characterization of the natural fracture system of the eagle ford formation val verde county texas the eagle ford formation has attracted considerable industry attention as a self sourced unconventional shale reservoir the productive interval in the eagle ford formation is the transgressive systems tract which contains parasequences whose lithologic content varies upward with increasing proportions of limestones optimum success in both exploration and production depends on the adequate characterization of fracture systems as a function of lithology the outcrops present along us highway 90 in val verde and terrell counties texas provide considerable insight into the regional natural fracture system of the eagle ford formation fracture orientation analysis reveals two sets of conjugate hybrid shear fractures and two sets of regional fractures abutting relationships suggest that hybrid shear fractures formed first followed by the thoroughgoing northeast striking fracture set and finally by a northwest striking set which tends to be confined to individual mechanical units the orientation of these fractures suggests that they formed during post laramide stress relaxation and progressive exhumation spacing frequency distribution analysis of the fracture population reveals a mature hypersaturated fracture system that likely formed at depth by overburden load and or fluid pressure near maximum burial our results indicate that the eagle ford formation displays a well developed fracture network regionally distributed in the val verde basin and likely present in the productive eagle ford play these observations provide evidence for pathways and vertical connectivity for potential fluid pathways throughout the eagle ford formation

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

connectivity permeability and channeling in randomly distributed and kinematically defined discrete fracture network models a major use of dfn models for industrial applications is to evaluate permeability and flow structure in hardrock aquifers from geological observations of fracture networks the relationship between the statistical fracture density distributions and permeability has been extensively studied but there has been little interest in the spatial structure of dfn models which is generally assumed to be spatially random i e poisson in this paper we compare the predictions of poisson dfns to new dfn models where fractures result from a growth process defined by simplified kinematic rules for nucleation growth and fracture arrest davy et al 2010 2013 this so called kinematic fracture model is characterized by a large proportion of t intersections and a smaller number of intersections per fracture several kinematic models were tested and compared with poisson dfn models with the same density length and orientation distributions connectivity permeability and flow distribution were calculated for 3d networks with a self similar power law fracture length distribution for the same statistical properties in orientation and density the permeability is systematically and significantly smaller by a factor of 1 5 to 10 for kinematic than for poisson models in both cases the permeability is well described by a linear relationship with the areal density p32 but the threshold of kinematic models is 50 larger than of poisson models flow channeling is also enhanced in kinematic dfn models this analysis demonstrates the importance of choosing an appropriate dfn organization for predicting flow properties from fracture network parameters

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

practicalities of extrapolating one dimensional fault and fracture size frequency distributions to higher dimensional samples previously published theory which extrapolates fault and fracture population statistics observed in a one dimensional sample to two and three dimensional populations is found to be of limited value in practical applications we demonstrate how significant the discrepancies may be and how they arise there are two main sources for the discrepancies 1 deviations from ideal spatial uniformity spatial poisson process of a fault or fracture pattern and 2 non power law scaling of the size frequency distributions of the population we show that even small fluctuations in spatial density combined with variance in the estimator of population statistics can lead to considerable deviations from the theoretical predictions ambiguity about power law scaling or otherwise of the underlying population is a typical characteristic of natural data sets and we demonstrate how this can affect the extrapolation of one dimensional data to higher dimensions in addition we present new theoretical approaches to the problem of extrapolation when clustering of faults and fractures is explicitly considered clustering is commonly observed in the field as en echelon arrays of fault or fracture segments and we show how this property of natural patterns can be quantified and included in the theory these results are relevant to building more realistic three dimensional models of the physical properties of fractured rocks such as fracture permeability and seismic anisotropy

correlation analysis of fracture arrangement in space abstract we present new techniques that overcome limitations of standard approaches to documenting spatial arrangement the new techniques directly quantify spatial arrangement by normalizing to expected values for randomly arranged fractures the techniques differ in terms of computational intensity robustness of results ability to detect anti correlation and use of fracture size data variation of spatial arrangement across a broad range of length scales facilitates distinguishing clustered and periodic arrangements opposite forms of organization from random arrangements moreover self organized arrangements can be distinguished from arrangements due to extrinsic organization traditional techniques for analysis of fracture spacing are hamstrung because they account neither for the sequence of fracture spacings nor for possible coordination between fracture size and position attributes accounted for by our methods all of the new techniques reveal fractal clustering in a test case of veins or cement filled opening mode fractures in pennsylvanian marble falls limestone the observed arrangement is readily distinguishable from random and periodic arrangements comparison of results that account for fracture size with results that ignore fracture size demonstrates that spatial arrangement is dominated by the sequence of fracture spacings rather than coordination of fracture size with position fracture size and position are not completely independent in this example however because large fractures are more clustered than small fractures both spatial and size organization of veins here probably emerged from fracture interaction during growth the new approaches described here along with freely available software to implement the techniques can be applied with effect to a wide range of structures or indeed many other phenomena such as drilling response where spatial heterogeneity is an issue

vein spacing in extending layered rock the effect of synkinematic cementation cemented fractures veins commonly show mm scale spacing even in layerbound arrangements in beds of cm to dm scale thickness the relief of tension around such layerbound fractures should preclude nearby fracture propagation and result in cm to dm scale fracture spacing we hypothesize that cement precipitated during vein opening could re establish tension across veins and lessen the effects of relieved tension thus decreasing fracture spacing we test this hypothesis using a computer based numerical model the model consists of a 2d triangular lattice of nodes connected by elastic springs the lattice is stretched by holding the left boundary stationary and moving the right boundary to the right at constant velocity the lattice consists of three layers springs within the middle fracturing layer fail upon stretching past a given critical length springs within the upper and lower matrix layers are indestructible we tune the model parameters to produce the familiar regular spacing of barren fractures joints then we perturb this system by adding cement within fractures as the fractures propagate and widen cementation is simulated by extending failed springs across the space between the nodes on which they are rooted and re attaching springs once they reach across re attached springs are assigned a new neutral length equal to their current length on re attachment the primary effect of cementation is to make fractures narrower and more closely spaced thus the model highlights the resistance to fracture widening by cement as a potential reason why veins can be more closely spaced than joints individual fractures open and seal multiple times when the stiffness of cemented springs is lower than that of the host rock springs suggesting that natural crack seal vein opening is associated with persistent mechanical weakness at extant fractures modeled veins are more irregularly spaced than modeled joints but the vein patterns remain more regularly spaced than would be expected for a random arrangement therefore the model does not explain systematic clustering of natural veins

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

a fractal relationship between vein thickness and gold grade in drill core from la codosera spain analysis of quartz vein thicknesses and gold grades in drill core from western spain indicates that both approximate a power law distribution suggesting an underlying fractal relationship log log plots of cumulative frequency n against vein thickness t show systematic changes in relation to gold grade the exponent d in a power law of the form n c t super d is lower for higher gold grades indicating that a greater proportion of the dilation is taking place on the thicker veins which are interpreted as providing interconnected conduits for gold bearing fluids thus monitoring the vein thickness distribution may provide a means of discriminating mineralized and barren structures in vein hosted mineral deposits

microstructural evolution in bitaxial crack seal veins a phase field study bitaxial crack sealing by epitaxial crystal growth is the most common vein forming process in earth s crust but the details of the microstructural processes in these are not well understood here we model the evolution of bitaxial crack seal quartz veins in two and three dimensions using the phase field method our numerical simulations show the influence of different parameters such as the obliquity of crack opening and crack location grain size and orientations on the evolving vein microstructure we examine the underlying growth competition observed during epitaxial growth of quartz results show many similarities with natural microstructures such as stretched crystals and compare well with the previous numerical findings as the ratio of crack aperture and matrix grain size for the present studies is chosen to be sufficiently large for growth competition to occur before complete sealing it leads to the formation of crystal fragments along the crack opening trajectory we explain how such fragment trails act as potential indicators of the opening of crack seal veins if they are confirmed to be common in natural microstructures finally we highlight the importance of accounting for the third dimension in the numerical simulations by analyzing the evolution of fluid connectivity in 2 d and 3 d during the sealing process

fracture aperture size frequency spatial distribution and growth processes in strata bounded and non strata bounded fractures cambrian meson group nw argentina abstract in cambrian meson group nw argentina small faults and three opening mode fracture sets defined by orientation and cement texture sets 1 3 formed sequentially in sandstone that most likely had constant mechanical properties throughout deformation yet the opening mode sets display contrasting fracture aperture size distributions spacing patterns and tendency to be bed bounded set 1 fractures are quartz filled or lined opening mode fractures with crack seal texture having a wide range of opening displacement kinematic aperture sizes they are irregularly spaced and non strata bounded fractures set 1 macro and microfracture opening displacement sizes are well described by a power law with slope 0 8 set 2 fractures are microscopic mostly quartz filled and have characteristic aperture sizes are probably not bed bounded and have either a near random or clustered spatial distribution set 3 fractures are quartz lined opening mode fractures with extensive open pore space having a narrow characteristic opening displacement size distribution they are regularly spaced and stratabounded differences between sets 1 and 3 can be accounted for by quartz deposition resisting fracture reopening to a greater extent for set 1 during repeated episodic growth where crack seal texture is present in fracture spanning quartz in contrast set 3 fractures are nearly barren with only trace cement deposits that did not resist opening power law opening displacement size distributions may be favored in cases where fracture growth is unequally partitioned amongst variably cemented fractures whereas a characteristic size is favored where growth is unaffected by cementation results imply that thermal history and diagenesis are important for fracture size distribution patterning

a 48 m y history of fracture opening temperature and fluid pressure cretaceous travis peak formation east texas basin quartz cement bridges across opening mode fractures of the cretaceous travis peak formation provide a textural and fluid inclusion record of incremental fracture opening during the burial evolution of this low porosity sandstone incremental crack seal fracture opening is inferred based on the banded structure of quartz cement bridges consisting of up to 700 cement bands averaging 5 m in thickness as observed with scanning electron microscope cathodoluminescence crack seal layers contain assemblages of aqueous two phase fluid inclusions based on fluid inclusion microthermometry and raman microprobe analyses we determined that these inclusions contain methane saturated brine trapped over temperatures ranging from 130 c to 154 c using textural crosscutting relations of quartz growth increments to infer the sequence of cement growth we reconstructed the fluid temperature and pore fluid pressure evolution during fracture opening in combination with published burial evolution models this reconstruction indicates that fracture opening started at ca 48 ma and above hydrostatic pore fluid pressure conditions and continued under steadily declining pore fluid pressure during partial exhumation until present times individual fractures opened over an 48 m y time span at rates of 16 23 m m y these rates suggest that fractures can remain hydraulically active over geologically long times in deep basinal settings

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

quantifying opening mode fracture spatial organization in horizontal wellbore image logs core and outcrop application to upper cretaceous frontier formation tight gas sandstones usa abstract the upper cretaceous frontier formation is a naturally fractured gas producing sandstone in wyoming regionally random and statistically more clustered than random patterns exist in the same upper to lower shoreface depositional facies east west and north south striking regional fractures sampled using image logs and cores from three horizontal wells exhibit clustered patterns whereas data collected from east west striking fractures in outcrop have patterns that are indistinguishable from random image log data analyzed with the correlation count method shows clusters 35 m wide and spaced 50 to 90 m apart as well as clusters up to 12 m wide with periodic inter cluster spacings a hierarchy of cluster sizes exists organization within clusters is likely fractal these rocks have markedly different structural and burial histories so regional differences in degree of clustering are unsurprising clustered patterns correspond to fractures having core quartz deposition contemporaneous with fracture opening circumstances that some models suggest might affect spacing patterns by interfering with fracture growth our results show that quantifying and identifying patterns as statistically more or less clustered than random delineates differences in fracture patterns that are not otherwise apparent but that may influence gas and water production and therefore may be economically important

spatial arrangement and size distribution of normal faults buckskin detachment upper plate western arizona abstract fault arrays typically include a wide range of fault sizes and those faults may be randomly located clustered together or regularly or periodically located in a rock volume here we investigate size distribution and spatial arrangement of normal faults using rigorous size scaling methods and normalized correlation count ncc outcrop data from miocene sedimentary rocks in the immediate upper plate of the regional buckskin detachment low angle normal fault have differing patterns of spatial arrangement as a function of displacement offset using lower size thresholds of 1 0 1 0 01 and 0 001 m displacements range over 5 orders of magnitude and have power law frequency distributions spanning four orders of magnitude from less than 0 001 m to more than 100 m with exponents of 0 6 and 0 9 the largest faults with 1 m displacement have a shallower size distribution slope and regular spacing of about 20 m in contrast smaller faults have steep size distribution slopes and irregular spacing with ncc plateau patterns indicating imposed clustering cluster widths are 15 m for the 0 1 m threshold 14 m for 0 01 m and 1 m for 0 001 m displacement threshold faults results demonstrate normalized correlation count effectively characterizes the spatial arrangement patterns of these faults our example from a high strain fault pattern above a detachment is compatible with size and spatial organization that was influenced primarily by boundary conditions such as fault shape mechanical unit thickness and internal stratigraphy on a range of scales rather than purely by interaction among faults during their propagation

measurement and characterisation of spatial distributions of fractures abstract a variety of line sample 1 d and map 2 d datasets of faults and joints has been used to investigate the spatial distributions of fractures and to test techniques of fractal analysis the natural fracture datasets have been supplemented by synthetic datasets with known characteristics the fault datasets investigated range in scale from regional to outcrop and the joint datasets are derived from outcrop 1 d datasets were analysed by the spacing population interval counting and fracture number interval counting techniques 2 d datasets were analysed by box counting and fracture number box counting techniques results indicate that fracture spacing can be characterised in line samples using either the 1 d interval counting technique or more simply by measuring the spacing population as the cumulative frequency distribution of spaces between adjacent fractures tectonic faults frequently show a power law spacing population indicating fractal dimensions of between 0 4 and 1 0 but except for outcrop data truncation effects degrade the analysis unrefined joint datasets commonly show negative exponential or lognormal spacing cumulative frequency distributions however single orientation joint sets are characterised by a regular spacing and are therefore non fractal for both fault and joint map data box counting techniques do not yield the power law relationship between box size and the number of boxes that is expected of fractal geometries it is concluded that 2 d box counting techniques are too insensitive to characterise the many different parameters of a fracture array the fracture density technique appears not to discriminate between very different fracture patterns and therefore does not provide useful results a fracture pattern incorporates several attributes e g size distribution orientation linkage density roughness each of which may scale independently and each of which may be fractal with its own fractal dimension full characterisation of fracture patterns will therefore require independent analysis of each of these attributes

from field data to fracture network modeling an example incorporating spatial structure this paper describes a technique for processing field data for a fracture network model which accounts for the observed spatial variability this has been done by generating a network subregion by subregion where the properties of each subregion are predicted through geostatistics once the geometry of a particular realization is specified flow through the network is studied we develop the method for a two dimensional analysis based on data from fanay augeres a uranium mine in france we plan to extend the analysis to three dimensions and compare the results with in situ test results in particular we have focused on the data collected in a long section of a drift where fractures have been mapped and steady state permeability tests have been performed in 10 boreholes in order to generate fractures in a statistically heterogeneous region we first divided the region into statistically homogeneous subregions in each subregion and for each fracture set we must specify the areal fracture density and the orientation length and aperture distributions we divided the fractures into five sets based on tectonic history and observed that for each set fractures spaced close together tended to have similar orientations this was built into the simulation an estimate of the aperture distribution for each set was made by assuming that the hydraulic apertures of the fractures intersecting a well test zone were proportional to the fracture opening observed in the core data input to the geostatistical analysis for each set consisted of 16 values of mean length and density of fractures in each 5 m by 2 m section of the drift wall the results of the simulation are tables of values for mean length and fracture density for each of the five sets the value of density simulated for each subregion was used directly to determine the number of fractures to be generated of that set in the subregion the value of mean length was used as the mean of the length distribution in the subregion the local coefficient of variation for length was the same in every subregion using the data described above we generated a 100 m by 100 m fracture network in a series of 100 statistically homogeneous subregions the fractured region generated has a total of 65 740 fractures a 70 m by 70 m region was isolated for directional permeability testing results show that the system is barely connected about 0 1 of the fractures essentially control permeability in the language of percolation theory this mesh is close to the critical density for which infinite clusters of fractures are found problems remaining include that 1 the analysis should be extended to three dimensions 2 the geostatistical simulation and network simulation should be combined rather than used as two separate operations and 3 the nature of flow through single fractures at fanay augeres should be investigated

mechanical stratigraphic controls on natural fracture spacing and penetration abstract fine grained low permeability sedimentary rocks such as shale and mudrock have drawn attention as unconventional hydrocarbon reservoirs fracturing both natural and induced is extremely important for increasing permeability in otherwise low permeability rock we analyze natural extension fracture networks within a complete measured outcrop section of the ernst member of the boquillas formation in big bend national park west texas results of bed center dip parallel scanline surveys demonstrate nearly identical fracture strikes and slight variation in dip between mudrock chalk and limestone beds fracture spacing tends to increase proportional to bed thickness in limestone and chalk beds however dramatic differences in fracture spacing are observed in mudrock a direct relationship is observed between fracture spacing thickness ratio and rock competence vertical fracture penetrations measured from the middle of chalk and limestone beds generally extend to and often beyond bed boundaries into the vertically adjacent mudrock beds in contrast fractures in the mudrock beds rarely penetrate beyond the bed boundaries into the adjacent carbonate beds consequently natural bed perpendicular fracture connectivity through the mechanically layered sequence generally is poor fracture connectivity strongly influences permeability architecture and fracture prediction should consider thin bed scale control on fracture heights and the strong lithologic control on fracture spacing

quantifying aperture spacing and fracture intensity in a carbonate reservoir analogue crato formation ne brazil abstract the characterization of fracture networks in carbonate reservoirs represents a primary approach to understand the processes of fluid flow the study of outcrop analogues has been used to enhance the understanding of the relative contribution of fracture networks to reservoir permeability the outcrop data can overcome scale limitations and or lack of consistency of subsurface datasets e g seismic well logs and cores this paper presents the fracture characterization of the aptian albian lacustrine laminites of crato formation araripe basin ne brazil which have been investigated as an analogue to some of the observed carbonate facies present in the pre salt reservoir sequence of the brazilian marginal basins in order to provide fracture attributes for discrete fracture network dfn modelling we investigate spacing and aperture distribution length and intensity of fractures based on the use of traditional scanline macro and microscale method the main fractures identified in the crato formation are shear and extensional fractures and with stylolites also present in this study we focused on vertical veins which strike in two principal directions nnw sse set 1 ne sw set 2 and are filled or partially filled by calcite or gypsum the relationship between length and aperture of vertical veins is linear apertures and spacings of fractures in both sets 1 and 2 follow power law and lognormal cumulative frequency distribution functions respectively fractures of set 2 are more likely to be clustered than are fractures of set 1 which have a wider kinematic aperture this structural database can be used to populate computational models that consider the widespread fracture system in the fluid flow simulation of carbonate reservoirs

natural fractures in shale a review and new observations natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low porosity and low permeability shale host rock many shale outcrops cores and image logs contain fractures or fracture traces and microseismic event patterns associated with hydraulic fracture stimulation have been ascribed to natural fracture reactivation here we review previous work and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization orientation and size patterns a wide range of shales have a common suite of types and configurations of fractures those at high angle to bedding faults bed parallel fractures early compacted fractures and fractures associated with concretions these fractures differ markedly in their prevalence and arrangement within each shale play however constituting different fracture stratigraphies differences that depend on interface and mechanical properties governed by depositional diagenetic and structural setting several mechanisms may act independently or in combination to cause fracture growth including differential compaction local and regional stress changes associated with tectonic events strain accommodation around large structures catagenesis and uplift fracture systems in shales are heterogeneous they can enhance or detract from producibility augment or reduce rock strength and the propensity to interact with hydraulic fracture stimulation burial history and fracture diagenesis influence fracture attributes and may provide more information for fracture prediction than is commonly appreciated the role of microfractures in production from shale is currently poorly understood yet potentially critical we identify a need for further work in this field and on the role of natural fractures generally

flexing is not stretching an analogue study of flexure induced fault populations abstract flexure induced fractures are predicted to form along the axis of maximum tensile stress within a bending brittle plate the mechanics of this process differ from extensional fault growth in response to lithosphere stretching where a distributed set of simultaneously growing fractures evolves through elastic interaction to simulate extensional fault growth during lithospheric flexure partially solidified plaster layers resting on a foam rubber substrate were depressed by a linear load and fractured in analogue models the length and spacing frequency distributions of the resulting crack populations were analyzed for a series of nine thin 5 mm and ten thick 15 mm layer experiments previous analogue stretching models predict power law length frequency distributions and clustered spacings c v 1 at low strains c v

fracpaq a matlab toolbox for the quantification of fracture patterns abstract the patterns of fractures in deformed rocks are rarely uniform or random fracture orientations sizes and spatial distributions often exhibit some kind of order in detail relationships may exist among the different fracture attributes e g small fractures dominated by one orientation larger fractures by another these relationships are important because the mechanical e g strength anisotropy and transport e g fluids heat properties of rock depend on these fracture attributes and patterns this paper describes fracpaq a new open source cross platform toolbox to quantify fracture patterns including distributions in fracture attributes and their spatial variation software has been developed to quantify fracture patterns from 2 d digital images such as thin section micrographs geological maps outcrop or aerial photographs or satellite images the toolbox comprises a suite of matlab scripts based on previously published quantitative methods for the analysis of fracture attributes orientations lengths intensity density and connectivity an estimate of permeability in 2 d is made using a parallel plate model the software provides an objective and consistent methodology for quantifying fracture patterns and their variations in 2 d across a wide range of length scales rock types and tectonic settings the implemented methods presented are inherently scale independent and a key task where applicable is analysing and integrating quantitative fracture pattern data from micro to macro scales the toolbox was developed in matlab and the source code is publicly available on github and the mathworks fileexchange the code runs on any computer with matlab installed including pcs with microsoft windows apple macs with mac os x and machines running different flavours of linux the application source code and sample input files are available in open repositories in the hope that other developers and researchers will optimise and extend the functionality for the benefit of the wider community

line sampling of fracture swarms and corridors abstract scanlines across a range of fracture networks are analysed using cumulative frequency plots spacing distribution and correlation integral plots synthetic samples from known distributions are used to distinguish uniform and random positioning of fractures from various types of clustering and the results used to interpret fracture corridors from the bergen area norway the kuiper test provides a means of establishing the statistical significance of clustering and the correlation integral allows assessment of clustering at different scales a simple workflow is proposed that uses quantitative measures for the definition and characterisation of the spatial organisation of fractures the form of fracture networks is shown to vary from uniform or random distributions through the development of simple corridors to clustering at various scales

algal uptake of u and some other metals implications for global geochemical cycling abstract freshwater green algae ankistrodesmus sp cultured in dilute solutions of metals chelated with edta revealed massive uptake of u examination of thames river waters and selected micro organisms of its aquatic ecosystem revealed an average dissolved u concentration of 1 46 ppb representing about twice the global mean riverine solute concentration of 0 6 ppb u suspended particulates principally algae contain u at 103 to 105 times that of their river habitat averaging 28 300 ppb u one community of the filamentous algae spirogyra sp contained intracellular crystals tentatively identified as the ca oxylate weddelite studies of algae from a variety of environments have shown large concentration factors for a wide range of elements algae from an acid u mine tailings site contain up to 2 7 u dry weight plus very high concentrations of other metals micro organisms may act as an important sink for metal solutes in the hydrosphere

fluid flow through carbonate platforms constraints from 234u 238u and cl in bahamas pore waters the geometry timing and rate of fluid flow through carbonate margins and platforms is not well constrained in this study we use u concentrations and isotope ratios measured on small volumes of pore water from bahamas slope sediment coupled with existing chlorinity data to place constraints on the fluid flow in this region and by implication other carbonate platforms these data also allow an assessment of the behaviour of u isotopes in an unusually well constrained water rock system we report pore water u concentrations which are controlled by dissolution of high u organic material at shallow depths in the sediment and by reduction of u to its insoluble 4c state at greater depths the dominant process influencing pore water 234 u 238 u is alpha recoil in holocene sediments the increase of pore water 234 u 238 u due to recoil provides an estimate of the horizontal flow rate of 11 cm year but with considerable uncertainty at depths in the sediment where conditions are reducing features in the u concentration and 234 u 238 u profiles are offset from one another which constrains the effective diffusivity for u in these sediments to be1 2 10 8 cm 2 s 1 at depths between the holocene and these reducing sediments pore water 234 u 238 u values are unusually low due to a recent increase in the dissolution rate of grain surfaces this suggests a strengthening of fluid flow probably due to the flooding of the banks at the last deglaciation and the re initiation of thermally driven venting of fluid on the bank top and accompanying recharge on the slopes interpretation of existing chlorinity data in the light of this change in flow rate constrain the recent horizontal flow rate to be 10 6 3 4 cm year estimates of flow rate from 234 u 238 u and cl are therefore in agreement and suggest flow rates close to those predicted by thermally driven models of fluid flow this agreement supports the idea that flow within the bahamas banks is mostly thermally driven and suggests that flow rates on the order of 10 cm year are typical for carbonate platforms where such flow occurs 1999 elsevier science b v all rights reserved

spatial arrangement of faults and opening mode fractures spatial arrangement is a fundamental characteristic of fracture arrays the pattern of fault and opening mode fracture positions in space defines structural heterogeneity and anisotropy in a rock volume governs how faults and fractures affect fluid flow and impacts our understanding of the initiation propagation and interactions during the formation of fracture patterns this special issue highlights recent progress with respect to characterizing and understanding the spatial arrangements of fault and fracture patterns providing examples over a wide range of scales and structural settings five papers describe new methods and improvements of existing techniques to quantify spatial arrangement one study unravels the time evolution of opening mode fracture spatial arrangement which are data needed to compare natural patterns with progressive fracture growth in kinematic and mechanical models three papers investigate the role of evolving diagenesis in localizing fractures by mechanical stratigraphy and nine discuss opening mode fracture spatial arrangement two papers show the relevance of complex cluster patterns to unconventional reservoirs through examples of fractures in tight gas sandstone horizontal wells and a study of fracture arrangement in shale four papers demonstrate the roles of folds in fracture localization and the development spatial patterns one paper models along fault friction and fluid pressure and their effects on fault related fracture arrangement contributions address deformation band patterns in carbonate rocks and fault size and arrangement above a detachment fault three papers describe fault and fracture arrangements in basement terrains and three document fracture patterns in shale this collection of papers points toward improvement in field methods continuing improvements in computer based data analysis and creation of synthetic fracture patterns and opportunities for further understanding fault and fracture attributes in the subsurface through coupled spatial size and pattern analysis

microfracture spacing distributions and the evolution of fracture patterns in sandstones abstract natural fracture patterns in sandstone were sampled using scanning electron microscope based cathodoluminescence sem cl imaging all fractures are opening mode and are fully or partially sealed by quartz cement most sampled fractures are too small to be height restricted by sedimentary layers at very low strains 100 datasets show spacings that are best fit by log normal size distributions compared to exponential power law or normal distributions the clustering of fractures suggests that the locations of natural factures are not determined by a random process to investigate natural fracture localization we reconstructed the opening history of a cluster of fractures within the huizachal group in northeastern mexico using fluid inclusions from synkinematic cements and thermal history constraints the largest fracture which is the only fracture in the cluster visible to the naked eye among 101 present opened relatively late in the sequence this result suggests that the growth of sets of fractures is a self organized process in which small initially isolated fractures grow and progressively interact with preferential growth of a subset of fractures developing at the expense of growth of the rest size dependent sealing of fractures within sets suggests that synkinematic cementation may contribute to fracture clustering

directional semivariogram analysis to identify and rank controls on the spatial variability of fracture networks abstract in this study the directional semivariogram is deployed to investigate the spatial variability of map scale fracture network attributes in the paradox basin utah the relative variability ratio r is introduced as the ratio of integrated anisotropic semivariogram models and r is shown to be an effective metric for quantifying the magnitude of spatial variability for any two azimuthal directions r is applied to a gis based data set comprising roughly 1200 fractures in an area which is bounded by a map scale anticline and a km scale normal fault this analysis reveals that proximity to the fault strongly influences the magnitude of spatial variability for both fracture intensity and intersection density within 1 2 km additionally there is significant anisotropy in the spatial variability which is correlated with trends of the anticline and fault the direction of minimum spatial correlation is normal to the fault at proximal distances and gradually rotates and becomes subparallel to the fold axis over the same 1 2 km distance away from the fault we interpret these changes to reflect varying scales of influence of the fault and the fold on fracture network development the fault locally influences the magnitude and variability of fracture network attributes whereas the fold sets the background level and structure of directional variability

intra seasonal oscillations observed in the mlt region above uk 52 n 2 w and esrange 68 n 21 e 1 analyses of intra seasonal oscillations i e oscillations with periods of 30 100 days in the mlt region made by meteor radars at middle uk five years of data and high latitudes esrange three years of data are presented in this work there is clear evidence of 75 day variability in the mean winds this intra seasonal oscillation appears to be a prominent feature of the mean zonal circulation and was observed quite regularly in the time intervals december 1996 september 2001 over uk and october 1999 december 2002 over esrange the intra seasonal oscillations observed over esrange are coherent in both amplitude and phase over the height range of 80 100 km covered by the radar they are especially strong in the range 90 97 km weaker near 87 88 km and stronger again near 81 km at these lower heights the intra seasonal oscillations are usually of longer period 80 days than and are out of phase with the oscillations at the upper heights we also note that this oscillation is a common feature in the meridional winds over the uk it is present but intermittent in the meridional winds over esrange

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

topological characterization of a polyphased gold bearing vein network abstract it is challenging to evaluate the connectivity of metal bearing hydrothermal vein systems i e the degree to which fractures or veins are connected within a network moreover many ancient ore bearing vein networks have been deformed and metamorphosed which hinders the reconstruction and quantification of their primary characteristics here we explore the reliability and limitations of the network distribution and topologic classification of vein terminations into i y and x nodes approaches to study deformed and metamorphosed mineral deposits we focus on the cheechoo gold deposit located in the eeyou istchee james bay area of quebec canada archean superior province this gold deposit shows a complex hydrothermal and magmatic polyphased gold bearing vein system hosted in a granodioritic tonalitic intrusion the cheechoo deposit is composed of quartz feldspar diopside qtz fsp di and quartz qtz veins that underwent ductile deformation and amphibolite facies metamorphism the high grade metamorphism led to the modification of vein shape relative position and spacing making these parameters unreliable for the characterization of the cheechoo vein system and similar metamorphosed networks however the number of veins per length unit and topology are reliable tools to characterize such networks in the gold bearing cheechoo system the gold mineralization appears to be mainly controlled by i the number of qtz and qtz fsp di veins per length unit and by ii the connectivity of these networks as shown by the increase of both parameters with the gold grade furthermore we introduce the y i ratio as a simple topological parameter to quantify the connectivity of two dimensional i y dominated systems this parameter is positively correlated with the number of connections per branch cb used to quantify network connectivity and ultimately represent the development of a fault or fracture zone

mechanical and fracture stratigraphy using examples from core studies this article shows that separate identification of mechanical stratigraphy and fracture stratigraphy leads to a clearer understanding of fracture patterns and more accurate prediction of fracture attributes away from the wellbore mechanical stratigraphy subdivides stratified rock into discrete mechanical units defined by properties such as tensile strength elastic stiffness brittleness and fracture mechanics properties fracture stratigraphy subdivides rock into fracture units according to extent intensity or some other observed fracture attribute mechanical stratigraphy is the by product of depositional composition and structure and chemical and mechanical changes superimposed on rock composition texture and interfaces after deposition fracture stratigraphy reflects a specific loading history and mechanical stratigraphy during failure because mechanical property changes reflect diagenesis and fractures evolve with loading history mechanical stratigraphy and fracture stratigraphy need not coincide in subsurface studies current mechanical stratigraphy is generally measurable but because of inherent limitations of sampling fracture stratigraphy is commonly incompletely known to accurately predict fractures in diagenetically and structurally complex settings we need to use evidence of loading and mechanical property history as well as current mechanical states

basement reservoir plumbing fracture aperture length and topology analysis of the lewisian complex nw scotland upfaulted ridges of neoarchean crystalline basement rocks formed in the faeroe shetland basin as a consequence of mesozoic rift processes and are an active target for oil exploration we carried out a comprehensive fault and fracture attribute study on the extensive exposures of geologically equivalent crystalline basement rocks onshore in nw scotland lewisian gneiss complex as an analogue for the offshore oil and gas reservoirs of the uplifted rona ridge basement high our analysis shows a power law distribution for fracture sizes aperture and length with random to clustered spacing and high connectivity indices regional variations between the scottish mainland and the outer hebrides are recognized that compare directly with variations observed along the rona ridge in the faeroe shetland basin here we develop a model for the scaling properties of the fracture systems in which variations in the aperture attributes are a function of the depth of erosion beneath the top basement unconformity more generally the combination of size spatial and connectivity attributes we found in these basement highs demonstrates that they can form highly effective well plumbed reservoir systems in their own right supplementary material additional methods and results are available at https doi org 10 6084 m9 figshare c 5017139 thematic collection this article is part of the the geology of fractured reservoirs collection available at https www lyellcollection org cc the geology of fractured reservoirs

pitfalls of using entrenched fracture relationships fractures in bedded carbonates of the hidden valley fault zone canyon lake gorge comal county texas characterizing natural fracture systems involves understanding fracture types faults joints and veins patterns orientations sets and spacing within sets size distributions penetration across layering aperture and trace length and timing relationships traditionally observation based relationships to lithology mechanical stratigraphy bed thickness structural position failure mode and stress history have been proposed for predicting fracture spacing along with the relative abundance of opening mode fracture versus faults in fractured rocks developing a conceptual fracture model from these relationships can be a useful process to help predict deformation in a fractured reservoir or other fractured rock systems a major pitfall when developing these models is using assumptions based on general relationships that are often site specific rather than universal in this paper we examine a mixed carbonate shale sequence that is cut by a seismic scale normal fault where fracture attributes do not follow commonly reported fracture relationships specifically we find 1 no clear relationship between frequency or spacing of opening mode fractures joints and veins and proximity to the main fault trace and 2 no detectable relationship between fracture spacing and bed thickness however we did find that 1 the frequency of small displacement faults is strongly and positively correlated with proximity to the main fault trace 2 fracture networks change pattern and failure mode extension versus shear fracture from pavement to pavement through the mechanically layered stratigraphic section and 3 faults are more abundant than opening mode fractures in many areas within the fracture network we interpret that the major fracturing initiated near maximum burial under relatively high differential stress conditions where shear failure dominated and that mode 1 extension fracturing occurred later under lower differential stress conditions filling in between earlier formed shear fractures we conclude that whenever possible site specific observations need to be carefully analyzed prior to developing fracture models and perhaps a different set of fracture network rules apply in rocks where shear failure dominates and mechanical stratigraphy influences deformation

fractal analysis of fractures in rocks the cantor s dust method abstract the cantor s dust method of fractal analysis has been used to describe the regularity of fracture patterns in geological materials seen on a two dimensional surface this linear method of analysis is well adapted to a determination of two dimensional phenomena the determination of the fractal dimension d indicates that it varies regularly as a function of the orientation of the analysis direction this is taken to indicate that there is a relation unexplained as yet between the regularity of the fracture pattern and the forces which produced it the use of the cantor s dust method on naturally fractured materials ranging in dimension from 3 mm to 45 m in maximum dimension indicates the fractal nature of the fractures found in natural materials the analysis method can also be used to identify local variations in the stress field which caused failure such as the fractures in a feldspar compared to those in the host granite etc the method appears to be a sensitive descriptive measure of the failure patterns in rocks

appraisal of fracture sampling methods and a new workflow to characterise heterogeneous fracture networks at outcrop abstract characterising fractures at outcrop for use as analogues to fractured reservoirs can use several methods four important fracture data collection methods are linear scanline sampling areal sampling window sampling and circular scanline sampling in regions of homogeneous fracture networks these methods are adequate to characterise fracture patterns for use as outcrop analogues however where fractures are heterogeneous it is more difficult to characterise fracture networks and a different approach is needed we develop a workflow for fracture data collection in a region of heterogeneous fractures in a fold and thrust belt which we believe has applicability to a wide variety of fracture networks in different tectonic settings we use an augmented circular scanline method along with areal sampling to collect a range of fracture attribute data including orientation length aperture spatial distribution and intensity this augmented circular scanline method more than halves the time taken for data collection provides accurate unbiased data that is representative of local fracture network attributes and involves data collection of a wider range of fracture attributes than other sampling techniques alone

fractal clustering of metamorphic veins metamorphic veins record the fracture controlled flow of fluids throughout the oceanic and continental crust i show that the spatial distributions of veins from three diverse metamorphic settings are fractal and self similar vein densities were measured by counting the number of macroscopic veins intersected along linear transects the localities included wol lastonite quartz veins in marbles for which the fractal dimension d is 0 46 actinolite chlorite veins in hydrothermally altered oceanic diabases d 0 81 and epidote quartz veins in contact metamorphosed basalts d 0 25 0 63 the fractal clustering of veins provides a geometric framework for understanding spatial and temporal patterns of fluid flow and mineral reaction during metamorphism