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

microstructural observations on natural syntectonic fibrous veins implications for the growth process abstract syntectonic antitaxial and ataxial fibrous veins were investigated using sem microprobe cathodoluminescence cl and optical microscopy in antitaxial calcite veins fibres and surrounding selvage grew simultaneously with similar growth rates of crystallographically differently oriented grains new material precipitated at the vein margin in antitaxial and bi mineralic ataxial microstructures bridges of country rock material formed during vein growth in an initial en echelon vein system in our antitaxial and bi mineralic ataxial samples the spacing of solid inclusions does not reflect individual crack seal openings

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

fault controlled sequential vein dilation competition between slip and precipitation rates in the austin chalk texas abstract multi layered calcite veins in a dilatant jog of a left stepping left slipping shallowly buried fault segment are composed of alternating millimeter to submillimeter thick calcite veinlets and host lithons forming a coarse crack seal texture the grain fabrics in calcite veinlets are mostly equant or irregular suggesting face controlled grain growth in a fluid filled cavity the relatively thick veinlets can be developed by progressive fault slip and veinlet opening simultaneously with calcite precipitation under low effective stress continuous changes in the oxygen isotopic compositions of the calcite veinlets along the length of veins suggest that the individual calcite veinlets were sequentially developed from the footwall to the hanging wall there is no particular evidence that these veins represent excursions in fluid pressure or instantaneous fracture opening related to episodic fault slip the fracture formation and filling cycle could have taken place along a continuously slipping fault contained within a porous rock with normal fluid pressure

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

subsurface fractures and their relationship to stress history in east texas basin sandstone abstract subsurface fractures were sampled with more than 400 m of core from the lower cretaceous travis peak formation a tabular sandstone and shale unit approximately 600 m thick that was deposited in the gradually subsiding gulf coast basin depth to the top of the formation in east texas now ranges between 1800 and 2900 m fractures in sandstone exhibit features that are due to intermittent fracturing and sealing crack seal which suggest that the fractures are the result of episodes of fluid pressure in excess of hydrostatic correlation of the vein mineral precipitation sequence with the diagenetic history and the results of previous 18 o studies of quartz cement indicates that fractures propagated at depths of between 900 and 1500 m during the migration of quartz precipitating fluids fracture permeability provides a mechanism for the passage of large fluid volumes recorded by the precipitation of extensive quartz cement in the formation fractures stopped propagating before maximum burial depth was achieved during approximately 1500 m of subsequent burial fractures either remained partly open or were sealed by postkinematic ankerite and clay minerals

structural framework of the edwards aquifer recharge zone in south central texas the edwards aquifer the major source of water for many communities in central texas is threatened by population growth and development over its recharge zone the location of the recharge and confined zones and the flow paths of the aquifer are controlled by the structure of and deformation processes within the balcones fault system a major system of predominantly down to the southeast normal faults we investigate the geologic structure of the edwards aquifer to assess the large scale aquifer architecture analyze fault offset and stratigraphic juxtaposition relationships evaluate fault zone deformation and dissolution and fault system architecture and investigate fault block deformation and scaling of small scale intrablock normal faults characterization of fault displacement shows a pattern of aquifer thinning that is likely to influence fault block communication and flow paths flow path constriction may be exacerbated by increased fault segment connectivity associated with large fault displacements also increased fault zone deformation associated with larger displacement faults is likely to further influence hydrologic properties overall faulting is expected to produce strong permeability anisotropy such that maximum permeability is subhorizontal and parallel to fault bedding intersections at all scales aquifer permeability is either unchanged or enhanced parallel to faults and in many cases decreased perpendicular to faults

quantifying uncertainties in multi scale studies of fractured reservoir analogues implemented statistical analysis of scan line data from carbonate rocks in this study we performed a fracture analysis on a cretaceous bedded carbonate succession well exposed in the sorrento peninsula the studied succession includes stratigraphic units that are very similar to the productive units of the buried apulian platform reservoir rocks in southern italy we analyzed eight carbonate beds including both limestones and dolomites the basic technique used in this study consisted of measuring fractures along bedding parallel scan lines for one limestone bed a microscale scan line about 15 cm long was also analyzed using a digital microcamera provided the cumulative distribution of fracture apertures is well described by a power law our analysis shows how the uncertainty in the estimate of fracture aperture cumulative frequencies grows for large aperture values this feature results in a large uncertainty in the estimate of the slope of the least squares line in a bi logarithmic diagram approximating the data distribution which is the exponent of the power law as the latter represents a fundamental parameter characterizing a fracture set and fracture distribution over different scales reducing the uncertainty in the estimate of the slope of the curve represents an important objective of quantitative fracture analysis this is obtained in this study by the application of multi scale analysis and by integrating micro scan line data with classic outcrop based scan line analysis the quantification of uncertainties in the cumulative distribution estimates of fracture apertures is performed by analyzing in detail the spacing distribution and consequently fracture density distribution for each aperture value our results suggests that a meaningful statistical analysis of fracture attributes such as aperture or opening displacement may be effectively carried out by using properly determined confidence intervals and by the integration of outcrop based and micro scan line data sets

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

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

quantitative study of the deformation at southern explorer ridge using high resolution bathymetric data abstract we present preliminary results of a morphological study of the summit of the southern explorer ridge ser the ser is an inflated intermediate rate spreading center located in the northeast pacific off the west coast of canada that hosts a large hydrothermal vent complex known as magic mountain a quantitative assessment of faulting on the axial summit graben floor close to the ridge summit is accomplished through the analysis of high resolution near bottom bathymetric data these data were acquired using a multibeam system mounted on an autonomous underwater vehicle operated a few tens meters above the seafloor structural mapping reveals numerous subvertical fissures and normal faults that nucleated from tension fissures the ratio between the length and the maximum scarp height of normal faults is not constant contrary to what is generally observed on subaerial faults highlighting the probable importance of fault segment linkage and fault growth processes within the relatively thin brittle layer populations of small faults exhibit an exponential size frequency distribution that reflects the importance of linkage in the fault growth history and the relatively large amount of tectonic strain 3 7 to 18 4 accommodated by the normal faults we propose that the 500 to 600 m wide and 60 m deep asymmetric axial summit graben of the ser formed due to magma chamber deflation as well as normal faulting that initiated on the present eastern border of the graben we find a well defined geographic distribution in the types of lava flows which indicates a general decrease of the eruption rate through time we also find that the magic mountain hydrothermal field is located in the vicinity of the large eastern axial summit graben bounding fault whose dimensions suggest it may reach to the brittle ductile transition depth this fault likely has provided an efficient physical pathway for fluids from the subsurface to the seafloor for a significant period of time thus allowing the hydrothermal system to grow and mature

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

general equations for modeling fluid rock interaction using trace elements and isotopes abstract general equations for modeling open and closed system fluid rock interaction using trace elements and isotopes are presented taylor s 1977 open system equation for stable isotopes is extended to account for a change in the value between rock and fluid during interaction it is shown that hydrothermal systems in which fractionation factors vary during fluid rock interaction will significantly alter the isotopic ratios of rocks compared to equivalent systems with invariant fractionation factors the equations for modeling trace element exchange are analogous to the stable isotope equations and permit the calculation of fluid rock ratios using changes in trace element concentrations in the rock similarly to stable isotope exchange the change of the trace element concentration in a rock during infiltration is a function of the trace element concentration in the fluid the fluid rock partition coefficient and the fluid rock ratio rather than simply the concentration of the trace element in the fluid

platform margin trajectory as a control on syndepositional fracture patterns canning basin western australia abstract syndepositional fractures are an important feature of high relief reef rimmed carbonate systems and exert a profound control on many facets of platform evolution including the timing and frequency of platform margin collapse events generation of an early diagenetic fluid flow system and subsequent karst and enhanced permeability development in the devonian reef complexes of the canning basin syndepositional fracturing increases significantly with progradation with a twofold increase in fracture intensity observed between the most retrogradational and progradational carbonate platforms this study demonstrates a statistically significant relationship between syndepositional fracture patterns and variations in stratigraphic architecture approximated here by platform margin trajectory in the platform margin syndepositional fracturing varies systematically with platform margin trajectory regardless of proximity to regional tectonic elements whereas in the platform interior tectonically active settings consistently display syndepositional fracture intensities significantly higher than predicted by platform margin trajectory alone the results presented here suggest a first order relationship between long term changes in stratigraphic architecture and syndepositional fracture development in high relief carbonate platforms and imply that whereas external regional tectonic drivers may enhance early fracturing they are not required

quantifying diagenetic and stratigraphic controls on fracture intensity in platform carbonates an example from the sierra madre oriental northeast mexico abstract normalized fracture intensity measured along 1d scanlines was used to compare fracture intensity for different sedimentary facies stratigraphic position bed thickness and degree of dolomitization in carbonate beds of the cupido and tamaulipas formations northeast mexico we calculated the fracture intensity for individual beds for single and combined fracture sets and the statistical significance of relationships using bivariate weighted regressions and multivariate methods results suggest that the degree of dolomitization is positively correlated with fracture intensity and has the strongest correlation followed by the position of the bed in the stratigraphic cycle and the mud content dolomite content normalized position in the parasequence environment of deposition and mud content show a significant degree of correlation among themselves and should be considered dependent variables for statistical purposes geological observations suggest that dolomite precipitation and fracturing occurred at least in part synchronously in these rocks a fracture intensity distribution model integrating sequence stratigraphic and diagenetic history is proposed for the cupido and tamaulipas formations with potential use in similar carbonate successions our analysis does not support the classic bed thickness fracture spacing relationship

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

aggregate properties of fracture populations abstract empirical studies indicate that the individual attributes of both faults and extension fractures follow power law scaling aggregate properties of fracture populations are important in a variety of problems and can be specified in terms of the scaling parameters of individual fracture attributes development of an expression for an aggregate property requires consideration of a number of independent factors including the topologic dimension of the aggregate property the topologic dimension of sampling and the possibility of scaling changes for fractures that span some mechanically significant layer the riemann zeta function provides an alternative to integration for the analytical and numerical solution of aggregate problems previous work regarding aggregate properties of fracture populations has focused on the strain due to faulting new expressions are developed here for other aggregate properties of interest fracture surface area fracture porosity fracture permeability and shear wave anisotropy a general characteristic of these aggregate properties is that for most values of scaling exponents the aggregate properties are dependent on the size of the sampling domain this implies that the aggregate properties are scale dependent additionally it appears that fracture surface area is concentrated in the smallest fractures of many populations fracture porosity is concentrated in the largest fractures of most populations but not as strongly as fracture permeability which probably derives almost entirely from the largest fractures in populations

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

chaotic breccia along the dent fault nw england implosion or collapse of a fault void a body of chaotic breccia along the reverse oblique dent fault zone is ascribed to hanging wall collapse into persistent voids created by geometric mismatch of fault walls although some implosion into transient voids is a possibility the breccia comprises a 20 m wide body of hanging wall lithologies with a chaotic clast supported fabric that contrasts with the fitted fabric breccias typical of the dent fault damage zone the breccia body has crude bedding defined by clast shape and size contrasts the void fill is cut by variscan fault strands which together with its ferroan calcite and barite cement prove its late carboniferous rather than recent age it is shown that any fault void transient or persistent had a smaller aperture than the final width of the breccia body and no more than 5 m a span that can be supported to depths of 2 or 3 km however cement zonation in the breccia fill suggests that the void opened in multiple increments each of an aperture compatible with the maximum displacement in any one event along the dent fault the dent fault example highlights the possible general importance of fault void collapse but also the problems in distinguishing it from implosion processes

growth of syndepositional faults in carbonate strata upper permian capitan platform new mexico usa abstract the upper permian capitan platform new mexico is cut by syndepositional faults and fractures these are parallel to platform margin and cluster within strata that steepen and thicken abruptly into the basin faults are steep dip slip have measurable displacements of up to 24 m are typically associated with growth strata and mostly tip out blindly within the platform basinward throwing faults are typically reverse and are steeper than shelfward throwing faults which have normal geometry the sense of displacement across some initially basinward throwing faults changed as they developed these patterns indicate that faults were rotated as they grew synkinematic rotation of faults is interpreted to have occurred due to syndepositional compaction driven down to basin tilting of the capitan platform faults grew through interaction with inherited fractures and by segment linkage vertically separated fault segments had a tendency to grow downward to link with underlying segments thus promoting accumulation of displacement across faults without vertical propagation of fault tips four fault types are distinguished on the basis of structures associated with their upper terminations i faults breaking free surface ii faults tipping out within non folded growth strata iii faults tipping out below growth monoclines and iv buried faults with no expression on free surface fault tips evolved from one type into another in response to variations in the rates of fault growth and deposition fault tip structures also vary laterally over distance of 750 1400 m inferably as a result of variable distances from fault centres

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

controls on fracture distribution in cretaceous sedimentary rocks from the isfahan region iran in this study relationships between fracture patterns lithology thickness diagenetic processes and grain size are evaluated within cretaceous sediments in two sections of dizlu and kolah ghazi of isfahan this study area was selected based on its outcrops of different rock units and its well developed tectonic fractures the fracture patterns within stratigraphic units of these sections are studied using geometrical and statistical analyses this study finds that variable fracture spacing and fracture spacing ratios can be affected by lithology thickness grain size of sediments and diagenetic processes a study of fracture stratigraphy based on fracture pattern evaluation within different cropped out sedimentary rocks can be used to improve understanding of the same types of sedimentary rock units below the surface or throughout other sedimentary basins consequently this could improve information regarding storage and fluid flow pattern throughout sedimentary rocks in different regions even for subsurface purposes

extracting olivine fo fa compositions from raman spectral peak positions the dominant feature of the olivine raman spectrum is a doublet that occurs in the spectral region of 815 825 cm 1 db1 and 838 857 cm 1 db2 these features arise from coupled symmetric and asymmetric stretching vibrational modes of the constituent sio4 tetrahedra the frequencies of both peaks show monotonic shifts following cation substitution between forsterite and fayalite we present a calibration for extracting olivine fo contents fo mg mg fe molar ratio fo0 100 from the peak positions of this doublet permitting estimates of chemical composition from raman spectra acquired in the laboratory or field as well as providing information on crystal structure distinction of polymorphs eight samples spanning the compositional range from forsterite to fayalite were used to develop the calibration equations for the db1 and db2 peaks individually and together the data indicate that the db1 peak is more reliable for calculating the compositions of fe rich olivine but that the db2 peak is better for magnesian compositions the two peak calibration overcomes the limitations of the single peak calibrations and is capable of calculating olivine compositions to within 10 fo units

silica diagenesis porosity evolution and mechanical behavior in siliceous mudstones mowry shale cretaceous rocky mountains u s a quartz in mudstones of the mowry shale cretaceous rocky mountain region takes several forms extrabasinal detrital silt some with transported overgrowths replacement of skeletal debris minor overgrowth on detrital quartz and replaced radiolaria pore filling in the intragranular pores of allochems and authigenic microquartz that is dispersed through the clay size matrix the matrix dispersed microquartz has cl character similar to the overgrowths with which it is intergrown supporting the interpretation that it is authigenic and pore filling radiolaria are widely observed in these mudrocks but are poorly preserved by partial mineral replacement quartz dolomite calcite pyrite we interpret dissolved radiolaria and possibly now cryptic diatoms as the likely source of silica for the overgrowths and microquartz through a reaction pathway that entailed the formation of opal ct lepispheres that subsequently recrystallized to microquartz a newly devised variation of the igv intergranular volume concept a parameter igvm is applied for partitioning the causes of porosity decline in mudstones all of the samples examined contain authigenic microquartz which averages around 40 of the rock volume the large volume of microquartz together with the volumes of surviving intergranular pores and pore filling solid hydrocarbon constrain the porosity to around 50 at the time of initial lepisphere formation despite the exceptional abundance of cement overall porosity decline is compaction dominated the presence of significant volumes of cement has implications for mechanical rock properties in the mowry shale which is expected to manifest brittle behavior where cements are most prominently developed

modeling fracture porosity evolution in dolostone abstract opening mode fractures in dolostones buried to depths of 1 5 km contain synkinematic dolomite cement the amount and internal structure of which has a systematic relationship to fracture size narrow fractures we developed a geometric crystal growth model for synkinematic dolomite fracture fill in fractured dolostones where periodic incremental fracture opening events are introduced with concurrent cement growth we assumed constant temperature and supersaturation with respect to dolomite a key assumption in the model is that rapid dolomite accumulation within bridges is governed by high cement growth rates on repeatedly broken grain surfaces during the process of crack seal slower cement growth rates occur on euhedral crystals this assumption is made on the basis of a comparison with quartz cement growth in fractured sandstones simulations with different fracture opening rates mimic bridge and lining cement morphologies including characteristic rhombic shapes of dolomite bridges

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

igneous petroleum systems in the malargue fold and thrust belt rio grande valley area neuquen basin argentina abstract the rio grande valley neuquen basin argentina is a large oil field where voluminous igneous sill complexes are emplaced in different levels of the sedimentary sequence in a fold and thrust belt setting these sills are predominantly emplaced in organic rich units such as vaca muerta and agrio formations and are critical elements of the defined petroleum system as they affect the thermal maturity of the surrounding source rocks and also act as naturally fractured reservoirs in this contribution we integrated structural sections borehole data core descriptions petrophysical laboratory tests and organic geochemistry measurements to characterize the impact of the magmatism in the petroleum system of the rio grande valley oil field especially we focused in evaluating the impact of the sill complexes in hydrocarbon generation migration and accumulation our data shows that 1 intrusions are preferably emplaced in organic rich units 2 a large number of intrusions is observed in areas intensely deformed by imbricated thrusts 3 an increase in the number of intrusions drives stronger and more extensive maturation processes in the vaca muerta formation enhancing notably the cracking of kerogen to hydrocarbon within this unit 4 where the number of intrusions increases the concentration of total gas and methane also increases given that all sills are suitable reservoirs 5 igneous rocks provide a low resistance path for fluids flow and act as the main reservoirs in the study area 6 the reservoir quality of igneous intrusions is dominated by the combination of matrix and fracture framework properties 7 zeolites are the most common type of cement in these reservoirs 8 deep sills located in the vaca muerta formation produce larger volumes of hydrocarbons relative to shallower intrusive reservoirs the rio grande valley oil field offers a world class example of the implications of igneous complexes in hydrocarbon generation migration and accumulation in a compressive tectonic setting of fold and thrust belt

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

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

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

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

sedimentary aspects and paleoenvironmental evolution of a rift basin salta group cretaceous paleogene northwestern argentina the rift history of the salta basin is related to the evolution of the central andes and to the activity of the pacific margin owing to its geographic location sedimentation occurred from the neocomian to the paleogene with deposits reaching up to 5 000 m in thickness paleoenvironmental analysis reveals an evolutionary history controlled by tectonic and climatic changes isolated grabens characterized the early synrift stage differential subsidence provoked distinct environments in the southern and northern subbasins in the southern subbasins alluvial fan fluvial fan and lacustrine deposits prevail whilst in the northern subbasins eolian and fluvial environments dominate during the maastrichtian two major factors controlled the basin fill the decrease in tectonic subsidence and a relative sea level rise as recorded in south america an extensive and shallow atlantic marine ingression installed a carbonate system coincident with mainly humid conditions until the danian until the middle eocene the fluvial and lacustrine environmental evolution of the sag basin was controlled especially by the alternation of temperate with dry and humid periods paleontological records reflect these climatic changes and show their relationship to the sedimentation regime

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

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

numerical simulations of polycrystal growth in veins vein microstructures contain a wealth of information on conditions during vein growth but correct interpretation requires an improved understanding of the processes involved in this paper we investigate the parameters controlling vein microstructures using numerical simulations of anisotropic crystal growth we focus on the effects of crystal growth rate anisotropy on growth competition and on the effects of the wall rock on vein microstructure during crack seal growth growth competition in a free fluid is controlled by the crystallographic orientation and growth anisotropy of the crystals we discuss the merits and limitations of different algorithms to simulate free growth in veins based on a detailed study of the crystal facets and grain boundaries produced microstructures in crack seal veins are influenced by additional parameters such as the width of individual crack seal increments and the fracture morphology we present a detailed study of the transition between free fluid growth and crack seal growth as a function of the relative rates of crack opening and crystal growth to illustrate how this induces the microstructural transition between fibrous and blocky veins

fracture controlled paleohydrology in a map scale detachment fold insights from the analysis of fluid inclusions in calcite and quartz veins abstract this study uses fluid inclusions in quartz and calcite veins to characterize the fracture controlled paleohydrology of a map scale evaporite cored detachment fold in the sierra madre oriental of northeastern mexico field observations indicate that the veins are tectonic in origin and that they formed in a general sequence that corresponds to four broad stages of progressive folding we collected samples from each vein stage in various structural and stratigraphic positions across the fold after determining the mineral paragenesis and origin of inclusions in each sample we used standard microthermometric techniques to measure the homogenization temperatures t h salinities and eutectic temperatures of the available two and three phase aqueous inclusions neither t h salinity nor eutectic temperature varied systematically with inclusion origin or mineral type the data are also not significantly correlated with structural position or vein type but are strongly partitioned by stratigraphy suggesting that the area was a vertically stratified hydrologic system consisting of three regional paleohydrostratigraphic units an upper unit comprised the indidura formation through difunta group and is characterized by t h near 150 c and salinity t h near 150 c and salinities near 12 or 22 wt nacl equivalent the lower unit comprised the zuloaga and la casita formations and is characterized by an average t h near 225 c and salinities in three groups near 12 22 or 36 wt nacl equivalent the preservation of multiple distinct fluid types in many veins suggests that fracture development during folding and uplift created a second paleohydrologic system that overprinted the vertically stratified system fracture controlled fluid migration in this second system occurred in periodic repeated and spatially variable pulses