faulting of rocks in three dimensional strain fields i failure of rocks in polyaxial servo control experiments abstract the dependence of the number of sets of faults and their orientation on the intermediate strain axis is investigated through polyaxial tests reported here and theoretical analysis reported in an accompanying paper in the experiments cubic samples of berea sandstone sierra white and westerly granites and candoro and solnhofen limestones were loaded on their three pairs of faces by three independent mutually perpendicular presses at room temperature two of the presses were servo controlled and applied constant displacement rates throughout the experiment most samples display three or four sets of faults in orthorhombic symmetry these faults form in several yielding events that follow a stage of elastic deformation in many experiments the maximum and the intermediate compressive stresses interchange orientations during the yielding events where the corresponding strains are constant the final stage of most experiments is characterized by slip along the faults

prediction of lateral variability in fracture intensity using multicomponent shear wave surface seismic as a precursor to horizontal drilling in the austin chalk summary a method of multicomponent multisource shear wave interpretation utilizing the output of the alford rotation for separating the fast and slow particle motion stacks in azimuthally anisotropic media yielded a qualitative prediction of lateral fracture intensity variation prior to horizontal drilling the method exploits a fundamental difference between the two particle motion stacks when vertical aligned fractures are the source of the shear wave splitting the fast shear wave s1 section is insensitive to the lateral variation in fracture intensity since its particle motion is parallel to the fractures hence reflectivity on the s1 cdp stacked section should be likewise insensitive to fractures the slow shear wave s2 section however possesses particle motion perpendicular to the fracture planes so variation in lateral fracture intensity will affect velocity and reflectivity examination of the reflectivity on the two sections for a seismic line reveals laterally consistent reflector strength on the s1 section for the austin chalk the horizon of interest but laterally inconsistent reflector strength for the same horizon on the s2 section in the study area reflector dimming on the s2 section was interpreted as indicating fracture intensity maxima horizontal drilling confirmed both fracture azimuth and intensity predictions and resulted in significantly higher initial hydrocarbon production than for surrounding wells

dependence of joint spacing on rock properties in carbonate strata joint density is studied in relation to petrographic and petrophysical parameters in two sedimentary carbonate formations characterized by different diagenetic histories the kimmeridgian limestones of the chay peninsula western france with a mean joint density of 6 37 fractures per meter fr m 1 94 fr ft and the bathonian limestones of the bouye outcrop western france with a mean joint density of 1 9 fr m 0 58 fr ft the chay carbonates are characterized by a lower caco3 content a higher average porosity and a lower sound velocity than values recorded in the bouye limestones the compressive strength and young s modulus of the bouye carbonates are respectively 10 and 3 times higher than in the chay carbonates a statistical analysis was used to identify relationships between joint density and carbonate rock properties when facies variations are marked the joint density at outcrop scale is related to the mean bed thickness the facies descriptors and the young s modulus when textural variations are more limited the joint density is controlled by the porosity at the scale of a sedimentary basin 63 8 of the variation in joint density may be accounted for by young s modulus and the sparite micrite ratio the decrease in the sparite micrite ratio reflects an increased number of grain boundaries in the carbonate rock which limits grain deformation and enhances joint density the variations in young s modulus depend essentially on the porosity and mineralogy of the studied rocks any increase in caco3 content or decrease in porosity is associated with an increase in the elastic properties of the rock and a reduction of joint density

assessment of the effects of tidal mixing in the kuril straits on the formation of the north pacific intermediate water abstract to assess accurately the effect of tidal mixing in the kuril straits on the formation of the north pacific intermediate water npiw the spatial distribution of diapycnal diffusivity recently obtained by the present authors is incorporated into an eddy permitting ogcm it is shown that the npiw is successfully reproduced although the diapycnal diffusivity averaged over the entire kuril straits is an order of magnitude less than has previously been assumed as a tuning parameter to reproduce the npiw in low resolution ogcms this strongly suggests that the effect of tidal mixing in the kuril straits on the formation of the npiw is relatively minor and that the physical processes omitted by the low resolution ogcms such as isopycnal mixing along the kuroshio extension region are much more important this suggestion gives warning of the danger that some misleading conclusions might be derived from ogcms that employ diapycnal diffusivity just as a tuning parameter to reproduce the observed features

mechanical stratigraphy and fault fold interaction absaroka thrust sheet salt river range wyoming abstract a distinct contrast in deformation style from tightly folded and faulted cambrian through ordovician strata to broadly folded and faulted mississippian and younger strata is displayed in the hanging wall of the absaroka thrust salt river range wyoming based on style the paleozoic section can be divided into two main structural lithic units each unit is characterized by a relatively weak ductile anisotropic lower section and a relatively strong brittle more isotropic upper section where the two units were stacked within the thrust sheet inverted fault propagation folds formed in the center of each unit and the overall transition upward from close to wide spaced folds and imbricate faults developed in the multilayer where the upper unit was isolated deformation was dominated by imbricate faulting with little associated folding and inverted fault propagation folds did not form observations illustrate that the mechanical interaction between the two units and boundary conditions imposed on them were significant to defining the deformation response it is note worthy that the boundary between the two structural lithic unts in part defined by the roof thrust of the stewart passive roof duplex occurs within a dolostone rather than in overlying shale of the darby formation i propose that the roof thrust and other detachments in dolostone reflect the relatively low frictional strength of dolostone under shallow crustal conditions cross cutting relations suggest that the absaroka sheet was cut by large imbricate faults during late stage movement over a major dip ramp a high deviatoric and low mean compressive stress state just beyond the upper ramp flat corner promoted break back imbrication and allowed tectonic thickening of the thrust wedge necessary to reestablish critical taper and for forward translation

3 d deformation pattern analysis and evolution of the anisclo anticline southern pyrenees understanding the genetic relationships of folding related deformation structures is of importance for both academic and industrial purposes we analysed the three dimensional deformation pattern of the anisclo anticline in the southern pyrenees the specifically designed statistical method includes sequential steps of statistical and spatial data analysis and classification our results indicate that pressure solution cleavage frequency exhibits a spatial distribution that depends on the position within the fold conversely such a dependence does not occur in joints and veins consequently our data suggest that pressure solution cleavage is the most appropriate deformation structure for unravelling fault fold kinematics in carbonate multilayer in particular we found that the spacing of solution cleavages s is related to the corresponding bed thickness h this supports the use of h s value for deformation intensity quantification instead of spacing in the anisclo anticline analysis of h s indicates the occurrence of three main deformation panels whose restoration suggests that fold evolution progressed from layer parallel shortening to decollement folding up to thrust breakthrough and fault propagation folding comparison of h s values with finite strain data indicates that line length was not preserved during fold evolution this evidence has important implications for the kinematic interpretation of layer parallel shortening and cross section balancing

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

stratigraphic controls on vertical fracture patterns in silurian dolomite northeastern wisconsin vertical opening mode fractures are mapped on quarry walls to assess the stratigraphic controls on fracture patterns in the relatively undeformed silurian dolomite of northeastern wisconsin our two stage study uses maps of vertical fractures to assess the effectiveness of various types of stratigraphic horizons e g organic partings or cycle bounding mud horizons in terminating opening mode fractures first the mechanical stratigraphy of the exposures is interpreted from the observed fracture pattern both visual inspection and a newly developed quantitative method are employed to identify effective mechanical interfaces the two methods show similar results confirming the validity of qualitative visual inspection the second stage of our study stochastically predicts mechanical stratigraphy and subsequent fracture pattern from empirical relationships between the observed sedimentary stratigraphy and the interpreted mechanical stratigraphy for example 63 of cycle bounding mud horizons within the inner middle and middle shelf facies associations serve as mechanical interfaces these empirical percentages are input to a monte carlo analysis of 50 stochastic realizations of mechanical stratigraphy comparisons of the stochastically predicted and interpreted mechanical stratigraphy yield errors ranging from 13 to 33 this method yields far better results than assuming that all stratigraphic horizons act as mechanical interfaces the methodology presented in this article demonstrates an improved prediction of fracture pattern within relatively undeformed strata from both complete characterization of sedimentary stratigraphy and understanding mechanical controls on fracturing

non linear growth kinematics of opening mode fractures abstract fracture aperture is commonly assumed to be a linear function of fracture length stress and elastic material properties under constant stress fracture opening displacement may change without concurrent length or height growth if the material effectively weakens after initial linear elastic fracture growth by changes in elastic properties or by non elastic deformation processes to investigate the kinematics of fracture opening and its dependence on length and height growth we reconstructed the opening history of three opening mode fractures that are bridged by crack seal quartz cement similar crack seal cement bridges had been interpreted to form by repeated incremental fracture opening and subsequent precipitation of quartz cement using scanning electron microscope cathodoluminescence imaging we determined the thickness and number of crack seal cement layers as a function of position along fracture length and height observed trends in crack seal cement layer thickness and number of fracture opening increments are consistent with non linear fracture growth kinematics consisting of an initial stage of fast fracture propagation relative to aperture growth followed by a stage of slow propagation and pronounced aperture growth consistent with earlier fluid inclusion observations indicating fracture opening and propagation occurring over 40 50 m y we interpret the second stage of pronounced aperture growth and slow propagation to result from fracture opening strain accommodated by solution precipitation creep and concurrent slow possibly subcritical fracture propagation

fracture development and mechanical stratigraphy of austin chalk texas the mechanical stratigraphy of the upper cretaceous austin chalk is established from study of fracture intensity along its outcrop trend from dallas to san antonio and westward to langtry texas and in the subsurface from study of cores and or fracture identification logs from 30 wells three mechanical stratigraphic units are recognized 1 an upper fractured massive chalk corresponding to the big house chalk member 2 a middle ductile chalk marl corresponding to the dessau chalk and burditt marl members and 3 a lower fractured massive chalk corresponding to the atco chalk member representative samples from the three mechanical stratigraphic units were experimentally shortened dry at 10 17 34 and 70 mpa confining pressure at 24 c and at a strain rate of 2 5 10 4 sec 1 to determine if the relative mechanical behavior observed at the surface could be extrapolated into the subsurface at different simulated burial depths the experimentally determined ductilities parallel those determined from outcrop and subsurface studies multiple linear regression analysis indicates that porosity is most strongly correlated with fracture strength smectite content has the second strongest correlation for low porosity specimens 9 13 5 the strength of specimens with 4 smectite is reduced 30 42 compared to specimens with no smectite the coefficient of internal friction at 17 mpa confining pressure decreases from 1 66 to 0 61 as smectite content increases from 0 to 4 sem photomicrographs of undeformed specimens show that smectite and other clays are distributed as large 30 m discrete amorphous concentrated masses throughout the chalk they are comminuted along the induced fracture surfaces where their grain size is 0 5 m or less these observations suggest that smectite acts as a soft inclusion localizing shear failure and correspondingly weakening the material

a techno functional perspective on quartz micro notches in sibudu s howiesons poort indicates the use of barbs in hunting technology abstract in this paper we present the results of a use wear study of quartz micro notches identified during a technological analysis of lithics from the howiesons poort layers of sibudu cave building on the technological analysis and preliminary functional screening of the archaeological material a series of experiments was designed to evaluate different hypotheses for notch formation blank production intentional notching hafting projectile use and trampling the experimental reference collection was compared with archaeological micro notches and a large sample of other archaeological quartz pieces including bladelets bipolar blanks flakes and retouched pieces this allowed us to evaluate the causes of micro notch formation in the studied assemblage results indicate two novelties in the howiesons poort hunting technology at sibudu the use of quartz barbs and non retouched quartz blanks it seems that in addition to backed pieces segments obliquely backed points etc unretouched pieces were mounted as elements in hunting weapons during the howiesons poort techno tradition seven probable and 29 tentative barbs were identified we thus present one of the strongest and oldest bodies of evidence for the use of barbs as projectile elements

geologic characterization of the type cored section for the upper cretaceous austin chalk group in southern texas a combination fractured and unconventional reservoir a rock based geologic characterization was completed on a continuous core through the austin chalk group section an active exploration target in southern texas because this core located in the pearsall field is continuous and includes both lower and upper contacts it can be used as the type cored section for the southern texas austin chalk several general lithofacies are defined two lithofacies lithofacies 1 and 2 are highly bioturbated and have low to moderate organic matter content and are poor to moderate source rocks two other lithofacies lithofacies 3 and 4 are laminated lithofacies 3 containing some burrows and lithofacies 4 containing no burrows both lithofacies are good source rocks organic matter in the laminated lithofacies 3 and 4 and some of the organic matter in the bioturbated lithofacies 2 are dominated by type ii kerogen suggesting the section can be self sourcing a general trend is observed whereby lithofacies that were deposited under more oxygenated conditions increase in abundance up section and is related to a decrease in total organic carbon and hence source rock quality the primary oil and gas production is from fractures with possible later production from the nano to microporous matrix after pressure drawdown pore types are predominantly interparticle nano to micropores and lesser intraparticle nanopores as defined by loucks et al 2012 porosity is less than 8 and permeability is less than 0 05 md and ranges down into the nanodarcy range the type cored section adds to the understanding of the southern texas austin chalk by providing a geological description of lithofacies stacking patterns source rock quality characterization and reservoir quality the type core can be used as a comparison section to other cored sections of the austin chalk

effects of simulated clay gouges on the sliding behavior of tennessee sandston abstract the effects of simulated fault gouge on the sliding behavior of tennessee sandstone are studied experimentally with special reference to the stabilizing effect of clay minerals mixed into the gouge about 30 specimens with gouge composed of pure clays of homogeneously mixed clay and anhydrite or of layered clay and anhydrite along a 35 precut are deformed dry in a triaxial apparatus at a confining pressure of 100 mpa with a shortening rate of about 5 10 4 sec and at room temperature pure clay gouges exhibit only stable sliding and the ultimate frictional strength is very low for bentonite mont morillonite intermediate for chlorite and illite and considerably higher for kaolinite anhydrite gouge shows violent stick slip at 100 mpa confining pressure when this mineral is mixed homogeneously with clays the frictional coefficient of the mixed gouge determined at its ultimate frictional strength decreases monotonically with an increase in the clay content the sliding mode changes from stick slip to stable sliding when the frictional coefficient of the mixed clay anhydrite gouge is lowered down below 90 95 of the coefficient of anhydrite gouge the stabilizing effect of clay in mixed gouge is closely related to the ultimate frictional strength of pure clays that is the effect is conspicuous only for a mineral with low frictional strength only 15 20 of bentonite suppresses the violent stick slip of anhydrite gouge in contrast violent stick slip occurs even if the gouge contains as much as 75 of kaolinite the behavior of illite and chlorite is intermediate between that of kaolinite and bentonite bentonite anhydrite two layer gouge exhibits stable sliding even when the bentonite content is only 5 thus the presence of a thin clay rich layer in a fault zone stabilizes the behavior much more effectively than do the clay minerals mixed homogeneously with the gouge this result brings out the mechanical significance of internal structures of a fault zone in understanding the effects of intrafault materials on the fault motion based on the present experimental results incorporated with some other experimental data it is argued that although the stabilizing effect of montmorillonite and vermiculite is indeed remarkable at room temperature the effect should be much less pronounced at elevated temperatures due perhaps to the dewatering of the clays in most geological environments where shallow earthquakes occur the stabilizing effect of clays is probably not so conspicuous as to completely suppress the unstable motion of a fault

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

from outcrop to flow simulation constructing discrete fracture models from a lidar survey terrestrial light detection and ranging lidar surveys offer potential enrichment of outcrop based research efforts to characterize fracture networks and assess their impact on subsurface fluid flow here we explore two methods to extract the three dimensional 3 d positions of natural fractures from a lidar survey collected at a roadcut through the cretaceous austin chalk 1 a manual method using the university of california davis keck center for active visualization in the earth sciences and 2 a semiautomated method based on mean normal and gaussian curvature surface classification each extraction method captures the characteristic frequencies and orientations of the primary fracture sets that we identified in the field yet they extract secondary fracture sets with varying ability after making assumptions regarding fracture lengths and apertures the extracted fractures served as a basis to construct a discrete fracture network dfn that agrees with field observations and a priori knowledge of fracture network systems using this dfn we performed flow simulations for two hypothetical scenarios with and without secondary fracture sets the results of these two scenarios indicate that for this particular fracture network secondary fracture sets marginally impact 10 change the breakthrough time of water injected into an oil filled reservoir our work provides a prototype workflow that links outcrop fracture observations to 3 d dfn model flow simulations using lidar data an approach that offers some improvement over traditional field based dfn constructions in addition the techniques we used to extract fractures may prove applicable to other outcrop studies with different research goals

rounding hinges to fault bend folding geometric and kinematic implications fault bend folding is a common folding mechanism in thrust and fold belts worldwide the widely used kink band geometric model of fault bend folding necessitates complex ramp segmentations to reproduce the rounded shape of many natural thrust related anticlines curvilinear hinge sectors provide a geometric and kinematic alternative solution to kink bands for modelling curved hinge folds we developed an analytical solution for modelling fault bend folding using circular hinge sectors the velocity field of this kinematic solution is different from that associated with the classical kink style model our solution predicts the development of curvilinear anticlines above staircase fault geometries the occurrence of limb rotation and consequently the development of rotational syngrowth wedges on both the forelimb and the crest conversely to the kink style kinematics curvilinear hinge sectors imply a dependence of deformation intensity from the fold shape and stratigraphic position of the folded layer application to natural thrust related anticlines validates the effectiveness of curvilinear fault bend folding

karst dissolution along fracture corridors in an anticline hinge jandaira formation brazil implications for reservoir quality abstract folded fractured and karstified structures have been recognized in several carbonate reservoirs however they are rarely fully identified from seismic or well data and have been poorly described the present study investigates the geological and structural controls that regional anticlines may exert on solution enlarged fracture corridors and epigenic karst concentrations in folded fractured carbonate units we performed regional mapping based on 106 2d seismic lines 51 well logs unmanned aerial vehicle uav imagery of four major outcrop sites i ii iii and iv and field investigations to parameterize fracture and karstic features density spacing length and aperture the study area is potiguar basin brazil where we identified the gentle ne trending apodi fold 10 km wide and 20 km long the fold formed along an inverted extensional fault during the latest miocene quaternary stress field and the envelope of the meandering trend of the apodi river coincides with the ne trending fold axis the fracture pattern in the fold hinge zone consists of two orthogonal syn folding fracture sets a ne sw striking hinge parallel set and an orthogonal nw se striking set these fractures and superposed karstification are mostly concentrated in the hinge zone along fracture corridors in an area 1 5 km wide and 20 km long we conclude that fracture corridors in anticline hinges are likely sites to have high fracture densities and wide apertures these corridors provide reservoir spaces that are favorable for the formation of karst cavities with significant increases in permeability and porosity along the fracture sets these features act as important fluid pathways and storage areas where matrix porosity is low the findings can be used for first order prediction in karstified carbonate reservoirs

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

what controls diffuse fractures in platform carbonates insights from provence france and apulia italy abstract fractures are widespread in rocks and regional opening mode arrays are commonly ascribed to major tectonic events however fractures occur in otherwise undeformed rocks some of these are early developed features independent of tectonics and forming a background network at regional scale to overcome this lack of understanding two hydrocarbon reservoir analogues from platform carbonates have been targeted the provence se france and the apulian platform se italy in both areas an early fracturing stage has been observed made of high angle to bedding opening mode fractures and bed parallel stylolites these features developed synchronously during the first burial stages and prior to major tectonic events the fracture sets are not genetically related to the present day layering contrarily fractures developed in a brittle media where facies transitions were not sharp and did not act as mechanical discontinuities carbonate facies distribution and early diagenetic imprint constrained the mechanical stratigraphy when fractures occurred in addition we observed that fractures related to late tectonic inversion were partly inhibited indeed rock mechanical properties change through time characterizing the temporal evolution of carbonate rocks has revealed that diagenesis and sedimentary facies are the prime actors for brittleness and mechanical layering in carbonates

sequential vein growth with fault displacement an example from the austin chalk formation texas to determine the opening and precipitation history and characteristics of vein forming fluids analyses of oxygen and carbon isotopes and trace elements were carried out on multilayered crack seal calcite veins in the austin chalk formation near san antonio texas the veins developed within the normal fault zones possessing unique chemical and textural characteristics which indicate sequential vein development they are composed of alternating millimeter to submillimeter thick calcite veinlets and host lithons occasionally crosscut by coarse equant grained secondary calcite veins regular changes in 18o e g 2 6 to 5 6 pee dee belemnite pdb of the calcite veinlets along the length of veins suggest that the individual calcite veinlets were sequentially developed a systematic 18o decrease in the vein opening direction primarily resulted from a continuous increase in temperature of the ascending fluids delivered to the austin chalk relatively constant 13c approximately 1 4 0 4 pdb for the multilayered veins and most secondary veins indicates that the composition of fluids from which the calcite veins precipitated was initially buffered by the bulk chalk there is no spatial variation in trace element composition of the calcite veinlets along the length of veins low sr concentrations in both calcite veinlets and secondary veins relative to those of the host chalk reflect a low partition coefficient of sr in calcite during vein formation normal faults in the cretaceous austin chalk were conduits to upwardly mobile vein forming fluids

variability of rock mechanical properties in the sequence stratigraphic context of the upper devonian new albany shale illinois basin abstract rock mechanical properties of tight shale reservoirs are important parameters in the exploration and development of unconventional shale oil and gas understanding the stratigraphic variability of rock mechanical properties in black shale successions is important for identifying promising intervals for hydraulic stimulation high resolution hardness tests 6 cm spacing with an equotip bambino 2 hardness tester were conducted on a new albany shale core to study the stratigraphic variability of rock mechanical properties combined with high resolution geochemical analyses 8 cm spacing acquired with a portable x ray fluorescence spectrometer it is possible to relate hardness variability to shale composition the results of our study suggest an average quartz content of about 28 for the entire succession approximately half of that amount 13 is interpreted to be of biogenic origin biogenic quartz mainly derived from dissolution of radiolaria occurs as authigenic quartz precipitated in tasmanites cysts and as microcrystalline quartz precipitated in the shale matrix petrographic observations indicate that the hardness of the new albany shale is critically controlled by biogenic quartz content not by total quartz content as suggested in multiple prior studies biogenic quartz forms an interconnected stiff framework which increases the hardness of shales in contrast dispersed hard grains e g detrital quartz feldspar and dolomite do not contribute to brittle behavior when examined under a scanning electron microscope hard shales are rich in biogenic quartz and contrast visibly with soft shales that are characterized by high contents of clay minerals negative correlation between hardness and al2o3 content as well as differential compaction of clay minerals around hard grains indicate that clay minerals are an important ductile component in the new albany shale although the general softness of kerogen suggests that there should be an influence of organic matter on hardness this effect is counteracted by biogenic quartz cementation which has a comparable stratigraphic distribution pattern we propose a new mineral composition based brittleness index on the basis of biogenic quartz content to characterize the brittleness of tight shale reservoirs within a sequence stratigraphic context the hardness of the new albany shale increases in transgressive systems tracts reaches a maximum at maximum flooding surfaces and decreases in highstand systems tracts with the exception of the blocher member maximum flooding surface intervals have the highest potential to develop brittle fractures and should be potential targets for hydraulic stimulation

distributed fracturing affecting isolated carbonate platforms the latemar platform natural laboratory dolomites north italy abstract isolated carbonate platforms are highly heterogeneous bodies and are typically composed of laterally juxtaposed first order domains with different sedimentological composition and organization i e a well stratified platform interior a massive margin and a slope with steeply dipping and poorly organized layers once subjected to overburden and tectonics forces directions and magnitudes of principal stresses inside such heterogeneous bodies are expected to deviate significantly from the regional trends potentially resulting in non obvious fracture patterns in addition characteristics such as fracture height and spacing distances are likely to change moving from one domain to the other as well as through time the middle triassic latemar platform dolomites n italy is affected by ubiquitous fractures typically joints orientation and height of more than 1500 fractures from 33 outcrops have been measured and their spatial characteristics were assessed with dedicated software fractures are organized in two directional sets nnw sse and ene wsw the orientation of each set is constant throughout the platform showing that the trajectories of causative stresses were not influenced by the platform architecture fracture spacings and heights of each set change moving from one domain to the other in the well bedded platform interior fractures are generally there are also significant differences in the characteristics of fractures of the two sets in all platform domains fractures of nnw sse set are less tall and more narrowly spaced than those of the ene wsw set in the platform interior fractures of the first set have a higher tendency to be confined to fracture units than those of the second set these differences are associated with diagenesis driven changes in bulk mechanical properties of the platform during its subsidence to maximum burial depth

fracture characterization in tight carbonates an example from the ozark plateau arkansas outcrop studies of fracture development used as analogs for subsurface fracture patterns are critical because of the importance of fractures as fluid flow pathways and the fact that most fracture networks exist at a smaller resolution than current seismic data can resolve fracture networks in carbonate units are typically controlled by the mechanical properties of the unit indicating that the mechanical stratigraphy as well as the fracture stratigraphy should be considered this study presents the results of a fracture analysis in the mississippian carbonates of the ozark plateau considering both mechanical and fracture network characteristics mechanical characteristics of the succession were defined using a combination of rebound values and thin section petrography fracture characteristics included orientation and intensity together with abutting relationships results indicate that fracture orientations show a distinct evolution throughout the measured succession including the appearance of early systematic sets followed by pervasive systematic fracture sets related to existing basement features fracture orientation changes do not correspond to changes in mechanical stratigraphy fracture intensity however is related to the thickness of the mechanical unit instead of the bed thickness and is greatest in less competent units mechanical control influences the fracture network on a smaller scale than that of regional tectonic stresses thus evaluations of carbonate reservoirs must account for both the large scale and the small scale investigations into fracture characteristic controls outcrop evaluations are of critical importance to properly assess characteristics that are challenging to recover from conventional subsurface data sets such as core and seismic reflection volumes