impact of early dolomitization on multi scale petrophysical heterogeneities and fracture intensity of low porosity platform carbonates albian cenomanian southern apennines italy abstract petrographic petrophysical and fracture analyses were carried out on middle cretaceous platform carbonates of the southern apennines italy that represent an outcrop analogue of the val d agri and tempa rossa reservoirs of the basilicata region the studied outcrops which are made of interlayered limestones and dolomites of inner platform environment were selected to study the impact of dolomitization on reservoir properties and the control of dolomite texture on fracture development two types of dolomites both formed during very early diagenesis were found interlayered at a metre scale with micrite rich limestones mainly mudstones and wackestones dolomite a is fine to medium crystalline and makes non planar mosaics dolomite b is coarse crystalline and makes planar s and planar e mosaics the intercrystalline space of the planar e subtype of dolomite b is either open or filled by un replaced micrite or by late calcite or saddle dolomite cement dolomite a and dolomite b have similar average porosities of 3 7 and 3 1 respectively which are significantly higher than the average porosity of limestones 1 4 their poro perm relationships are similar with the notable exception of planar e type b dolomites which generally display higher permeability values the intensity of top bounded fractures is distinctly lower in coarse crystalline dolomites than in fine crystalline dolomites and limestones both at the macro and the micro scale on the other hand neither lithology i e limestone vs dolomite nor dolomite crystal size control the intensity of perfect bed bounded fractures which is strictly controlled by the fracture layer thickness our results provide information that could be used as guidance for the characterization and modelling of fractured carbonate reservoirs made of interlayered limestones and dolomites

practical approaches to identifying sealed and open fractures for one essential ingredient of permeable fracture networks degree of fracture pore space preservation in large fractures i show how the characterization challenge presented by sparse fracture sampling can be overcome by measuring a surrogate the abundance of rock mass cement that precipitated after fractures ceased opening sampling limitations are overcome because the surrogate is readily measured in small rock samples including sidewall cores and cuttings permitting site specific diagnosis of the capacity of fractures to transmit fluid over a wider range of sample depths than conventional methods allow a diverse core database shows that this surrogate correctly predicts where large fractures are sealed information on timing of fracture opening relative to cement sequence can be obtained in two ways first evidence of fracture movement history and cement sequences in sparse large fractures can be extrapolated to areas having only cement data alternately evidence of fracture timing can be acquired from sealed micrometer scale fractures distribution of porosity reducing cement is commonly heterogeneous from bed to bed and location to location in siliciclastic and carbonate rocks however because patterns of sealed or open fractures cannot be delineated using fracture observations alone surrogates have practical value for production fairway mapping and other applications in which identifying open fractures is essential this study highlights the vital interplay among structural and diagenetic processes for fracture porosity preservation or destruction

scaling and connectivity of joint systems in sandstones from western norway abstract the scaling properties of a joint system in devonian sandstones in western norway have been investigated using seven maps covering areas from 18 to 720 m across which were generated by mapping in the field and from low level aerial photography taken from different heights each map represent a scale window on the fracture population bounded by resolution at small scales and the sample size at large scales a power law relationship between fracture trace length and critical observation height maximum height at which a trace can be identified is derived and used to create a statistical model for the resolution effects the model indicates that a continuous smooth curve without a straight line segment on a log log cumulative frequency distribution plot does not necessarily rule out a power law as the underlying population distribution together the maps indicate a powerlaw trace length distribution with an exponent of 2 1 this power law may be valid over four or more orders of magnitude with natural lower cut off of around 1 m the exponent is significantly different from 2 0 strictly selfsimilar case and is reflected in a decrease in the abundance of fractures with length comparable to map size as map scale decreases the fracture trace length distribution results in a decrease in apparent connectivity with decreasing scale high resolution large scale maps are well connected while the lowest resolution map smallest scale is unconnected fractures in the smallest scale map are however connected by small fractures below the limit of resolution represented by the largest scale map the variation in apparent connectivity with scale has implications for fluid flow when fractures are open to fluid flow the scaling properties of apparent connectivity imply that beyond a certain scale the size of fracture controlling flow will be scale independent in this fracture system this appears to occur in sample areas of around 300 m across

extent of power law scaling for natural fractures in rock new data sets from natural faults and extension fractures exhibit simple power law scaling across 3 4 4 9 orders of magnitude regardless of rock type or movement mode the data show no evidence of natural gaps or scaling changes each data set consists of independent measurements made at different observational scales a power law regression to the subset of smaller fractures in each case provides an extrapolation that accurately predicts associated larger fractures consequently data representing a limited range of fracture sizes may be used to characterize a much broader spectrum of fracture sizes

ice shelf water overflow and bottom water formation in the southern weddell sea 1 cold shelf waters flowing out of the filchner depression in the southern weddell sea make a significant contribution to the production of weddell sea bottom water wsbw a precursor to antarctic bottom water aabw we use all available current meter records from the region to calculate the flux of cold water 1 9 c over the sill at the northern end of the filchner depression 1 6 0 5 sv and to determine its fate the estimated fluxes and mixing rates imply a rate of wsbw formation referenced to 0 8 c of 4 3 1 4 sv we identify three pathways for the cold shelf waters to enter the deep weddell sea circulation one path involves flow constrained to follow the shelf break the other two paths are down the continental slope resulting from the cold dense water being steered northward by prominent ridges that cross the continental slope near 36 w and 37 w mooring data indicate that the deep plumes can retain their core characteristics to depths greater than 2000 m probably aided by thermobaricity the plume water at this depth can flow at a speed approaching 1 m s 1 implying that the flow is occasionally supercritical we postulate that such supercriticality acts to limit mixing between the plume and its environment the transition from supercritical to slower more uniform flow is associated with very efficient mixing probably as a result of hydraulic jumps

integrating borehole image logs with core a method to enhance subsurface fracture characterization image logs provide critical data for characterizing fractured reservoirs however the nature of such logs coupled with borehole modified stresses creates challenges that may lead to erroneous fracture interpretations we developed a methodology to integrate image logs core and geomechanical models to obtain high confidence fracture data sets derived from image logs the methodology starts with a detailed comparison of overlapping image logs and core first to properly shift them into alignment core to log shift and second to determine which features interpreted as fractures on image logs picks can be validated as fractures in core then we derive guidelines for interpreting image logs without core control to reduce the likelihood of false fracture picks geomechanical modeling is used to assist with quality control and interpretation of image logs where drilling induced tensile fractures occur we use a fractured carbonate reservoir to demonstrate the application of this methodology comparison of image log fracture picks with core led to an average rejection rate of greater than 50 of fractures picked on the image logs these high rejection rates are mostly caused by overinterpretation of discontinuous sinusoids and misidentification of sedimentary features and induced fractures we derived rules for interpreting high quality fractures 1 only sinusoids with continuity greater than 50 should be interpreted and 2 sinusoids with dip angles less than 55 should be excluded from the interpretation application of this methodology has provided much higher quality fracture orientation and frequency data than that resulting from interpretation of borehole images alone the improved fracture data set provides a more reliable basis for testing hypotheses and deriving models of fracture distribution

statistics of microstructure patchiness in a stratified lake 1 statistics of microstructure patches in a sheared strongly stratified metalimnion of lake banyoles catalonia spain which occupied 40 of the total lake depth of 12 m are analyzed light winds 0 25 m the ratio ltp hp depends on the patch richardson and mixing reynolds numbers following the parameterization of lozovatsky and fernando 2002 analysis of the dynamics of mixing reveals that averaged vertical diffusivities ranged between 1 10 4 m2 s 1 and 5 10 5 m2 s 1 depending on the phase of the internal waves episodic wind gusts wind speed above 6 m s 1 transfer 1 6 of the wind energy to the metalimnion and 0 7 to the hypolimnion generating large microstructure patches with hp of several meters

numerical simulation of the filchner overflow the plume of ice shelf water isw flowing into the weddell sea over the filchner sill contributes to the formation of antarctic bottom water the filchner overflow is simulated using a hydrostatic primitive equation three dimensional ocean model with a 0 5 2 sv isw influx above the filchner sill the best fit to mooring temperature observations is found with influxes of 0 5 and 1 sv below a previous estimate of 1 6 0 5 sv based on sparse mooring velocities the plume first moves north over the continental shelf and then turns west along slope of the continental shelf break where it breaks up into subplumes and domes some of which then move downslope other subplumes run into the eastern submarine ridge and propagate along the ridge downslope in a chaotic manner the next western ridge is crossed by the plume through several paths despite a number of discrepancies with observational data the model reproduces many attributes of the flow in particular we argue that the temporal variability shown by the observations can largely be attributed to the unstable structure of the flow where the temperature fluctuations are determined by the motion of the domes past the moorings our sensitivity studies show that while thermobaricity plays a role its effect is small for the flows considered smoothing the ridges out demonstrate that their presence strongly affects the plume shape around the ridges an increase in the bottom drag or viscosity leads to slowing down and hence thickening and widening of the plume

a scale independent approach to fracture intensity and average spacing measurement fracture intensity the number of fractures per unit length along a sample line is an important attribute of fracture systems that can be problematic to establish in the subsurface lack of adequate constraints on fracture intensity may limit the economic exploitation of fractured reservoirs because intensity describes the abundance of fractures potentially available for fluid flow and the probability of encountering fractures in a borehole traditional methods of fracture intensity measurement are inadequate because they ignore the wide spectrum of fracture sizes found in many fracture systems and the consequent scale dependence of fracture intensity an alternative approach makes use of fracture size distributions which allow more meaningful comparisons between different locations and allow microfractures in subsurface samples to be used for fracture intensity measurement comparisons are more meaningful because sampling artifacts can be recognized and avoided and because common thresholds of fracture size can be enforced for counting in different locations additionally quantification of the fracture size distribution provides a mechanism for evaluation of uncertainties estimates of fracture intensity using this approach for two carbonate beds in the sierra madre oriental mexico illustrate how size cognizant measurements cast new light on widely accepted interpretation of geologic controls of fracture intensity

internal tides and baroclinicity in the southern weddell sea 1 model description near the continental shelf break in the southern weddell sea warm deep water and western shelf water meet mixing mechanisms such as internal tides have the potential to mix these water masses and form antarctic bottom water a modified version of the princeton ocean model was utilized to investigate the internal tidal fields generated by the interaction of the m 2 barotropic tide with topography for transects across the continental shelf and slope in the southern weddell sea internal tides were generated over the upper continental slope as predicted by linear internal wave theory although the essentially two dimensional domain resulted in differences between the model elevations and the observations exceeding the observational uncertainties the cross slope velocities agreed well with differences less than the uncertainties for 78 of the existing observations

dynamics of the wind driven sea level variation around antarctica coastal sea level variation around antarctica is characterized by a coherent circumpolarly in phase fluctuation correlated with the antarctic oscillation aao this study addresses the dynamics of the winddriven sea level variation around antarctica a realistic barotropic numerical model reproduced well the observed sea level around antarctica from numerical model experiments the authors demonstrate that the forcing responsible for the coastal sea level is the wind stress at the coastal boundary both the dominant coherent signal and westward propagating signals are identified in the model and these signals are trapped over the shelf and slope around antarctica as a mechanism of these trapped signals the authors consider analytical solutions of the oceanic response to alongshore wind stress over the shelf and slope in the circumpolar domain in these solutions besides the shelf wave mode a wavenumber zero mode appears and characterizes the coastal dynamics around antarctica at periods from 10 to 200 days the coherent sea level can be explained quantitatively by the solution of this wavenumber zero mode with a 5 10 day damping time scale the spectral peaks of the westward propagating signals can be explained by the resonance of the shelf wave mode the wavenumber zero mode can respond to the wavenumber zero forcing at any frequency and the degree of response increases with decreasing frequency in addition the wavenumber zero component of wind stress corresponding to the aao variation is a dominant forcing therefore the coherent sea level variation around antarctica is preferably generated and becomes a dominant feature in the circumpolar domain particularly at lower frequencies

a permeability model for naturally fractured carbonate reservoirs abstract based on a detailed structural study performed on a reservoir surface analogue a fracture permeability model for carbonate reservoirs is proposed this involves four hierarchical systems each one assumed to convey fluids exclusively to that of immediately higher order new basic equations are then provided simulating hydrodynamic reservoir response the large scale first order structures consist of faults to which high permeability structures damage zones are associated at the meter scale stratabound joint systems together with bedding joints bounding the mechanical layers form a connected network transporting fluids to the fault system at a lower scale down to crystal size non stratabound joints constitute a pervasive and capillary fracture network which conveys fluids within the rock mass the non fractured host rock constitutes the lower permeability system at the crystal scale as no published works include an effective integration of detailed structural and numerical models the present study aims at covering a significant gap in the literature also providing appropriate theoretical basis for subsequent studies on fracture network modeling and reservoir simulation besides its application in the field of reservoir development and management the illustrated model may also be useful in environmental studies involving ground fluids such as e g in the field of special toxic nuclear waste management

baroclinic instability of density current along a sloping bottom and the associated transport process numerical experiments with a three dimensional nonhydrostatic ocean model have been carried out to investigate the dynamic processes of downslope density current and associated transport of dense water focusing on the effects of bottom slope for the range of slope inclination s from 0 to 0 04 bottom slope has two effects on baroclinic instability of the flow one is the stabilizing effect due to the topographic effect and to offshoreward increase in water depth the other is the destabilizing effect due to steepened inclination of isopycnal surface which increases available potential energy since the destabilizing effect over the upper slope overcomes the stabilizing one in the early stage unstable waves develop more rapidly with increasing s nonlinear energy conversion due to reynolds stresses also affects the flow stability and then the growth rate of instability increases linearly with s in this stage after the unstable waves have finite amplitudes in the mature stage the stabilizing effect of bottom slope becomes dominant especially over the lower slope thus vigorous unstable eddies are confined to the upper slope region in steep slope cases while they cover the whole slope region in gentle slope cases and this difference in the eddy field causes a drastic change in downslope transport of dense water in gentle slope cases efficiency of the transport becomes higher with increasing s since the destabilizing effect overcomes the stabilizing one as a whole on the contrary the efficiency becomes lower with increasing s in steep slope cases since the stabilizing effect surpasses the destabilizing one as a result the most effective transport is realized in the case with s 0 005 although the transport due to eddies in steep slope cases becomes weak over the lower slope it is still larger than that due to the bottom ekman current since the eddy transport occurs in the bottom layer with a thickness of 100 200 m which is 4 8 times the ekman layer thickness

outcrops as guides to subsurface natural fractures example from the nikanassin formation tight gas sandstone grande cache alberta foothills canada abstract outcrop studies are essential for understanding subsurface fractures because some key attributes such as length and height distribution cannot be measured using wellbores discrepancies between outcrop and subsurface fracture attributes arise because loading paths due to burial exhumation and thermal paths differ consequently even where rock type and structural setting are close matches differences may exist in mechanical properties and fracture growth here we investigate petrologic criteria and disseminated arrays of quartz filled microfractures as tools for comparing and contrasting fractures in outcrop with those as sampled by core for tight gas sandstones fracture cements are key diagnostic criteria for proving correspondence between outcrop and core we offer guidelines on selecting suitable outcrop analogs that differ markedly from current practice our illustration uses outcrops and core samples of late jurassic to early cretaceous nikanassin formation a tight gas sandstone but aspects of the outcrop vetting procedure can be applied to other rock types in some core and outcrops nikanassin formation fractures contain quartz and ankerite cement assemblages crack seal and rind textures and aperture size distributions that show that the outcrop and subsurface localities we examined share the same fracture stratigraphy and similar diagenesis these outcrops can thus be used to measure attributes including those that are unlikely to be captured in core such as fracture height and length distributions relative and absolute fracture intensity with structural position and degree of fill by synkinematic cement in our example absolute fracture intensity and distribution of postkinematic carbonate cements are site specific and differ between outcrop and core but in ways that can be quantified

modeling tidal currents beneath filchner ronne ice shelf and on the adjacent continental shelf their effect on mixing and transport a depth averaged tidal model has been applied to the southern weddell sea the model domain covers the southern continental shelf including the ocean cavity beneath filchner ronne ice shelf reasonable agreement with the available current meter data has been achieved our results confirm that in areas with shallow water and large topographic gradients tidal oscillations with peak velocities up to 1 m s 1 play a significant role in the vertical mixing and transport of water masses the estimated energy dissipation beneath filchner ronne ice shelf due to surface friction is 25 gw approximately 1 of the world s total tidal dissipation tidally induced lagrangian residual currents converging at the ice front an area of strong mixing draw together water masses from the continental shelf and sub ice shelf cavity the model indicates that lagrangian residual currents have fluxes of up to 250 000 m3 s 1 and speeds of over 5 cm s 1 along the ice front with over 350 000 m3 s 1 being exchanged between the sub ice shelf cavity and adjacent continental shelf these currents are particularly efficient in ventilating the sub ice shelf cavity within 150 km of ronne ice front such strong tidal mixing will significantly modify the properties of water masses that flow through this region particularly to the west of berkner island the model predictions indicate that tidal processes strongly influence the oceanographic conditions in the vicinity of ronne ice front shipborne observations along the ice front support many of the model predictions concerning the effect of tides on the hydrography

scanning electron microscope cathodoluminescence analysis of quartz reveals complex growth histories in veins from the butte porphyry copper deposit montana scanning electron microscope cathodoluminescence sem cl analysis of quartz reveals textures that cannot be observed using optical microscopy or backscattered electrons these cryptic textures yield insight into timing and physical conditions of quartz growth especially in environments with multiple quartz precipitation events hydrothermal quartz from quartz sulfide veins in the porphyry copper deposit in butte montana was analyzed by sem cl revealing the following textures euhedral growth zones wide nonluminescing bands that cut across multiple quartz grains rounded luminescent quartz grain cores with euhedral overgrowths nonluminescing splatters of quartz connected by networks of cobweb like nonluminescing quartz in otherwise luminescent quartz concentric growth zones and wide nonluminescent grain boundaries these textures indicate that many veins have undergone fracturing dilation growth of quartz into fluid filled space quartz dissolution and recrystallization of quartz precipitation and dissolution textures indicate that early quartz molybdenite veins formed as a result of pressure fluctuations between lithostatic and hydrostatic at high temperatures and later pyrite quartz veins formed near hydrostatic pressure in response to temperature decrease through and beyond the field of retrograde quartz solubility

power law vein thickness distributions and positive feedback in vein growth the thicknesses of veins of a single generation from the kodiak accretionary complex show a power law distribution with an exponent of d 1 33 this value d 1 indicates that a significant portion of the vein related extension is accommodated by the thinnest veins thick vein segments are typically confined to southeast dipping brittle ductile shear zones textural observations of veins within these zones indicate a progressive evolution from crack seal to euhedral growth of quartz with an increase over time in the duration that fractures remained open model simulations of vein thickness distributions show that a constant growth model results in a negative exponential distribution and that it is necessary to assume size proportional growth to produce a power law distribution textures and power law thickness distributions for the kodiak veins are thus consistent with an increasing time averaged growth rate as vein thickness increased comprising a positive feedback the positive feedback resulted in the progressive evolution from fractures to brittle ductile shear zones and ultimately to faults

regional fractures ii fracturing of mesaverde reservoirs in the piceance basin colorado 1 cretaceous strata in the east central piceance basin of northwestern colorado were subjected to several phases of stress due to horizontal tectonic compression burial and uplift these rocks contain a regional set of west northwest extension fractures observed in abundant vertical and deviated core from the u s department of energy s multiwell experiment and slant hole completion test wells time depth relationships fracture orientation and fluid inclusion analyses indicate that the fractures formed at about 36 40 ma during a phase of increased laramide west northwest compression and in a pressure temperature regime compatible with the geologically reconstructed maximum burial depths this regional fracturing is an example of load parallel extension fracturing and basinwide dilatancy at depth under conditions of high pore pressure and anisotropic tectonically created horizontal stress

fracture porosity creation and persistence in a basement involved laramide fold upper cretaceous frontier formation green river basin usa fracture hosted porosity and quartz distribution along with crack seal texture and fluid inclusion assemblage sequences in isolated bridging quartz deposits show that open fractures can persist through protracted burial and uplift in foreland basins fractures oriented at a high angle to current maximum compressive stress remain open and were weak mechanical discontinuities for millions of years even at great depth upper cretaceous frontier formation sandstones in the basement involved laramide table rock anticline eastern greater green river basin wyoming sampled by two horizontal wells cut parallel or nearly parallel to bedding and at a high angle to steeply dipping fractures have 41 5 m of rock in four cores at depths of 4538 4547 m cores intersect older e striking set 1 fractures are abutted by or locally cross cut by n striking set 2 fractures both sets contain quartz and porosity sequenced using quartz crack seal cement texture maps set 1 fluid inclusion assemblage fia trapping temperatures increase progressively from 140 to 165 c then decrease to c 150 c compatible with fracture opening over c 15 ma during rapid burial followed by uplift in eocene oligocene time set 2 opened at c 160 c probably near maximum burial after a period of quiescence set 2 reopened at c 5 ma at c 140 c on a cooling trajectory intermittent set 2 movement could reflect local basement involved fault movement followed after a pause by further set 2 reactivation in the modern stress field during uplift interpretations are sensitive to available burial thermal histories which have considerable uncertainty

rapid digital quantification of microfracture populations abstract populations of microfractures are a structural fabric in many rocks deformed at upper crustal conditions in some cases these fractures are visible in transmitted light microscopy as fluid inclusion planes or cement filled microfractures but because sem based cathodoluminescence cl reveals more fractures and delineates their shapes sizes and crosscutting relations it is a more effective structural tool yet at magnifications of 150 300 at which many microfractures are visible sem cl detectors image only small sample areas 0 5 0 1 mm 2 relative to fracture population patterns the substantial effort required to image and measure centimeter size areas at high magnification has impeded quantitative study of microfractures we present a method for efficient collection of mosaics of high resolution cl imagery a preparation method that allows samples to be any size while retaining continuous imagery of rock no gaps and software that facilitates fracture mapping and data reduction although the method introduced here was developed for cl imagery it can be used with any other kind of images including mosaics from petrographic microscopes compared with manual measurements the new method increases several fold the number of microfractures imaged without a proportional increase in level of effort increases the accuracy and repeatability of fracture measurements and speeds quantification and display of fracture population attributes we illustrate the method on microfracture arrays in dolostone from ne mexico and sandstone from nw scotland we show that key aspects of microfracture population attributes are only fully manifest at scales larger than a single thin section

fracture abundance and patterns in the subandean fold and thrust belt devonian huamampampa formation petroleum reservoirs and outcrops argentina and bolivia abstract opening mode fractures in devonian sandstones in outcrop and in several fields in the subandean ranges of southern bolivia and northern argentina compose two fracture sets i and ii that strike west northwest and north northeast respectively abundant set i fractures are at a high angle to local andean structural trends and set ii is aligned with fold axes crosscutting relations and quartz textures in fractures suggest that although set i is locally older these sets may have opened partly contemporaneously sets comprise both macroscopic fractures and more abundant millimeter scale microfractures fractures with opening displacement of less than 0 1 mm are typically sealed with quartz but wider fractures are lined with quartz and contain connected fracture porosity microfractures are more abundant than macrofractures and size distributions can be interpreted to approximate power laws microfracture strain is an efficient method of quantifying fracture abundance both sets record small fracture strains of 0 00016 0 0083 in backlimbs set i strain is higher in brittle quartz rich sandstones set ii strain varies markedly and is generally high in hinges and steep forelimbs of asymmetric anticlines for individual samples set i ii strains in fold hinges and forelimbs are comparable consistent with concurrent set i ii growth open fractures that could augment permeability are present both on and off structure but microfractures document fracture abundance that varies with structural position and sandstone quartz content that could account for variations in production outcomes

diagenesis in porosity evolution of opening mode fractures middle triassic to lower jurassic la boca formation ne mexico abstract opening mode fractures joints in middle triassic to lower jurassic la boca formation sandstones northeastern mexico have patterns of fracture porosity mineral fill structures and size distributions not previously described from outcrop patterns match those found in cores from many basins we used aperture measurements along lines of observation scanlines fracture trace maps petrography high resolution scanning electron microscope sem based cathodoluminescence and fluid inclusions to characterize fracture populations open fractures are lined by quartz that precipitated while fractures were opening whereas sealed fractures additionally contain calcite deposited after fractures ceased opening large fractures and arrays of contemporaneous microfractures have consistent power law aperture size scaling over approximately three orders of magnitude our results imply that open fractures and fracture sizes depend on diagenetic state the interplay of fracture mechanics and diagenetic history is a determinant on effective porosity within fractures and thus open fracture persistence connectivity and fluid flow

low frequency currents and continental shelf waves in the southern weddell sea abstract the salient features of low frequency current fluctuations obtained from an analysis of eight current meter records from the continental shelf and slope of the southern weddell sea are compared to baroclinic and barotropic theories a simple baroclinic theory of internal waves is used successfully to predict high frequency spectral cutoff values from low frequency velocity ellipse calculations made from the continental slope mooring data the success of this theory indicates that the higher spectral energy levels observed over the slope compared to the shelf are probably due to baroclinic motions a barotropic model of free continental shelf waves probably by saint guily 1976 is adapted for the local topography and the predictions of the model compared to observations coherences and phases between moorings separated by 10 and 160 km in the along shelf direction provide substantial evidence of the existence of shelf waves as predicted by the theory for periods of 3 60 days and for the lowest

coherent sea level response to the antarctic oscillation 1 large scale sea level variations around antarctica were studied and their relationship with atmospheric variations was investigated tide gauge data at five coastal stations were used coherent sea level variations were clearly detected for all stations on intraseasonal time scales the coherent variations had significant negative correlations with an index of the atmospheric annular mode variation antarctic oscillation coherence was significant for periods from 10 to 100 days with negligible time lag the negative correlation is consistent with the mechanism that a high low westerly anomaly leads to a stronger weaker northward ekman drift and causes divergence convergence around antarctica

laurentian palaeostress trajectories and ephemeral fracture permeability cambrian eriboll formation sandstones west of the moine thrust zone nw scotland abstract cambrian eriboll formation sandstones of the ardvreck group that crop out west of the moine thrust zone contain joints and quartz filled or quartz lined fractures that resemble cemented joints of the fractures containing quartz five sets strike north nw to wnw ne west and north according to crosscutting relations this is a progression from the oldest to the youngest set sets include opening mode microfractures partly visible in transmitted light as fluid inclusion planes and sharply defined as microveins using sem based cathodoluminescence cl dating the oldest north striking set using inferred quartz accumulation rates fluid inclusions and burial history suggests that these fractures mark a palaeozoic east west least horizontal stress trajectory in laurentia the youngest two sets of porous fractures are associated with faults that cut and postdate the moine thrust zone data indicate that at depth in basins pervasive fracture systems arising from discrete loading events are ephemeral owing to fracture porosity destruction by cementation

observations of the antarctic slope undercurrent in the southeastern weddell sea 1 the antarctic slope front presents a dynamical barrier between the cold antarctic shelf waters in contact with ice shelves and the warmer subsurface waters offshore two hydrographic sections with full depth current measurements were undertaken in january and february 2009 across the slope and shelf in the southeastern weddell sea southwestward surface intensified currents of 30 cm s 1 and northeastward undercurrents of 6 9 cm s 1 were in thermal wind balance with the sloping isopycnals across the front which migrated offshore by 30 km in the time interval between the two sections a mid depth undercurrent on february 23 was associated with a 130 m uplift of the main pycnocline bringing warm deep water closer to the shelf break this vertical displacement comparable to that caused by seasonal variations in wind speed implies that undercurrents may affect the exchanges between coastal and deep waters near the antarctic continental margins

current variability under landfast sea ice in lutzow holm bay antarctica current measurements were made under landfast sea ice in lutzow holm bay antarctica in 1991 in spite of no direct wind forcing and negligible thermohaline forcing currents with a magnitude of 0 1 0 3 m s 1 were observed it is suggested that current variability strongly depends on the extent of the landfast ice on the eastward continental shelf before the fast ice edge extends to the shelf break the current at ongul strait located near the continental coast is highly coherent with the wind with a time lag of 1 1 5 days and somewhat coherent with the adjusted sea level with no time lag at periods of 6 8 days after the fast ice edge is stably located along the shelf break the current is not at all coherent with the wind or sea level we infer that the variability at periods of 6 8 days is mainly due to the first mode shelf wave excited by the wind stress over the eastward continental shelf the direction of the prevailing current at ongul strait changed drastically from southward the same direction of the wind to northward in may although the prevailing wind direction did not change this change in timing coincides with the rapid development period of the fast ice extent

a method to detect natural fracture strike in sandstones in siliciclastic hydrocarbon reservoir rocks economic gas and oil production may depend on the attributes of natural fractures and with the advent of horizontal drilling fractures are increasingly exploration and development targets yet reliable information on such key fracture attributes as orientation strike is sparse because few fractures intersect vertical well bores this paper describes how fracture strike can be documented on a bed by bed basis even in well bores where few or no visible fractures are directly sampled quartz lined opening mode microfractures lengths of microns to millimeters in quartz cemented sandstones commonly are not visible using standard petrographic methods but systematic mapping of these microfractures is possible using photomultiplier based electron beam induced luminescence scanned cathodoluminescence imaging as shown by observations primarily from three natural gas plays and one oil play in the united states microfracture strike is a good guide to the strike of large fractures macrofractures that formed concurrently because microfractures are widespread and small specimens can be used to get accurate fracture strike data this approach can be applied to samples obtained from wireline conveyed rotary drilled sidewall coring devices as well as to samples from full diameter core

provenance interpretation of quartz by scanning electron microscope cathodoluminescence fabric analysis we used a cathodoluminescence cl detector attached to a scanning electron microscope sem to study patterns of variable intensity cl in quartz grains from a variety of igneous metamorphic sedimentary and shock deformed meteorite impact rocks distinctive fabrics in quartz grains revealed by sem imaged differential cl include zoning healed fractures complex shears planar features shocked quartz dark cl streaks and patches indistinct mottled texture and nondifferential low contrast cl zoning is common in volcanic quartz and some plutonic quartz zoned plutonic quartz is distinguished from volcanic quartz by the presence of closed fractures and dark cl streaks and patches metamorphic quartz displays either an indistinct mottled texture or nearly uniform nondifferential cl quartz from rocks severely deformed by tectonism displays a complex pattern of multiple small scale shears quartz from meteorite impact sites and some system boundaries is characterized by intricate patterns of planar features presumably created by shock metamorphism thus the sem cl fabric analysis technique provides a rapid method for distinguishing quartz from a variety of source rocks

relationships between sem cathodoluminescence response and trace element composition of hydrothermal vein quartz laser ablation icp ms data of hydrothermal vein quartz with zonation in scanning electron microscope cathodoluminescence reveal two groups of trace elements one that co varies in concentration with luminosity and another that remains uniform throughout bright luminosity correlates with a high total abundance of trace elements in early quartz including al up to 410 g g ti up to 240 g g k up to 330 g g li up to 8 g g na up to 36 g g and fe up to 20 g g up to 20 times lower concentrations of these elements are associated with dark luminosity in late quartz concentrations of p 21 g g ga 0 3 g g ge 1 3 g g sn 1 5 g g cu 0 3 g g and ag 0 1 g g demonstrate no relationship with luminosity charge balance cannot be achieved for bright luminescent quartz unless the presence of up to 6 g g h not analyzed by la icpms or interstitial al3 is invoked interestingly the inferred 6 g g h remains constant for quartz containing more than 10 mol total trace elements and might represent the solubility of h in hydrothermal quartz at temperatures between 450 and 700 c la icpms results indicate that the fluid chemistry determines the quartz trace element pattern which may serve as a monitor for the chemical environment from which quartz crystallized ratios of na al and ti al are uniform whereas k al evolves toward lower values with decreasing luminosity combined evidence including quartz vein textures silica solubility data and estimates of the temperature of quartz growth suggest that early quartz crystallized fast but late quartz grew more slowly at 350 to 425 c it is speculated that the higher the quartz growth rate is the more trace elements are incorporated into quartz besides the temperature dependence known from the literature clearly growth rate temperature and fluid chemistry are important parameters affecting luminosity however their relative importance may vary from case to case

provenance analysis by single quartz grain sem cl optical microscopy abstract the integration of panchromatic scanning electron microscopy cathodoluminescence sem cl with optical microscopy analysis on single quartz grains is a new technique to interpret provenance of quartz rich sediments the combination of information gained from sem cl on textural features and cl response with information from optical microscopy allows distinction of different quartz types much more easily then with conventional microscopy or color cl analysis features visible by sem cl include zoning microcracks deformation fractures and lamellae diagenetic crushing recrystallization and strength and texture of cl response features visible by optical microscopy include embayments open fractures extinction behavior and mono poly and microcrystallinity the key is to integrate cl and optical properties of the same quartz grains in order to reduce ambiguities inherent in both techniques used on their own our technique is well suited for distinguishing between plutonic volcanic and metamorphic quartz in coarse to medium grained sand or sandstone this simple and inexpensive approach can easily be combined with other provenance techniques in this paper we outline the combined sem cl optical microscopy technique and demonstrate its application with examples from the south island of new zealand

effect of pore structure on the producibility of tight gas sandstones the estimated ultimate recovery eur is one of the most significant properties of tight gas sandstone reservoirs but it remains difficult to predict estimated ultimate recovery is dependent on the success of stimulation by hydraulic fracturing the existence and connectivity of natural fractures and as illustrated in this article the pore structure of the matrix here we analyze the lab measurements that are indicative of the pore structure and then we predict the effect of pore structure on producibility we develop a relationship between the eur of tight gas sandstones and their petrophysical properties measured by drainage and imbibition tests mercury intrusion withdrawal and porous plate and by resistivity analyses we use the ratio of residual mercury saturation after mercury withdrawal s gr to initial mercury saturation s gi which is the saturation at the start of withdrawal as a measure of gas likely to be trapped in the matrix during production and hence a proxy for eur a multitype pore space model is required to explain mercury intrusion capillary pressures in these rocks implications of this model are supported by other available laboratory measurements the model comprises a conventional network model and a treelike pore structure an acyclic network that mimic the intergranular and intragranular void spaces respectively the notion of the treelike pore structure is introduced here for the first time in the context of tight gas sandstones applying the multitype model to porous plate data we classify the pore spaces of rocks into intergranular dominant intermediate and intragranular dominant this pore space classification is topological and is not based on scale or size these classes have progressively less drainage and imbibition hysteresis which leads to the prediction that significantly more hydrocarbon is recoverable from intragranular porosity than intergranular porosity available field data production logs corroborate the higher producibility of intervals with intragranular porosity although the data are not sufficient to eliminate the possible contribution of other factors such as size and shape of the volume contacted by hydraulic fractures or the presence and attributes of natural fractures the superior recovery of hydrocarbon from intragranular dominant pore structures is despite its inferior initial production rate

how to overcome imaging problems associated with carbonate minerals on sem based cathodoluminescence systems on sem based cathodoluminescence systems scanned cl methods are needed to overcome image quality problems caused by persistent luminescence of carbonate minerals an effective solution to the persistence problem is to acquire images using only the shorter wavelengths most easily done by using a broadband short wavelength uv blue range filter the filter used provides 80 to 90 transmissivity in the range of 385 to 495 nm and some transmissivity as low as 350 nm the filter allows transmission of the relatively nonpersistent uv violet luminescence present in most carbonates in the range of 350 to 425 nm but blocks the common orange red wavelength luminescence found in carbonates the lack of imaging problems in subsequent images shows that persistent luminescence in carbonates is primarily in the orange red wavelengths cathodoluminescence images produced using a short wavelength filter are comparable in detail to those obtained from conventional light microscope based cathodoluminescence systems in almost all examples features visible in the orange red wavelengths show corresponding variations in luminescence in the shorter wavelengths

aperture size scaling variations in a low strain opening mode fracture set cozzette sandstone colorado the aperture size distribution of a set of opening mode fractures in the cozzette sandstone is complex and cannot be described uniquely by a single equation this study of horizontal core includes aperture size data of macrofractures measured using a hand lens and microfractures measured using scanning electron microscope based cathodoluminescence sem cl macrofractures are poorly cemented and total fracture strain is an order of magnitude lower than the smallest strain values of most previously described vein sets macrofractures in this set therefore resemble typical joints as opposed to veins in most respects although intragranular microfractures compose about 98 of the microfracture population orientation fracture fill and fracture trace characteristics suggest that only transgranular microfractures can be shown to be genetically related to macrofractures aperture size data below 0 012 mm and above 1 mm follow a log normal distribution intermediate size data though relatively sparse suggest a power law aperture size distribution these results support the notion that this fracture set is an intermediate case between typical veins and joints the results also suggest that processes leading to size distributions commonly associated with veins i e power laws may be present in joint sets but over a more limited range bound at small size scales by grain scale effects and above by mechanical layering effects

scaling relationships of joint and vein arrays from the burren co clare ireland abstract we present a study of the systematics of veins and joints in carboniferous limestones of the burren ireland scaling relationships were established for fracture arrays mapped from low elevation aerial photographs that image fractures on numerous limestone pavements for areas up to ca 1 km2 the veins and joints occur in the same sequence but have contrasting scaling properties the veins strike north south and cut many beds to form vertically persistent non stratabound arrays they are strongly clustered and have scale invariant geometric properties vein geometries suggest they grew sub critically under relatively high differential stresses during north south directed variscan compression the joints form stratabound arrays with regular spacings that scale with bed thickness they show greater strike variation than the veins and have lognormal length distributions the joints formed during uplift under low differential stress conditions the contrasting scaling properties of the joints and veins are attributed to different overburden stresses at the time of formation the veins formed at greater depths than the joints in conditions that favoured fracture propagation across mechanical discontinuities resulting in the development of non stratabound scaling properties

prediction of macrofracture properties using microfracture information mesaverde group sandstones san juan basin new mexico abstract a better understanding of relationships between microfractures and macrofractures is prerequisite to the extrapolation of observed microfracture properties as a basis for inferring properties of associated macrofractures the hypothesis that micro and macrofractures represent different size fractions of the same fracture sets and consequently have linked attributes was tested on sandstones of the mesaverde group san juan basin new mexico the use of a cathodoluminescence detector attached to a scanning electron microscope sem allowed observation of microfractures not readily visible using conventional microscopic techniques the orientations of microfractures reproduce the orientations recorded at the macroscopic scale from cores and outcrops fracture size distributions are best modeled using power laws and extrapolation of microfracture frequencies to the scale of macrofractures successfully predicts the macrofracture frequencies fracture size distributions change at the length scale of the thickness of the mechanical layer typically by increasing the exponent of their power law distribution this change is independent of censoring biases and reflects changes in sampling topology and or fracture growth mechanisms

quantifying vein attributes in massive mudstones triassic sw england implications for progressive evolution of opening mode fracture networks abstract attributes of gypsum vein arrays in the triassic massive mudstones of sw england were analyzed to infer the process of vein network evolution field data demonstrate that there is no clear correlation between vein orientation and thickness however sub horizontal veins have a higher average thickness than the inclined veins vein thickness distribution is best characterized by a log normal distribution the fractal dimensions d of vein networks analyzed range from 1 44 to 1 71 vein networks which contain abundant bridging veins between neighbouring sub horizontal veins to form an anastomosing pattern exhibit a higher d value the fraction of vein volume derived from image analysis ranges from 6 75 to 17 94 a positive correlation is revealed by plotting the fractal dimensions of the vein networks versus the number of vein intersection points suggesting that an anti clustering distribution of veins could contribute to more interactions and also intersections of the gypsum veins the vein attribute data presented indicate that mineralization in the massive mudstones is mainly localized to some of the largest veins as a result of mechanical interaction of neighbouring veins during vein propagation hindering the nucleation of new veins as a consequence accommodation of the bulk rock strain could be facilitated mainly by expansion of the large backbone veins which follow a positive feedback mechanism of growth this study reveals a strong link between the geometric parameters of opening mode fractures with the history of their formation propagation and intersection

strain accommodated by brittle failure in adjacent units of the monterey formation u s a scale effects and evidence for uniform displacement boundary conditions abstract extensional strain accommodated by brittle deformation was measured in adjacent mudstone and dolostone units at arroyo burro beach california the dolostone failed in effective tension whereas the mudstone failed in shear both in response to the same extensional tectonic event in the mudstone unit initial bed length and fault displacement methods document extensional strains of 6 6 0 3 and 6 0 0 3 respectively upon adjusting the displacement estimates according to theoretical fault displacement population analysis the corrected strain becomes 9 7 0 3 for the mudstone measurements of vein apertures in the dolostone document extensional strain that varies according to scale with outcrop vein scanlines indicating a strain of 3 4 0 1 and thin section scanlines yielding a strain of 5 8 0 2 applying theoretical fault displacement population analysis to vein apertures in dolostone shows that small veins below the detection limit of outcrop surveys contribute significantly to fracture related strain within the dolostone this difference in extensional strain may arise because veins measured in outcrop extend across the entire bed height and thus are controlled by the dolostone mechanical layer thickness whereas microscopic veins measured in thin section terminate without regard to a bounding layer the corrected dolostone strain becomes 10 2 1 0 matching the revised strain calculated in the adjacent mudstone unit and indicating uniform displacement boundary conditions for the two markedly different lithologies the 10 extensional strain at arroyo burro indicates significant strike parallel nw se extension accommodated by brittle failure during development of the western transverse ranges fold and thrust belt

formation and growth of extensional fracture sets naturally occurring extension fractures including some joints and veins form sets consisting of numerous subparallel fractures each with well defined ends in order to correctly interpret these fractures in the field it is necessary to determine which factors control the geometries of fracture sets this requires consideration of the conditions leading to both the initiation and the cessation of fracture propagation a fracture begins to propagate when the crack extension force reaches a critical value which is a property of the rock and of environmental conditions after propagation initiates the crack extension force varies owing to 1 increase in crack length 2 change in remote strain 3 elastic interaction with nearby cracks 4 change in internal fluid pressure and 5 stress relaxation due to growth of cracks in the surrounding rock each fracture continues to propagate as long as its crack extension force exceeds the critical value and terminates when this condition is no longer met neglecting fluid effects the final geometry of the fracture set is a function of the remote strain history and the initial crack geometry which can be characterized by the initial crack density for low initial densities large increases in crack length occur with little or no increase in remote strain however for large initial densities crack lengths increase only if the applied strain increases this analysis is used to estimate the variation in remote strain during formation of a fracture set in granitic rock from observations of the final fracture geometry

geometry formation and development of tectonic fractures on the reykjanes peninsula southwest iceland abstract a study was made of 903 fractures most of which are of postglacial age on the reykjanes peninsula some 120 fractures belonging to the vogar fissure swarm he completely within the same postglacial lava flow and were studied in great detail for the vogar fractures 75 are pure tension fractures and 25 are normal faults the average length of the vogar fractures is 611 m the average width is 0 6 m the average spacing measured perpendicular to the average strike n54e of fractures is 456 m the estimated average depth is of the order of several hundreds of meters and the average throw of the faults is 2 3 m all major fractures on the peninsula appear to have formed by coalescence of smaller fractures as a result of absolute tensile stress the fractures group into four fissure swarms each being a part of a volcanic system which it is proposed is fed by an elongated partially molten magma reservoir located at a depth of about 8 km beneath the peninsula it is suggested that a slight magmatic pressure increase in these reservoirs and resulting uplift and tensile stress at the surface is the main cause of postglacial fracture formation

marine controls on modern sedimentation on the antarctic continental shelf abstract the extreme depth and rugged topography of the antarctic continental shelf its seasonal to perennial sea ice cover and its large slowly calving ice shelves exert major influences on the adjacent ocean and sediments regionally variable exchanges of heat and momentum between the atmosphere and ocean result in net off shelf transport of sea ice and net downward salt flux this buoyancy partitioning drives a vertical convection that is followed by westward and northward flows of ventilated dense water across the continental shelves shelf waters meet mix with and modify circumpolar deep water at a dynamic slope front near the continental shelf break locally or periodically forming antarctic bottom water nutrient rich deep water upwells along this front regionally intruding onto the continental shelf in long thermally attenuated filaments these intrusions are frequently linked to mass balance processes at the base of ice tongues and ice shelves melting freezing and tidal currents beneath glacial ice are related to the larger scale circulation and to the fate of basal debris in the ice sheet the ocean circulation near ice fronts and decadal variability in ice front positions may account for several features of the sediment record time series 1 yr current measurements in the ross and weddell seas have revealed moderate to strong currents that are seasonally variable vertically coherent and dominated by the diurnal tide and longer term events few long term current measurements exist in the shelf and slope bottom boundary layers that are most important to sediment deposition and erosion icebergs may frequent certain routes e g in association with currents and fronts near the edge of the continental shelf and are capable of scouring and resuspending sediments down to shelf break depths sea ice plays a role in shelf sedimentation by rafting eolian and bottom debris away from the nearshore regions in addition generally offshore winds produce numerous leads and polynyas along the coastline and within the sea ice fields higher biological productivity in these polynyas may be linked to higher local sedimentation rates biogenic marine sediments apparently comprise a major percentage of the regionally variable modern shelf deposits despite the impressive glacial setting and polar environment

modeling ice shelf water overflow and bottom water formation in the southern weddell sea 1 descent of ice shelf water isw overflowed from the filchner depression in the southern weddell sea and consequent weddell sea bottom water wsbw production is investigated through high resolution numerical experiments the model reproduces the part of isw which interacts with the submarine ridge near 36 w the baroclinic instability wave developed near the exit of the filchner depression leads to intermittent outflow of isw and the intermittent passage of isw origin dense water induces 6 day period oscillation of velocity at the east side of the ridge which coincides well with mooring data the existence of the ridge induces two different mechanisms for cross isobath transport of the isw origin dense water a strong downslope current at the east side of the ridge and offshore transport by eddies which are generated when a gravity current makes a detour around the tip of the ridge over a wide region to the west of the ridge in a sensitivity experiment where thermobaricity is removed from the equation of state there is almost no transport of the isw origin water below 3000 m depth wsbw production rate as a result of descent of the isw origin dense water in the model domain is about 0 15 sv