an experimental study of the relationship between joint spacing and layer thickness abstract two methods for measuring joint spacing are described and compared the area method is a constant for a given outcrop area and is not affected by joint distribution within that area in contrast the line method depends on the location of the linear traverse two kinds of joint sets are distinguished on bedding surfaces 1 a poorly developed set represents the early stages of development when typical joint lengths are less than typical spacing 2 a well developed set represents later stages when lengths are much greater than spacing the area method is applicable to both poorly and well developed sets whereas the line method produces inconsistent results for poorly developed joint sets surface textures on many joints indicate point fracture origins and propagation parallel to bedding whereas most numerical model studies of spacing assume linear origins and propagation perpendicular to bedding the laboratory experiments described in this paper do not suffer from these restrictions an important concept confirmed during these experiments is fracture saturation when the applied strain reaches a certain value fracture spacing stops evolving and remains nearly constant these fracture sets are well developed spacing at saturation is a function of layer thickness but is independent of strain whereas spacing before saturation varies strongly with applied strain thus plotting spacing vs thickness and comparing the slopes of lines fit to such data for poorly developed joint sets in different layers is unlikely to be a diagnostic test for differences in material properties on the other hand spacing may be a sensitive indicator of strain for layers with poorly developed joint sets assessing fracture saturation is a lirst order consideration when gathering spacing data

ultrasonic velocity and anisotropy of hydrocarbon source rocks an experimental study of the physical properties of black kerogen rich shales also including maturation analysis scanning electron microscope sem observations and physical modeling revealed fairly peculiar petrophysical parameters specifically these rocks have very low porosity and density but most importantly both p and s ultrasonic velocities normal to bedding are extremely low whereas they are much higher parallel to bedding giving rise to a strong anisotropy even at high confining pressures we found that these parameters primarily reflect kerogen content microstructure and maturation level of these rocks we found also that microcracks inferred from ultrasonic velocity measurements occur only in mature shales these microcracks are parallel to the bedding plane and further enhance strong intrinsic anisotropy notably at low effective pressure our results show that on a small scale kerogen rich shales are transversely isotropic rocks and can be effectively modeled using the thin laye

brittle ductile transition shear failure and leakage in shales and mudrocks abstract this paper presents an experimental study on brittle ductile transition shear behaviour the formation of shear fractures in shales and mudrocks and the hydraulic properties of shales and mudrocks with shear induced fractures the experimental study is based on triaxial tests on different mudrocks and shales from north sea reservoirs and adjacent areas sheared at different effective confining stresses it is shown that during burial mudrocks behave as normally consolidated materials and exhibit ductile response to increased load however several processes like uplift chemical diagenesis and overpressure build up turn mudrocks into overconsolidated materials which exhibit brittle behaviour during loading triaxial tests with different loading conditions have been performed to establish the brittle to ductile transition of various mudrocks based on the results of the tests it is shown that the brittle to ductile transition can be related to the overconsolidation ratio a relationship between normalized undrained shear strength and overconsolidation ratio is established in combination with this relationship a correlation between compressional wave velocity and apparent pre consolidation stress which accounts for both mechanical and chemical diagenesis may be used as a tool to evaluate possible leakage of hydrocarbon seals

mechanics of fluid driven fracture growth in naturally fractured reservoirs with simple network geometries 1 a numerical model has been developed for fluid driven opening mode fracture growth in a naturally fractured formation the rock formation contains discrete deformable fractures which are initially closed but conductive because of their preexisting apertures fluid flow that develops along fractures depends on fracture geometry defined by preexisting aperture distribution offsets along a fracture path and intersections of two or more fractures the model couples fluid flow elastic deformation and frictional sliding to obtain the solution which depends on the competition between fractures for permeability enhancement the fractures can be opened by fluid pressure that exceeds the normal stress acting on them and by interactions with intersecting closed fractures experiencing coulomb type frictional slip the newtonian fluid is assumed to flow through the conductive fractures according to a lubrication equation that relates the cube of an equivalent hydraulic aperture to fracture conductivity the rock material is assumed to be impermeable and elastic this paper provides the governing equations for the multiple fracture systems and the solution methods used flow distribution and fracture growth in conductive fracture sets are simulated for a range of geometric arrangements and hydraulic properties numerical results show that elastic interaction between fracture branches plays a controlling role in fluid migration although initial apertures can give rise to a preferential fluid flow direction during the early stage in the presence of offsets fracture segments subject to strong compression are difficult to open hydraulically and their resulting smaller permeability can increase overall upstream fracture pressure and opening the patterns of fluid flow become more complicated if fractures intersect each other a portion of injected fluid is lost into closed empty fractures that cut across the main hydraulic fracture and this delays the pressure increases required for fracture growth past the crosscutting fracture the nonlinear fluid loss rate depends on the geometric complexities of the fracture sets and on the fluid viscosity sometimes fracture growth can be accelerated by the fast fluid transport along an intersected relatively conductive natural fracture

stress dependent permeability of a de mineralised fracture in shale abstract the paper presents the results of laboratory direct shear tests on the hydro mechanical behaviour of an extensional fracture in shale the tests were conducted on fracture specimens obtained by mechanically splitting blocks of a naturally fractured shale along the cemented fracture and dissolving the calcite fracture filling with a strong acid the experiments consisted of radial fluid flow tests from a single fluid source along the fracture surface fracture samples were tested under different levels of effective normal stress and shear displacement the experimental results show significant reduction of fracture permeability during increasing contact normal stress across the fracture and after shearing of the fracture under constant high normal stress however the tested fractures never completely closed even under normal stresses close to or higher than the unconfined compressive strength of the intact shale the fracture permeability remained much higher than the intact shale or matrix permeability even when the fracture surface has undergone some gouge formation from local asperity failure during shearing the results indicate that fractures once created are difficult to close by mechanical loading in tight formations like shales fractures will always be conduits for fluid flow unless closed by cementation currently it is widely assumed that fractures in sedimentary basins stay open only when there is an overpressure which could keep the fractures hydraulically parted however the results presented here indicate that given a sufficient hydraulic gradient fluid can still flow along a fracture even in the absence of large overpressures

estimating average fracture spacing in subsurface rock knowledge of the spacing of fractures in reservoir rocks i e the distance between parallel fractures in a subsurface joint set can lead to a better understanding of the production characteristics of a reservoir and serve to quantify the relative degree of deformation in subsurface rocks in this paper i present a new method for estimating the spacing of subsurface fractures this new method is easy to use from the standpoint of both data collection and data analysis the average fracture spacing method can be applied with boreholes of any orientation relative to a fracture set the method is especially powerful when it is used for the relatively common case of a borehole nearly parallel to a fracture set e g vertical borehole intersecting vertical fractures average fracture spacing is estimated from an analytical solution based on observed borehole fracture intersections and observed fracture porosity the only data required are the dimensions of the core or imaged borehole and the total height of all sampled fractures because the likelihood of intersecting fractures increases when a well is deviated perpendicular to the fractures of a set fracture reservoirs commonly are candidates for deviated boreholes an informed decision on borehole deviation requires predicting the fracture intersection frequency as a function of both deviation magnitude and direction a new method based on probabilities of borehole fracture intersections uses spacing and height data from subsurface joint like fractures and the borehole diameter to predict fracture intersection frequencies for all possible well deviations fracture intersection frequency solutions are presented with respect to a conventional geographic reference frame thus simplifying even the most complex three dimentional situations

characteristics and origins of coal cleat a review abstract cleats are natural opening mode fractures in coal beds they account for most of the permeability and much of the porosity of coalbed gas reservoirs and can have a significant effect on the success of engineering procedures such as cavity stimulations because permeability and stimulation success are commonly limiting factors in gas well performance knowledge of cleat characteristics and origins is essential for successful exploration and production although the coal cleat literature spans at least 160 years mining issues have been the principal focus and quantitative data are almost exclusively limited to orientation and spacing information few data are available on apertures heights lengths connectivity and the relation of cleat formation to diagenesis characteristics that are critical to permeability moreover recent studies of cleat orientation patterns and fracture style suggest that new investigations of even these well studied parameters can yield insight into coal permeability more effective predictions of cleat patterns will come from advances in understanding cleat origins although cleat formation has been speculatively attributed to diagenetic and or tectonic processes a viable mechanical process for creating cleats has yet to be demonstrated progress in this area may come from recent developments in fracture mechanics and in coal geochemistry

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

basement faults and seismicity in the appalachian basin of new york state abstract landsat lineaments identified by earth satellite corporation earthsat 1997 can be groundtruthed across the appalachian basin of new york state nys both fracture intensification domains fids and faults are observed in outcrop along the lineaments confirmation of deep structure associated with the surface structure is provided by both well log analyses and seismic reflection data primarily proprietary additional faults are proposed by comparing the lineament locations with gravity and magnetic data the result is a web of basement faults that crisscross new york state by overlaying epicenter locations on the fault lineament maps it is possible to observe the spatial correlation between seismic events and the faults every seismic event in the appalachian basin portion of nys lies on or near a known or suspected fault it thus appears that not only are there more faults than previously suspected in nys but also many of these faults are seismically active

are open fractures necessarily aligned with maximum horizontal stress abstract fluid flow in fractured rock is an increasingly central issue in recovering water and hydrocarbon supplies and geothermal energy in predicting flow of pollutants underground in engineering structures and in understanding large scale crustal behaviour conventional wisdom assumes that fluids prefer to flow along fractures oriented parallel or nearly parallel to modern day maximum horizontal compressive stress or s hmax the reasoning is that these fractures have the lowest normal stresses across them and therefore provide the least resistance to flow for example this view governs how geophysicists design and interpret seismic experiments to probe fracture fluid pathways in the deep subsurface contrary to these widely held views here we use core stress measurement and fluid flow data to show that s hmax does not necessarily coincide with the direction of open natural fractures in the subsurface 3 km depth consequently in situ stress direction cannot be considered to predict or control the direction of maximum permeability in rock where effective stress is compressive and fractures are expected to be closed chemical alteration dictates location of open conduits either preserving or destroying fracture flow pathways no matter their orientation

anisotropic effective medium modeling of the elastic properties of shales shales are complex porous materials normally consisting of percolating and interpenetrating fluid and solid phases the solid phase is generally comprised of several mineral components and forms an intricate and anisotropic microstructure the shape orientation and connection of the two phases control the anisotropic elastic properties of the composite solid we develop a theoretical framework that allows us to predict the effective elastic properties of shales its usefulness is demonstrated with numerical modeling and by comparison with established ultrasonic laboratory experiments the theory is based on a combination of anisotropic formulations of the self consistent sca and differential effective medium dem approximations this combination guarantees that both the fluid and solid phases percolate at all porosities our modeling of the elastic properties of shales proceeds in four steps first we consider the case of an aligned biconnected clay fluid composite composed of ellipsoidal inclusion

stress anisotropy and velocity anisotropy in low porosity shale abstract shales are known for often marked intrinsic anisotropy of many of their properties including strength permeability and velocity for example in addition it is well known that anisotropic stress fields can also have a significant impact on anisotropy of velocity even in an isotropic medium this paper sets out to investigate the ultrasonic velocity response of well characterised low porosity shales from the officer basin in western australia to both isotropic and anisotropic stress fields and to evaluate the velocity response to the changing stress field during consolidated undrained multi stage triaxial tests on core plugs cut normal to bedding vpv increases monotonically with increasing effective stress and vs1 behaves similarly although with some scatter vph and vsh remain constant initially but then decrease within each stage of the multi stage test although velocity from stage to stage at any given differential stress increases this has the impact of decreasing both p wave e and s wave anisotropy through application of differential stress within each loading stage however increasing the magnitude of an isotropic stress field has little effect on the velocity anisotropies the intrinsic anisotropy of the shale remains reasonably high at the highest confining pressures the results indicate the magnitude and orientation of the stress anisotropy with respect to the shale microfabric has a significant impact on the velocity response to changing stress fields

eocene early miocene foreland basin development and the history of himalayan thrusting western and central nepal sedimentologic petrographic and u pb detrital zircon ages from middle eocene through early miocene sedimentary rocks in the lesser himalayan zone of western and central nepal indicate that a peripheral foreland basin system had developed in the eastern himalayan collision zone by middle eocene time the shallow marine eocene bhainskati formation accumulated in a back bulge depozone between a southward migrating forebulge and the indian craton migration of the forebulge through this region during eocene oligocene time produced a regional unconformity that spans 15 20 myr by early miocene time the forebulge unconformity was onlapped by the distal fringes of the southward migrating foredeep depozone represented by fluvial deposits of the dumri formation continued southward migration of the foredeep during the neogene accommodated the fluvial siwalik group light mineral provenance data and u pb detrital zircon ages suggest that the bhainskati was derived partly from tethyan sedimentary rocks of the tibetan himalayan zone during initial growth of the himalayan fold thrust belt the dumri was derived from metasedimentary and crystalline rocks of the greater himalayan zone during emplacement of the main central thrust and contemporaneous tectonic unroofing by normal faulting along the south tibetan detachment system the lesser himalayan crystalline thrust sheets were emplaced soon after deposition of the dumri formation 15 10 ma paleocurrent and lithofacies data from the dumri formation indicate deposition by west southwestward flowing rivers that drained into the indus portion of the himalayan foreland basin system during the early miocene thick channel sandstones in the lower dumri may represent the early miocene counterpart of the modern ganges river eastward diversion of the ganges drainage system to near its present location had occurred by 15 ma as the high standing aravalli range on the northern indian shield approached the front of the fold thrust belt assuming reasonable values for the flexural rigidity of indian lithosphere the time span of the forebulge unconformity yields a velocity of 14 33 mm yr for the southward migration of the fold thrust belt relative to india this range of values is consistent with neogene and present day estimates and suggests that only one third to one half of india eurasia convergence has been accommodated by shortening in the himalayan fold thrust belt since the onset of collision

the effect of anisotropy on the young s moduli and poisson s ratios of shales young s modulus and poisson s ratio are required for geomechanics applications such as hydraulic fracture design analysis of wellbore stability and rock failure determination of in situ stress and assessment of the response of reservoirs and surrounding rocks to changes in pore pressure and stress shales are usually anisotropic and models that neglect shale anisotropy may fail to describe geomechanical behaviour correctly anisotropy in shales results from a partial alignment of anisotropic clay particles kerogen inclusions microcracks low aspect ratio pores and layering for shales the young s modulus measured parallel to bedding e1 is usually greater than the young s modulus measured perpendicular to bedding e3 however the poisson s ratio 31 corresponding to stress applied perpendicular to bedding and strain measured parallel to bedding can be greater than equal to or less than the poisson s ratio 12 for stress applied parallel to bedding and strain measured parallel to bedding for transverse isotropy the elastic anisotropy resulting from a partial alignment of clay particles can be written in terms of the coefficients w200 and w400 which describe the impact of clay particle orientation on the anisotropy of shales disorder in the orientation of clay particles acts to reduce w400 faster than w200 since w400 is a higher order moment of the clay particle orientation distribution function than w200 this is confirmed by analysis of measured anisotropy parameters for shales a partial alignment of clay particles is consistent with the measured young s moduli for shales and with values of poisson s ratio 31 12 but not with values 31 12 these values can be explained if there exist kerogen inclusions microcracks or low aspect ratio pores aligned parallel to the bedding plane

fracture development in paleozoic shale of chongqing area south china part one fracture characteristics and comparative analysis of main controlling factors abstract natural macro and micro fractures in the longmaxi shale lower silurian from yuye 1 well and the niutitang shale lower cambrian from yuke 1 well are characterized regional geological structural evolution and shale geochemistry including toc and mineral composition etc are analyzed the controlling factors of natural fractures in palaeozoic shale in the southeast of chongqing have been confirmed which have been analyzed qualitatively or semi quantitatively the results show that fractures are abundant in both the longmaxi shale and niutitang shale due to the multiphase strong tectonic movement in the southeast of chongqing therefore tectonic factors are the principal factors that control fracture development in the paleozoic shale of this region followed by the mineral composition of the shale vertically the mineral composition of shale in yuke 1 well has a high dispersion degree and a strong heterogeneity the dispersion coefficients of quartz and dolomite were 0 48 and 1 respectively therefore the development and distribution of fractures are controlled by quartz and dolomite content in the niutitang shale moreover the quartz content shows positive correlation with the fractures quantity and fractures are most abundant where dolomite content up to 64 7 while the mineral composition of shale in yuye 1 well has a low dispersion degree and a strong homogeneity in a vertical position the dispersion coefficients of quartz and plagioclase were 0 15 and 0 26 respectively which shows unconspicuous control effect for fractures therefore the development and distribution of fractures are mainly controlled by the toc content moreover the toc content shows positive correlation with the fractures quantity fractures are sealed by calcite quartz and pyrite although sealed fractures do not contribute to the permeability of the reservoir they are important planes of weakness that tend to be reactivated by hydraulically induced fractures sem images show that intergranular pore is connected by microfractures thereby the validity of porosity has been improved the mixed network of microfractures and micropore provide enhanced permeability or storage capacity for the reservoir

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

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

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

the origin of tensile fracture lineaments abstract fracture controlled lineaments commonly seen where brittle basement is exposed at the earth s surface are generally restricted to a small number of sets with angles of 45 90 between sets the length frequency distribution of lineaments in each set follows a truncated poisson function such lineaments usually show almost no shearing offset suggesting a tensile origin a simple mechanical model of tensile fracturing is used to explain the spacing directions and length of lineaments as well as their depth frequency distribution results suggest that the penetration depth of tensile fractures which produce lineaments at the earth s surface is directly related to their length and that the fracture density is inversely proportional to fracture depth finally the angles between lineament sets may be controlled by the ratio of strength of unfractured rock to that of pre existing fractures which might heal with time the most likely source of tension is tectonic uplift fractures due to typical uplifts of 0 5 1 km over distances of 10 100 km may penetrate as brittle fractures to several kilometres into the crust perhaps to the depth at which seismic activity ceases

hydrocarbon generation induced microcracking of source rocks laboratory measurements of ultrasonic velocity and anisotropy in kerogen rich black shales of varying maturity suggest that extensive bedding parallel microcracks exist in situ in most mature source rocks undergoing the major stage of hydrocarbon generation and migration given the normal faulting regime with the vertical stress being the maximum principal stress typical of most sedimentary basins this microcrack alignment cannot be accounted for using simplified fracture mechanics concepts this subhorizontal microcrack alignment is consistent with 1 a model of local principal stress rotation and deviatoric stress reduction within an overpressured formation undergoing hydrocarbon generation and with 2 a strong mechanical strength anisotropy of kerogen rich shales caused by bedding parallel alignment of kerogen microlayers microcracks originate within kerogen or at kerogen illite interfaces when pore pressure exceeds the bedding normal total stress by only a few mpa due to the extremely low fractu

elastic anisotropy of shales accompanying the resource potential of gas shales is a new interest in understanding the physical and petrophysical properties of shales shale by geochemical or stratigraphic measures is arguably the most common lithology encountered in sedimentary basins despite this shales remain little studied while engineers and explorationists focused on conventional reservoirs geophysicists did this knowing full well that often a reflection coefficient from a reservoir was controlled by the shale properties of the cap rock we compensated for this ignorance by arguing that shales are deposited in deepwater environments in which lateral and vertical changes are slow and therefore inconsequential we further compounded this ignorance by assuming that the shales were isotropic an example of the consequences of this ignorance was clearly documented by margesson and sondergeld 1999 engineers share culpability for this ignorance too since most of the drilling problems occur in shales and most of the lithologies d

intermittent stability and frontogenesis in an area influenced by land runoff a frontal structure in liverpool bay was studied using conductivity temperature depth ctd and current meter data at times of weak vertical mixing by wind and tides a density driven gravity flow induced by river discharge distorts the thermohaline field so that an expanding stratified region develops bounded offshore by a surface front expansion of this frontal structure is increasingly curbed by rotation as a near geostrophic balance is attained owing to friction the space scale and time scale of frontogenesis are somewhat greater than two internal rossby radii and an inertial period respectively at times of increased mixing the frontal structure is destroyed and the enhanced friction between layers inhibits gravity flow hampering the frontogenesis mechanism large variability in mixing conditions results in an intermittent frontal structure

natural sealed fractures in mudrocks a case study tied to burial history from the barnett shale fort worth basin texas usa abstract the mississippian barnett shale texas usa consisting of organic rich shales and limestones hosts the largest gas fields of north america this study examines sealed fractures from core and outcrop samples of the barnett shale of the fort worth basin and aims to 1 characterize the phases occurring in the fractures from samples having experienced different burial histories 2 establish a paragenetic sequence to relate the timing of fracture origin and sealing with the burial history of the basin and 3 contribute to the understanding of the mechanisms of fracture formation in shales including overpressure origin four fracture generations were distinguished in the most deeply buried core samples by characterizing the sealing minerals petrographically and geochemically the generations were inserted into the framework of a reconstructed burial history for the fort worth basin which allowed a time sequence for fracture development to be established this in turn allowed inference of conditions of fracture development and consideration of fracture mechanisms as well as the origin of the parent fluids of sealing minerals type 1 fractures formed during early mechanical compaction at a few 10 s to 100 m of depth of still not fully cemented sediments type 2 fractures formed during moderate burial 2 km from slightly modified seawater their timing is consistent with overpressure generated during rapid deposition and differential compaction of pennsylvanian lithologies during the onset of the ouachita compressional event type 3 fractures formed during deep burial 3 km from silica rich basinal brines possibly derived from clay diagenesis type 4 fractures formed at very deep burial 4 km from hot and 18 o rich fluids carrying light oil 20 30 api and record the opening of the fluid system after hydrocarbon migration differences are highlighted between the timing and thermal regimes under which fractures formed in barnett lithologies from different areas of the basin this suggesting that extrapolation of outcrop observations to subsurface must be used with due care

water sill hypothesis for the origin of certain veins in the meguma group nova scotia canada certain bedding parallel quartz veins some gold bearing in the cambrian ordovician meguma group nova scotia are believed to indicate the former presence of water sills the veins contain primary subhedral quartz crystals which tend to be oriented normal to the vein walls and display brazil twins twinned crystals commonly show hexagonal sections with dark solid inclusion rich centers and white fluid inclusion rich rims viewed in a plane perpendicular to bedding the dark centers of crystals exhibit millimetre thick quartz plates bounded by bedding parallel micrometre thick inclusion bands of white micas the inclusion bands are aligned parallel to the vein walls and terminate within tapered crystals that have an outer zone of milky quartz the quartz veins are on limbs and hinges of folds particularly anticlines from these observations we conclude that 1 the solid inclusion banded quartz resulted from episodic hydraulic extension fracturing and incremental growth of dispersed quartz bridges across water sills 2 the milky quartz filled the vein by growing in optical continuity with the solid inclusion banded quartz 3 localization of certain bedding parallel quartz veins is due to ponding of water beneath aquacludes in very broad domes formed by differential compaction in the meguma group 4 during tectonic shortening these intensely veined rocks localized some of the major anticlines

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 simple analysis of oil induced fracturing in sedimentary rocks abstract geometric form of kerogen patches control oil induced fracturing in impermeable source rocks thin elongate kerogen flakes cause lateral fracturing kerogen has to occur in spherical or cylindrical forms to induce vertical fractures if length to width ratio of kerogen flakes is sufficiently large then lateral fractures are definitely initiated during oil generation in impermeable source rock oil and rock compressibilities and lateral to vertical stress ratio are decisive factors in vertical fracturing in impermeable rocks lateral to vertical stress ratio is generally higher than the maximum for oil induced vertical fracturing in tectonically relaxed sedimentary basins oil generation cannot initiate vertical fractures except when the lateral to vertical stress ratio is reduced below critical level either by tectonic tension or erosion of overburden when the source rock is permeable then the rate of oil seepage from kerogen into the surrounding rock is crucial in fracture initiation oil induced fractures cannot form unless permeability and hence rate of oil seepage from kerogen into the surrounding rock is non existent or negligible most source rocks have some permeability therefore oil induced fracture initiation should be a rare phenomenon

fracture network of the ferron sandstone member of the mancos shale east central utah usa abstract the fracture network at the outcrop of the ferron sandstone member of the mancos shale was studied to gain an understanding of the tectonic history of the region and to contribute data to studies of gas and water transmissivity related to the occurrence and production of coal bed methane about 1900 fracture readings were made at 40 coal outcrops and 62 sandstone outcrops in the area from willow springs wash in the south to farnham dome in the north of the study area in east central utah two sets of regional vertical to nearly vertical systematic face cleats were identified in ferron coals a northwest striking set trends at a mean azimuth of 321 and a northeast striking set has a mean azimuth of 55 cleats were observed in all coal outcrops examined and are closely spaced and commonly coated with thin films of iron oxide two sets of regional systematic joint sets in sandstone were also identified and have mean azimuths of 321 and 34 the joints of each set are planar long and extend vertically to nearly vertically through multiple beds the northeast striking set is more prevalent than the northwest striking set in some places joints of the northeast striking set occur in closely spaced clusters or joint zones flanked by unjointed rock both sets are mineralized with iron oxide and calcite and the northwest striking set is commonly tightly cemented which allowed the northeast striking set to propagate across it all cleats and joints of these sets are interpreted as opening mode mode i fractures abutting relations indicate that the northwest striking cleats and joints formed first and were later overprinted by the northeast striking cleats and joints burial curves constructed for the ferron indicate rapid initial burial after deposition the ferron reached a depth of 3000 ft 1000 m within 5 2 million years m y and this is considered a minimum depth and time for development of cleats and joints the sevier orogeny produced southeast directed compressional stress at this time and is thought to be the likely mechanism for the northwest striking systematic cleats and joints the onset of the laramide orogeny occurred at about 75 ma within 13 7 m y of burial and is thought to be the probable mechanism for development of the northeast striking systematic cleats and joints uplift of the ferron in the late tertiary contributed to development of butt cleats and secondary cross joints and probably enhanced previously formed fracture sets using a study of the younger blackhawk formation as an analogy the fracture pattern of the ferron in the subsurface is probably similar to that at the surface at least as far west as the paradise fault and joe s valley graben farther to the west on the wasatch plateau the orientations of ferron fractures may diverge from those measured at the outcrop

separation and imaging of seismic diffractions using migrated dip angle gathers abstractcommon reflection angle migration can produce migrated gathers either in the scattering angle domain or in the dip angle domain the latter reveals a clear distinction between reflection and diffraction events we derived analytical expressions for events in the dip angle domain and found that the shape difference can be used for reflection diffraction separation we defined reflection and diffraction models in the radon space the radon transform allowed us to isolate diffractions from reflections and noise the separation procedure can be performed after either time migration or depth migration synthetic and real data examples confirmed the validity of this technique