess horizontal and vertical permeability and porosity variations in deposits of braided river systems the samples are variably cemented with porosities ranging from 5 to 20 and permeabilities range from 0 1 to 1200 md the permeability of the hawkesbury sandstone does not show any obvious relationship to its porosity the permeability variations are closely related to sedimentary facies type while the porosity distribution primarily reflects postdepositional diagenesis of the sandstone statistical analysis of all plug data shows a strong azimuth directional permeability in the horizontal plane the horizontal semivariogram correlation length ranges for two adjacent orthogonal outcrops are 4 5 and 2 5 m respectively the permeability correlation length ranges in the vertical direction for the two outcrops are similar ranging from 1 3 to 1 6 m permeability is much more variable in the vertical direction than in the horizontal direction the high degree of spatial variability observed in the permeability data is not consistent with fractal models based on the gaussian normal probability distribution subdivision of the sandstone into genetically related sedimentary facies significantly improves the predictability of the permeability distribution alternatively the high degree of the permeability heterogeneity can be described using a new model based on the levy stable family of probability distributions

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses